A stable quantum key distribution system

Abstract

A stable quantum key distribution system. The transmitting end includes a laser, an intensity modulator, a circulator, a transmitting end interferometer, a wavelength division multiplexer, a photodiode, a third polarization beam splitter, a first single-photon detector, and a second single-photon detector. The laser is sequentially connected to the intensity modulator and the first port of the circulator. The receiving end includes a demultiplexer, a synchronous laser, an attenuator, and a receiving end interferometer. The input end of the demultiplexer is connected to the output end of the wavelength division multiplexer. Compared with the prior art, the system of the present invention is very stable, can automatically compensate for polarization changes, and at the same time, since the two pulses travel exactly the same path, it can automatically compensate for phase drift. Therefore, phase compensation and polarization compensation are not required, and it can resist the influence of the external environment of the device and the channel, and can achieve continuous and stable operation. The present invention can increase the utilization rate of photon energy to 1, that is, increase the system coding rate to twice that of the original scheme, and is easy to manufacture.

Description

Technical Field

[0001] The present invention relates to the technical field of quantum polarization encoding, and particularly to a stable quantum key distribution system. Background Art

[0002] With the development of technologies such as the Internet of Things, cloud computing, big data, and artificial intelligence, the world will enter the Internet of Everything (IoE) era from the Internet Plus era in the next decade. Along with the explosion of information volume, information encryption and security technologies have been fully embedded in the political, economic, and cultural fields to ensure the information security of governments, enterprises, and individuals. In recent years, with the proposal and development of quantum computers, a huge threat has been posed to the current classical cryptosystems based on computational complexity. Quantum key distribution can provide unconditional secure key distribution for two communication parties over long distances, and its information-theoretic security is guaranteed by the basic principles of quantum mechanics. For a practical quantum key distribution system, it is able to continuously and stably output quantum secure keys to meet the encryption needs of users. The optical and electronics stability of the system is the most basic requirement. The commonly used encoding methods include phase encoding and polarization encoding. Due to the birefringence effect of the optical fiber channel, the polarization state of photons randomly changes in polarization encoding, and polarization compensation is required. The stability of the phase encoding system is higher than that of the former, but due to the phase drift in the interferometer, the system still needs to perform phase compensation. The implementation of polarization and phase compensation requires additional hardware or software, increasing the complexity of the system. Summary of the Invention

[0003] Aiming at the above-mentioned defects existing in the prior art, the present invention provides a stable quantum key distribution system as follows:

[0004] The technical solution of the present invention is realized as follows:

[0005] A stable quantum key distribution system, comprising a transmitting end and a receiving end. The transmitting end includes a laser, an intensity modulator, a circulator, a transmitting end interferometer, a wavelength division multiplexer, a photodiode, a third polarization beam splitter, a first single-photon detector, and a second single-photon detector. The laser is sequentially connected to the intensity modulator and the first port of the circulator. The second port of the circulator is connected to the input port of the transmitting end interferometer and is fusion spliced at 45°. The output port of the transmitting end interferometer is connected to the second input port of the wavelength division multiplexer. The first input port of the wavelength division multiplexer is connected to the photodiode. The third port of the circulator is connected to the first port of the third polarization beam splitter. The third port and the fourth port of the third polarization beam splitter are respectively connected to the first single-photon detector and the second single-photon detector. The receiving end includes a demultiplexer, a synchronization laser, an attenuator, and a receiving end interferometer. The input end of the demultiplexer is connected to the output end of the wavelength division multiplexer. The first output port of the demultiplexer is connected to the synchronization laser. The second output port of the demultiplexer is connected to the input port of the attenuator. The output port of the attenuator is connected to the receiving end interferometer.

[0006] Preferably, the transmitting end interferometer includes a first polarization beam splitter and a first phase modulator. One port of the first polarization beam splitter is connected to the input end of the first phase modulator. The second port is connected to the second port of the circulator and is fusion spliced at 45°. The third port is connected to the output end of the first phase modulator. The fourth port is connected to the second input port of the wavelength division multiplexer. The receiving end interferometer includes a second polarization beam splitter, a 90° Faraday selector, and a second phase modulator. One port of the second polarization beam splitter is connected to one end of the 90° Faraday selector. The second port is connected to the output port of the attenuator. The third port is connected to the output end of the second phase modulator. The input end of the second phase modulator is connected to the other end of the 90° Faraday selector.

[0007] Preferably, the wavelength division multiplexer and the demultiplexer are connected by a polarization-maintaining optical fiber.

[0008] Compared with the prior art, the present invention has the following beneficial effects:

[0009] The system of the present invention is very stable. Due to the Sagnac structure of the receiving end interferometer, it can automatically compensate for polarization changes. At the same time, since the two pulses travel exactly the same path, it can automatically compensate for phase drift. Therefore, phase compensation and polarization compensation are not required, and it can resist the influence of the external environment of the device and the channel, and can achieve continuous and stable operation. The present invention can improve the utilization rate of photon energy to 1, that is, double the system coding rate to that of the original scheme. The structures of the transmitting end interferometer and the receiving end interferometer are simple. The transmitting end interferometer is only composed of a 2x2 polarization beam splitter and a phase modulator, and the receiving end interferometer is only composed of a 1x2 polarization beam splitter, a 90° Faraday rotator, and a phase modulator, which are easy to manufacture. Description of the Drawings

[0010] Figure 1 This is a schematic block diagram of the stable quantum key distribution system of the present invention.

[0011] In the figure: transmitting end 100, laser 110, intensity modulator 120, circulator 130, transmitting end interferometer 140, first polarization beam splitter 141, first phase modulator 142, wavelength division multiplexer 150, photodiode 160, third polarization beam splitter 170, first single-photon detector 180, second single-photon detector 190, receiving end 200, demultiplexer 210, synchronization laser 220, attenuator 230, receiving end interferometer 240, second polarization beam splitter 241, 90° Faraday selector 242, second phase modulator 243. Detailed Embodiment

[0012] Next, the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention.

[0013] As Figure 1 shown, a stable quantum key distribution system includes a transmitting end 100 and a receiving end 200. The transmitting end 100 includes a laser 110, an intensity modulator 120, a circulator 130, a transmitting end interferometer 140, a wavelength division multiplexer 150, a photodiode 160, a third polarization beam splitter 170, a first single-photon detector 180, and a second single-photon detector 190. The laser 110 is sequentially connected to the intensity modulator 120 and the first port of the circulator 130. The second port of the circulator 130 is connected to the input port of the transmitting end interferometer 140 and is fusion-spliced at 45°. The output port of the transmitting end interferometer 140 is connected to the second input port of the wavelength division multiplexer 150. The first input port of the wavelength division multiplexer 150 is connected to the photodiode 160. The third port of the circulator 130 is connected to the first port of the third polarization beam splitter 170. The third port and the fourth port of the third polarization beam splitter 170 are respectively connected to the first single-photon detector 180 and the second single-photon detector 190. The receiving end 200 includes a demultiplexer 210, a synchronization laser 220, an attenuator 230, and a receiving end interferometer 240. The input end of the demultiplexer 210 is connected to the output end of the wavelength division multiplexer 150. The first output port of the demultiplexer 210 is connected to the synchronization laser 220. The second output port of the demultiplexer 210 is connected to the input port of the attenuator 230. The output port of the attenuator 230 is connected to the receiving end interferometer 240.

[0014] The transmitting - end interferometer 140 includes a first polarization beam splitter 141 and a first phase modulator 142. One port of the first polarization beam splitter 141 is connected to the input end of the first phase modulator 142, the second port is connected to the second port of the circulator 130 and is fusion - spliced at 45°, the third port is connected to the output end of the first phase modulator 142, and the fourth port is connected to the second input port of the wavelength - division multiplexer 150. The receiving - end interferometer 240 includes a second polarization beam splitter 241, a 90° Faraday selector 242, and a second phase modulator 243. One port of the second polarization beam splitter 241 is connected to one end of the 90° Faraday selector 242, the second port is connected to the output port of the attenuator 230, the third port is connected to the output end of the second phase modulator 243, and the input end of the second phase modulator 243 is connected to the other end of the 90° Faraday selector 242.

[0015] The wavelength - division multiplexer 150 and the demultiplexer 210 are connected by polarization - maintaining optical fiber.

[0016] At the transmitting end, the optical pulse P0 generated by the laser LD1 enters the intensity modulator IM. By loading different voltages, different output optical intensities are obtained to generate signal states and decoy states to defend against photon - number - splitting attacks. Subsequently, the optical pulse enters from the first port (input port) of the circulator CIR and exits from the second port. After passing through the 45° fusion - spliced part, the polarization state becomes 45°. Its horizontal component H propagates along the slow axis of the polarization - maintaining optical fiber, and the vertical component V propagates along the fast axis of the polarization - maintaining optical fiber. After entering the transmitting - end interferometer, it is split into two optical pulses P1 and P2 by the first polarization beam splitter PBS1. Among them, P1 is the H - component transmitted by the first polarization beam splitter PBS1 and directly exits from the output port of the interferometer. P2 is the V - component, which travels around in the interferometer, is modulated by the first phase modulator PMA, and then exits from the output port of the interferometer. Therefore, after the optical pulse P0 passes through the transmitting - end interferometer, two optical pulses P1 and P2 with a time interval of T (determined by the internal fiber length of the interferometer) and perpendicular polarization directions are generated. Among them, P1 (polarization state is horizontal polarization H, and the optical power is 1 / 2 of the total power of P0) precedes P2 (polarization state is vertical polarization V, and the optical power is 1 / 2 of the total power of P0). There is a phase difference between these two pulses, which is obtained by modulating P2 by the first phase modulator PMA. By circuit control, the first phase modulator PMA only modulates the phase of the optical pulse P2, and the phase difference between P1 and P2 can be randomly modulated to 0, π / 2, π, 3π / 2. Subsequently, it enters the optical - fiber channel through the wavelength - division multiplexer WDM.

[0017] The optical pulses P1 and P2 reach the receiving end through the channel QC. First, they are demultiplexed by the wavelength division multiplexer demultiplexer WDM, and then enter the receiving end interferometer after passing through the attenuator ATT. At this time, due to the birefringence effect of the optical fiber channel, the polarization state of the photons propagating in it will change with the environment. Therefore, the polarization states of P1 and P2 will become unpredictable after reaching the receiving end. Since the receiving end interferometer has a Sagnac structure and a 90° Faraday rotator mirror, this structure is equivalent to the Faraday mirror FM, which rotates the polarization states of both P1 and P2 by 90°. And the propagation path lengths of P1 and P2 inside the interferometer are the same and the directions are opposite. When exiting the interferometer, P1 still leads P2 by a time T. The second phase modulator PMB is controlled by a circuit to perform phase modulation on the optical pulse P2, randomly loading phases 0, π / 2, π, 3π / 2. After P1 and P2 pass through the attenuator ATT and the wavelength division multiplexer demultiplexer WDM again, they return to the optical fiber channel and further return to the sending end.

[0018] Since the Sagnac loop structure is equivalent to the polarization modulator FM, when the polarization state of the photons passes through the optical fiber channel twice and returns to the sending end interferometer again, the polarization states of P1 and P2 are rotated by 90° compared to when they exit the sending end interferometer. That is, the polarization state of P1 becomes V, and the polarization state of P2 becomes H. When re-entering the sending end interferometer, P2 will directly transmit through the first polarization beam splitter PBS1, and P1 will enter the interferometer and make a circle before exiting from the input port of the interferometer at the same time as P2. Since P1 propagated less time T than P2 at the sending end and more time T than P2 when returning to the sending end, P1 and P2 finally arrive at the input port of the interferometer at the same time, and the two are superimposed to form an optical pulse P3. After the synthesized optical pulse is rotated by 45°, it is output from the third port of the circulator CIR and enters the third polarization beam splitter PBS3 for polarization state analysis. The two components are finally respectively detected by the first single photon detector SPD1 and the second single photon detector SPD2. The detection results can generate a secure key between the transceiver parties through the post-processing process.

[0019] Ignoring the loss of the phase modulator, the 45° polarization state of the optical pulse P1 incident on the sending end interferometer can be written as The state after passing through the sending end interferometer can be written as where represents the phase modulated by the first phase modulator PMA on the optical pulse P2, and the subscript T indicates that the V component lags the H component by a time T. After passing through the receiving end interferometer, the phase of P1 is modulated by applying a voltage to the second phase modulator PMB The polarization state of the optical pulse P3 that returns to the sending end interferometer and is output from its input port becomes where It can be seen that the optical power of the synthesized optical pulse P3 is equal to that of P1, i.e., there is no loss. And the polarization state of P3 is determined by the phase difference modulated by the transmitter and the receiver.

[0020] When the first phase modulator PMA and the second phase modulator PMB modulate different phases to obtain different phase differences, the polarization states of the obtained optical pulse P3 are shown in Table 1:

[0021] Table 1: Polarization states of optical pulse P6 obtained with different phase differences

[0022]

[0023] After passing through the 45° rotation structure, the polarization states |+> and |-> become |H> and |V>, while |R> and |L> remain unchanged. Therefore, the two sets of bases can be distinguished by the first polarization beam splitter PBS1.

[0024] The transmitter modulates 4 phases and the receiver modulates 2 phases. The corresponding single-photon detector response probabilities are shown in Table 2:

[0025] Table 2: Detector response probability table

[0026]

[0027] Based on the structure and principle of the present invention, it can be seen that the system of the present invention is very stable. Due to the Sagnac structure of the receiver interferometer, polarization changes can be automatically compensated. At the same time, since the two pulses travel exactly the same path, phase drift can be automatically compensated. Therefore, phase compensation and polarization compensation are not required, and the influence of the external environment of the device and the channel can be resisted, enabling continuous and stable operation. The present invention can increase the utilization rate of photon energy to 1, that is, double the system coding rate compared to the original scheme. The structures of the transmitter interferometer and the receiver interferometer are simple. The transmitter interferometer is only composed of a 2x2 polarization beam splitter and a phase modulator, and the receiver interferometer is only composed of a 1x2 polarization beam splitter, a 90° Faraday rotator and a phase modulator, which are easy to fabricate.

Claims

1. A stable quantum key distribution system, comprising a sending end and a receiving end, characterized in that, The transmitting end includes a laser, an intensity modulator, a circulator, a transmitting-end interferometer, a wavelength division multiplexer, a photodiode, a third polarization beam splitter, a first single-photon detector, and a second single-photon detector. The laser is sequentially connected to the intensity modulator and the first port of the circulator. The second port of the circulator is connected to the input port of the transmitting-end interferometer and is fusion-spliced at 45°. The output port of the transmitting-end interferometer is connected to the second input port of the wavelength division multiplexer. The first input port of the wavelength division multiplexer is connected to the photodiode. The third port of the circulator is connected to the first port of the third polarization beam splitter. The third port and the fourth port of the third polarization beam splitter are respectively connected to the first single-photon detector and the second single-photon detector. The receiving end includes a demultiplexer, a synchronization laser, an attenuator, and a receiving-end interferometer. The input end of the demultiplexer is connected to the output end of the wavelength division multiplexer. The first output port of the demultiplexer is connected to the synchronization laser. The second output port of the demultiplexer is connected to the input port of the attenuator. The output port of the attenuator is connected to the receiving-end interferometer. The transmitting-end interferometer includes a first polarization beam splitter and a first phase modulator. One port of the first polarization beam splitter is connected to the input end of the first phase modulator. The second port is connected to the second port of the circulator and is fusion-spliced at 45°. The third port is connected to the output end of the first phase modulator. The fourth port is connected to the second input port of the wavelength division multiplexer. The receiving-end interferometer includes a second polarization beam splitter, a 90° Faraday selector, and a second phase modulator. One port of the second polarization beam splitter is connected to one end of the 90° Faraday selector. The second port is connected to the output port of the attenuator. The third port is connected to the output end of the second phase modulator. The input end of the second phase modulator is connected to the other end of the 90° Faraday selector. The optical pulse P0 generated by the laser enters the intensity modulator to generate a signal state and a decoy state. After entering the transmitting-end interferometer, the optical pulse P0 will generate two optical pulses P1 and P2 with a time interval of T and perpendicular polarization directions. Only the optical pulse P2 is phase-modulated by the first phase modulator, and then enters the optical fiber channel through wavelength division multiplexing. After the optical pulses P1 and P2 enter the receiving end, they are demultiplexed and attenuated and then enter the receiving-end interferometer. The receiving-end interferometer is of Sagnac structure, which rotates the polarization states of P1 and P2 by 90°. The optical pulse P2 is phase-modulated by the second phase modulator. P1 and P2 return to the optical fiber channel again and further return to the transmitting end. When the photon polarization states of P1 and P2 return to the transmitting-end interferometer again, the two are superimposed to form an optical pulse P3. After the synthesized optical pulse rotates by 45°, it is output from an output end of the circulator and enters the third polarization beam splitter for polarization state analysis. The two components are finally respectively detected by the first single-photon detector and the second single-photon detector. The detection results generate a secure key between the transmitter and the receiver through a post-processing process.

2. The stable quantum key distribution system according to claim 1, wherein The wavelength division multiplexer and the demultiplexer are connected by a polarization-maintaining optical fiber.

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