RU2754758C1 - Optical circuit of the receiver with one detector and the system for quantum key distribution (options) - Google Patents

Abstract

FIELD: quantum cryptography.SUBSTANCE: invention relates to quantum cryptography and means for quantum key distribution over an open space. The effect is achieved due to the implementation of the optical scheme of the receiver for quantum key distribution, comprising: a detector of single photons; a switch made with the possibility of rotation or linear movement, providing the substitution of optical elements on the path of the detected radiation in front of the single photon detector; and optical elements located on the switch.EFFECT: decrease in noise pulses and, as a consequence, a decrease in the level of quantum errors in the key due to the use of a single detector of single photons in the circuit.14 cl, 5 dwg

Description

FIELD OF TECHNOLOGY

[0001] The claimed technical solution generally relates to the field of computing, and in particular to quantum cryptography and means for quantum key distribution in an open space based on polarization coding with one detector.

LEVEL OF TECHNOLOGY

[0002] JP 5492255 B2 "Quantum communication system" (TOSHIBA CORP., Published: 05.14.2014) is known in the art. This solution discloses a quantum key distribution (QKD) system using the standard BB84 protocol. The system contains a quantum receiver, which is used for signal polarization coding.

[0003] The receiver optical circuit includes a detector subsystem consisting of two single photon detectors connected to two interferometer outputs and an electronic control circuit.

[0004] In this case, the interferometer includes an input beam splitter (BS) with a division of 50% by 50%.

[0005] Self-differential avalanche photodiodes or sinusoidal gate avalanche photodiodes are used to detect single photons in this system. In the receiver circuit, a polarizer can be used, which is installed in front of the phase modulator.

[0006] In addition, the prior art solution uses four phase shifts in the BB84 protocol when encoding quantum states at 0 °, 90 °, 180 °, and 270 °.

[0007] Also known from the prior art is the solution US 9935721 B2 "Optical communication method and optical communication system" (Furukawa Electric Co Ltd, published: 03.04.2018). This solution discloses an example of an optical scheme in which, instead of a half-wave plate, the receiving unit uses gates, one of which is installed in the path of receiving a signal from the transmitter, and the second is installed in front of the polarizer of the avalanche LED.

[0008] A disadvantage of the known solutions in this field of technology is the complicated design of the receiver, which leads to an increase in noise pulses, affecting the appearance of quantum errors in the key.

DISCLOSURE OF THE INVENTION

[0009] In the claimed technical solution, a new optical scheme of the receiver for quantum key distribution is proposed, which differs from the classical scheme in a simplified design.

[0010] The main technical result is the reduction of noise pulses and, as a consequence, a decrease in the level of quantum errors in the key due to the use of a single detector of single photons in the circuit.

[0011] The specified technical result is achieved through the implementation of the optical scheme of the receiver for quantum key distribution, containing:

- single photon detector;

- a switch providing the substitution of optical elements on the path of the detected radiation in front of the single photon detector; and

- optical elements located on the switch.

[0012] In one of the particular embodiments of the scheme, the optical elements are selected from the group: polarizers, wave plates with a polarizer.

[0013] In another particular embodiment of the circuit, the switch is rotatable or linearly movable.

[0014] In another particular embodiment of the circuit, the switch further comprises reflective elements.

[0015] In another particular embodiment of the circuit, the switch is configured to deflect the beam by a reflective element.

[0016] In another particular embodiment of the circuit, the reflective element is a mirror or a prism.

[0017] In another particular embodiment of the circuit, the switch is configured with an acousto-optic deflector.

[0018] Also, the specified technical result is achieved through the implementation of the optical scheme of the receiver for quantum key distribution, containing:

- single photon detector;

- at least one polarizer installed in front of the single photon detector;

- a switch providing the substitution of optical elements on the path of the registered radiation in front of the polarizer; and

- optical elements located on the switch.

[0019] In one of the particular embodiments of the circuit, the optical elements are wave plates.

[0020] In another particular embodiment of the circuit, the switch is rotatable or linearly movable.

[0021] In another particular embodiment of the circuit, the switch further comprises reflective elements.

[0022] In another particular embodiment of the circuit, the switch is configured to deflect the beam by the reflective element.

[0023] In another particular embodiment of the circuit, the reflective element is a mirror or a prism.

[0024] In another particular embodiment of the circuit, the switch is configured with an acousto-optic deflector.

[0025] In addition, the claimed solution is implemented using a system for quantum key distribution containing, a quantum communication channel between a transmitter and a receiver, a transmitter unit configured to generate single photons and transmit them to the input of the receiver unit containing the above optical circuits.

[0026] In another particular implementation of the system, the data channel is a channel through an open space.

BRIEF DESCRIPTION OF DRAWINGS

[0027] Features and advantages of the present invention will become apparent from the following detailed description of the invention and the accompanying drawings.

[0028] FIG. 1 illustrates a prior art example.

[0029] FIG. 2A illustrates an example of an optical receiver circuit for quantum key distribution without a polarizer.

[0030] FIG. 2B illustrates an example of an optical scheme of a receiver for quantum key distribution with a wave plate and without a polarizer.

[0031] FIG. 3 illustrates an example of an optical scheme of a receiver for quantum key distribution with a polarizer.

[0032] FIG. 4 illustrates an example of a system for quantum key distribution.

CARRYING OUT THE INVENTION

[0033] Consider the basic principles of quantum key generation based on the BB84 protocol [1-4]. The transmitting side (traditionally referred to in the literature as Alice) prepares single-photon states with linear polarization in two bases that are not orthogonal to each other (Fig. 1).

[0034] One vertical-horizontal "Basis 1" - with photon polarization 0 and 90 degrees. The second is the diagonal "Basis 2" - with polarization 45 and -45 degrees.

[0035] Alice and the receiver (traditionally called Bob) agree on a binary code for each polarization, for example, photons with polarizations of 0 and 45 degrees represent the number 0, and photons with polarizations of 90 and -45 degrees represent the number 1.

[0036] During the transmission, Alice sends a sequence of photons, the polarization of which is randomly selected, and can be 0, 45, 90 and 45 degrees. Bob registers the arrived photons, and for each of them randomly chooses the basis of measurement "Basis 1" or "Basis 2".

[0037] Through the open supplemental communication channel, Bob informs Alice in which basis the measurements were taken, but does not report the result of this measurement. Since a photon can have a value of both "0" and "1", the message about the fact of registration of a photon through an open channel does not provide any information in the case of the presence of an extraneous eavesdropping unit called Eve (from the English Eavesdropper).

[0038] Alice responds by telling Bob if the correct measurement basis has been chosen for each photon. Keeping in the series only measurements made in the correct basis, Alice and Bob create a unique random sequence of zeros and ones, from which they then form a secret key.

[0039] FIG. 1 depicts a typical receiver node used for quantum key distribution based on polarization coding (Bob node). At the entrance to Bob's receiving unit there is a polarization controller based on wave plates. In some devices, in practice, it is absent [1, 3].

[0040] Such a widespread scheme employs a λ / 2 half-wave plate and a polarizing prism (PBS). The λ / 2 half-wave plate and the PBS polarizing prism require high accuracy, adherence to complex technology, and are used for a rather narrow wavelength range (a half-wave plate is made for a specific wavelength, with a tolerance for the λ / 300 phase difference). When assembling the optical scheme, the λ / 2 plate and the PBS prism require precise alignment in order to correctly separate the different polarizations of the light beam. These elements are interconnected, for example, when changing the position of the λ / 2 plate, an adjustment of the PBS prism is required.

[0041] FIG. 2A shows the optical scheme (10) of the Bob receiver (100) for quantum key distribution, which contains: a single photon detector APD (101), a switch (102) that provides the substitution of optical elements (103) on the path of the detected radiation in front of the single photon detector APD ( 101) and optical elements (103) located on the switch (102).

[0042] The optical elements (103) can be polarizers or wave plates with a polarizer. The switch (102) is made with the possibility of rotation around its axis or linear movement to ensure the substitution of optical elements (103) under the optical beam. The switch (102) may additionally contain reflective elements (not shown) that provide beam deflection at a predetermined angle. A mirror or a prism can be used as a reflective element. Also, in another embodiment, the switch (102) can be configured with an acousto-optic deflector that deflects the initial direction beam when scattered on an acoustic wave and directs it to the optical element.

[0043] FIG. 3 shows the second variant of the optical scheme (11) of the Bob receiver (100) for quantum key distribution, which in its design additionally contains at least one polarizer P (104) installed in front of the ADP single photon detector (101). In this version of the circuit (11), the switch (102) provides the substitution of optical elements (103) on the path of the detected radiation in front of the polarizer P (104).

[0044] As seen in FIG. 2A to FIG. 2B and Fig. 3, in the declared optical schemes (10, 11) of the receiving unit (100), there is no BS beam splitting cube and technologically complex PBS polarization prisms and a λ / 2 half-wave plate. Instead of four ADP detectors, one (101) is used. Since the ADP single photon detectors have their own noise level, these noise (spurious) pulses contribute to the level of quantum errors in the key. When using schemes (10, 11) with one ADP detector (101), fewer noise pulses are formed, which leads to a decrease in the level of quantum errors in the key.

[0045] For correct operation of the receiving unit (100), in front of the single photon detector APD (101), the switch (102) substitutes optical elements (103) that allow decoding a signal consisting of various quantum states of a photon. As mentioned above, the switch (102) can be different in design based on rotation, movement, deflection, etc., or it can be composed of several switches. In this case, the number of optical elements (103) can be varied based on their need to form the required deflection of the optical beam at the required angle (for example, 4, 6, 8 elements, etc.).

[0046] By the time a photon arrives from the transmitter unit to the receiver (100), the switch (102) randomly places one of the optical elements (103) in front of the APD (101). The key generation rate is proportional to the operating speed of the switch (102).

[0047] The most commonly used protocol for polarization coding is the BB84 protocol [1-4]. When encoding quantum states in the BB84 protocol, four polarizations of radiation (photons) 0, -45, +45 and +90 degrees are used. The transmitter unit (200) Alice (Fig. 4) generates such photons and directs them to a communication channel, in this case, through an open space, for example, a quantum channel. These single photons arrive at the input of Bob's declared receiver (100).

[0048] Consider the case when photons with polarization 0, -45, +45 and +90 degrees arrive at the input of the receiving node (Fig. 1). For correct operation of the receiving unit (100), the optical elements (103) in the scheme (10) can be film polarizers with a polarization separation ratio above 1: 100.

[0049] In particular, the BB84 protocol requires three polarizers. In this case, one window in the switch (102) remains empty, in the other windows there are polarizers with an orientation of -45, +45 and +90 degrees. The "Basis 1" will include a window without a polarizer and with a polarizer of 90 degrees. "Basis 2" will include polarizers -45 or +45 degrees. A total of four optical elements are required for the BB84 protocol (103).

[0050] By the time a photon arrives from the transmitter (200) to the receiver (100), the switch (102) randomly places one of the optical elements (103) in front of the detector. The key generation rate is proportional to the operating speed of the switch. The switch (102) is controlled by a software algorithm implemented by a computing device associated with the receiver (100). The operation of the algorithm is related to the operation of the random key distribution program.

[0051] If a photon with vertical polarization - 0 degrees came to the input of the receiver (100), then this photon has a value of "0" in the vertical-horizontal basis "Basis 1". There are four options for the arrangement of optical elements (103) in front of the APD (101). If before the APD (101) the switch (102) has installed an optical element (103) without a polarizer, then the APD (101) will trigger this photon.

[0052] Hereinafter, for simplicity, it is assumed that the quantum efficiency of the single photon detector APD (101) is 100%. If before the APD (101) the switch (102) sets at this moment the polarizer with the orientation of 90 degrees, then the APD (101) will not work. If before the APD (101) switch (102) sets the polarizer -45 or +45 degrees, then APD (101) will register a photon with a probability of 50%.

[0053] If APD (101) registered a photon, then Alice (200) informs through the open communication channel in which basis this photon was prepared and sent to Bob (100). If Alice (200) used "Basis 1" for coding, and APD (101) triggered with an optical element (103) without a polarizer, then this triggering of APD (101) is taken into account in the quantum key with the value "0". If the APD (101) triggered when the switch (102) substituted the optical element (103) with polarizers of -45 or +45 degrees (referring to "Basis 2"), then this triggering of the APD (101) is not taken into account.

[0054] Thus, if APD (101) registered a photon, then the value of this photon is included in the quantum key, provided that the bases of emission by the Alice (200) unit and the receiver Bob (100) coincide. If the basis of emission by Alice's block (200) does not coincide with the basis of Bob's receiving unit (100), then this APD triggering (101) is not taken into account and does not participate in the formation of the quantum key.

[0055] In one of the special cases shown in FIG. 2B, if photons with polarizations other than 0, -45, +45, and +90 degrees arrive at the input of the receiving unit (100), then a polarization controller (105) is additionally used in the optical scheme (10). The polarization controller (105) is used to change the polarization to the desired values of 0, -45, +45 and +90 degrees.

[0056] In addition to the BB84 protocol, the Six-state protocol (SSP) [5] is also known from the prior art [5], which can also be used to distribute a quantum key over an open space. When encoding quantum states in the SSP protocol, six polarizations of 0, -45, +45 and +90 degrees are used, circular right-handed and circular left-handed. These states are combined into three bases.

[0057] The states with polarizations of 0 and 90 degrees constitute the first basis - "Basis 1". States with polarization -45, + 45 degrees make up the second basis - “Basis 2”. The states of photons with polarization circular dextrorotatory and circular dextrorotatory can be combined into the third basis "Basis 3".

[0058] For the correct operation of the declared optical scheme (10) of the receiving node in this protocol, one hole in the switch (102) remains empty, in three windows there are elements (103) in the form of polarizers with an orientation of -45, +45 and +90 degrees , in the fifth and sixth windows there are quarter-wave plates with a polarizer.

[0059] The algorithm for obtaining a quantum key is similar to the BB84 protocol. The switch (102) randomly substitutes one of the optical elements (103) for the incoming photon. If APD (101) registered a photon, then the value of this photon is included in the quantum key, provided that the bases of emission and reception coincide. If the emission basis (for Alice) does not coincide with the reception basis (for Bob), then this APD trigger (101) is discarded and does not participate in the formation of the quantum key.

[0060] Example.

Suppose that Alice's block emitted a photon with a polarization of 0 degrees in basis 1. Bob's block, upon receiving this photon, substituted an optical element that belongs to basis 2. If the APD triggered, then this APD trigger is removed from the quantum key sequence. If during the operation of the switch in the Bob block, the substitution of the optical element from basis 1 is carried out and the APD is triggered, then such a triggering of the detector is taken into account when forming the key.

[0061] When encoding quantum states by polarization based on other protocols, the optical elements (103) may consist of wave plates and polarizers required for this protocol.

[0062] Consider the case where there is no polarization controller (105) at the input of the receiver (100). In addition, on the way from the transmitter to the receiver, polarization distortions are introduced in the optical channel, and photons with a polarization different from that required for the correct operation of the receiver (100) arrive at the input of the receiver (100). For the BB84 protocol, this is the polarization of photons 0, -45, +45 and +90 degrees.

[0063] Then, for this case, a variant of the optical scheme (11) shown in FIG. 3. In such a case, the optical elements (103) may consist of wave plates or a set of wave plates. Each such optical element (103) is a polarization controller that restores the polarization of incoming photons to linear (for example, to 0, -45, +45 and +90 degrees) and rotates the polarization plane so that it corresponds to the polarization plane of the polarizer P (104 ) in front of the single photon detector (101). These elements are combined into bases that correspond to the bases of the transmitter.

[0064] Consider an example of the operation of the optical circuit (11) of the receiver (100).

[0065] Let the polarizer P (104) of FIG. 3 is oriented to provide 0 degree polarization transmission and excludes 90 degree polarization transmission.

[0066] Consider a photon that is emitted by a linearly polarized transmitter from Alice's node (200), has a polarization of 0 degrees in basis 1. It changes its polarization along the propagation path from node (200) to the transmitter due to the properties of the medium. The corresponding optical element (103) in the receiver on the switch for this polarization of the photon from Alice (102) must restore the linear polarization of this radiation and rotate it so that the direction is equal to 0 degrees.

[0067] Consider a photon that is emitted by a linearly polarized transmitter from Alice's node (200), has a polarization of 90 degrees in basis 1. It changes its polarization along its propagation path from node (200) to the transmitter due to the properties of the medium. The corresponding optical element (103) in the receiver on the switch for this polarization of the photon from Alice (102) must restore the linear polarization of this radiation and rotate it so that the direction is equal to 0 degrees.

[0068] Consider a photon that is emitted by a linearly polarized transmitter from Alice's node (200), has a polarization of -45 degrees in basis 2. It changes its polarization along the propagation path from node (200) to the transmitter due to the properties of the medium. The corresponding optical element (103) in the receiver on the switch for this polarization of the photon from Alice (102) must restore the linear polarization of this radiation and rotate it so that the direction is equal to 0 degrees.

[0069] Consider a photon that is emitted by a linearly polarized transmitter from Alice's node (200), has a polarization of +45 degrees in basis 2. It changes its polarization along the propagation path from node (200) to the transmitter due to the properties of the medium. The corresponding optical element (103) in the receiver on the switch for this polarization of the photon from Alice (102) must restore the linear polarization of this radiation and rotate it so that the direction is equal to 0 degrees.

[0070] When encoding quantum states by polarization based on other protocols, the optical elements (103) can be composed of waveplates required for this protocol.

[0071] The presented application materials disclose preferred examples of the implementation of the technical solution and should not be construed as limiting other, particular examples of its implementation, not going beyond the scope of the claimed legal protection, which are obvious to specialists in the relevant field of technology.

Sources of information:

1. Bennett et al. Bennett, C. H., Bessette, F., Brassard, G. et al. Experimental quantum cryptography. J. Cryptology 5,3-28 (1992). https://doi.org/10.1007/BF00191318.

2. Quantum cryptography Nicolas Gisin, Gregoire Ribordy, Wolfgang Tittel, and Hugo Zbinden Rev. Mod. Phys. 74, 145 DOI: https://doi.org/10.1103/RevModPhys.74.1453.

3. Optics spectroscopy. Generation of a quantum key based on the coding of polarization states of photons Optics and Spectroscopy, 2004, vol. 96, no. 5, p. 772 776. V. L. Kurochkin, I. I. Ryabtsev, I. G. Unknown.

4. Liao, SK., Cai, WQ., Liu, WY. et al. Satellite-to-ground quantum key distribution. Nature 549, 43-47 (2017). https: //doi.ora/10.1038/nature23655.

5. Bruss, D., 1998, "Optimal eavesdropping in quantum cryptography with six states," Phys. Rev. Lett. 81, 3018 3021.

Claims (21)

1. Optical scheme of the receiver for quantum key distribution, containing: - single photon detector; - a switch made with the possibility of rotation or linear movement, providing the substitution of optical elements on the path of the registered radiation in front of the single photon detector; and - optical elements located on the switch. 2. Optical scheme according to claim 1, characterized in that the optical elements are selected from the group: polarizers, wave plates with a polarizer. 3. Optical circuit according to claim 1, characterized in that the switch further comprises reflective elements. 4. Optical system according to claim 3, characterized in that the switch is configured to deflect the beam by the reflecting element. 5. Optical arrangement according to claim 3, characterized in that the reflecting element is a mirror or a prism. 6. Optical circuit according to claim 1, characterized in that the switch is configured with an acousto-optic deflector. 7. Optical scheme of the receiver for quantum key distribution, containing: - single photon detector; - at least one polarizer installed in front of the single photon detector; - a switch made with the possibility of rotation or linear movement, providing the substitution of optical elements on the path of the registered radiation in front of the polarizer; and - optical elements located on the switch. 8. Optical arrangement according to claim 7, characterized in that each optical element is made in the form of at least one wave plate. 9. Optical circuit according to claim 7, characterized in that the switch further comprises reflective elements. 10. Optical system according to claim 9, characterized in that the switch is configured to deflect the beam by the reflecting element. 11. Optical arrangement according to claim 9, characterized in that the reflecting element is a mirror or a prism. 12. Optical circuit according to claim 7, characterized in that the switch is configured with an acousto-optic deflector. 13. A system for quantum key distribution, comprising a quantum communication channel between a transmitter and a receiver, a transmitter unit configured to generate single photons and transmit them to the input of a receiver unit containing an optical circuit according to any one of claims 1-12. 14. The system of claim 13, wherein the data transmission channel is a channel through an open space.

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