KULIK SERGEY (MY)
WAHIDDIN MOHAMED RIDZA (MY)
KULIK SERGEY (MY)
| US20020097874A1 | 2002-07-25 | |||
| US6522749B2 | 2003-02-18 | |||
| US20050135620A1 | 2005-06-23 | |||
| US7304314B2 | 2007-12-04 |
Claims
1. A system for quantum key distribution comprising of
a preparation block for generating a plurality of biphotons having polarisation states;
a transformation block for encoding information in polarisation degree of freedom after receiving said biphotons from said preparation block;
a measurement block for encoding said information after receiving said information from said transformation block
wherein a quantum channel is provided as a free space for allowing said biphotons and said information transferred and received among said preparation block, said transformation block and said measurement block.
2. The system as claimed in Claim 1 , wherein a Bob Station or receiver comprises said preparation block and said measurement block.
3. The system as claimed in Claim 1 , wherein an Alice Station or sender comprises said transformation block.
4. The system as claimed in Claim 1, wherein said quantum channel allows said biphoton propagated without noticeable losses.
5. The system as claimed in Claim 1 , wherein said preparation block further comprising of
a non-linear crystal having periodical structure for generating said biphotons by a laser into said crystal;
a first multiplexer for splitting said biphotons generated into two single-mode fibres; a plurality of polarisation controllers for applying polarisation transformations to said two single-mode fibres; a second multiplexer for combining said single-mode fibres into a single node; and
a fibre coupler having a telescope for forming a beam and emitting into said quantum channel after receiving said single node from said second multiplexer.
6. The system as claimed in Claim 5, wherein said crystal is a periodically polled nonlinear crystal with periodical structure.
7. The system as claimed in Claim 5, wherein said crystal is cut to form said biphotons at different frequencies satisfying energy conservation and propagating different directions for satisfying momentum conservation.
8. The system as claimed in Claim 5, wherein said biphotons are produced via a Spontaneous Parametric Down Conversion (SPDC).
9. The system as claimed in Claim 5, wherein said crystal is cut according to non-collinear and frequency -non-degenerate Type 1 of SPDC.
10. The system as claimed in Claim 5, wherein said biphotons having a same polarisation.
11. The system as claimed in Claim 5, wherein said polarisation controllers are operated by applying voltage to a piezoelectric transformer.
12. The system as claimed in Claims 5 and 11 , wherein said polarisation transformations are proportional to said applied voltage.
13. The system as claimed in Claim 5, wherein a state is produced by said fibre coupler to be sent to said Alice Station or sender.
14. The system as claimed in Claim 1 , wherein said transformation block further comprising of a first fibre coupler having a first telescope for receiving said state from said Bob Station or receiver and transforming into said biphotons; a first multiplexer for splitting said biphotons into two single-mode fibres to form single photons with different wavelengths;
a plurality of polarisation controllers for performing transformations on said single photons;
a second multiplexer for combining said single photons into a single node; and
a second fibre coupler having a second telescope for forming a beam and emitting into said quantum channel after receiving said single node from said second multiplexer.
15. The system as claimed in Claim 14, wherein said transformations are according to two- way deterministic protocol.
16. The system as claimed in Claim 14, wherein said transformations allow said Alice Station or sender to encode information in polarisation degree of freedom.
17. The system as claimed in Claim 14, wherein a state is produced by said second fibre coupler to be sent to said measurement block of said Bob Station or reciever.
18. The system as claimed in Claim 1 , wherein said measurement block further comprising of a fibre coupler having a telescope for receiving said state from said Alice Station or sender and transforming into said biphotons;
a multiplexer for splitting said biphotons from said fibre coupler into single photons with different wavelengths;
a pair of polarisation beam splitters for separating said single photons with orthogonal polarisation; and a plurality of polarisation controllers and detectors for detecting different polarisation states. |
A system for preparation and unitary transformation of quantum states for quantum key distribution
Technical Field of the Invention
The present invention relates to the field of quantum key distribution particularly to a system for preparation and unitary transformation of quantum states.
Background of the Invention
Quantum key distribution involves establishing a key between a sender ("Alice") and a receiver ("Bob") by using weak (e.g. 0.1 photon on average) pulsed optical signals transmitted over a "quantum channel". The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper ("Eve") that attempts to intercept or otherwise measure the quantum signal will introduce errors into transmitted signals and reveal her presence.
A straightforward way of particular ququart state preparation was recently discussed by Bogdanov et al. (2006) on Polarisation states of four-dimensional systems based on biphotons. The alternative device discussed in these papers is based on the polarisation transformations under the biphotons performed in a single mode simultaneously on both wavelengths. This method has nevertheless, serious disadvantage caused by the necessity to insert in the beam retardant plates with fixed thickness. So each transformation can be done with particular retardant plate only and limits the speed of information encoding. Even if combined retardant plate was used, as suggested in the above-mentioned citation, the speed of information will be limited by the slow mechanical switching of the angle between the parts of the plate. This is principal limitation that cannot be overcome in practical implementation of quantum information protocols based on polarisation ququarts.
It is an object of present invention to overcome the drawbacks of the above-mentioned conventional method.
Summary of the Invention
The present invention provides a system for quantum key distribution comprising of
a preparation block for generating a plurality of biphotons having polarisation states;
a transformation block for encoding information in polarisation degree of freedom after receiving the biphotons from the preparation block;
a measurement block for encoding the information after receiving the information from the transformation block
wherein a quantum channel is provided as a free space for allowing the biphotons and the information transferred and received among the preparation block, the transformation block and the measurement block.
According to one aspect of the present invention wherein a Bob Station comprises the preparation block and the measurement block.
According to another aspect of the present invention wherein an Alice Station comprises the transformation block.
According to another aspect of the present invention wherein the quantum channel allows the biphoton propagated without noticeable losses.
According to another aspect of the present invention wherein the preparation block further comprising of
a non-linear crystal having periodical structure for generating the biphotons by a laser into the crystal;
a first multiplexer for splitting the biphotons generated into two single-mode fibres;
a plurality of polarisation controllers for applying polarisation transformations to the two single-mode fibres;
a second multiplexer for combining the single-mode fibres into a single node; and
a fibre coupler having a telescope for forming a beam and emitting into the quantum channel after receiving the single node from the second multiplexer.
According to another aspect of the present invention wherein the crystal is a periodically polled non-linear crystal with periodical structure.
According to another aspect of the present invention wherein the non-linear crystal is cut to form the biphotons at different frequencies satisfying energy conservation and propagating different directions for satisfying momentum conservation.
According to another aspect of the present invention wherein the biphotons are produced via a Spontaneous Parametric Down Conversion (SPDC).
According to another aspect of the present invention wherein the non-linear crystal is cut according to non-collinear and frequency -non-degenerate Type 1 of SPDC.
According to another aspect of the present invention wherein the biphotons having a same polarisation.
According to another aspect of the present invention wherein the polarisation controllers are operated by applying voltage to a piezoelectric transformer.
According to another aspect of the present invention wherein the polarisation transformations are proportional to said applied voltage.
According to another aspect of the present invention wherein a state is produced by the fibre coupler to be sent to said Alice Station.
According to another aspect of the present invention wherein the transformation block further comprising of
a first fibre coupler having a first telescope for receiving the state from the Bob Station and transforming into the biphotons;
a first multiplexer for splitting the biphotons into two single-mode fibres to form single photons with different wavelengths;
a plurality of polarisation controllers for performing transformations on the single photons;
a second multiplexer for combining the single photons into a single node; and
a second fibre coupler having a second telescope for forming a beam and emitting into the quantum channel after receiving the single node from the second multiplexer.
According to another aspect of the present invention wherein the transformations are according to two-way deterministic protocol.
According to another aspect of the present invention wherein the transformations allow the Alice Station to encode information in polarisation degree of freedom.
According to another aspect of the present invention wherein a state is produced by the second fibre coupler to be sent to the measurement block of the Bob Station.
According to another aspect of the present invention wherein the measurement block further comprising of
a fibre coupler having a telescope for receiving the state from the Alice Station and transforming into the biphotons;
a multiplexer for splitting the biphotons from the fibre coupler into single photons with different wavelengths;
a pair of polarisation beam splitters for separating the single photons with orthogonal polarisation;
a plurality of polarisation controllers and detectors for detecting different polarisation states.
Brief Description of the Drawings
The novel features that are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its structure and operation together with the additional objects and advantages thereof are best understood through following description of exemplary embodiments of the present invention when read in conjunction with accompanying drawings.
Figure 1 illustrates a block diagram showing a system of quantum key distribution in accordance with one embodiment of the present invention;
Figure 2 illustrates a schematic diagram showing a preparation block of a Bob Station in accordance with another embodiment of the present invention;
Figure 3 illustrates a schematic diagram showing a transformation block of an Alice Station in accordance with another embodiment of the present invention; and
Figure 4 illustrates a schematic diagram showing a measurement block of a Bob Station in accordance with another embodiment of the present invention.
Detailed Description of the Invention
For the purposes of the present invention, the term biphoton is referred to as an entangled- photon pair, or two photons quantum-mechanically entangled together,
The present invention provides a combination of all-fibre preparation, transformation and measurement of the quantum states needed for two-way 6- state deterministic protocol and their transmitting through free-space. The system of the present invention allows controlling and switching states by means of available optical components, which is a crucial point in the systems.
Referring to Figure 1, the system of the present invention 122 comprises two stations namely a Bob Station 110 and an Alice Station 112 connected by a free-space quantum channel 114. Quantum channel 114 is a space between the two stations 110,112 where biphoton polarisation states are propagated without noticeable losses and depends on the positions where both stations 110,112 located.
A Bob Station 110 of the system of the present invention comprises two main blocks namely a preparation block 116 and a measurement block (decoding) 118 while an Alice Station 112 of the system of the present invention having a transformation (encoding) block 120.
Figure 2 illustrates a schematic diagram showing a preparation block 116 of a Bob Station 110 in accordance with another embodiment of the present invention. A laser 210 is used to pump a periodically polled non-linear crystal 212 with periodical structure. During this process, Spontaneous Parametric Down Conversion (SPDC) occurs in the crystal 212. Therefore, at the output of the crystal 212 pairs of correlated photons 214 appear. The crystal 212 used in the system of the present invention is cut for collinear, frequency-non- degenerate Type I SPDC regime. This will emit pairs of photons with the same polarisation but with different frequencies satisfying to energy conservation
ω = ω λ +ω 2
and propagating along different directions θ x and θ 2 satisfying to momentum conservation:
k p =k v + k 2 + Q,
where k ,k v k 2 are the wave vectors of the pump, signal and idler photons, and
Q = n — is a vector,
λ
characterising the spatial periodical structure of the crystal to satisfy quasi-phase matching, λ is a spatial period of the structure, and n is a unit vector, perpendicular to the layers.
Such performance of nonlinear crystal 212 is chosen in the system of the present invention to increase the efficiency of Spontaneous Parametric Down-Conversion (SPDC) as well as to couple with the laser and SPDC modes into the single-mode fibres as described by Tanzilli et al., 2002.
The polarization state of the biphoton field generated in such process is:
This state belongs to two single spatial modes while being distributed among two frequency modes due to collinear and non-degenerate regime of the Spontaneous Parametric Down- Conversion (SPDC). The correlated photons 214 enter a first wavelength-division multiplexer (WDM1 ) 216, which splits these photons 214 with different frequencies into two, single-mode fibres 218. This can be done by applying polarisation transformations using polarisation controllers 220, 222 (PC1 and PC2) acting on two fibres 218 independently. In particular an example of a set of output states, which can be prepared using initial state
IV 1 V 2 ) and two polarisation controllers 220, 222 are shown in Table 1 below:
Table 1
The polarisation controllers 220, 222 of the system of the present invention are operated by applying voltage to piezoelectric transformers. The introduced phase shift between two main polarisations is proportional to the applied voltage.
Then, these photons will meet a second wavelength-division multiplexer 224 (WDM2) which combine two spatial modes into a single node 226 and enter into a first fibre coupler 228 (C1) provided with a first telescope 230 (T1) are used to form good quality beam emitted into quantum channel (free-space) 114. Hence, a state ψf| 232 is produced and then sent to the Alice Station 112.
The quantum state jψ™ } 232 sent by the preparation block 116 of the Bob Station 110 is coupled to the fibre using input a second telescope 310 (T2) and a second fibre coupler 312 (C2) in the transformation block 120 of the Alice Station 112 as shown in Figure 3. Biphotons are formed and entered into a third wavelength-division multiplexer 314 (WDM3). These biphotons are then splitted into two single-mode fibres 318 in a way to form single photons with different wavelengths propagate along different fibres. Polarisation controllers 320, 322 (PC3 and PC4) are used to perform necessary transformation X, Y, Z, and / for these fibres 318 according to the two-way deterministic protocol, shown in the Table 2 below:
Table 2
wherein
These transformations allow the Alice Station 112 to encode information in polarisation degree of freedom. After transformations by polarisation controllers 320, 322 (PC3 and PC4), these photons will meet a fourth wavelength-division multiplexer 324 (WDM4) which combines two spatial modes into a single node 326 as similar with the preparation block 116 of the Bob Station 110 of the system of the present invention. The single node 326 enters into a second fibre coupler 328 (C2) provided with a second telescope 330 (T2) are used to form good quality beam emitted into quantum channel (free-space) 114. Hence, a state ψ 4 0 '") 332 is produced and then sent to the measurement block (decoding) 118 of the Bob Station 110 for decoding.
The state ψf) 332 sent by the transformation block 120 of the Alice Station 112 enters the measurement block (decoding) 118 of the Bob Station 110 and received by a fourth telescope 410 (T4) and a fourth fibre coupler 412 (C4). Biphotons 414 are formed and separated by a Fifth Wavelength-Division Multiplexer 416(WDM5) into photons 417, 419 with different wavelengths. A pair of polarisation beamsplitters 418, 420 is provided, to separate these photons with orthogonal polarisation.
The four-input double-coincidence scheme linked with the outputs of single-photon detectors D1, D2, D3, D4 registers the biphoton-based ququarts. The scheme works as follows:
1. If state IH 1 H 2 ) comes, then detectors D4, D2 will fire;
2. If state U 1 V 2 ) comes, then detectors D4, D 1 will fire;
3. If state IV 1 H 2 ) comes, then detectors D3, D2 will fire;
4. If state IV 1 V 2 ) comes, then detectors D3, D1 will fire.
The choice of the appropriate state under protocol is provided by polarisation controllers PC5, PC6, PC7, and PC8 introduced in front of single-photon detectors D1 , D2, D3, and D4. According to the protocol, Bob 110 switches between polarisations each time when he sends a state to Alice 112. Thus, if his measurement is performed in the correct basis and this will allow Bob 110 to learn deterministically which transformation was chosen by Alice 112 to encode the information for example when registered in the measurement block mean coincidences between pairs of detectors. The input identification allows one to determine which pair is fired.
One of the advantages of the system of the present invention performs transformations quickly in time by regular polarisation transformers, which are commercially available. Another advantage of the system of the present invention is able to control all polarisation states, which are necessary to implement the protocol and can be realised by regular polarisation elements, which are commercially available.
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.
References:
"PPLN waveguide for quantum communication" by S. Tanzilli; W. Tittel, H. De Riedmatten, H. Zbinden, P. Baldi, M. De Micheli, D.B. Ostrowsky, and N. Gisin, THE EUROPEAN PHYSICAL JOURNAL D 18, 155-160 (2002)].
Yu.l.Bogdanov, R.F.Galeyev, S.P.Kulik, G.A.Maslennikov, E.V.Moreva, "Polarization four- dimensional systems based on biphotons", Phys. Rev. A 1 73, 063810 (2006).
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