A quantum key distribution system with polarization and phase composite encoding

Abstract

A quantum key distribution system with polarization and phase composite coding includes a transmitting end and a receiving end, wherein the transmitting end includes a laser, an intensity modulator, a polarization coding module, a phase coding module and an electrically adjustable attenuator connected in sequence, and the receiving end includes a phase decoding module, a polarization decoding module and a single-photon detector connected in sequence, and the transmitting end and the receiving end are connected through a single-mode optical fiber. Compared with the prior art, the present invention can improve the efficiency of the protocol by performing phase and polarization composite coding on single-photon bits, and the efficiency can be increased to 4 times of the original protocol by using a partial selection basis; the polarization coding and decoding structure is simple and the performance is stable, which not only reduces the complexity of the system, but also improves the security of the system; the phase coding and decoding module has the characteristic of being polarization-independent, so it will not interact with the polarization coding, and is not affected by channel disturbances, so that the phase coding and decoding process is very stable, and the stability of the overall system is increased.

Description

Technical Field

[0001] The present invention relates to the technical field of quantum polarization coding, and in particular to a quantum key distribution system of polarization and phase composite coding. Background Art

[0002] Quantum key distribution (QKD) can ensure unconditional and secure key distribution between two parties in long-distance communication. Its information theory security is guaranteed by the basic principles of quantum mechanics. After more than 30 years of research and development, quantum key distribution has gradually been put into practical use. Among them, the BB84 QKD protocol is the most mature and widely used in current technology. The typical BB84 protocol only encodes bit information in one dimension of a single photon, such as phase, polarization or frequency. By extending the encoding of single photons, that is, encoding bit information in multiple dimensions of single photons, single photons carry multiple bits of information, and then decoding these dimensions separately, the security code rate can be improved, thereby improving the overall efficiency of the system. However, the key generation rate of the current QKD system is low and cannot meet the encryption requirements of existing traditional optical fiber communications. Some solutions are to add a depolarizer at the transmitting end to randomize the polarization state before the photon enters the optical fiber channel, so as to eliminate the effect of optical fiber birefringence and the influence of environmental disturbance on the polarization state. Adding a polarization beam splitter at the receiving end for polarization can obtain stable interference results. But this solution will double the losses and reduce the efficiency of the system by half. Summary of the invention

[0003] In view of the above defects in the prior art, the present invention provides a quantum key distribution system with polarization and phase composite coding as follows:

[0004] The technical solution of the present invention is achieved in this way:

[0005] A quantum key distribution system with polarization and phase composite coding includes a transmitting end and a receiving end, wherein the transmitting end includes a laser, an intensity modulator, a polarization encoding module, a phase encoding module and an electrically adjustable attenuator connected in sequence, and the receiving end includes a phase decoding module, a polarization decoding module and a single-photon detector connected in sequence, and the transmitting end and the receiving end are connected via a single-mode optical fiber.

[0006] Preferably, the polarization encoding module includes a first circulator, a first phase modulator and a first Faraday rotator connected in sequence, the phase encoding module includes a first optical fiber beam splitter, a 2X2 polarization beam splitter, a second phase modulator and a Faraday rotator, the three ports and the four ports of the first optical fiber beam splitter are respectively connected to the one port and the second port of the 2X2 polarization beam splitter through long and short arm optical fibers, the three ports and the four ports of the 2X2 polarization beam splitter are respectively connected to the Faraday rotator and the second phase modulator through polarization-maintaining optical fibers, and the second phase modulator is connected to the Faraday rotator through polarization-maintaining optical fibers, the one port of the first optical fiber beam splitter is connected to the three ports of the first circulator, and the two ports are connected to the electrically adjustable attenuator; the structure of the phase decoding module Consistent with the phase encoding module, the polarization decoding module includes a second circulator and a second optical fiber splitter, wherein one port of the second circulator is connected to the electrically controllable attenuator at the transmitting end through an optical fiber, two ports are connected to one port of the first optical fiber splitter in the phase decoding module, and three ports are connected to two ports of the second optical fiber splitter through a delay line. The second port of the first optical fiber splitter in the phase decoding module is connected to one port of the second optical fiber splitter, and the third and fourth ports of the second optical fiber splitter are both connected to one port of a 1X2 polarization beam splitter. Both 1X2 polarization beam splitters are provided with a correction module, and the second and third ports of the two 1X2 polarization beam splitters are connected to single-photon detectors. The first and second optical fiber beam splitters are both 2X2 single-mode optical fiber splitters.

[0007] Preferably, the polarization encoding module includes a first circulator, a first phase modulator and a first Faraday rotator connected in sequence, the phase encoding module includes a first fiber beam splitter, a 2X2 polarization beam splitter, a second phase modulator and a Faraday rotator, the three ports and the four ports of the first fiber beam splitter are respectively connected to the one port and the second port of the 2X2 polarization beam splitter through long and short arm optical fibers, the three ports and the four ports of the 2X2 polarization beam splitter are respectively connected to the Faraday rotator and the second phase modulator through polarization-maintaining optical fibers, and the second phase modulator is connected to the Faraday rotator through polarization-maintaining optical fibers, the one port of the first fiber beam splitter is connected to the three ports of the first circulator, and the two ports are connected to the electrically adjustable attenuator; the structure of the phase decoding module is consistent with that of the phase encoding module, and the polarization decoding module includes a second circulator, a third circulator and A fourth circulator, wherein one port of the second circulator is connected to an electrically controllable attenuator at a transmitting end through an optical fiber, two ports are connected to one port of a first optical fiber beam splitter in a phase decoding module, and three ports are connected to one port of a third circulator through a delay line. The second port of the first optical fiber beam splitter in the phase decoding module is connected to one port of a fourth circulator. A correction module is provided at the front end of each of the third and fourth circulator ports. The second port of the third circulator is sequentially connected to a third phase modulator and a second Faraday rotator mirror. The second port of the fourth circulator is sequentially connected to a fourth phase modulator and a third Faraday rotator mirror. The third port and the fourth port are both connected to one port of a 1X2 polarization beam splitter. The second port and the third port of the two 1X2 polarization beam splitters are both connected to single-photon detectors. The first optical fiber beam splitter is a 2X2 single-mode optical fiber beam splitter.

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

[0009] 1. The quantum key distribution system with polarization and phase composite coding of the present invention can improve the efficiency of the protocol by performing phase and polarization composite coding on single photon bits. The efficiency can be increased to 4 times of the original protocol by using a partial selection basis;

[0010] 2. Polarization encoding and decoding have a simple structure and stable performance. Compared with traditional multi-laser encoding and passive basis decoding, there is no side channel quantum state preparation and measurement information leakage, which not only reduces the complexity of the system, but also improves the security of the system;

[0011] 3. The phase encoding and decoding modules are polarization-independent, so they will not interact with polarization encoding and are not affected by channel disturbances, making the phase encoding and decoding process very stable and increasing the stability of the overall system. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a principle block diagram of a quantum key distribution system with polarization and phase composite encoding according to the present invention;

[0013] Figure 2 This is a principle block diagram of Embodiment 1 of the present invention;

[0014] Figure 3 This is a principle block diagram of the second embodiment of the present invention.

[0015] In the figure: a transmitting end 100, a laser 110, an intensity modulator 120, a polarization encoding module 130, a first circulator 131, a first phase modulator 132, a first Faraday rotator 133, a phase encoding module 140, a first fiber beam splitter 141, a 2X2 polarization beam splitter 142, a second phase modulator 143, a Faraday rotator 144, an electrically adjustable attenuator 150, a receiving end 200, a correction module 210, a phase decoding module 220, a polarization decoding module 230, a second circulator 231, a second fiber beam splitter 232, a 1X2 polarization beam splitter 233, a third circulator 234 with three ports, a fourth circulator 235, a third phase modulator 236, a second Faraday rotator 237, a fourth phase modulator 238, a third Faraday rotator 239, and a single photon detector 240. DETAILED DESCRIPTION

[0016] The present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0017] like Figure 1 As shown, a quantum key distribution system with polarization and phase composite encoding includes a transmitting end 100 and a receiving end 200, wherein the transmitting end 100 includes a laser 110, an intensity modulator 120, a polarization encoding module 130, a phase encoding module 140 and an electrically adjustable attenuator 150 connected in sequence, and the receiving end 200 includes a phase decoding module 220, a polarization decoding module 230 and a single photon detector 240 connected in sequence, and the transmitting end 100 and the receiving end 200 are connected via a single-mode optical fiber.

[0018] like Figure 2As shown, in the first embodiment, the polarization encoding module 130 includes a first circulator 131, a first phase modulator 132 and a first Faraday rotator 133 connected in sequence, and the phase encoding module 140 includes a first fiber beam splitter 141, a 2X2 polarization beam splitter 142, a second phase modulator 143 and a Faraday rotator 144. The three ports and the four ports of the first fiber beam splitter 141 are respectively connected to the one port and the second port of the 2X2 polarization beam splitter 142 through long and short arm optical fibers, and the three ports and the four ports of the 2X2 polarization beam splitter 142 are respectively connected to the Faraday rotator 144 and the second phase modulator 143 through polarization-maintaining optical fibers, and the second phase modulator 143 is connected to the Faraday rotator 144 through polarization-maintaining optical fibers. The first port of the first fiber beam splitter 141 is connected to the three ports of the first circulator 131, and the second port is connected to the electrically adjustable attenuator 150; the structure of the phase decoding module 220 Consistent with the phase encoding module 140, the polarization decoding module 230 includes a second circulator 231 and a second optical fiber splitter 232, wherein one port of the second circulator 231 is connected to the electrically controllable attenuator 150 of the transmitting end 100 through an optical fiber, two ports are connected to one port of the first optical fiber splitter 141 in the phase decoding module 220, and three ports are connected to two ports of the second optical fiber splitter 232 through a delay line. The two ports of the first optical fiber splitter 141 in the phase decoding module 220 are connected to one port of the second optical fiber splitter 232, and the three ports and the four ports of the second optical fiber splitter 232 are both connected to one port of a 1X2 polarization beam splitter 233. Both 1X2 polarization beam splitters 233 are provided with a correction module 210, and the two ports and the three ports of the two 1X2 polarization beam splitters 233 are both connected to a single photon detector 240. The first optical fiber splitter 141 and the second optical fiber splitter 232 are both 2X2 single-mode optical fiber splitters.

[0019] Specific implementation process:

[0020] The optical fiber at the second port of the first circulator 131 is fused at 45° to the input end of the first phase modulator 132. The optical pulse emitted by the laser 110 enters the first port of the first circulator 131 after the intensity is modulated by the intensity modulator 120. The polarization state is rotated by 45° and is then divided into two mutually perpendicular polarization components |H〉 and |V〉. The components enter the first phase modulator 132 and are reflected by the first Faraday rotator 133 and then pass through the first phase modulator 132 again. The phase difference between |H〉 and |V〉 can be changed by modulating the voltage of the first phase modulator 132. The resulting polarization state is When the phase difference When , the corresponding four polarization states are shown in Table 1

[0021] Table 1: Four polarization states generated by the transmitter

[0022]

[0023] After polarization encoding, the optical pulse enters the phase encoding module 140 (MZSI) for phase encoding. The phase encoding module 140 (MZSI) consists of a 2X2 single-mode fiber beam splitter BS, a 2X2 polarization beam splitter PBS, a phase modulator PM and a Faraday rotator FR. It can be seen that the MZSI is essentially composed of an unequal-arm Mach-Zehnder (MZ) interferometer and a Sagnac ring. Among them, the long arm (l) of the unequal-arm MZ interferometer has a delay line (DL) to ensure that the arm length difference between it and the short arm (s) is Δl. The optical fiber in the Sagnac ring is a polarization-maintaining fiber, and the rest is a single-mode optical fiber. The second phase modulator 143 is required to allow both TM polarized light and TE polarized light to pass through (such as a commercial titanium diffused phase modulator). The Faraday rotator 144 introduces the Faraday effect, which can rotate the polarization state of light by 90°. The polarization-encoded optical pulse enters the 2X2 fiber beam splitter and is divided into two optical pulses, of which pulse P1 passes through the long arm of the unequal-arm MZ interferometer, and pulse P2 passes through the short arm of the unequal-arm MZ interferometer. The long-arm pulse P1 is decomposed by the polarization beam splitter PBS into two mutually orthogonal polarization state optical pulses P1x and P1y. These two polarization state optical pulses pass through the Sagnac ring structure clockwise and counterclockwise respectively, and finally return to the polarization beam splitter at the same time and synthesize into a pulse P11, and return to the long arm of the unequal-arm MZ interferometer. Since pulses P1x and P1y arrive at the phase modulator at the same time from opposite directions, they are modulated with the same phase θ l , and the polarization state of the final synthesized light pulse P11 is perpendicular to the polarization state of the incident light pulse P1. After the light pulse P11 returns to the long arm, it is split into two light pulses for output through the fiber beam splitter BS again. Similarly, the light pulse P2 passing through the short arm of the unequal-arm MZ interferometer will also be decomposed by the polarization beam splitter PBS into two mutually orthogonal polarization state light pulses P2x and P2y, which pass through the Sagnac ring and reach the phase modulator at the same time and are modulated by phase θ s , and then synthesized into a light pulse P22 at the polarization beam splitter PBS, whose polarization state is perpendicular to P2, and finally returns to the short arm of the unequal-arm MZ interferometer and is split into two light pulses by the fiber beam splitter BS. Finally, the phase encoder outputs two time intervals of 2Δl / v (where v is the propagation speed of light in the optical fiber) with a phase difference of The phase difference is randomly controlled by adjusting the phase modulator PM. Phase encoding can be performed when the optical pulse is 0, π / 2, π, or 3π / 2. Finally, the optical pulse is attenuated to the single photon level by an electrically adjustable attenuator.

[0024] After the optical pulse enters the receiving end 200 through the channel, it first needs to pass through the second circulator 231 to enter the phase decoding module 220. After decoding, the pulse will enter the two input ports of the beam splitter respectively. The optical pulse emitted from the third port of the circulator will pass through a delay DL, which is equivalent to time division multiplexing of two signals. The two output ports of the beam splitter are respectively connected to the polarization controller, and the polarization state disturbed by the channel is restored by the polarization controller combined with the polarization compensation algorithm. Finally, they pass through a polarization beam splitter for polarization analysis and finally enter the single photon detector for detection.

[0025] The workflow of this quantum key distribution system is summarized as follows:

[0026] 1. Laser triggering: The pulsed laser generates a series of pulsed light at a certain repetition frequency through a trigger signal;

[0027] 2. Decoy state modulation: The light pulse is randomly intensity modulated by the intensity modulator and becomes a signal state, a decoy state or a vacuum state;

[0028] 3. Transmitter encoding: The optical pulse modulated by the intensity modulator enters the polarization encoding module for encoding, and the polarization states generated are |+>, |->, |R>, |L>. Then the optical pulse passes through the phase encoding module MZSI and is randomly phase modulated by the phase modulator, so that the phase difference between the two pulses output from the MZSI is 0, π / 2, π, 3π / 2 respectively;

[0029] 4. Electrically variable attenuator (EVOA): EVOA attenuates optical pulses to the single photon level;

[0030] 5. Decoding at the receiving end: After being transmitted through the optical fiber channel, the optical signal enters the receiving end and sequentially enters the phase decoding unit and the polarization decoding unit to complete the decoding process, where the phase decoding PM modulation phase is 0, π / 2, π, 3π / 2, and the polarization decoding PM modulation phase is 0, π / 2;

[0031] Measurement: Use a single-photon detector to measure the system results for subsequent processing to generate security keys.

[0032] like Figure 3As shown in the second embodiment, the polarization encoding module 130 includes a first circulator 131, a first phase modulator 132 and a first Faraday rotator 133 connected in sequence, and the phase encoding module 140 includes a first fiber beam splitter 141, a 2X2 polarization beam splitter 142, a second phase modulator 143 and a Faraday rotator 144. The three ports and the four ports of the first fiber beam splitter 141 are respectively connected to the one port and the second port of the 2X2 polarization beam splitter 142 through long and short arm optical fibers. The third port and the fourth port of the phase encoder 142 are respectively connected to the Faraday rotator 144 and the second phase modulator 143 through the polarization-maintaining optical fiber, and the second phase modulator 143 is connected to the Faraday rotator 144 through the polarization-maintaining optical fiber. The first port of the first optical fiber beam splitter 141 is connected to the third port of the first circulator 131, and the second port is connected to the electrically adjustable attenuator 150. The structure of the phase decoding module 220 is consistent with that of the phase encoding module 140. The polarization decoding module 230 includes a second circulator 231, a third circulator 234 and The fourth circulator 235, the second circulator 231 has one port connected to the electrically controllable attenuator 150 of the transmitting end 100 through an optical fiber, the second port is connected to the first port of the first optical fiber beam splitter 141 in the phase decoding module 220, the third port is connected to the first port of the third circulator 234 through a delay line, the second port of the first optical fiber beam splitter 141 in the phase decoding module 220 is connected to the first port of the fourth circulator 235, the third circulator 234 and the fourth circulator 235 are both provided with a correction module 210 at the front end of the first port. The second port of the fourth circulator 235 is connected to the fourth phase modulator 238 and the third Faraday rotator 239 in sequence, the third port of the third circulator 234 and the third port of the fourth circulator 235 are connected to one port of a 1X2 polarization beam splitter 233, the second port and the third port of the two 1X2 polarization beam splitters 233 are connected to a single photon detector 240, and the first optical fiber beam splitter 141 is a 2X2 single-mode optical fiber beam splitter.

[0033] The second embodiment adopts the active base selection method to avoid the safety problems caused by passive base selection.

[0034] Based on the structure and principle of the present invention, the quantum key distribution system with polarization and phase composite encoding of the present invention can improve the efficiency of the protocol by performing phase and polarization composite encoding on single photon bits, and the efficiency can be increased to 4 times that of the original protocol by adopting a partial selection basis method; the polarization encoding and decoding structure is simple and the performance is stable. Compared with traditional multi-laser encoding and passive basis selection decoding, there is no side channel quantum state preparation and measurement information leakage, which not only reduces the complexity of the system but also improves the security of the system; the phase encoding and decoding module has the characteristic of being polarization-independent, so it will not interact with the polarization encoding, and is not affected by channel disturbances, so that the phase encoding and decoding process is very stable, increasing the stability of the overall system.

Claims

1. A quantum key distribution system with polarization and phase composite coding, comprising a transmitting end and a receiving end, characterized in that: The transmitting end includes a laser, an intensity modulator, a polarization encoding module, a phase encoding module and an electrically adjustable attenuator connected in sequence, the receiving end includes a phase decoding module, a polarization decoding module and a single-photon detector connected in sequence, the polarization encoding module includes a first circulator, a first phase modulator and a first Faraday rotator mirror connected in sequence, the phase encoding module includes a first optical fiber beam splitter, a 2X2 polarization beam splitter, a second phase modulator and a Faraday rotator, the three ports and the four ports of the first optical fiber beam splitter are respectively connected to the one port and the second port of the 2X2 polarization beam splitter through long and short arm optical fibers, the three ports and the four ports of the 2X2 polarization beam splitter are respectively connected to the Faraday rotator and the second phase modulator through polarization-maintaining optical fibers, and the second phase modulator and the Faraday rotator are connected through polarization-maintaining optical fibers, the first optical fiber beam splitter One port is connected to the three ports of the first circulator, and the second port is connected to the electrically adjustable attenuator; the structure of the phase decoding module is consistent with that of the phase encoding module, and the polarization decoding module includes a second circulator and a second optical fiber splitter, one port of the second circulator is connected to the electrically controllable attenuator at the transmitting end through an optical fiber, two ports are connected to one port of the first optical fiber splitter in the phase decoding module, and three ports are connected to two ports of the second optical fiber splitter through a delay line. The second port of the first optical fiber splitter in the phase decoding module is connected to one port of the second optical fiber splitter, and the third port and the fourth port of the second optical fiber splitter are both connected to one port of a 1X2 polarization beam splitter, and the two 1X2 polarization beam splitters are both provided with a correction module, and the second port and the third port of the two 1X2 polarization beam splitters are both connected to a single-photon detector, and the first optical fiber splitter and the second optical fiber splitter are both 2X2 single-mode optical fiber splitters.

2. A quantum key distribution system with polarization and phase composite coding, comprising a transmitting end and a receiving end, characterized in that: The transmitting end includes a laser, an intensity modulator, a polarization encoding module, a phase encoding module and an electrically adjustable attenuator connected in sequence, the receiving end includes a phase decoding module, a polarization decoding module and a single photon detector connected in sequence, the polarization encoding module includes a first circulator, a first phase modulator and a first Faraday rotator mirror connected in sequence, the phase encoding module includes a first optical fiber beam splitter, a 2X2 polarization beam splitter, a second phase modulator and a Faraday rotator, the three ports and the four ports of the first optical fiber beam splitter are respectively connected to the one port and the two ports of the 2X2 polarization beam splitter through long and short arm optical fibers, the three ports and the four ports of the 2X2 polarization beam splitter are respectively connected to the Faraday rotator and the second phase modulator through polarization-maintaining optical fibers, and the second phase modulator is connected to the Faraday rotator through polarization-maintaining optical fibers, the one port of the first optical fiber beam splitter is connected to the three ports of the first circulator, and the two ports are connected to the electrically adjustable attenuator; the phase decoding module The structure of the block is consistent with that of the phase encoding module. The polarization decoding module includes a second circulator, a third circulator and a fourth circulator. One port of the second circulator is connected to the electrically controllable attenuator at the transmitting end through an optical fiber, and two ports are connected to one port of the first optical fiber beam splitter in the phase decoding module. The three ports are connected to one port of the third circulator through a delay line. The second port of the first optical fiber beam splitter in the phase decoding module is connected to one port of the fourth circulator. The front ends of the third circulator and the fourth circulator are both provided with a correction module. The second port of the third circulator is connected to the third phase modulator and the second Faraday rotator mirror in sequence. The second port of the fourth circulator is connected to the fourth phase modulator and the third Faraday rotator mirror in sequence. The third port and the fourth port of the third circulator are both connected to one port of a 1X2 polarization beam splitter. The second port and the third port of the two 1X2 polarization beam splitters are both connected to single-photon detectors. The first optical fiber beam splitter is a 2X2 single-mode optical fiber beam splitter.

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