TECHNOLOGICAL FIELD

The present application is generally in the field of quantum key distribution (QKD) applications, and particularly relates to the calibration and/or testing of QKD transmitters.

BACKGROUND

This section intends to provide background information concerning the present application, which is not necessarily prior art.

In order to enable mass deployment of QKD systems and mass manufacturing (e.g., factory level automation), the QKD system must be robust enough so that on power-up the system would automatically optimize its parameters and start performing the QKD sequence. This system initialization stage needs to be done in an environment which is not as controlled and sterile as a lab environment, under a wide range of possible temperature levels, after shipping and storing in unknown conditions, and possibly after repairing and/or component changing e.g., replacement of system components, such as the transmitter and/or of the receiver units, and/or pairing to a new/unfamiliar receiver. Therefore, the conventional factory calibration of a QKD receiver-transmitter pair of units is typically insufficient and may be prone to instabilities and malfunctions after deployment and/or under unexpected working conditions.

Since QKD systems have a high signal bandwidth, often higher than 10GHz, analyzing the signal quality without lab equipment and without a receiver becomes a challenging task. Particularly, in case of QKD systems, in order to prevent side channel attacks, the system calibration should not rely on feedbacks from any device or person located outside the transmitter (or receiver) system, as it exposes the system to attacks. Rather, it is necessary that the system initialization stages be self-contained.

Wang et al ("Integrated electronics in 130 nm CMOS process for quantum key distribution sender device", Rev. Sci. Instrum. 91,034701 (2020)) presents an integrated electronic system in the 130 nm complementary metal oxide semiconductor process for the QKD sender device. The electronics provide driving signals for the optics at the sender terminal of the quantum channel in QKD and consist mainly of three key modules, namely, a laser diode driver with a high slew rate, a high-speed physical random number generator, and a pre-driver for the electro-optic modulator. The electronic system is designed to operate at frequencies as high as 625-MHz to accommodate the frequency of the QKD system. The high degree of integration is advantageous for miniaturizing QKD sender devices.

GENERAL DESCRIPTION

The calibration/testing of QKD transmitter systems after deployment, and/or in between operations, thereof, should be carried out with minimal, or no, external intervention, to eliminate tampering and/or eavesdropping attempts. The present application provides QKD transmitter configurations capable of self-calibration/testing with minimal, or altogether without requiring, receiving feedback indications from external unit(s) e.g., from a QKD receiver or an oscilloscope. For this purpose, QKD transmitter configurations hereof are configured to generate a light-modulating signal by combining a plurality of selectively obtained and/or adjusted electric/EM pulse signals, measure optical signals generated by light modulation utilizing the light-modulating signal and generating measurement data/signals indicative thereof, and adjust one or more of the electric/EM pulse signals based on the measurement data to accordingly adjust the light modulation signal to obtain desired quantum communication states by the light modulation.

Accordingly, in embodiments hereof the QKD transmitter comprises a modulator driving portion having a plurality of electric/EM pulse signal sources, at least one of which configured to generate electric/EM pulse signals being a substantial inversion (e.g., having an opposite amplitude direction i.e., opposite voltage polarity) of electric/EM pulse signals generated by at least another one of the plurality of signal pulse sources, and a plurality of analog combiners configured to generate the light-modulating signal that is substantially proportional to a summation/superposition of the pulse signals generated by the plurality of electric/EM pulse signal sources. The light-modulating signal is used for modulating quantum communication states in coherent (e.g., laser) light, and the QKD transmitter is configured to measure the modulated quantum communication states and generate measurement data indicative thereof, and carry out from time-to-time (or periodically) self-testing/calibration procedures to adjust the electric/EM pulse signals from one or more of the electric/EM pulse signal sources based on the generated measurement data.

One or more of the electric/EM pulse signal sources can be coupled to signal manipulation means configured to controllably adjust at least one of the amplitude and time delay of the pulse signals thereby generated, before it is input to the analog combiner. A control unit is used to process the measurement data and determine based thereon a gain value and/or time delay value for one or more of the signal manipulation means for adjusting the light- modulating signal to produce the required quantum communication states. The control unit can be configured to scan for the signal manipulation means, of the one or more of the electric/EM pulse signal sources, permissible gain test values and/or time delay test values, generate control signals for producing optical signals utilizing the light-modulating signal as adjusted with the scanned test values, process respective measurement data obtained for the adjusted lightmodulating signal, and determine based thereon optimal gain and/or time delay values for one or more of the signal manipulation means.

Optionally, but in some embodiments preferably, the electrical pulse signal output from at least one of the electric/EM pulse signal sources is passed through a tuneable gain unit configured to controllably adjust the amplitude of the electric/EM pulse signals generated by the electric/EM pulse signal source. Optionally, but in some embodiments preferably, the pulse signal output from at least one of the electric/EM pulse signal sources is passed through a tuneable time delay unit configured to controllably apply a time delay to the electric/EM pulse signals generated by the electric/EM pulse signal source. The output of each one of the electric/EM pulse signal sources may be coupled to a respective pair of serially coupled gain and time delay units. The present disclosure exemplifies passing the electric/EM pulse signals through a gain unit and thereafter through the time delay unit, but the order of these units may be changed for one or more of the electric/EM pulse signal sources, if so required.

In some possible embodiments the QKD transmitter comprises 3 (three) electric/EM pulse signal sources and 2 (two) analog combiners configured to generate the light modulating signal from the adjusted or non-adjusted electric/EM pulse signals. A first analog combiner can be configured to superposition/summate the adjusted (or non-adjusted) output of two of the electric/EM pulse signal sources having opposite voltage polarities. A second analog combiner can be configured to superposition/summate the output from the first analog combiner with the adjusted (or non-adjusted) output from the third electric/EM pulse signal source for producing the light modulating signal. The light modulating signal produced by the second analog combiner is passed in some embodiments through a tuneable output gain unit configured to controllably adjust the amplitude of the light-modulating signal driving the electrooptical modulator.

In other possible embodiments the QKD transmitter comprises 4 (four) electric/EM pulse signal sources and three analog combiners configured to generate the light modulating signal from the adjusted or non-adjusted electric/EM pulse signals. A first analog combiner can be configured to superposition/summate the adjusted (or non-adjusted) output of two of the electric/EM pulse signal sources having opposite voltage polarities. A second analog combiner can be configured to superposition/summate the output from the first analog combiner with the adjusted (or non-adjusted) output from a third electric/EM pulse signal source. The third analog combiner can be configured to superposition/summate the output from the second analog combiner with the adjusted (or non-adjusted) output from the fourth electric/EM pulse signal source, for producing the light modulating signal. The light modulating signal produced by the third analog combiner is passed in some embodiments through a tuneable gain unit configured to controllably adjust the amplitude of the light-modulating signal driving the electrooptical modulator.

The exact order of the superposition/summation of the electric/EM pulse signals is not critical, and it can be changed per application specific design considerations/requirements. The present disclosure provides various calibration and/or testing procedures for setting various parameters of the QKD transmitter. Since at least some of the parameter determined by the calibration and/or testing procedures depends on the particular order of the units/components of the QKD transmitter, the same order used for carrying out the calibration and/or testing procedures should be thereafter maintained.

The calibration procedures disclosed herein can be performed once e.g., during factory calibration, on installation, on power up, during software (SW), firmware (FW), and/or FPGA code update, and/or when changing the operating frequency (e.g., upon replacement of the receiver unit), or intermittently or periodically/routinely during system operation in between QKD sequences. In some embodiments the quantum communication states are used as calibration patterns during the system operation.

According to one aspect there is provided a QKD transmitter system comprising: a plurality of pulse signal sources, at least one of which configured to generate pulse signals which voltage polarity is opposite to a voltage polarity of amplitudes' of pulse signals of at least another one of the plurality of signal pulse sources; a plurality of signal manipulating units, each configured to controllably adjust at least one of amplitude and delay time of at least some of the pulse signals generated by the plurality of pulse signal sources; and a plurality of analog combiner units configured to combine the adjusted and non-adjusted pulse signals and generate therefrom a light-modulating signal for modulating quantum communication states in light generated by a light source. The light-modulating signal can be substantially proportional to a summation of the adjusted and non-adjusted pulse signals.

The QKD transmitter can comprise an output tuneable gain unit configured to controllably adjust the light-modulating signal before modulating the quantum communication states. Optionally, but in some embodiments preferably, the QKD transmitter comprises three pulse signal sources, respective three signal manipulating units, and two analog combiner units. In some embodiments the QKD transmitter is configured such that the pulse signals generated by the pulse signal source configured to generate the pulse signals with the opposite voltage polarity are not combined with pulse signals from the other pulse signal sources for generation of at least one main quantum states of the quantum communication states. The first analog combiner (Si) of the two analog combiners can be configured to combine pulse signals generated by second and third pulse signal sources (Pi and P2) of the three pulse signal sources having the opposite voltage polarities, and a second analog combiner (So) of the two analog combiners is configured to combine signals produced by the first analog combiner (Si) with pulse signals generated by a first pulse signal source (Po) of the three pulse signal sources. Alternatively, the QKD transmitter is configured to combine the first, second and third, pulse signal sources (Po, Pi and P2) by a single analog combiner.

The QKD transmitter is configured in some embodiments to cause the respective signal manipulating unit of the first pulse signal source (Po) and the output tuneable gain unit (Gm) to adjust the amplitudes of signals thereby received such that power/intensity I of the modulated light responsive to pulse signals generated by the first pulse signal source (Po) is attenuated by a predefined upper-level intensity setting factor a to about a-I, wherein 0.95<α<1. The QKD transmitter can be further configured to cause the respective signal manipulating unit of the second pulse signal source (Pi) to adjust the amplitudes of signals thereby received such that power/intensity of the modulated light I responsive to pulse signals generated by the first pulse signal source (Po) is attenuated by a predefined mid-intensity level setting factor > to about ?•!, wherein P<a.

In some embodiments the QKD transmitter is configured to cause the respective signal manipulating unit of the first pulse signal source (Po) to adjust the amplitude of signals thereby received such that power/intensity of the modulated light I responsive to pulse signals generated by the first and second pulse signal sources (Po and Pi) is attenuated by an upper- mid-level intensity setting factor y to an upper-mid-level power/intensity 1^, wherein f><v<a. The QKD transmitter can be configured to cause the respective signal manipulating unit of the third pulse signal source (P2) to adjust the amplitude of signals thereby received such that power/intensity of the modulated light I responsive to pulse signals generated by the first, second and third, pulse signal sources (Po, Pi and P2) is attenuated to the upper-mid-level power/intensity Ip.

The QKD transmitter can be configured to cause the respective signal manipulating unit of the third pulse signal source (P2) to adjust the amplitude of signals thereby received such that power/intensity of the modulated light / responsive to pulse signals generated by the second and third pulse signal sources (Pi and P2) is attenuated to the about half of the upper- mid-level power/intensity Ip. The QKD transmitter can be configured to cause the respective signal manipulating unit of the first and/or second pulse signal sources (Po and/or Pi) to adjust the time delay of signals thereby received such that power/intensity of the modulated light / responsive to pulse signals generated by the first and second pulse signal sources is minimized. The QKD transmitter is configured in some embodiments to cause the respective signal manipulating unit of the third pulse signal source (P2) to adjust the time delay of signals thereby received such that power/intensity of the modulated light / responsive to pulse signals generated by the second and third pulse signal sources is minimized.

The QKD transmitter comprises in some embodiments two pairs of the pulse signal sources, respective four signal manipulating units each coupled to one of the pulse signals sources, and three analog combiner units, wherein the pulse signal sources of each pair of the pulse signal sources are configured to generate pulse signals of opposite voltage polarities. One pair of the pulse signals sources can be configured to generate main quantum states of the quantum communication states, and the other pair of the pulse signals sources can be configured to generate decoy states of the quantum communication states.

In possible embodiments, a first analog combiner (Σ ₂ ) of the three analog combiners is configured to combine pulse signals generated by fourth and third pulse signal sources (P3 and P2) forming one of the pairs of the pulse signal sources, a second analog combiner (Si) of the three analog combiners can be configured to combine signals produced by the first analog combiner (Σ ₂ ) with pulse signals generated by a second pulse signal source (Pi) of the four pulse signal sources, a third analog combiner (So) of the three analog combiners can be configured to combine signals produced by the second analog combiner (Si) with pulse signals generated by a first pulse signal source (Po) of the four pulse signal sources, the first and second pulse signal sources (Po and Pi) are forming the other pair of pulse signal sources. Alternatively, the QKD transmitter is configured to combine the first, second, third and fourth, pulse signal sources (Po, Pi, P2 and P3) by a single analog combiner.

The QKD transmitter is configured in some embodiments to cause the respective signal manipulating unit of the first pulse signal source (Po) and the output tuneable gain unit (G _m ) to adjust amplitudes of signals thereby received such that power/intensity / responsive to pulse signals generated by the first pulse signal source (Po) of the modulated light is attenuated by a predefined upper-level intensity setting factor a to an upper-level power/intensity I _P attemi=a-I, wherein 0.95<α<1. The QKD transmitter can be configured to cause the respective signal manipulating unit of the second pulse signal source (Pi) to adjust amplitudes of signals thereby received such that power/intensity of the modulated light I responsive to pulse signals generated by the second pulse signal source (Pl) is attenuated to a mid-intensity level of about , wherein is predefined mid-intensity level setting factor.

In possible embodiments the QKD transmitter is configured to cause the respective signal manipulating unit of the second pulse signal source (Pi) to adjust amplitudes of signals thereby received such that power/intensity of the modulated light I responsive to pulse signals generated by the first and second pulse signal sources (Po and Pi) is attenuated to a fine-tuned- intensity level of about The QKD transmitter can be configured to cause the respective signal manipulating unit of the third pulse signal source (P2) to adjust amplitudes of signals thereby received such that power/intensity of the modulated light I responsive to pulse signals generated by the third pulse signal source (P2) is attenuated to about wherein μ and μd are predefined uniform average number of photons for main and decoy quantum states respectively.

The QKD transmitter is configured in some embodiments to cause the respective signal manipulating unit of the fourth pulse signal source (P3) to adjust amplitudes of signals thereby received such that power/intensity of the modulated light I responsive to pulse signals generated by the fourth pulse signal source (P3) is attenuated to a mid-decoy level wherein is predefined mid-intensity level setting factor. The QKD transmitter can be configured to cause the respective signal manipulating unit of the fourth pulse signal source (P3) to adjust amplitudes of signals thereby received such that power/intensity of the modulated light I responsive to pulse signals generated the third and fourth pulse signal source (P2 and P3) is attenuated to the mid-decoy level

The QKD transmitter is configured in some embodiments to cause the respective signal manipulating unit of the first and/or second pulse signal sources (Po and/or Pi) to adjust time delay of signals thereby received such that power/intensity of the modulated light I responsive to pulse signals generated by the first and second pulse signal sources is minimized. The QKD transmitter can be configured to cause the respective signal manipulating unit of the third pulse signal sources (P2) to adjust time delay of signals thereby received such that power/intensity of the modulated light I responsive to pulse signals generated by the second and third pulse signal sources (Pi and P2) is minimized. The QKD transmitter can be configured to cause the respective signal manipulating unit of the fourth pulse signal sources (P3) to adjust time delay of signals thereby received such that power/intensity of the modulated light I responsive to pulse signals generated by the third and fourth pulse signal sources (P2 and P3) is minimized. In possible embodiments the quantum communication states comprise four main quantum states, four decoy states, and one vacuum state. In other possible embodiments the quantum communication states comprise four main quantum states, a different of quantum states (e.g., any number of decoy states) as may be required by other QKD protocols, and one vacuum state. The QKD transmitter comprises in some embodiments a power meter (e.g., comprising a photodiode) configured to measure optical output power/intensity of the modulated light signals and generate measurement data/signals indicative thereof, and a control unit configured and operable to adjust at least one of the plurality of signal manipulating units and/or the output tuneable gain unit based on the measurement data/signals.

The system can be configured to receive feedback data/signals indicative of interference (destructive or constructive) visibility obtained in response to transmitted optical signals, and the control unit can be configured adjust at least one of a frequency of a clock unit thereof and pulse time difference of the pulse signals based on the feedback data/signals.

In another aspect there is provided a QKD transmitter's calibration method comprising: generating a plurality pf pulse signals, voltage polarity of at least one of the plurality of pulse signals is opposite to a voltage polarity of at least another one of the plurality of pulse signals; combining the plurality of pulse signals and generating therefrom a light-modulating signal; modulating light signal by the light-modulating signal; measuring the modulated light signals and generating measurement data indicative thereof; and adjusting at least one of amplitude and delay time of at least some of the plurality of pulse signals to obtain/communicate a desired quantum communication state. The method comprises in some embodiments combining first, second and third, pulse signals to generate the light-modulating signal, wherein the second pulse signal (Pi) having the opposite voltage polarity.

The method can comprise generating at least one main quantum state by the pulse signal source configured to generate the pulse signals with the opposite voltage polarity without combining it with the other pulse signals. The method can comprise adjusting amplitudes of the first pulse signal and of the light-modulating signal such that power/intensity I of the modulated light signals responsive to the first pulse signal is attenuated by a predefined upper- level intensity setting factor a to about a-I, wherein 0.95<α<1.

The method comprises in some embodiments adjusting amplitude of the second pulse signal (Pi) such that power/intensity of the modulated light signals I responsive to the first pulse signal (Po) is attenuated by a predefined mid-intensity level setting factor f> to about β.I, wherein β<α. The methos can comprise adjusting the first pulse signal (Po) such that power/intensity of the modulated light signals I responsive to the first and second pulse signals (Po and Pi) is attenuated by an upper-mid-level intensity setting factor y to an upper-mid-level power/intensity 1^, wherein The methos can further comprise adjusting amplitude of the third pulse signal (P2) such that power/intensity of the modulated light signals I responsive to the first, second and third, pulse signals (Po, Pi and P2) is attenuated to the upper-mid-level power/intensity The method may also comprise adjusting amplitudes of the third pulse signal (P2) such that power/intensity of the modulated light signals I responsive to the second and third pulse signals (Pi and P2) is attenuated to about half of the upper-mid-level power/intensity Ip.

In some embodiments the method comprises adjusting time delay of the first and/or second pulse signals (Po and/or Pi) such that power/intensity of the modulated light signals I responsive to the first and second pulse signals is minimized. The method can comprise adjusting time delay of the third pulse signal source (P2) such that power/intensity of the modulated light signals I responsive to the second and third pulse signals is minimized.

The method comprises in some embodiments generating the light modulating signal by combining two pairs of the pulse signals, wherein each of the pairs pulse signals are of opposite voltage polarities. The method can comprise generating main quantum states from one pair of the pulse signals, and generating decoy states from the other pair pulse signals. The method can further comprise adjusting amplitudes of the first pulse signal (Po) and of the lightmodulating of signals such that power/intensity of the modulated light signals I responsive to the first pulse signal (Po) is attenuated by a predefined upper-level intensity setting factor a to an upper-level power/intensity wherein 0.95<α<1.

The method may comprise adjusting amplitude of the second pulse signal (Pi) such that power/intensity of the modulated light signal I responsive to the second pulse signal (Pi) is attenuated to a mid-intensity level wherein β<α is predefined midintensity level setting factor. Optionally, the method comprises adjusting amplitude of the second pulse signal (Pi) such that power/intensity of the modulated light signals I responsive to the first and second pulse signal (Po and Pi) is attenuated to a fine-tuned-intensity level

The method comprises in some embodiments adjusting amplitude of the third pulse signal (P2) such that power/intensity of the modulated light signals I responsive to the third pulse signal (P2) is attenuated to about wherein μ and μd are predefined uniform average number of photons for main and decoy quantum states respectively. The method can comprise adjusting amplitude of the fourth pulse signal (P3) such that power/intensity of the modulated light signals / responsive to the fourth pulse signal (P3) is attenuated to a mid-decoy level is predefined mid-intensity level setting factor. Optionally, the method comprises adjusting amplitude of the fourth pulse signal (P3) such that power/intensity of the modulated light signal I responsive to the third and fourth pulse signal (P2 and P3) is attenuated to the mid-decoy level

The comprises in some embodiments adjusting a time delay of the first and/or second pulse signal (Po and/or Pi) such that power/intensity of the modulated light signals / responsive to the first and second pulse signal is minimized. The method can comprise adjusting time delay of the third pulse signal (P2) such that power/intensity of the modulated light signals / responsive to the second and third pulse signal (Pi and P2) is minimized. The method can further comprise adjusting a time delay of the fourth pulse signal (P3) such that power/intensity of the modulated light signals / responsive to the third and fourth pulse signal (P2 and P3) is minimized.

The method may comprise receiving feedback data/signals indicative of interference obtained in response to transmitted optical signals, and adjusting at least one of a frequency of a clock unit of the transmitter and pulse time difference of the pulse signals based on the feedback data/signals.

It is noted that calibration techniques disclosed herein advantageously aligns/synchronise pulse signals from multiple pulse source using relatively slow photodiode power meter device to align the pulse signals in substantially high (picoseconds) resolution, and combine them into the light-modulating signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings. Features shown in the drawings are meant to be illustrative of only some embodiments of the invention, unless otherwise implicitly indicated. In the drawings like reference numerals are used to indicate corresponding parts, and in which:

Figs. 1A to 1C are block diagrams schematically illustrating a QKD transmitter system according to some possible embodiments, wherein Fig. 1A depicts a QKD transmitter utilizing three pulse signals sources, Fig. IB depicts a QKD transmitter utilizing four pulse signals sources, and Fig. 1C shows a possible implementation of a signal manipulation unit;

Fig. 2 graphically illustrates optical signals of the 4 (four) main quantum communication states and the 5 (five) decoy states according to some possible embodiments; Fig. 3 graphically illustrates calibration/testing pulse signals according to some possible embodiments;

Fig. 4 is a block diagram schematically illustrating a QKD communication system according to some possible embodiments; and

Figs. 5A and 5B are block diagrams schematically illustrating calibration sequences usable in some embodiments for calibration of the transmitter systems of Figs. 1A and IB respective.

DETAILED DESCRIPTION OF EMBODIMENTS

One or more specific and/or alternative embodiments of the present disclosure will be described below with reference to the drawings, which are to be considered in all aspects as illustrative only and not restrictive in any manner. It shall be apparent to one skilled in the art that these embodiments may be practiced without such specific details. In an effort to provide a concise description of these embodiments, not all features or details of an actual implementation are described at length in the specification. Emphasis instead being placed upon clearly illustrating the principles of the invention such that persons skilled in the art will be able to make and use the configurations hereof, once they understand the principles of the subject matter disclosed herein. This invention may be provided in other specific forms and embodiments without departing from the essential characteristics described herein.

The QKD transmitter systems of the present application are configured to generate light-modulating signals by combining a plurality of electric/EM pulse signals passed through signal manipulation units configured to adjust at least one of amplitude and time delay of at least some of the electric/EM pulse signals. Optionally, at least the amplitude of the light modulating signal is adjusted before it is input to an electrooptic modulator of the QKD transmitter for encoding the quantum communication states to light signal received from a coherent (e.g., laser) light source. System calibration and/or testing procedures can be carried out from time to time, or periodically /routinely, in between QKD sequences performed during the system operation, by scanning for at least some of the electric/EM pulse signal sources test values for at least one of gain and time delay, measuring the optical signals generated by the electrooptical modulator responsive to the scanned test values, and determining based thereon optimal gain and/or time delay for the electric/EM pulse signals of at least some of the electric/EM pulse signal sources.

For an overview of several example features, process stages, and principles of the invention, the transmitter examples illustrated schematically and diagrammatically in the figures are intended for QKD communication. These transmitter systems are shown as one example implementation that demonstrates a number of features, processes, and principles used to provide self-calibratable QKD transmitter implementations, but they are also useful for other applications and can be made in different variations. Therefore, this description will proceed with reference to the shown examples, but with the understanding that the invention recited in the claims below can also be implemented in myriad other ways, once the principles are understood from the descriptions, explanations, and drawings herein. All such variations, as well as any other modifications apparent to one of ordinary skill in the art and useful in QKD communication applications may be suitably employed, and are intended to fall within the scope of this disclosure.

Fig. 1A schematically illustrates a QKD transmitter system 10 according to some possible embodiments of the present application. The QKD transmitter system 10 comprises a modulator driving portion lOw, wherein electric/electromagnetic (EM e.g., radiofrequency - RF) signals generated by three or more pulse signal sources Po, Pi, P2,... (e.g., electric/EM signal sources such as produced by FPGA's Serializer/Deserializer (SerDes) or digital-to- analog converters, or other pulse generators) propagate over electromagnetic signal guiding elements (designated by double-lined arrowed lines e.g., RF/microwave waveguides), and an optical light modulating portion lOp wherein optical signals generated by a coherent (e.g., laser) light source 12 propagate over optical signal guiding elements (designated by dashed arrowed lines e.g., optical fibers).

In this non-limiting example the QKD transmitter system 10 comprises in the modulator driving portion lOw three pulse signal sources Po, Pi and P2, and two analog signal combiners Eo and Ei configured to produce desired combinations of electric pulse signals generated by the pulse signal sources Po, Pi and P2. Particularly, each one of the combiners Eo and Ei (e.g., implemented by dedicated IC and/or electrical resistors circuitry) is configured to combine (superposition) the two electromagnetic signals received at its input terminals into a single electromagnetic output signal that is substantially proportional to summation of the electromagnetic signals inputted thereto. In some embodiments each one of the analog combiners Eo and Ei has a 6dB (i.e., half voltage amplitude attenuation) insertion loss.

For example, in some embodiments the pulse signal sources Po, Pi and P2, and the analog combiners Eo and Ei, are configured as follows: • Pulse signal source Po: configured to generate positive electric/EM pulse signals that passes through the analog combiner Eo, which output towards the EOM 13 thus includes ½ (half) of the initial voltage level received from the pulse signal source Po;

• Pulse signal source Pi: configured to generate negative electric/EM pulse signals that passes through the two analog combiners, Eo and Ei, which output towards the EOM 13 thus includes *4 (quarter) of the initial voltage level received from the pulse signal source Pi; and

• Pulse signal source P2: configured to generate positive electric/EM pulse signals that passes through the two analog combiners, Eo and Ei, which output towards the EOM 13 thus includes *4 (quarter) of the initial voltage level received from the pulse signal source P2.

In some embodiments the electric/EM pulse signals generated by one or more of the pulse signal sources Pz (where 0<z<2 is an integer number) is passed through a respective tuneable gain unit Gz configured to controllably adjust the amplitude of the electric/EM pulse signals generated by the pulse signal sources Pz, before it reaches the analog combiner(s). The combined signal generated by the last analog combiner Eo in the modulator driving portion lOw is used as a light-modulating signal for modulation of optical signals by the electrooptic modulator (EOM, or modulators in some implementations) 13 provided in the light modulating portion lOp of the transmitter system 10.

The light-modulating signal generated by the last combiner Eo is controllably adjusted in some embodiments by a tuneable output gain unit (e.g., amplifier, or amplifier chain, having a tuneable total gain) Gm of the modulator driving portion lOw, before it is input into the EOM 13. Optionally, the controllable gain unit Gm is an integral part of the EOM 13, and in such possible embodiments it can be omitted from the transmitter system 10. In possible embodiments the gain of the tuneable gain units Gz is high enough to render the output gain unit Gm redundant and discardable.

Optionally, but in some embodiments preferably, the electric/EM signals produced by one or more of the tuneable gain units Gz is passed through a respective tuneable delay unit Dz configured to controllably apply a desired time delay to the electric pulse signal thereby received, before it reaches the analog combiner(s). The tuneable gain Gz and delay Dz units of each pulse signal source Pz are also referred to herein as a signal manipulation unit, which may be implemented as a single integrated device/unit, or inside the respective pulse signal source Pz. Optionally, but in some embodiments preferably, functionalities of this signal manipulation sequence are distributed over two or more separate devices/units (e.g., microchips), as exemplified in Fig. 1C.

In the specific and non-limiting example of Fig. 1C, the pulse signal units P _; are implemented in a first device/unit (e.g., FPGA microchip), the tuneable gain units Gj are implemented in a second device/unit (e.g., application- specific integrated circuit - ASIC/IC microchip) which may be (e.g., 30-50 times) faster, or slower, than the first device/unit. Optionally, but in some embodiments preferably, functionalities of one or more of the pulse signal Pj, tuneable delay D _i , and/or gain Gj units, is distributed between the first and second device/units (e.g., FPGA and IC) in accordance with their performance requirements.

It is noted that the embodiments disclosed herein can be advantageously exploited for other applications requiring fast analog signal generation, and/or for fast generation of accurate analog signals. Due to the ability to use standard digital SerDeses to generate positive and negative analog signals levels at the same or similar rate.

For example, in possible embodiments, each tuneable delay unit D _i can be configured to apply a “coarse” delay part Di ^C e.g., having a defined (logical) 1 (one) bit resolution, and a “fine” delay part Di ^F e.g., having a defined (physical) sub-bit resolution. The “fine” (sub-bit) delay part Di ^F is achieved in some embodiments utilizing a dedicated integrated circuit (IC e.g., allowing 1 to 5, optionally about 3, picoseconds increments) with different conduction paths e.g., formed on a printed circuit board (PCB), by controllably adjusting conducting path/wire lengths, and/or by controllably adjusting the electronic paths, and the “coarse” delay part Di ^C is implemented by logical delay units implemented in a field-programmable gate array (FPGA e.g., allowing 80 to 120, optionally about 100, picoseconds increments) chip.

It is noted that the delay units Do and Di can have the same role in possible calibration processes of the system 10, and thus, in such embodiments, one of them can be omitted. However, in the embodiments exemplified in Fig. 1A both delay units Do and Di are used in the QKD transmitter system 10 in order to keep the electric/EM signal shape identical in all of the paths of the modulator driving portion lOw. In some embodiments one or more of the tuneable delay units D _i is implemented by a digital regenerative IC, and can act as an amplifier and/or inverter, if so required.

The light modulating portion lOp of the system 10 comprises a coherent (e.g., laser) light source 12 powered by the electrical current source 11, the EOM 13 that modulates the desired quantum communication states in the light signal generated by the light source 12, and a variable optical attenuator (VOA) unit 14 configured to controllably set a desired attenuation factor to the modulated optical pulse signals produced by the EOM 13. As seen, the power/intensity of the modulated optical pulse signals from the EOM 13, or after passage through the VOA 14, or some portion thereof, can be further attenuated by a fixed optical attenuation unit 17 e.g., to the single-photon power level.

An optical splitter (or a signal tap) 15 is used in some embodiments to branch some predefined portion (e.g., half power signal) of the modulated optical pulse signals e.g., from the EOM 13 or after passage through the VOA 14, into a power meter unit 16. The power meter unit 16 can be implemented by a photodiode configured to measure the power/intensity of the optical signal thereby received. Unless specifically specified, for most stages of the initialization procedures disclosed herein, there is no need to calibrate the power meter unit 16 in the embodiments wherein it is implemented with a photodiode. In some of the disclosed embodiments there is also no need to calibrate the VOA 14.

Optionally, the electric current generated by the current source 11 is passed through a tuneable delay unit DLD configured to controllably delay the electric driving current of the light source 12, as required in some embodiments for direct modulation of the generated light signal relative to all other pulse signal sources in the system e.g., for ON/OFF periodic switching the light source in order to adapt the light signal timing such that the signal modulation is applied by the EOM 13 during the “ON” time intervals correctly.

A control unit 18 having one or more processors 18p and memories 18m is used in possible embodiments to operate the different units of the system 10 and set one or more parameters of its different components/units. The control unit 18 can be configured to process the measurement data/signals 16o generated by the power meter 16 and determine based thereon calibration values for one or more of the components of the system 10 e.g., using one or more of the calibration sequences disclosed hereinbelow or hereinabove. The calibration values can be set by one or more of the control signals CP/, CGj, CGm, CD/, CDLD, CVOA, (designated by dotted arrowed lines, collectively referred to herein as control signals 18c) generated by the control unit 18. This way, a closed feedback loop is obtained and utilized for calibrating and/or testing the QKD transmitter system 10.

For example, the control unit 18 can be configured to generate (e.g., ON/OFF) control signals CP _i for operating one or more of the electric pulse sources P _i , and/or control signals CG _i and/or CGm to set gain factor/value of at least one of the tunable gain units Gj and/or Gm for thereby applying a desired gain to the electric/EM pulse signals thereby received, and/or control signals CD _i and/or CDLD to set time delay value of at least one of the tuneable delay units D _i and/or DLD for thereby applying a desired time delay to the electric/EM pulse signals thereby received, and/or control signals CVOA to set an attenuation factor/value of the tuneable attenuation unit VOA for thereby applying a desired attenuation factor to the optical signals thereby received.

In some possible embodiments the delay time applied by the tuneable delay unit DLD is implemented internally by the current source 11, which may render the tuneable delay unit DLD redundant and discardable. Accordingly, the current source 11 can also receive a control signal Ccs from the control unit 18 for setting its internal time delay.

It is noted that other arrangements utilizing one or more analog combiner units can be used to obtained substantially the same functionality of the QKD transmitter system 10. For example, Fig. 1A further demonstrates a possible embodiment of the modulator driving portion lOw* of the QKD transmitter system 10 utilizing a single (3:1) analog combiner unit E to produce the light modulating signal.

Fig. IB schematically illustrates a QKD transmitter system 10' according to other possible embodiments of the present application. The QKD transmitter 10' resembles in many aspects the QKD transmitter 10 of Fig. 1, but inter alia differs therefrom in that its modulator driving portion 10w' comprises an additional electric/EM pulse signal source , an additional analog signal combiner E2, an additional tuneable gain unit G3 configured to controllably adjust the amplitude of the electric/EM pulse signals generated by the additional pulse signal source P3, an additional tuneable delay unit D3 for applying a desire delay time to the electric/EM pulse signals from the additional tuneable gain unit G3, and respective additional electromagnetic signal guiding elements for connecting these units.

The pulse signal sources Po, Pi, P2 and P3, and the analog combiners Eo, Ei and E2, are configured as follows:

• Pulse signal source Po: configured to generate positive pulse signals that passes through the analog combiner Eo, which output towards the EOM 13 includes ½ (half) of the initial voltage level received from the pulse signal source Po;

• Pulse signal source Pi: configured to generate negative pulse signals that passes through the analog combiners Eo and Ei, which output towards the EOM 13 thus includes *4 (quarter) of the initial voltage level received from the pulse signal source Pi;

• Pulse signal source P2: configured to generate positive pulse signals that passes through the analog combiners Eo, Ei and E2, which output towards the EOM 13 thus includes (eighth) of the initial voltage level received from the pulse signal source P2; and • Pulse signal source P3: configured to generate negative pulse signals that passes through the analog combiners Lo, Li and E2, which output towards the EOM 13 thus include 14 (eighth) of the initial voltage level received from the pulse signal source P3.

Similarly, Fig. IB illustrates using both of the delay units Do and Di in order to keep the electric/EM signal shape identical in all of the paths of the modulator driving portion 10w' of the system 10, but as mentioned hereinabove, the delay units Do and Di can have the same role in possible calibration processes of the system 10', and thus in such embodiments one of them can be omitted. The light modulating portion 10p' of the system 10' similarly comprises the coherent (e.g., laser) light source 12 powered by the electrical current source 11, the EOM 13 used to modulate the desired quantum communication states in the light signal generated by the light source 12, and the variable optical attenuator (VOA) unit 14 for controllably setting a desired attenuation factor to the modulated optical pulse signals produced by the EOM 13. The power/intensity of the modulated optical pulse signals from the EOM 13, or after passage through the VOA 14, or some portion thereof, can be similarly further attenuated by a fixed optical attenuation unit 17 e.g., to the single-photon power level.

The optical splitter (or a signal tap) 15 can be similarly used in some embodiments to branch some predefined portion (e.g., half power signal) of the modulated optical pulse signals from the EOM 13, or from the VOA 14, into the power meter unit 16. Correspondingly, calibration of the power meter unit 16 in embodiments wherein it utilizes a photodiode is required in only some of the initialization procedures disclosed herein.

The control unit 18' is similarly configured to operate components of the transmitter system 10'. The control unit 18' can be configured to process the measurement data/signals generated by the power meter 16 and determine based thereon calibration values for one or more of the components of the system 10' e.g., using one or more of the calibration/testing sequences disclosed herein. The control unit 18' can be accordingly configured to set calibration values by one or more of the control signals CP _; (where 0<z<3 is an integer number), CGz, CGm, CD/, CDLD, CVOA, (designated by dotted arrowed lines, collectively referred to herein as control signals 18c') thereby generated. In this way, a closed feedback loop is obtained, that can be used for calibrating and/or testing the transmitter system 10'.

It is noted that other arrangements utilizing one or more analog combiner units can be used to obtained substantially the same functionality of the QKD transmitter system 10'. For example, Fig. IB further demonstrates a possible embodiment of the modulator driving portion 10w'* of the QKD transmitter system 10' utilizing a single (4:1) analog combiner unit E' to produce the light modulating signal.

It is noted that losses in the transmitter system 10 and/or 10' are considered as having “ideal” losses in the present disclosure, for the sake of simplicity. Excess insertion losses, though addressed by the initialization procedures hereof, are generally ignored in the present disclosure for the sake of clarity. The electrical/EM pulses combined to form the lightmodulating signal are configured to "carve" the light signal from the light source 12 into optical pulses forming the quantum communication states of the system. The optical power/intensity of the modulated optical pulses depends on the transfer function of the EOM 13 (e.g., proportional to sin ² ), as exemplified in Table 1. The EOM 13 can be implemented by a Mach Zehnder modulator. Optionally, but in some embodiments preferably, the EOM 13 is implemented by a single-drive (or dual-drive) Mach Zehnder modulator using any of the embodiments disclosed in International Patent Publication No. WO 2022/003704 of the same applicant hereof, the disclosure of which is incorporated herein by reference.

For example, in order to generate time-bin encoding of the 9 (nine) quantum communication states required for operation of a two-decoy state BB84 protocol, or other derivatives of the BB84 protocol the quantum states |0), 11), |p), |m), the decoy states |0 _d ), | Id), |Pd), l ^m d), and the vacuum state |P _d ), the following electrical/EM pulses are injected in some embodiments into the EOM 13:

|0): Pulse at to with voltage amplitude vo and no pulse at ti;

11): No pulse at to and a pulse at ti with voltage amplitude of vo;

|p): Pulse at to with voltage amplitude -vo/2 and pulse at ti with voltage amplitude -vo/2;

|m): Pulse at to with voltage amplitude vo/2 and pulse at ti with voltage amplitude -vo/2 (inverted polarity);

|0 _d ): Pulse at to with voltage amplitude va and no pulse at ti;

| l _d ): No pulse at to and a pulse at ti with voltage amplitude va;

|p _d ): Pulse at to with voltage amplitude -Vd/2 and pulse at ti with voltage amplitude -Vd/2; |m _d ): Pulse at to with voltage amplitude va/2 and pulse at ti with voltage amplitude -Vd/2 (inverted polarity); and

|P _d ): No pulse is sent. wherein to is the first time slot of the transmission frame of the qubit, ti is the second time slot of the transmission frame of the qubit, r is the time interval between to and ti, vo is a voltage level of the combined electrical/EM pulse signal required for producing a desired intensity /power Io of the optical signal of the |0) and | 1) main quantum states, and v is a voltage level of the combined electrical/EM pulse signals required for producing a desired intensity /power Id of the optical signal of the decoy states, which can be greater or smaller than and in some embodiments Vd<vo).

Fig. 2 graphically illustrates optical signals of the 4 (four) main quantum communication states, and the 5 (five) decoy states, according to some possible embodiments. It is understood that the optical pulse illustrations provided in Fig. 2 are of the proportional optical fields, as optical signals having negative power/intensity are not actually feasible. In practice, it is the opposite phase of the modulated optical signals 21 that gives the optical field the negative ("-") sign.

It is noted in this respect that though the transmitter system 10 of Fig. 1A has the advantage of requiring less system resources, on the other hand it requires superposition of all 3 (three) pulse signal sources Po, Pi and P2, to generate the 9 (nine) quantum communication states. Thus, in the transmitter system 10 configuration of Fig. 1A, the optical power/intensity Id characterizing the decoy states depends on the optical power/intensity Io characterizing the main quantum states. On the other hand, the transmitter system 10' of Fig. IB is flexible in determining the levels of the decoy states, as the main quantum states can be generated with the two pulse signal sources Po and Pi, and the decoy states can be generated separately/independently using two different pulse sources, P2 and P3. Thus, in the transmitter system 10' of Fig. IB, the optical power/intensity Id characterizing the decoy states does not depend on the optical power/intensity Io characterizing the main quantum states.

The 9 (nine) quantum communication states can be generated in the transmitter system 10 configurations of Fig. 1A by setting the tuneable gain units Gj and Gm such that:

• the amplitude of the pulse signal generated by the Po pulse signal source after passing through the tuneable gain unit

• The amplitude of the pulse signal generated by the Pi pulse signal source after passing through the tuneable gain unit and

• The amplitude of the pulse signal generated by the P2 pulse signal source after passing through the tuneable gain unit wherein V _n designates pulse voltage level for full transmission by the EOM 13. It is however noted that the embodiments disclosed herein do not necessitate setting the EOM 13 into full transmittance, and will actually work properly also if a voltage level that is close to (not exactly) V _n is used for producing the optical power/intensity Io characterizing the main quantum states by the EOM 13. As seen in Figs. 1A and IB, the pulse signals produced by the tuneable gain unit Gm are used to encode the main quantum and decoy states by the EOM 13. Using the aboveindicated amplitudes with the correct pulse signals' superposition allow an efficient implementation of decoy state BB84 protocol, or other derivatives of the BB84 protocol. It is noted that this is the only amplitude ratio that can generate the required states with the same optical power/intensity for the main states, and the same optical power/intensity for the decoy states (except the vacuum state) for the transmitter system 10 of Fig. 1A (other than inverting all the electric signs).

Table 1: generating the 9 (nine) quantum states by the transmitter system 10 of Fig. 1A

The 9 (nine) quantum communication states can be generated in the transmitter system 10' configurations of Fig. IB by setting the tuneable gain units G _i and Gm such that:

• the amplitude of the pulse signal generated by the Po pulse signal source after passing through the tuneable gain unit Gm substantially equals

• the amplitude of the pulse signal generated by the Pi pulse signal source after passing through the tuneable gain unit Gm substantially equals

• the amplitude of the pulse signal generated by the P2 pulse signal source after passing through the tuneable gain unit Gm substantially equals V _d ; and

• the amplitude of the pulse signal generated by the P3 pulse signal source after passing through the tuneable gain unit Gm substantially equals wherein V _d is the bias voltage level for the decoy states (optionally, but not necessarily, V _d <V _n ). n is a positive real number configured such that the optical output of the EOM 13 when applying the V _d voltage level thereto is twice the optical power/intensity obtained when applying thereto e.g., according to the EOM's non-linear response (see

Table 2), wherein I(V) is the transmission function of the EOM 13, so the n parameter can be determined by solving the equation

Table 2: generating the 9 (nine) quantum states by the transmitter system 10' of Fig. IB

Other optically transmitted patterns/signals are used in some embodiment for system calibration and testing. For example, the pulse signal sources P _; can be configured to transmit predefined electric/EM signal patterns repeatedly/periodically during system initialization stages, for calibrating the transmitter system 10 (0<z<2), and/or the transmitter system 10' (0<z<3). The signal patterns graphically illustrated in Fig. 3 exemplify possible patterns that can be generated by the pulse signal sources P _i for the systems' calibration procedures. The amplitude of the signal patterns used in such calibration depends on the pulse signal source used, as every pulse signal source may have different gain parameter and its pulse signals may propagate via different number of combiners of the EM/RF path of its modulator driving portion 10w/10w'.

Fig. 4 is a block diagram schematically illustrating a QKD communication system 20 according to some possible embodiments. In the QKD communication system 20 a standard (classical) communication channel SC (e.g., optical fiber, parallel/serial data/signal communication bus, wireless data/signal communication) is used to exchange data/signals between the QKD transmitter (Alice) 10 (or 10') and the QKD receiver (Bob) 19 for controlling/orchestrating (e.g., clock synchronization/recovery, sifting, data processing) the QKD protocol steps carried out over the quantum communication channel QC (e.g., optical fiber, free space).

Respective optical transceivers OT can be used in the QKD transmitter 10 (or 10') and in the QKD receiver 19 for communication data/signals between Alice and Bob over the SC. The modulated optical pulse signals 17o produced by the optical light modulating portion lOp of the QKD transmitter are transmitted over the QC to the QKD receiver 19. The modulated optical pulse signals from the QC are passed through an unbalanced interferometer UNBI e.g., asymmetric Mach Zehnder or Michelson interferometer, of the QKD receiver 19 and therefrom detected by the light detector(s) e.g., one or more single photon detectors DETi and/or DET2 for detection of different interference patterns. A control unit CTRL of the QKD receiver 19 receives and processes the measurement data/signal generated by the light detector DET, communicates data/signals with the QKD transmitter 10 (or 10') over the SC, and optionally also synchronizes its clock CLKR with clock signals of the QKD transmitter clock CLKT. A random number generator RNG is also provided in the QKD transmitter 10 (or 10') and in the QKD receiver 19 for carrying out the QKD protocol.

Optionally, but in some embodiments preferably, the initialization of the QKD system 20 is configured to optimize the frequency of the clock CLKT of the QKD transmitter system 10 (or 10') to parameters of the receiver's interferometer UNBI. In a time-bin qubit implementation of the QKD protocol, the measurement outcome of the \p) and |m) main quantum states depend on the interference of the unbalanced interferometer UNBI at the QKD receiver (i.e., Bob) 19. In some embodiments this is the only step that requires a connection to an external component e.g., Bob/receiver system 19, for receipt of feedback data/signals.

On the other hand, if the Bob receiver system 19 is not connected to a quantum Alice transmitter 10 (or 10'), all the other system initialization steps can be performed with a nominal or other pre-defined clock frequency of the QKD system 20. In this case, however, when connecting the QKD transmitter 10 (or 10') to a Bob receiver system 19, the frequency of Alice’s clock CLKT needs to be adapted to the frequency of Bob’s interferometer UNBI, either by measurements as performed in the following initialization step, or to a known, pre-measured frequency. Fig. 5A is a flowchart illustrating an automatic calibration sequence 50 of the transmitter system 10 of Fig. 1A according to some possible embodiments. The calibration sequence 50 may start with an initialization stage (optional step si designated by dashed-line box) in which the frequency of clock CLKT of the transmitter system 10 (or 10') is optimized to parameter(s) of the Bob's receiver system 19. As the arms of Bob's interferometer UNBI are fixed in length for the interference of consecutive optical pulse signals, the available adjustable parameter to maximize this interference is the time difference between the peaks of the amplitudes of the |0) and | 1) main quantum states. This is done in some embodiments by changing this parameter at the QKD transmitter side (Alice) 10 (or 10') according to feedback data/signals received from the QKD receiver side (Bob) 19 over the SC. In addition, the length difference between the arms of Bob's interferometer UNBI could be regarded as a selffrequency of Bob's interferometer UNBI. In some embodiments the interference visibility (as measured by either the DETi or DET2 detector, or ratio of power signals thereby detected) is maximized at Bob's interferometer UNBI by adjusting the frequency of the clock CEKT of Alice’s QKD transmitter system 10 (or 10'), and its repetition rate, which can be achieved utilizing the following procedure:

Transmited pattern-, pattern 1 from the pulse signal source Po (as exemplified in Fig. 3).

Procedure-. For pattern 1 generated by the pulse signal source Po measure the interference visibility at Bob's interferometer UNBI, for optimal constructive and destructive interference. During this procedure the time difference ti-to between the qubit's pulses ti and to can be changed directly at Alice's QKD transmitter 10 (or 10') by changing the pulse timing of its clock CLKT, and/or by changing the frequency the clock CLKT of the Alice’s QKD transmitter. In this specific and non-limiting example a test frequency value for Alice's clock CLKT is scanned starting from a predefined permissible minimal frequency value Frequencymin, and up to a predefine permissible maximal frequency value Frequency _ma x. The frequency of the clock CLKT (or the time difference ti-to between peaks) that returns the best visibility at Bob's QKD receiver 19 is then chosen for use by the Alice's QKD transmitter 10 (or 10').

Pseudo code-.

Pattern 1

For Frequencyj= Frequency _m in to Frequencymax OptimizeVisibility( ) BobMeasureVisibility(Visibilityj)

End Frequency= Frequencyj@( OplimalVisibilily(Visibililyj))

The predefined permissible minimal and maximal frequency values, Frequency _m in and Frequencymax, can be determined based on manufacturing tolerances of the UNBI interferometer.

The OptimizeVisibility() function is a sequence in which the Alice transmitter changes the wavelength of the (e.g. , laser) light source 12 over the free spectral range (FSR) of the UNBI interferometer, with or without feedback from the Bob receiver, which causes the photons to constructively interfere as reflected at one output of the UNBI interferometer (e.g., at DETi), and to destructively interfere as reflected at the other output of the UNBI interferometer (e.g., at DET ₂ ), and vice versa. It is noted that thermal or mechanical control of the interferometer can be alternatively used to optimize the visibility, and other techniques can be similarly used. With this information the Bob receiver can calculate the visibility of the UNBI interferometer. The BobMeasureVisibility() function causes Bob's receiver system 19 to measure the visibility of optical signals received from the transmitter system 10 and transmit the same to the transmitter system 10 over the SC channel e.g., Bob can continuously transmit to Alice over the SC channel the measured visibility, and Alice can then decide which frequency is the optimal frequency to carry out the QKD sequence.

Result'. The clock CLK _T of Alice's QKD transmitter system and/or the pulse time difference ti-to are set for optimal interference at Bob’s interferometer UNBI.

The configuration of the QKD Transmitter 10 has the minimal number of degrees of freedom for power setting for the pulse signal sources P _; to generate power balanced decoy state BB84 protocol (or other derivatives of the BB84 protocol), in the sense that the configuration of the QKD Transmitter 10 hereof is the only way to generate the 4 (four) main quantum states with a uniform average number of photons per state p, and the 4 (four) decoy quantum states with a uniform average number of photons per pulse p _d . Based on the limited number of degrees of freedom of the QKD transmitter 10 (or 10') it can be determined that p = 2μ _d . This ratio is efficient for the two-decoy state BB84 protocol (or other derivatives of the BB84 protocol), with the addition of the vacuum state, in which no optical pulses are generated.

Optionally, but in some embodiments preferably, the calibration sequence 50 further comprises a self-calibration procedure (step s2) for the initial setting of the gain factors of the Gm and Go gain units. This self-calibration procedure is configured to set the electric/EM pulse signal entering the EOM 13 to be close to V _n e.g., to about a • V _n , where 0.95<a<l is an upperlevel intensity setting parameter. The Gm gain unit is configured to amplify different electric/EM signal levels, and therefore in some embodiments its working point needs to be far from saturation and/or in a linear working region thereof, and/or configured such that it compensates for other non-linearities of the transmitter system 10, such as of the EOM 13 e.g., a Mach-Zehnder electro-optic modulator (MZ EOM). The Go gain unit can also reach saturation, as it is typically similar to the gain unit Gi, and the signal pulses generated by the Po and Pi signal pulse sources should keep the same signal shape with different input signal levels/amplitudes. These objectives can be achieved with the following procedure:

Transmited pattern-, pattern 1 from the pulse signal source Po (as exemplified in Fig. 3).

Procedure'. For a test gain value Gm/ of the gain unit Gm scanned starting from a predefined permissible minimal gain value Gmin and up to a predefine permissible maximal gain value Gmax, record the intensity /power 1/ (16o) of the optical output measured by the power meter 16 for a range of test values G _oj of the gain unit Go, starting from a predefined permissible minimal gain value G _0min and up to a predefined permissible maximal gain value Gomax- If optical power saturation of the EOM 13 is recognized, then record the measured saturation power/intensity I/=I _sat and the test gain values for which the measured optical output power/intensity is about 0.95-I _sat (where 0.95 is provided as one non-limiting example of a value close to 1, which can provide a desired V _π ).

Pseudo code'.

Result: the gain value of the gain unit Gm is set, a temporary gain value of the gain unit Go is determined, and the determined Gm and Imax values are recorded. The function Saturated(I) is configured to recognize that the slope of the output optical power/intensity I vector is close to 0 (zero) for the newly measured optical intensity values which means that the output optical signal is saturated (not to be confused with the amplifier gain saturation) i.e., in optical saturation increasing the amplitude of the electrical/EM signal supplied to the EOM 13 does not result in an increase in the optical power/intensity of the light signals thereby generated. The predefined permissible minimal and maximal gain values, and can be determined by choosing a suitable linear operation regimes of the gain units e.g., according to manufacturer's datasheets.

Optionally, but in some embodiments preferably, the calibration sequence 50 further comprises a self-calibration procedure (step s3) for setting an initial gain factor of the gain unit Gi. As seen in Table 7, the electric/EM pulse signal generated by the pulse signal source Pi is the only signal transmitted without superposition with other pulses for generation of the main quantum states. Therefore, it is convenient to set the gain value of the gain unit Gi before setting the gain values of the Go and G2 gain units. The gain unit Gi is configured to set the electric/EM signal entering the EOM 13 to be close to wherein is a mid-intensity level setting parameter. In some embodiments the mid- voltage level setting parameter is set to in order to obtain an optical output power/intensity of about when transmitting pattern 2 by the pulse signal source Pi i.e., carried out without operating the Po and/or P2 signal pulse sources.

Transmited pattern-, pattern 2 from the Pi pulse signal source.

Procedure'. Record the optical output power/intensity measured by the power meter 16 for a range of Gi test values, starting from a permissible minimal gain value Gimin and up to a permissible maximal gain value Gimax- The gain value of the Gi gain unit is set such that the peak of the output optical power/intensity of the measured pulse is ½ (half) of I _m ax. This is the value for which the average optical output power/intensity of pattern 2 is ½ (half) of the optical output power/intensity of Imax.

Pseudo code'. End

Result', the gain value of the gain unit Gi is set, and the determined gain value Gi and the measured mid-intensity optical output level I,s=Ii/2 value and recorded.

The predefined permissible minimal and maximal gain values, Giminn and Gimax can be determined by choosing a suitable linear operation regime of the gain unit e.g. , according to the manufacturer's datasheet.

Optionally, but in some embodiments preferably, the calibration sequence 50 further comprises a self-calibration procedure (step s4) for setting the pulse signal overlap parameters utilizing the Do and/or Di time delay units. The following procedure sets the delay time parameters of the delay unit Do and/or Di, such that the positive electric/EM pulse signals generated by the Po signal pulse source and the negative electric/EM pulse signals generated by the Pi signal pulse source overlap in the Σ _o analog combiner.

Transmited pattern', pattern 1 generated by the Po pulse signal source, and pattern 2 generated by the Pi pulse signal source. In this way it can be guaranteed that the correct pulse signals overlap i.e., to prevent overlapping pulse n from the Po pulse signal source with pulse n+1 from the Pi pulse signal source, for example.

Procedure'. When the overlap between the electric/EM pulse signals generated by the Po and Pi signal pulse sources is acceptable or ideal, the electric/EM pulse power (e.g., root mean square - RMS) from the analog combiners is minimal, and the measured optical output power/intensity I _j is minimal as well. The following procedure scans the pulse position, by scanning test time delay values starting from a predefined permissible minimal delay time value D _0min and up to a predefine permissible maximal delay time value Domax, to get an optimal overlap. The test time delay value can be defined by either by the Do or Di delay unit i.e., setting the same delay time value for both the Do and Di delay units is equivalent to setting a 0 (zero) delay time value for both delay units.

Pseudo code'. Result', the time delay values of the Do and/or Di delay units are set so that electric/EM pulse signals from the Po and Pi pulse signal sources substantially overlap in the analog combiners.

The predefined permissible minimal and maximal time delay values, Domin and Domax can be determined by the range of possible time delays between the pulse sources e.g., as estimated by the system design.

Optionally, but in some embodiments preferably, the calibration sequence 50 further comprises a self-calibration procedure (step s5) for fine tuning the Go gain unit. The Go gain unit is set in some embodiments to have a total output optical power/intensity of yl _m ax for the Po pulse signal source, when operated with test pattern 1, and without the Pi pulse signal source or without the P2 pulse signal source, wherein fi<y<a is an upper- mid-level intensity setting parameter e.g., for a~^l and /?->0.5. In some embodiments this procedure is configured to determine a gain value for the Go gain unit to provide a total voltage of %• V _n (i.e., y=%) for the Po pulse signal source when operated with test pattern 1 and without the Pi or P2 pulse signal sources.

Transmited pattern', pattern 1 from the Po pulse signal source and pattern 2 from the Pi pulse signal source.

Procedure', the power of the merged/combined electric/EM pulse signals produced from the Gm gain unit is set such that the optical output power/intensity is about *4 of I _m ax, by scanning test values for the Go gain unit, starting from a predefined permissible minimal gain value Gomin and up to a predefine permissible maximal gain value Gomax- Pseudo code'.

For Go ⁼ Gomin tO Gomax

MeasurePower( Ij )

End

Go = Gj(Ij == Imax/4)

Result: set the gain value for the Go gain unit.

The predefine permissible minimal and maximal gain values, Gomin and Gomax, can be determined by choosing a suitable linear operation regime of the gain unit e.g., according to the manufacturer's datasheet.

Optionally, but in some embodiments preferably, the calibration sequence 50 further comprises a self-calibration procedure (step s6) for fine tuning the D2 delay unit. The D2 delay unit sets the delay of electric/EM pulse signals generated by the P2 pulse signal source, such that the electric/EM pulse signals from the P2 pulse signal source overlap with the electric/EM pulse signals from the Pi pulse signal source, which thus also overlap with the electric/EM pulse signals from the Po pulse signal source.

Pattern-, pattern 2 from the Pi pulse signal source, and pattern 1 from the P2 pulse signal source.

Procedure-, for the purpose of this procedure the gain value of the G2 gain unit is set to be about *Gi, wherein d<[! is a lower-mid-level intensity setting parameter. In some embodiments the lower-mid-level intensity setting parameter is set such that G2= Gi/3 ( =’/3). The setting procedure of D2 is similar to the setting procedure of the Do delay unit i.e., test values for the D2 delay unit are scanned starting from a predefined permissible minimal time delay value D2min and up to a predefine permissible maximal time delay value D2max, and the optical output power/intensity is measured. The optimal value of the D2 delay unit is set such that the measured optical output power/intensity is minimal.

Pseudo code-, like the Do delay unit procedure but with setting G2=<5*GI.

Result-, the time delay of the D2 delay unit is set such that the electric/EM pulses from the Po, Pi and P2, pulse signal sources overlap in the analog combiners.

The predefined permissible minimal and maximal delay time values, D2min and D2max, can be determined based on the range of possible time delay between the pulse sources e.g., as estimated by the system design.

Optionally, but in some embodiments preferably, the calibration sequence 50 further comprises a self-calibration procedure (step s7) for fine tuning the G2 gain unit. The G2 gain unit is set in some embodiments to optimize the visibility of the |m) main quantum state, so that the power/intensities of the two optical half-pulses (formed by positive and negative voltage pulses) forming the |m) main quantum state are substantially identical, and also substantially identical to the power/intensities of the half-pulses forming the \p) main quantum state. The P2 pulse signal source is configured in some embodiments to produce a total electric/EM signal of about %-Vjt when operated without the Po pulse signal source, or without the Pi pulse signal source, with test pattern 1.

Transmitted pattern-, pattern 1 from the Po pulse signal source, pattern 2 from the Pi pulse signal source and pattern 1 from the P2 pulse signal source.

Procedure-, the optical output power/intensity is set to about Ijg=Ii/2 e.g., the optical power/intensity obtained for the combined electric/EM signals from all of the Gj pulse signal sources should substantially equal to the optical power/intensity obtained for pattern 2 transmitted from the Pi pulse signal source only (as measured in step s3), by scanning test values for the G2 gain unit starting from a predefined permissible minimal gain value G2min and up to a predefined permissible maximal gain value G2max- Pseudo code'.

For G2 = G2min tO G2 max

MeasurePower( Ij ) end

G ₂ = Gj(Ij == Ii/2)

Result', the gain value of the G2 gain unit is set for an optimal |m) main quantum state.

The predefined permissible minimal and maximal gain values, G2min and G2max, can be determined by choosing a suitable linear operation regime e.g., according to manufacturer's datasheet.

Alternatively, the fine tuning of the G2 gain unit is configured to optimize the visibility of the |m _d ) decoy state, so that the optical power/intensity of the two half -pulses forming the |m _d ) state is substantially identical, and also substantially identical to the optical power/intensity of the two half-pulses forming the \p _d ) decoy state. In some embodiments the gain value of the G2 gain unit is set to provide combined electric/EM output of about %• V _Jl [/7on’ is it measured?], when pattern 1 is generated by the P2 pulse signal source, and without additional pulse source.

Transmited pattern', pattern 2 from the Pi pulse signal source and pattern 1 from the P2 pulse signal source.

Procedure', the combined electric/EM output signal is set to about (e.g., for A-Vu 1^%-Imax optical output power/intensity), by scanning test values for the G2 gain unit starting from the predefine permissible minimal gain value G2min and up to the predefine permissible maximal gain value G2max.

Pseudo code'.

For G2 = G2min tO G2 max

MeasurePower( Ij )

End

G2 = Gj(Ij = = Imax/4)

Result', the gain value of the G2 gain unit is set for optimal |m _d ) decoy state.

Optionally, but in some embodiments preferably, the calibration sequence 50 further comprises a self-calibration procedure (optional step s8, designated by dashed line box) for fine tuning the Gm gain unit. In this optional procedure the non-linear response of the EOM 13 can be used to balance the intensity of the various quantum communication states, if so needed e.g., when the total optical output power/intensity of the 4 (four) main quantum states is not uniform, or when the optical output power/intensity of the 4 (four) decoy states is not uniform. Changing the gain value of the Gm gain unit will change values close to V _n /2 the most, while keeping values close to V _n almost unchanged, which thus enables changing the balance between the different quantum communication states e.g., by scanning the test values for the Gm gain unit and measuring the power of 10), | 1) , \p), |m) for each Gm value i.e., a Gm value can be set for which the power of all main quantum states is substantially equal. Alternatively, the other gain units Gj can be adapted accordingly, without using Gm.

Fig. 5B is a flowchart illustrating an automatic calibration sequence 51 of the transmitter system 10' of Fig. IB according to some possible embodiments. The calibration sequence 51 may start with an initialization stage (optional step ql designated by dashed-line box) in which the frequency of clock CLKT of the transmitter system 10 is optimized to parameter(s) of the Bob's receiver system 19. The initialization stage (ql) is similar to the initialization stage (si) of the automatic calibration sequence 50 of Fig. 1A, and thus will not be described herein again for the sake of brevity.

Optionally, but in some embodiments preferably, the calibration sequence 51 further comprises a self-calibration procedure (step q2) for initial setting of the gain factors of the Gm and Go gain units. This procedure can be configured to set the electric/EM pulse signals inputted to the EOM 13 to be close to V _n e.g., to about a - V _n where 0.95<a<l. The Gm gain unit amplifies different signal levels and therefore its working point should be far from saturation, and/or at a linear response working regime thereof, and/or configured to compensate for other non-linearities of the system 10', such as of the EOM 13. The Go gain unit is also sensitive to saturation, as it is substantially identical to the Gi, G2 and G3, gain units, and the electric/EM pulse signals generated by the Po, Pi and P2, pulse signal sources should keep the same signal shape with different intensities.

Transmited pattern-, pattern 1 from the Po pulse signal source.

Procedure-. For a test gain value Gm/ of the gain unit Gm scanned starting from the predefine permissible minimal gain value Gmin and up to the predefine permissible maximal gain value Gmax, record the optical output intensity/power 1/ (16o) of the optical output measured by the power meter 16 for a range of test values Go/ of the gain unit Go, starting from a predefined permissible minimal gain value Gomin and up to a predefined permissible maximal gain value Gomax- If optical power saturation of the EOM 13 is recognized, then record the measured saturation power/intensity I ₇ =I _sat and the test gain values G _m ^"G _m j and Go^"Go/ for which the measured optical output is about 0.95-I _sat (where 0.95 is provided as one non-limiting example of a value close to 1, which can provide a desired V _n ).

Result', the gain values of the Gm and Go gain units are set, the gain values of the Gm and Go gain units, and the measured optical output power/intensity Ipatternl are recorded.

The function Saturated(I) is configured to recognize that the slope of the optical output I vector is close to 0 (zero) for the new values, which means that the optical signal is saturated.

Optionally, but in some embodiments preferably, the calibration sequence 51 further comprises a self-calibration procedure (step q3) for initial setting gain factor for the Gi gain unit. This procedure sets the electric/EM signal entering the modulator to be close to close to . This stage is helpful but not essential (designated by dashed line box).

Transmitted pattern', pattern 2 from the Pi pulse signal source.

Procedure'. Record the output power for a range of test values for the G ₁ gain unit, starting from the predefine permissible minimal gain value G _1min and up to a predefine permissible maximal gain value Gimax- The gain value of the Gi gain unit is set such that the peak of the pulse obtained for the Pi pulse signal source is ½ (half i.e. of peak of the pulses obtained for the Po pulse signal source. This is the value for which the average optical output power/intensity obtained for pattern 2 from the Pi pulse signal source is ½ (half) of the optical output power/intensity obtained for pattern 1 from the Po pulse signal source. Assuming that the gain unit Go is not saturated, then the gain unit Gi is also not saturated, as it is a similar component. If this is not a valid assumption, saturation test for the gain unit Gi may be required, and the gain should be set to keep the linear correlation of the optical output power/intensities, while no signal is saturated.

Pseudo code'.

End

Result', the gain value of the Gi gain unit is set, and the gain value for Gi and the measured optical output (vector of all gain range) values are recorded.

Optionally, but in some embodiments preferably, the calibration sequence 51 further comprises a self-calibration procedure (step q4) for setting the pulse overlap parameters for the Do and Di delay units. This procedure sets the delay time parameters of the Do and Di delay units such that the positive electric/EM pulse signals from the Po pulse signal source and the negative electric/EM pulse signals from the Pi pulse signal source substantially overlap in the analog combiners.

Transmitted pattern', pattern 1 from the Po pulse signal source and pattern 2 from the Pi pulse signal source, which are combined to form pattern 3 when overlapping. This way it is assured that the right pulses overlap i.e.. to prevent overlap of pulse n from the Po pulse signal source with pulse n+1 from the Pi pulse signal source, for example.

Procedure'. When the overlap is acceptable or ideal, the power (RMS) of the electric/EM pulse signal is minimal, and the optical output power/intensity is minimal as well. The pulse position is scanned to get an optimal overlap. The entire time delay scan range is defined by either the Do or Di delay unit i.e.. setting the same delay for both the Do and Di delay units is equivalent to setting a zero (0) delay time for both delay units. It is noted that this will however shift the combined pulse relative to the pulse generated from the P2 pulse signal source.

Pseudo code'.

Result', the delay value of the Do delay unit is set such that the output optical power/intensity obtained is minimal.

Optionally, but in some embodiments preferably, the calibration sequence 51 further comprises a self-calibration procedure (step q5) for fine tuning the Gi gain unit. Differences in response of the electro optical chain (e.g., amplifier gain and EOM response to different polarity) and/or other non-linearities, may lead to different optical power/intensity of the two half-pulses forming the |m) main quantum state. In order to get high visibility interference between the first and the second half-pulses of the |m) main quantum state, it is essential to fine tune the Gi gain unit.

Transmited pattern', pattern 1 from the Po pulse signal source and pattern 2 from the Pi pulse signal source, which are combined to form pattern 3.

Procedure', the optical output power/intensity is set to substantially equal to the power/intensity obtained for pattern 2 only (e.g., the optical output power/intensity previously measured in step q3) by scanning test values for the Gi gain unit starting from the permissible minimal gain value G _1min and up to the permissible maximal gain value Gimax- As is monotonically increasing (until saturation) and i is monotonically decreasing, there will be a gain value of the Gi gain unit for which which is used in some embodiments as the optimal gain value for the Gi gain unit.

Pseudo code'.

Result: the gain value of the Gi gain unit is set for high visibility of the main quantum state, with the same power as the \p) main quantum state, and the gain value for the Gi gain unit, and the measured optical output power/intensity values are recorded.

Optionally, but in some embodiments preferably, the calibration sequence 51 further comprises a self-calibration procedure (step q6) for fine tuning the G2 gain unit. In the transmitter system 10', the P2 and P3 signal pulse sources act similar to the Po and Pi signal pulse source, but generate the decoy states and not the main quantum states. Therefore, the target bias voltage of the P2 and P3 signal pulse sources is not ut a different (normally lower) voltage value, V _d , intended to get an output optical power of Id (e.g., Id < Io). Therefore, the parameter tuning of the G2 gain unit, the D2 delay unit, the G3 gain unit and the D3 delay unit, utilizes in some embodiments a procedure that is similar to the procedure used for tuning the Go gain unit, the Do delay unit, the Gi gain unit, and the Di delay unit, respectively. The Gm gain unit can be kept at the gain value previously set in the phase of tuning Go and Gi (in step q2). The P2 pulse signal source is used to generate the decoy states.

Transmitted pattern-, pattern 1 from the P2 pulse signal source.

Procedure-, the optical output power/intensity for the merged/combined electric/EM signal is set to substantially equal to (wherein are defined by the user, and is the optical output power/intensity measured and recorded in step q2), by scanning gain test values for the gain unit G2 starting from the predefine permissible minimal gain value and up to the predefine permissible maximal gain value G2max-

Pseudo code:

End

Result: the gain value of the G2 gain unit is set such that the decoy states have optical power/intensity of about ■ 1 patternl-

Optionally, but in some embodiments preferably, the calibration sequence 51 further comprises a self-calibration procedure (step q7) for fine tuning the D2 delay unit. In order for the decoy states to be indistinguishable from the main quantum states, their timings should be substantially the same. This is obtained in some embodiments by first tuning the pulse signals from the P2 pulse signal source to overlap with the pulse signals from the Pi and the Po pulse signal sources (which are already overlapping at this stage).

Pattern-, pattern 2 from the Pi pulse signal source and pattern 1 from the P2 pulse signal source.

Procedure-. The procedure for setting the delay time values of the D2 delay unit is substantially similar to the procedure used for setting the Do (or Di) delay unit (step q4). A test value for the D2 delay unit is scanned starting from the predefine permissible minimal delay value D2min and up to the predefine permissible maximal gain value D2max, and the optical output power/intensity is measured. The delay time value of the D2 delay unit is set such that the measured optical output power/intensity is minimal.

Result', the delay time value for the D2 delay unit is set such that the pulse signals from the P2 pulse signal source overlap with the pulse signals form the Pi and Po pulse signal sources.

Optionally, but in some embodiments preferably, the calibration sequence 51 further comprises a self-calibration procedure (step q8) for initial setting a gain values for the G3 gain unit. The pulse signal source P3 has in some embodiments a similar role as that of the Pi pulse signal source, but for the decoy states, and setting its gain value can thus be similar to the initial setting of the gain value of the Gi gain unit (step q3), or to the fine tuning of the gain value of the Gi gain unit (step q5).

Transmited pattern', pattern 2 from the P3 pulse signal source.

Procedure'. Record (and keep for later use) the optical output power/intensity obtained for a range of test values for the G3 gain unit, starting from a predefined permissible minimal gain value and up to a predefined permissible maximal gain value Gsmax- The initial gain value for the G3 gain unit is set such that the peak optical output power/intensity of the pulse is abou e.g., about ½ (half) of optical output power/intensity of the pulses transmitted from the P2 pulse signal source recorded in step q3). The parameter is selected in some embodiments such that the value for which the average optical output power/intensity obtained for the pattern 2 from the P3 pulse signal source is about ½ (half) the optical output power/intensity obtained for the pattern 1 from the P2 pulse signal source i.e., P=V2. Assuming that the G2 gain unit is not saturated, the G3 gain unit is also expected to be unsaturated, as it is a similar component tuned to a lower gain. If this is not a valid assumption, saturation test for the G3 gain unit is required, and the gain can be set to keep the ratio of optical output power/intensity, while no signal is saturated.

Pseudo code'.

End

Result', the initial gain values of the G3 gain unit is set, and the initial gain value for the G3 gain unit and the measured optical output power/intensity (vector of all gain range) values are recorded. The predefined permissible minimal and maximal gain values, Gsmm and Gsmax can be determined by choosing a suitable linear operation regime e.g., according to manufacturer's datasheet.

Optionally, but in some embodiments preferably, the calibration sequence 51 further comprises a self-calibration procedure (step q9) for setting the pulse overlap parameters for the D3 delay unit. This procedure is configured to set the delay value parameter of the D3 delay unit such that the pulse signals from the P3 pulse signal source overlap in the analog combiner Σ ₀ with the pulse signals from the Po, Pi and P2, pulse signal sources.

Transmited pattern-, pattern 1 from the P2 pulse signal source and pattern 2 from the P3 pulse signal source, which are combined to form pattern 3 when overlapping.

Procedure'. When the overlap is acceptable or ideal, the electric pulse power (RMS) is minimal, and the measured optical output power/intensity is minimal as well. This procedure scans the pulse position delay time values for the D3 delay unit starting from a predefined permissible minimal delay time value Dsmin and up to a predefine permissible delay time value D3max, to get an optimal overlap.

Pseudo code'.

Result', the delay time value of the D3 delay unit is set such that the optical output power/intensity is minimal, such that all electric/EM pulse signal channels overlap.

The predefined permissible minimal and maximal delay time values, can be determined by the range of possible time delay between the pulse sources e.g. , as estimated by the system design.

Optionally, but in some embodiments preferably, the calibration sequence 50 further comprises a self-calibration procedure (step qlO) for fine tuning the G3 gain unit. Differences in response of the electro optical chain (e.g., amplifier gain and/or EOM response to different polarities) and/or other non-linearities may lead to different pulse power/intensities for the two optical half-pulses forming the decoy state. In order to obtain high visibility interference between the first and second optical half-pulses, it is required to fine tune the gain value of the G3 gain unit.

Transmited pattern-, pattern 1 from the P2 pulse signal source and pattern 2 from the P3 pulse signal source, which are combined to form the pattern 3.

Procedure', the optical output power/intensity for the merged/combined electric/EM pulse signals is set to substantially equal to the optical output power/intensity obtained for pattern 2 generated only by the P3 pulse signal source (as measured in step q8 as by scanning test values for the G3 gain unit starting from the predefined permissible minimal gain value and up to the predefined permissible maximal gain value Gsmax- Pseudo code'.

Result', the gain value of the G3 gain unit is set to provide high visibility for the optical output power/intensity of the decoy state, with substantially the same optical output power/intensity as that obtained for the decoy state, the gain value for the G3 gain unit and the measured optical output power/intensity value are recorded.

Optionally, but in some embodiments preferably, the calibration sequence 50 and/or 51 further comprises a self-calibration procedure for setting direct modulation of the light source signal delay and/or duty cycle. In some embodiments all prior settings of the pulse signals and optical output power/intensity measurements are conducted while the light source 12 is in a continuous-wave (CW) operation mode. However, in QKD systems, as a countermeasure for coherent phase attacks, the phase between qubits needs to be randomized. One way to randomize the phase is to directly modulate the electric supply current of the (e.g., laser) light source 12.

Direct modulation of the laser light source 12 is performed in some embodiments by changing its electric driving current from the current source 11 to different levels e.g., the electric supply current level 1^^ in which the laser is “ON” and an electric supply current level I _rnd for which the phase of the (e.g., laser) light signal output is randomized, where I _rnd can be lower than the laser operating (lasing) threshold current. The goal of this calibration procedure is to position the optical output pulses produce by the EOM 13 within the stable power regime of the “ON” time of the laser light source 12. This can ensure that equal optical pulse amplitudes are obtained in time to and ti, (as exemplified in Fig. 2).

Another advantage of the direct modulation of the electric supply current of the light source 12 is that it can improve the contrast of the optical output signal and the system’s signal to noise ratio (SNR), so there is advantage to keep the "ON" time to a minimal duration (minimal duty cycle).

. In some embodiments the “ON” time is set from three parts: (i) the rise time, wherein the optical output power/intensity obtained for the electrical/EM pulse signal pattern 5 is lower than that obtained for the electrical/EM pulse signal pattern 6; (ii) the stable regime of the “ON” time, wherein the optical output powers/intensities obtained for pattern 5 and pattern 6 are equal; and (iii) the fall time in which the optical output power/intensity obtained for pattern 5 is higher than that obtained for pattern 6.

Transmited pattern-, pattern 5 from the Po pulse signal source, and thereafter pattern 6 also from the Po pulse signal source. The repetition rate of the laser direct modulation is the same as that of the pattern 5 and pattern 6 repetition electric/EM pulse signal rates, such that each 1^^ regime (“ON” time) of the laser light source 12 should contain an optical output pulse that is responsive to a respective electric/EM pulse signal pattern.

Procedure'. When the position of the electric/EM pulse signals in the direct modulation “ON” time is acceptable or ideal, the optical output power responsive to the pattern 5 and pattern 6 electric/EM pulse signals is substantially equal. The laser driver direct modulation timing can be scanned by changing the delay time value of the DLD delay unit i.e.. the delay of the light signal modulation relative to the pulse signals, starting from a predefined permissible minimal delay time value DLOH™ and up to a predefine permissible delay time value Dujmax, to get an optimal setting for the pattern 5 electric/EM pulse signal and then for the pattern 6 electric/EM pulse signal.

The minimal time duration of the “ON” time interval can be set according to the measurement of the optical output pulse signals by the power meter unit 16. The initial time duration of the “ON” time internal TLDO is determined first, which is >> ti-to- Thereafter, the excess time TtDexcess, in which the optical output power/intensity of the pattern 5 and pattern 6 electric/EM pulse signals is substantially equal, is determined. TtDexcess is reduced from the initial duration time TLDO in order to obtain the minimal value TEDIUM. The delay time position is redefined in order to set the optical pulses in the new minimal “ON” time location.

Pseudo code'. Patern 5

Result', the delay time value for the DLD delay unit is set such that the optical pulses are substantially at the same output power/intensity of the directly modulated laser light source 12, and the "ON" time is minimized.

The predefined permissible minimal and maximal delay time values, can be determined by the period time (set by the repetition rate) of the pattern 5 and pattern 6 signals.

Optionally, but in some embodiments preferably, the calibration sequence 50 and/or 51 further comprises a self-calibration procedure for setting gain value for the VOA unit 14 for required single photon amplitude μ. This procedure requires factory calibration value of a few passive components of the transmitter system 10/10'. A QKD system according to some possible embodiments may employ a weak coherent pulse (WCP) from a modulated laser light source 12 as the photon source. In such embodiments it is important that the mean photon number of the quantum states p is determined precisely and monitored continuously. A possible way to achieve a WCP is by attenuating the coherent light generated by light source 12 such that the output to the QKD channel (i.e., the QC) will have values of p < 1. However, variations e.g., due to thermal instability or other factors, may vary the value of p, making the system vulnerable to attacks or reduce the system efficiency.

Accordingly, it is essential to keep the value of p constant and monitored. This requires factory calibration value of a few passive components (e.g., fixed attenuators, and/or optical splitters and every other optical element on the optical path) in addition to an active automated monitor and correction actions.

Transmitted pattern-, pattern 5 from the Po pulse signal source during the calibration process, and random states (main and decoy) with a known distribution and measured power (power measurement of every state was done in previous stages) during the continuous operation of the system.

Procedure'. Factory calibration of the attenuation and splitting ratios of the optical path after the VOA is required (including temperature dependence if exists) e.g., of the fixed attenuator 17. The attenuation value of the VOA 14 is set to a minimum attenuation. For this stage only, the photodiode of the power meter 16 needs to be calibrated. This procedure measures the optical output power/intensity l _PD measured by the calibrated internal photodiode or power meter 16. The optical output signal i.e., the power/intensity measured for the desired is calculated from all relevant system parameters, for example wavelength, background noise level, repetition rate, decoy protocol parameters, fixed attenuators, splitting ratio etc. In possible embodiments the attenuation value of the VOA 14 is set such that I _PD = 1^ e.g., by using single-photon avalanche diode (SPAD) in the power meter 16.

Pseudo code'.

(Fixed Attenuation is the attenuation resulting from all components with a fixed attenuation, including attenuators, splitters, isolators, splices etc.) Result', the attenuation value of the VOA 14 is adjusted such that are set to the desired target value.

Optionally, but in some embodiments preferably, the calibration sequence 50 and/or 51 further comprises a self-calibration procedure for estimating the crosstalk of the |0) and | 1) main quantum states. In the QKD protocol every signal that is measured in the wrong state is represented by the quantum bit error rate (QBER) and regarded as it may arise from an eavesdropper. To overcome this, the total QBER of the system is taken into account and the maximal possible system secure key rate is calculated accordingly. One possible contribution to this noise is crosstalk caused by non-ideal pulse generation. Specifically, what is the magnitude of the photon leakage from the |0) main quantum state into the time slot of the 11) main quantum state.

Transmited pattern', pattern 5 from the Po pulse signal source and pattern 1 from the Po pulse signal source.

Procedure'. For pattern 5 from the Po pulse signal source measure the optical output power/intensity. Then measure the optical output power/intensity for the pattern 1 from the Po pulse signal source. After subtracting the optical vacuum power/intensity, the Pattern 1 electric/EM pulse signal should produce about twice the optical output power/intensity produced by the pattern 5 electric/EM pulse signal (2 pulses compared to 1). If there is however a difference, it is the crosstalk that is translated to the QBER, as the modulator optical output is non-linear with respect to the electrical input.

Pseudo code'.

MeasurePower( Io, vacuum)

Pattern 5

Result'. Crosstalk is measured and used as parameter for maximal possible secure rate.

Measures to reduce the crosstalk, such as pre-emphasis configurations can be applied accordingly.

Optionally, but in some embodiments preferably, the calibration sequence 50 and/or 51 further comprises a self-calibration procedure for summarizing all of the tests/calibration procedures (Alice benchmarking and parameter estimation). As a final step the intrinsic QBER of Alice is calculated. This allows benchmarking of the QKD transmitter systems 10/10' without the need for a QKD Bob receiver system 19. This can assist in pairing Alice and Bob systems' for specific performance requirements. In addition, it can also help recognizing malfunctions and degradation in the transmitter quality. These parameters can also assist in precise determination of the decoy state parameters. This is achieved in some embodiments by crosstalk estimation, based on differences in the optical output power/intensity measured for the |p) and |m) main quantum states, which affects the QBER directly

Using the testing/calibration procedures disclosed herein, nearly arbitrary electric and optical signals can be generated with standard FPGA and electronics, and versatile programmable pulse generators can be implemented at relatively low costs. Nowadays electric/EM signal generators capable of generating lOOps pulses may cost over 100,000 USD, and this solution can be built for less than 2000 USD.

It should also be understood that throughout this disclosure, where a process or method is shown or described, the steps/acts of the method may be performed in any order and/or simultaneously, and/or with other steps/acts not-illustrated/described herein, unless it is clear from the context that one step depends on another being performed first. In possible embodiments not all of the illustrated/described steps/acts are required to carry out the method.

Functions of the system described hereinabove may be controlled through instructions executed by a computer-based control system 18/18'. The control system 18/18' suitable for use with embodiments described hereinabove may include, for example, one or more processors connected to a communication bus, one or more volatile memories (e.g., random access memory - RAM) or non-volatile memories (e.g., Flash memory). A secondary memory (e.g., a hard disk drive, a removable storage drive, and/or removable memory chip such as an EPROM, PROM or Flash memory) may be used for storing data, computer programs or other instructions, to be loaded into the computer system.

The flowcharts and block diagrams in the different depicted embodiments may represent a module, segment, function, and/or a portion of an operation or step. For example, one or more of the blocks may be implemented as program code, in hardware, or as a combination of the two. Items, such as the various illustrative blocks, modules, elements, components, methods, operations, steps, and algorithms described herein may be implemented as hardware or a combination of hardware and computer software. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application. In an embodiment where the features of the disclosed embodiments are implemented by software, the software can be stored in a computer program product and loaded into the computer system using the removable storage drive, the memory chips or the communications interface. The control logic (software), when executed by a control processor, causes the control processor to perform certain functions of the invention as described herein. In another embodiment, features of the invention are implemented primarily in hardware using, for example, hardware components such as application specific integrated circuits (ASICs) or field-programmable gated arrays (FPGAs). Implementation of the hardware state machine so as to perform the functions described herein will be apparent to persons skilled in the relevant art(s). In yet another embodiment, features of the invention can be implemented using a combination of both hardware and software.

As described hereinabove and shown in the associated figures, the present application provides transmitter systems for QKD applications and related methods. While particular embodiments of the invention have been described, it will be understood, however, that the invention is not limited thereto, since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, all without exceeding the scope of the claims.