FIELD OF INVENTION

The present invention relates to an auto compensating quantum key distribution (QKD) network.

BACKGROUND OF THE INVENTION

Quantum cryptography represents a recent technological development that provides assured privacy of a communication link. Quantum cryptography is based on the laws of quantum physics and permits the detection of eavesdropping across the link in real time. Quantum cryptography technique has been conventionally applied to distribute keys from a single photon source to a single photon detector, either through fiber optic strands or through the air.

In the fiber-based quantum key distribution (QKD) network, a photon has to transmit information from an Alice setup to a Bob setup using optical fiber. Since the protocol encodes information in the phase or in the polarization of the photon, any change in the phase or the polarization state due to mechanical and thermal stresses on the fiber or due to fiber imperfections, must be eliminated or compensated to a degree that permits reliable interferometry, for improved performance with reduced noise.

Certain approaches followed in the art for such elimination or compensation have the drawback that they require use of Faraday mirrors in the Alice setup. Faraday mirrors are nonlinear components, causing nonlinear polarization rotation (NPR). An inherent problem with these approaches is in the stability of the output polarization state. Fluctuations in the linear birefringence caused by temperature changes, drafts in the fiber environment and vibrations generally disturb a proper functioning.

SUMMARY OF THE INVENTION

The present invention overcomes the drawbacks of the existing apparatus and methods by utilizing linear components such as an optical circulator and a spinning profile splicing means in an Alice setup, in order to auto-compensate the signal from a Bob setup, by controlling the birefringence effect with better polarization stability. The configuration proposed under the present invention provides better interference results in the Bob setup and is easy to implement in the field work. This configuration produces a normal interference pattern, as opposed to the interference pattern by Faraday mirror that is inverting. The coding herein is based on phase modulation since the polarization inside the fiber changes through out fiber direction.

According to an embodiment of the present invention, an apparatus for auto compensating Quantum Key Distribution (QKD) network is proposed. The apparatus comprises an Alice setup for receiving a transmitted signal from a Bob setup to form a received signal. The Alice setup further comprises a first phase modulator, a first spinning profile splicing means and a first optical circulator. The arrangement is such that the received signal is forwardable to the first splicing means after a first phase modulation in the first phase modulator, for undergoing a first change in polarization in the first splicing means by a first angle. The received signal is forwardable to the first optical circulator after the first change, for transmitting back the signal to the first splicing means for undergoing a second change in polarization by a second angle for constructing a total change of the first angle added to the second angle. The signal then is returnable to the Bob setup as a returned signal after a second phase modulation in the first phase modulator. The first and second phase modulations are encoded at the first phase modulator for achieving a total phase modulation in all possible directions of polarization. According to an embodiment, the transmitted signal is generated starting from an optical source which is a laser source of a coherent signal, that is provided in the Bob setup.

According to an embodiment the Alice setup further comprises a first attenuator, for attenuating the intensities of the received signal and the returned signal.

According to an embodiment, an unbalanced interferometer in the Bob setup undergoes a flip wherein a long path signal and a short path signal that together form the transmitted signal get reversed for the returned signal.

According to an embodiment, a first photodetector in the Bob setup is capable of receiving destructive interference results from the returned signal arriving from the interferometer, while a second photodetector in the Bob setup is capable of receiving constructive interference results from the returned signal arriving from the interferometer.

According to an embodiment, at least one of the first and the second photodetectors is an avalanche type.

According to an embodiment, the coherent signal is polarized in a first direction and any unpolarized component in the coherent signal is substantially filtered out by a polarizer in the Bob setup, before the signal enters the interferometer.

According to an embodiment, the polarization in the first direction is changed to a second direction in the long path signal by a second spinning profile splicing means, the first direction and the second direction being mutually perpendicular.

According to an embodiment, each of the first and the second angles in the Alice setup is 45 degrees. According to another aspect of the invention, a method for auto compensating Quantum Key Distribution (QKD) network using the apparatus as described above is proposed. The method comprises the steps of receiving the transmitted signal from the Bob setup by the Alice setup to form the received signal; forwarding the received signal to the first splicing means after the first phase modulation in the first phase modulator, for undergoing the first change in polarization by the first angle in the first splicing means; forwarding the received signal to the first circulator after the first change, for transmitting back the signal to the first splicing means for undergoing the second change in polarization by the second angle for constructing the total change of the first angle added to the second angle; returning the signal to the Bob setup as the returned signal after the second phase modulation in the first phase modulator, the first and the second phase modulations being encoded at the first phase modulator for achieving a total phase modulation in all possible directions of polarization.

The method may further comprise a step of generating the transmitted signal starting from the optical source.

The method may further comprise the steps of attenuating the intensities of the received signal and the returned signal by the first attenuator in the Alice setup.

The method may further comprise the steps of receiving destructive interference results from the returned signal arriving from the unbalanced interferometer, by the first photodetector in the Bob setup and receiving constructive interference results from the unbalanced interferometer by the second photodetector in the Bob setup.

The method may further comprise the steps of polarizing the coherent signal in a first direction and substantially filtering out any unpolarized component in the coherent signal by a polarizer in the Bob setup, before the coherent signal enters the interferometer. The method may further comprise a step of changing the polarization in the first direction to the second direction in the long path signal by the second spinning profile splicing means, the first direction and the second direction being mutually perpendicular.

The present invention consists of certain novel features and a combination of parts hereinafter fully described and illustrated in the accompanying drawings andparticularly pointed out in the appended claims; it being understood that various changes in the details may be possible without departing from the scope of the invention or sacrificing any of the advantages of the present invention.

BRIEF DESCRIPTION OF T HE DRAWINGS

In the following drawings, same reference numbers generally refer to the same parts throughout. The drawings are not necessarily to scale, instead emphasis is placed upon illustrating the principles of the invention. The various embodiments and advantages of the present invention will be more fully understood when considered with respect to the following detailed description, appended claims and accompanying drawings wherein:

Figure 1 : A block diagram of an auto-compensating setup for the QKD network, using a circulator and a spinning profile splicing means in the Alice setup, according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description presents several preferred embodiments of the present invention in sufficient detail such that those skilled in the art can make and use the invention. Before describing in detail embodiments that are in accordance with the present invention, it should be noted that all of the figures are drawn for ease of explanation of the basic teachings of the present invention only. The extension of the figures with respect to the number, position, relationship and dimension of the parts of the preferred embodiment will be within the skill of the art after the following teachings of the present invention have been read and understood. Further, the exact dimensions and dimensional proportions to conform to specific force, weight, strength and similar requirements will likewise be within the skill of the art after the following teachings of the present invention have been read and understood.

According to an embodiment of the present invention, as shown in Figure 1, an apparatus (100) for auto compensating Quantum Key Distribution (QKD) network, comprises a Bob setup (2) for transmitting a polarized optical signal to form a transmitted signal and an Alice setup (1) for receiving the transmitted signal to form a received signal. The Alice setup (1) uses a first circulator (12) and a first spinning profile splicing means (10). A first phase modulator (11) is preferably used for changing the phase of the received signal. In this embodiment, the arrangement is such that the received signal is forwardable to the first spinning profile splicing means (10) after a first phase modulation in the first phase modulator (11), for undergoing a first change in polarization in the first spinning profile splicing means (10) by a first angle. The received signal is forwardable to the first optical circulator (12) after the first change, for transmitting back the signal to the first spinning profile splicing means (10) for undergoing a second change in polarization by a second angle for constructing a total change of the first angle added to the second angle. Each of the first and the second angles may preferably be 45 degrees, causing the total change by 90 degrees. The signal is then returnable to the Bob setup (2) as a returned signal after a second phase modulation in the first phase modulator (11). The first and second phase modulations are encoded at the first phase modulator (11) for achieving a total phase modulation in all possible directions of polarization. The Alice setup (1) may further comprise a first attenuator (9), for attenuating the intensity of the received signal preferably before the first phase modulation and after the second phase modulation. The received signal will have a change in phase in the first phase modulator (11), thus creating the state for the QKD. The first spinning profile splicing means (10) provides a full coverage of polarization modulation. It may have a 45 degrees setting, such that the polarization of the received signal changes by the first angle of 45 degrees. Thereafter the received signal enters the first optical circulator (12) which creates a reflective effect in the signal by transmitting back the signal through the first spinning profile splicing means (10) and the first phase modulator (11) and then back to Bob setup (2). Thus, the phase modulation at Alice setup (1) is passed twice by the signal during this process, for achieving a total phase modulation in all possible direction of polarization. The function of the first spinning profile splicing means (10) and the first circulator (12) is similar to the function of the conventionally used Faraday mirror.

The conventional Bob setup (2) may be maintained using an optical source (3), a first and a second photodetectors APD 1, APD 2 (13 a, 13 b), a second phase modulator (6), a second circulator (5), a first and a second polarization beam splitters (PBS) (8a, 8b), a second optical attenuator (16), an unbalanced interferometer (4) and polarizer (13). The optical source (3) may be a laser source. This laser source is normally a weak coherent signal to be used as the single photon source for the QKD network. The scope of the invention is equally applicable to other suitable types of optical sources. The optical source (3) commonly emits a vertically polarized signal and a polarizer (13) filters out un- polarized signal if any before the signal enters the interferometer (4). Preferably, after the filtration, the coherent signal may be attenuated by the second optical attenuator (16), before being forwarded to the second circulator (5). The interferometer (4) undergoes a flip wherein a long path signal (15a) and a short path signal (15b) within the interferometer (4) that together form the transmitted signal get reversed for the returned signal, wherein the state is already encoded on one of the two signals. The property of polarization on both signals is already flipped, due to the first spinning profile splicing means (10) and the first circulator. (12) at Alice setup (1). This will ensure that the long path signal (15a) which goes through a longer arm of the interferometer (4) will return to its shorter arm and vice versa for the returned signal. The second PBS (8b) will direct both the long path signal (15a) and the short path signal (15b) based on the polarization which is already compensated by returning the signal within the same fiber. Thus, the network works in the bidirectional direction where the signal is reflected back to this interferometer (4), from the Alice setup (1). The signal from the optical source (3) enters through a first arm of the circulator (5) and goes through a second arm of the circulator (5) to the interferometer (4) via the first PBS (8a). The second photo detector APD 2 (13b) collects the signal from a third arm of the circulator (5). For the long path signal (15 a), the vertically polarized signal passes through the second phase modulator (6) and then through a second spinning profile splicing means (7) where the vertical polarization is tuned into a horizontal polarization. The second spinning profile splicing means (7) is preferably placed after second phase modulator (6) in order to enable use of any type of phase modulator, whether polarization dependent or independent. The purpose of the second spinning profile splicing method is to identify the path of the optical signal passing through, which can be further directed by the second PBS (8b) to form the transmitted beam. The short path signal (15b) is directly connected to the second PBS (8b) from the first PBS (8a). Both the long path signal (15a) and the short path signal (15b) are combined at the second PBS (8b) and forwarded by single mode fiber to the Alice setup (1). This method makes the interference perfect with no adjacent signals. The APD 1 (13a) is capable of receiving destructive interference results from the returned signal arriving from the interferometer (4) and through the PBS (8a), while the APD 2 (13b) is capable of receiving constructive interference results arriving from the interferometer (4) and through the second optical circulator (5). Conventionally, the APD 1 (13a), which is connected to the first PBS (8a), collects the constructive interference results in the network, while APD 2 (13b), which is connected to the circulator (5) collects the destructive interference results in the network. However, according to an embodiment of the present invention, APD 1 (13a) and the APD 2 (13b) function inversely when compared to the conventional configuration. Due to the polarization rotation splicing point in Alice setup (1) and Bob setup (2) which gives a 90 degrees and a 45 degrees rotation respectively, the function of both APD 1 (13a) and APD 2 (13b) is inversed. The APD 1 and APD 2 (13a,13b) may be avalanche type photodetectors. However, any other type of suitable photodetectors may be used. The Bob setup (2) may as well have some other arrangement as the scope of the invention is limited to the Alice setup (1). Therefore, with the proposed configuration, the security level is maintained for the performance in the QKD network. It can be constructed in a wavelength-division multiplexing (WDM) environment supporting at least three communication links within a single fiber. For the present invention, the coding is based on phase modulation since the polarization inside the fiber changes throughout the fiber direction. The polarization mode fibers fix the polarization into two polarization axis: vertical and horizontal. The purpose of this arrangement is to achieve auto- compensating technique, where the interference of two signals is achieved without the interference of any side signal. As to further discussion of the manner of usage ahd operation of the present invention, the same should be apparent from the above description. Accordingly, no further discussion relating to the manner of usage and operation will be provided.

invention along with mp^ -d taiTs ^' set forth for purpose of illustration, it will be imde?3£©¾rd" by ^" those skilled in the art that many variations or modifications in details of design, construction and operation may be made without departing from the present invention as defined in the claims. The scope of the invention is as indicated by the appended claims and all changes that come withiii the meaning . and range of equivalency of the claims are intended to be embraced therein.