A quantum key distribution system

Abstract

The present invention belongs to the technical field of optical secure communication, and discloses a quantum key distribution system, wherein the transmitting end includes a laser, a bidirectional polarization interferometer, a first polarization beam splitter, a first photodetector, a first beam splitter, a first polarization analysis module and an adjustable attenuator, and the receiving end includes a polarization controller, a second beam splitter and a second polarization analysis module; the polarization controller is used to perform polarization compensation on the received quantum state; the second beam splitter is used to split the quantum state after polarization compensation; the second polarization analysis module is used to perform polarization measurement on the two-way quantum state components respectively. Compared with the prior art, the present invention utilizes the random phase generated by the spontaneous radiation of the laser, and by passing the optical pulse forward and backward twice through the bidirectional polarization interferometer, the entrapped state and the polarization state can be passively prepared respectively, without the need for active modulation devices, and only one laser and one interferometer are needed, which not only eliminates the side channel information leakage, but also greatly reduces the system complexity.

Description

Technical Field

[0001] The present invention relates to the technical field of optical secure communication, and in particular to a quantum key distribution system. Background Art

[0002] With the rapid development and application of the Internet of Things and big data, the importance of communication security has become increasingly prominent. Quantum key distribution can provide unconditionally secure key distribution for communication and has very broad application prospects in the field of information security. However, the actual quantum key distribution system is imperfect, which will lead to security vulnerabilities in the system. For example, active decoy state preparation and active quantum state preparation will have modulation errors and are vulnerable to Trojan horse light attacks, which reduces the security of the key.

[0003] In order to solve the above problems, passive quantum key distribution schemes have been proposed. For example, in the paper "Passive sources for the Bennett-Brassard 1984 quantum-key-distribution protocol with practical signals," Phys. Rev. A 82(5), 052325 (2010), four lasers and multiple beam splitters, polarization beam splitters and photodetectors are used to simultaneously realize the passive preparation of entrapped states and polarization quantum states. Figure 1 As shown. This scheme does not require active modulation devices, eliminates modulation errors and is immune to Trojan horse attacks. However, it is difficult to ensure wavelength consistency for multiple lasers, there are side channel vulnerabilities, and the system is relatively complex. The paper Y. Kurochkin et al. "A practical transmitter device for passive state BB84 quantumkey distribution." arxiv preprint arxiv:2405.08481 (2024) uses a single laser and uses a ring structure composed of a beam splitter and a polarization measurement module to achieve passive preparation of polarization quantum states, reducing the number of lasers and detectors. However, this scheme does not have a passive preparation module for the entrapped state, which reduces the security of the system, and also requires an unequal-arm interferometer at the receiving end, which increases the complexity of the system. Summary of the invention

[0004] In view of the above defects in the prior art, the present invention proposes a quantum key distribution system.

[0005] The technical solution of the present invention is achieved in this way:

[0006] A quantum key distribution system includes a transmitting end and a receiving end connected by an optical fiber channel, wherein the transmitting end includes a laser LD, a bidirectional polarization interferometer, a first polarization beam splitter PBS1, a first photodetector PD1, a first beam splitter BS1, a first polarization analysis module and an adjustable attenuator VOA.

[0007] The first port, the second port and the third port of the bidirectional polarization interferometer are respectively connected to the input port and the output port of the LD and PBS1 through a 45° polarization rotation structure;

[0008] The input port and two output ports of BS1 correspond to the fourth port of the bidirectional polarization interferometer, the first polarization analysis module and the input port of the variable attenuator VOA respectively;

[0009] Another output port of PBS1 is connected to PD1;

[0010] The output port of the VOA is connected to the optical fiber channel as the quantum state output port of the transmitting end;

[0011] LD is used to generate horizontally polarized light pulses with a period of T and random phases;

[0012] The arm length delay difference of the bidirectional polarization interferometer in the transmission direction from the third port to the fourth port is an integer multiple of T, and is twice the arm length delay difference in the transmission direction from the first port to the second port;

[0013] The first polarization analysis module is used to perform polarization analysis on the optical signal emitted from the fourth port of the bidirectional polarization interferometer and split by BS1;

[0014] The receiving end includes a polarization controller PC, a second beam splitter BS2 and a second polarization analysis module;

[0015] The polarization controller is used to perform polarization compensation on the received quantum state;

[0016] BS2 is used to split the quantum state after polarization compensation to generate two quantum state components;

[0017] The second polarization analysis module is used to perform polarization measurement on the two quantum state components respectively.

[0018] Preferably, the bidirectional polarization interferometer polarization-splits the optical signal incident to its first port into two polarization components that are transmitted along the short arm and the long arm with an arm length delay difference of T, respectively, and emits them from its second port; and polarization-splits the optical signal incident to its third port into two polarization components that are transmitted along the short arm and the long arm with an arm length delay difference of 2T, respectively, and emits them from its fourth port.

[0019] Preferably, the bidirectional polarization interferometer comprises a second polarization beam splitter PBS2, a third polarization beam splitter PBS3, a fourth polarization beam splitter PBS4 and a fifth polarization beam splitter PBS5;

[0020] Two output ports of PBS2 are connected to one input port of PBS3 and one input port of PBS4 respectively;

[0021] The two output ports of PBS4 are connected to the two input ports of PBS5 respectively;

[0022] The output port of PBS5 is connected to another input port of PBS3;

[0023] PBS2 and PBS3 constitute a first unequal-arm Mach-Zehnder interferometer;

[0024] PBS4 and PBS5 constitute a second unequal-arm Mach-Zehnder interferometer;

[0025] The second unequal-arm Mach-Zehnder interferometer is located on the long arm of the first unequal-arm Mach-Zehnder interferometer, and the arm length delay difference between the two is T; and the optical signal transmitted along the slow axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer enters the short arm of the second unequal-arm Mach-Zehnder interferometer, and the optical signal transmitted along the fast axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer enters the long arm of the second unequal-arm Mach-Zehnder interferometer.

[0026] Preferably, the bidirectional polarization interferometer comprises a second polarization beam splitter PBS2, a third polarization beam splitter PBS3 and a sixth polarization beam splitter PBS6;

[0027] Two output ports of PBS2 are connected to one input port of PBS3 and one input port of PBS6 respectively;

[0028] One output port of PBS6 is directly connected to another input port thereof through an optical fiber, forming a ring structure;

[0029] Another output port of PBS6 is connected to another input port of PBS3;

[0030] PBS2 and PBS3 form a first unequal-arm Mach-Zehnder interferometer with an arm length delay difference of T;

[0031] The ring structure is located on the long arm of the first unequal-arm Mach-Zehnder interferometer, and the introduced delay is T; and the optical signal transmitted along the slow axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer does not enter the ring structure, and the optical signal transmitted along the fast axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer enters the ring structure.

[0032] Preferably, the first polarization analysis module includes a third beam splitter BS3, a first polarizer POL1, a second polarizer POL2, a first quarter wave plate QWP1, a second photodetector PD2 and a third photodetector PD3.

[0033] The angle between the polarization transmission direction of POL1 and the horizontal polarization direction is 45°;

[0034] The polarization transmission direction of POL2 is parallel to the horizontal polarization direction;

[0035] The main axis direction of QWP1 is at an angle of 45° to the horizontal polarization direction;

[0036] The input port of BS3 is connected to an output port of BS1;

[0037] An output port of BS3 is connected to POL1 and PD2 in sequence;

[0038] The other output port of BS3 is QWP1, POL2 and PD3 in sequence.

[0039] Preferably, the first polarization analysis module comprises a second quarter wave plate QWP2, a seventh polarization beam splitter PBS7 and a fourth photodetector PD4.

[0040] The main axis direction of QWP2 is at an angle of 45° to the horizontal polarization direction;

[0041] The input port of PBS7 is connected to an output port of BS1 through QWP2;

[0042] The output port of PBS7 is connected to PD4.

[0043] Preferably, the second polarization analysis module includes an eighth polarization beam splitter PBS8, a ninth polarization beam splitter PBS9, a first single photon detector SPD1, a second single photon detector SPD2, a third single photon detector SPD3, and a fourth single photon detector SPD4.

[0044] One output port of BS2 is connected to the input port of PBS8; the other output port is connected to the input port of PBS9 through a 45° polarization rotation structure;

[0045] The two output ports of PBS8 and the two output ports of PBS9 are connected to SPD1 to SPD4 respectively.

[0046] Preferably, the second polarization analysis module includes a tenth polarization beam splitter PBS10, a first single photon detector SPD1, and a second single photon detector SPD2.

[0047] The two output ports of BS2 are connected to the two input ports of PBS10 through optical fibers with a delay difference of T / 2; a 45° polarization rotation structure is provided on the longer optical fiber;

[0048] The two output ports of PBS10 are connected to SPD1 and SPD2 respectively.

[0049] Preferably, the splitting ratio of BS1 is 90:10, wherein the output port with greater intensity is connected to the first polarization analysis module.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] The present invention proposes a quantum key distribution system, which utilizes the random phase generated by the spontaneous radiation of the laser. By passing the light pulse forward and backward twice through a bidirectional polarization interferometer, the entrapped state and the polarization state can be passively prepared respectively. No active modulation device is required, and only one laser and one interferometer are required. This not only eliminates the side channel information leakage, but also greatly reduces the complexity of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is a schematic diagram of the technical solution principle of the existing quantum key distribution technology;

[0053] Figure 2 It is a schematic diagram of the structural principle of the quantum key distribution system of the present invention;

[0054] Figure 3 This is a schematic diagram of the principle of the first structural embodiment of the quantum key distribution system of the present invention;

[0055] Figure 4 This is a schematic diagram of the principle of the second structural embodiment of the quantum key distribution system of the present invention. DETAILED DESCRIPTION

[0056] The present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0057] like Figure 2 As shown, a quantum key distribution system includes a transmitting end and a receiving end connected by an optical fiber channel, the transmitting end includes a laser LD, a bidirectional polarization interferometer, a first polarization beam splitter PBS1, a first photodetector PD1, a first beam splitter BS1, a first polarization analysis module and an adjustable attenuator VOA,

[0058] The first port, the second port and the third port of the bidirectional polarization interferometer are respectively connected to the laser, the input port and an output port of the first polarization beam splitter PBS1 through a 45° polarization rotation structure;

[0059] The input port and two output ports of the first beam splitter BS1 are respectively connected to the fourth port of the bidirectional polarization interferometer, the first polarization analysis module and the input port of the variable attenuator VOA;

[0060] Another output port of PBS1 is connected to PD1;

[0061] The output port of the VOA is connected to the optical fiber channel as the quantum state output port of the transmitting end;

[0062] The laser LD is used to generate horizontally polarized light pulses with a period of T and a random phase;

[0063] The first polarization analysis module is used to perform polarization analysis on the optical signal emitted from the fourth port of the bidirectional polarization interferometer and split by BS1;

[0064] The receiving end includes a polarization controller PC, a second beam splitter BS2 and a second polarization analysis module;

[0065] The polarization controller is used to perform polarization compensation on the received quantum state;

[0066] The second beam splitter BS2 is used to split the quantum state after polarization compensation to generate two quantum state components;

[0067] The second polarization analysis module is used to perform polarization measurement on the two quantum state components respectively.

[0068] The bidirectional polarization interferometer polarizes and splits the optical signal incident to its first port into two polarization components that are transmitted along the short arm and the long arm with a time delay difference of T, and emits them from its second port; and polarizes and splits the optical signal incident to its third port into two polarization components that are transmitted along the short arm and the long arm with a time delay difference of 2T, and emits them from its fourth port.

[0069] The specific working process is as follows:

[0070] LD generates horizontally polarized light pulses, which first pass through a 45° polarization rotation structure to become 45° linearly polarized light, enter the first port of the bidirectional polarization interferometer, and are polarized and split into horizontal polarization components and vertical polarization components. Both are transmitted along the slow axis of the polarization-maintaining optical fiber on the short arm and long arm in the bidirectional polarization interferometer, and are successively emitted from the second port, with a time difference equal to the period T of the light pulse. Therefore, the vertical polarization component of one light pulse is transmitted through the long arm of the bidirectional polarization interferometer, and the horizontal polarization component of the latter light pulse is transmitted through the short arm of the bidirectional polarization interferometer. The two are simultaneously emitted from the second port of the bidirectional polarization interferometer, and polarization beam combination is performed to generate light pulses with a period of T. The polarization state can be written as

[0071] ,

[0072] in, is the phase difference between two adjacent light pulses. After passing through the 45° polarization rotation structure, the polarization state becomes

[0073] ,

[0074] After being polarized and split by PBS1, the horizontal polarization component is transmitted into PD1 for detection, and the light intensity is

[0075] ,

[0076] Since the light pulses generated by the laser have random phases, the phase difference between the two light pulses is also random, making is also random. The vertical polarization component is reflected with an intensity of

[0077] ,

[0078] According to the law of energy conservation, the intensity of the vertical polarization component can be calculated by measuring the intensity of the horizontal polarization component through PD1. By setting a threshold and post-selecting the result of PD1, the passive preparation of the decoy state can be achieved.

[0079] The vertical polarization component is transformed into 45° linear polarization light through the 45° polarization rotation structure, and enters the third port of the bidirectional polarization interferometer again, and is polarized into horizontal polarization component and vertical polarization component. Both are transmitted along the fast axis of the polarization-maintaining fiber on the short arm and long arm in the bidirectional polarization interferometer, and are successively emitted from the fourth port with a time difference of 2T. Therefore, the vertical polarization component of the i-th light pulse is transmitted in the reverse direction through the long arm of the bidirectional polarization interferometer, and the horizontal polarization component of the i+2-th light pulse is transmitted in the reverse direction through the short arm of the bidirectional polarization interferometer. The two are simultaneously emitted from the fourth port of the bidirectional polarization interferometer, and polarization beam combining is performed to generate light pulses with a period of T. The polarization state can be written as

[0080] ,

[0081] in, are the phase difference between the first two light pulses and the phase difference between the last two light pulses in the four adjacent pulses. After BS1 beam splitting and adjustable attenuation to single photon level, the polarization quantum state can be obtained as

[0082] ,

[0083] in, It can be seen that the generated polarization quantum state corresponds to any point on the Bloch sphere and is completely determined by The only certainty.

[0084] The polarization states of most optical pulses are measured after the first polarization analysis module BS1 splits the beam, and the four polarization states of +45° linear deflection (D), -45° linear deflection (A) under the X basis and right-hand circular deflection (R), left-hand circular deflection (L) under the Y basis can be selected through post-selection, so as to realize the BB84 protocol. The quantum state reaches the receiving end through the optical fiber channel, and first passes through the polarization controller PC for polarization compensation to offset the random polarization changes caused by the birefringence effect of the optical fiber channel, and converts the polarization states D, A, R, L into R, L, H, V. Then, two quantum state components are generated through BS2 splitting, and enter the second polarization analysis module respectively. The second polarization analysis module is used to measure the polarization of the two quantum state components respectively. Finally, after post-processing processes such as basis alignment, error correction, and confidentiality amplification, secure quantum key distribution can be achieved between the transmitter and the receiver.

[0085] like Figure 3 As shown, embodiment 1:

[0086] The bidirectional polarization interferometer includes a second polarization beam splitter PBS2, a third polarization beam splitter PBS3, a fourth polarization beam splitter PBS4 and a fifth polarization beam splitter PBS5;

[0087] Two output ports of PBS2 are connected to one input port of PBS3 and one input port of PBS4 respectively;

[0088] The two output ports of PBS4 are connected to the two input ports of PBS5 respectively;

[0089] The output port of PBS5 is connected to another input port of PBS3;

[0090] PBS2 and PBS3 constitute a first unequal-arm Mach-Zehnder interferometer;

[0091] PBS4 and PBS5 constitute a second unequal-arm Mach-Zehnder interferometer;

[0092] The second unequal-arm Mach-Zehnder interferometer is located on the long arm of the first unequal-arm Mach-Zehnder interferometer, and the arm length delay difference between the two is T; and the optical signal transmitted along the slow axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer enters the short arm of the second unequal-arm Mach-Zehnder interferometer, and the optical signal transmitted along the fast axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer enters the long arm of the second unequal-arm Mach-Zehnder interferometer.

[0093] The first polarization analysis module includes a third beam splitter BS3, a first polarizer POL1, a second polarizer POL2, a first quarter wave plate QWP1, a second photodetector PD2 and a third photodetector PD3.

[0094] The angle between the polarization transmission direction of POL1 and the horizontal polarization direction is 45°;

[0095] The polarization transmission direction of POL2 is parallel to the horizontal polarization direction;

[0096] The main axis direction of QWP1 is at an angle of 45° to the horizontal polarization direction;

[0097] The input port of BS3 is connected to an output port of BS1;

[0098] An output port of BS3 is connected to POL1 and PD2 in sequence;

[0099] The other output port of BS3 is QWP1, POL2 and PD3 in sequence.

[0100] The second polarization analysis module includes an eighth polarization beam splitter PBS8, a ninth polarization beam splitter PBS9, a first single photon detector SPD1, a second single photon detector SPD2, a third single photon detector SPD3, and a fourth single photon detector SPD4.

[0101] One output port of BS2 is connected to the input port of PBS8; the other output port is connected to the input port of PBS9 through a 45° polarization rotation structure;

[0102] The two output ports of PBS8 and the two output ports of PBS9 are connected to SPD1 to SPD4 respectively.

[0103] The specific working process of the first embodiment is as follows:

[0104] LD generates a horizontally polarized light pulse, which first passes through a 45° polarization rotation structure to become a 45° linearly polarized light, enters an input port of PBS2, and is polarized and split into a horizontal polarization component and a vertical polarization component. The former is transmitted along the slow axis of the short-arm optical fiber of the first unequal-arm Mach-Zehnder interferometer, arrives at PBS3 and is emitted from one of its output ports; the latter is transmitted along the slow axis of the long-arm optical fiber of the first unequal-arm Mach-Zehnder interferometer, enters the short arm of the second unequal-arm Mach-Zehnder interferometer from the input port of PBS4, and arrives at PBS3 after being emitted from the output port of PBS5, and is emitted from one of its output ports. The time difference between the two when they are emitted from PBS3 is equal to the period T of the light pulse. Therefore, the vertical polarization component of one light pulse and the horizontal polarization component of the next light pulse are emitted from one of the output ports of PBS3 at the same time, and polarization combination is performed to generate a light pulse with a period of T. The polarization state can be written as

[0105] ,

[0106] in, is the phase difference between two adjacent light pulses. After passing through the 45° polarization rotation structure, the polarization state becomes

[0107] ,

[0108] After being polarized and split by PBS1, the horizontal polarization component is transmitted into PD1 for detection, and the light intensity is

[0109] ,

[0110] Since the light pulses generated by the laser have random phases, the phase difference between the two light pulses is also random, making is also random. The vertical polarization component is reflected with an intensity of

[0111] ,

[0112] According to the law of energy conservation, the intensity of the vertical polarization component can be calculated by measuring the intensity of the horizontal polarization component through PD1. By setting a threshold and post-selecting the result of PD1, the passive preparation of the decoy state can be achieved.

[0113] The vertical polarization component is transformed into 45° linear polarized light through the 45° polarization rotation structure, and is incident from the other output port of PBS3. It is polarized and split into horizontal polarization component and vertical polarization component. The latter is transmitted in the opposite direction along the fast axis of the short arm optical fiber of the first unequal-arm Mach-Zehnder interferometer, and reaches PBS2 and exits from one of its output ports; the former is transmitted in the opposite direction along the fast axis of the long arm optical fiber of the first unequal-arm Mach-Zehnder interferometer, enters the long arm of the second unequal-arm Mach-Zehnder interferometer from the output port of PBS5, and arrives at PBS2 after exiting from the input port of PBS4, and exits from its other input port. The time difference between the two when they exit from PBS2 is equal to 2T. Therefore, the vertical polarization component of the i-th optical pulse and the horizontal polarization component of the i+2-th optical pulse are simultaneously emitted from the other input port of PBS2, and polarization combination is performed to generate optical pulses with a period of T. The polarization state can be written as

[0114] ,

[0115] in, are the phase difference between the first two light pulses and the phase difference between the last two light pulses in the four adjacent pulses. After BS1 beam splitting and adjustable attenuation to single photon level, the polarization quantum state can be obtained as

[0116] ,

[0117] in, It can be seen that the generated polarization quantum state corresponds to any point on the Bloch sphere and is completely determined by The only certainty.

[0118] Most of the light pulses after BS1 splitting enter the input port of BS3 and are split into two components with the same amplitude and polarization state for polarization analysis. When the polarization state of the quantum state is D, one component passes through POL1 and enters PD2. At this time, the detection result of PD2 is the maximum light intensity; the other component becomes circularly polarized light after passing through QWP1, and enters PD3 after passing through POL2. At this time, the detection result of PD3 is half of the maximum light intensity.

[0119] When the polarization state of the quantum state is A, one component passes through POL1 and enters PD2. At this time, the detection result of PD2 is the minimum light intensity; the other component becomes circularly polarized light after passing through QWP1, and enters PD3 after passing through POL2. At this time, the detection result of PD3 is half of the maximum light intensity;

[0120] When the polarization state of the quantum state is R, one component passes through POL1 and enters PD2. At this time, the detection result of PD2 is half of the maximum light intensity. The other component becomes horizontally polarized after passing through QWP1, and all passes through POL2 and enters PD3. At this time, the detection result of PD3 is the maximum light intensity.

[0121] When the polarization state of the quantum state is L, one component passes through POL1 and enters PD2. At this time, the detection result of PD2 is half of the maximum light intensity; the other component becomes vertically polarized after passing through QWP1, and all passes through POL2 and enters PD3. At this time, the detection result of PD3 is the minimum light intensity.

[0122] According to the detection results of PD2 and PD3, the four polarization states of D, A under X basis and R, L under Y basis can be selected through post-selection, and the BB84 protocol can be implemented. The quantum state reaches the receiving end through the optical fiber channel. It first passes through the polarization controller PC for polarization compensation to offset the random polarization changes caused by the birefringence effect of the optical fiber channel, and converts the polarization states D, A, R, L to R, L, H, V. Then, two quantum state components are generated through BS2 beam splitting. One path enters the input port of PBS8, and is measured by SPD1 and SPD2 respectively after polarization beam splitting; the other path enters the input port of PBS9 after passing through the 45° polarization rotation structure, and is measured by SPD3 and SPD4 respectively after polarization beam splitting. Finally, after post-processing processes such as basis alignment, error correction, and confidentiality amplification, secure quantum key distribution can be achieved between the transmitter and the receiver.

[0123] like Figure 4 As shown, Example 2:

[0124] The bidirectional polarization interferometer includes a second polarization beam splitter PBS2, a third polarization beam splitter PBS3 and a sixth polarization beam splitter PBS6;

[0125] Two output ports of PBS2 are connected to one input port of PBS3 and one input port of PBS6 respectively;

[0126] One output port of PBS6 is directly connected to another input port thereof through an optical fiber, forming a ring structure;

[0127] Another output port of PBS6 is connected to another input port of PBS3;

[0128] PBS2 and PBS3 form a first unequal-arm Mach-Zehnder interferometer with an arm length delay difference of T;

[0129] The ring structure is located on the long arm of the first unequal-arm Mach-Zehnder interferometer, and the introduced delay is T; and the optical signal transmitted along the slow axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer does not enter the ring structure, and the optical signal transmitted along the fast axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer enters the ring structure.

[0130] The first polarization analysis module includes QWP2, PBS7 and a fourth photodetector PD4,

[0131] The main axis direction of QWP2 is at an angle of 45° to the horizontal polarization direction;

[0132] The input port of PBS7 is connected to an output port of BS1 through QWP2;

[0133] The output port of PBS7 is connected to PD4.

[0134] The second polarization analysis module includes a tenth polarization beam splitter PBS10, a first single photon detector SPD1, and a second single photon detector SPD2.

[0135] The two output ports of BS2 are connected to the two input ports of PBS10 through optical fibers with a delay difference of T / 2; a 45° polarization rotation structure is provided on the longer optical fiber;

[0136] The two output ports of PBS10 are connected to SPD1 and SPD2 respectively.

[0137] The specific working process of the second embodiment is as follows:

[0138] LD generates a horizontally polarized light pulse, which first passes through a 45° polarization rotation structure to become a 45° linearly polarized light, enters an input port of PBS2, and is polarized and split into a horizontal polarization component and a vertical polarization component. The former is transmitted along the slow axis of the short-arm optical fiber of the first unequal-arm Mach-Zehnder interferometer, and arrives at PBS3 and is emitted from one of its output ports; the latter is transmitted along the slow axis of the long-arm optical fiber of the first unequal-arm Mach-Zehnder interferometer, directly transmitted from PBS6, and arrives at PBS3 and is emitted from one of its output ports. The time difference between the two when they are emitted from PBS3 is equal to the period T of the light pulse. Therefore, the vertical polarization component of one light pulse and the horizontal polarization component of the next light pulse are emitted from an output port of PBS3 at the same time, and polarization combination is performed to generate a light pulse with a period of T. The polarization state can be written as

[0139] ,

[0140] in, is the phase difference between two adjacent light pulses. After passing through the 45° polarization rotation structure, the polarization state becomes

[0141] ,

[0142] After being polarized and split by PBS1, the horizontal polarization component is transmitted into PD1 for detection, and the light intensity is

[0143] ,

[0144] Since the light pulses generated by the laser have random phases, the phase difference between the two light pulses is also random, making is also random. The vertical polarization component is reflected with an intensity of

[0145] ,

[0146] According to the law of energy conservation, the intensity of the vertical polarization component can be calculated by measuring the intensity of the horizontal polarization component through PD1. By setting a threshold and post-selecting the result of PD1, the passive preparation of the decoy state can be achieved.

[0147] The vertical polarization component is transformed into 45° linear polarized light through the 45° polarization rotation structure, and is incident from the other output port of PBS3. It is polarized and split into horizontal polarization component and vertical polarization component. The latter is transmitted in the opposite direction along the fast axis of the short-arm optical fiber of the first unequal-arm Mach-Zehnder interferometer, and reaches PBS2 and exits from one of its output ports; the former is transmitted in the opposite direction along the fast axis of the long-arm optical fiber of the first unequal-arm Mach-Zehnder interferometer, and is reflected from PBS6 and enters the ring structure, and then reaches PBS2 after exiting from PBS6, and exits from its other input port. The time difference between the two when they exit from PBS2 is equal to 2T. Therefore, the vertical polarization component of the i-th optical pulse and the horizontal polarization component of the i+2-th optical pulse are simultaneously emitted from the other input port of PBS2, and polarization combination is performed to generate optical pulses with a period of T. The polarization state can be written as

[0148] ,

[0149] in, are the phase difference between the first two light pulses and the phase difference between the last two light pulses in the four adjacent pulses. After BS1 beam splitting and adjustable attenuation to single photon level, the polarization quantum state can be obtained as

[0150] ,

[0151] in, It can be seen that the generated polarization quantum state corresponds to any point on the Bloch sphere and is completely determined by The only certainty.

[0152] Since the light intensity detected by PD1 contains the intensity information of the horizontal polarization component and the vertical polarization component of the quantum state, it can be used to detect the polarization state. When the polarization state of the quantum state is D, the detection result of PD1 for the i-th pulse is the maximum light intensity, and the detection result for the i+2-th pulse is the minimum light intensity; when the polarization state of the quantum state is A, the detection result of PD1 for the i-th pulse is the minimum light intensity, and the detection result for the i+2-th pulse is the maximum light intensity; when the polarization state of the quantum state is R, the detection result of PD1 for odd-numbered pulses is half of the maximum light intensity; at the same time, the detection result of PD4 is the maximum light intensity; when the polarization state of the quantum state is L, the detection result of PD1 for odd-numbered pulses is half of the maximum light intensity; the detection result of PD4 is the minimum light intensity.

[0153] According to the detection results of PD1 and PD4, the four polarization states of D, A under X basis and R, L under Y basis can be selected through post-selection, and the BB84 protocol can be implemented. The quantum state reaches the receiving end through the optical fiber channel. It first passes through the polarization controller PC for polarization compensation to offset the random polarization changes caused by the birefringence effect of the optical fiber channel, and converts the polarization states D, A, R, L into R, L, H, V. Then, two quantum state components are generated through BS2 beam splitting. One path enters an input port of PBS10. After polarization beam splitting, it is measured by SPD1 and SPD2 respectively; the other path passes through a 45° polarization rotation structure and delays T / 2 before entering another input port of PBS10. After polarization beam splitting, it is measured after time division multiplexing by SPD1 and SPD2 respectively. Finally, after post-processing processes such as basis alignment, error correction, and confidentiality amplification, secure quantum key distribution can be achieved between the transmitter and the receiver.

[0154] It can be seen from the embodiments of the present invention that the present invention proposes a quantum key distribution system, which utilizes the random phase generated by the spontaneous radiation of the laser. By passing the light pulse forward and backward twice through a bidirectional polarization interferometer, the entrapped state and the polarization state can be passively prepared respectively. No active modulation device is required, and only one laser and one interferometer are required, which not only eliminates the side channel information leakage, but also greatly reduces the complexity of the system.

Claims

1. A quantum key distribution system, characterized in that: It includes a transmitting end and a receiving end connected by an optical fiber channel, wherein the transmitting end includes a laser LD, a bidirectional polarization interferometer, a first polarization beam splitter PBS1, a first photodetector PD1, a first beam splitter BS1, a first polarization analysis module and an adjustable attenuator VOA, The first port, the second port and the third port of the bidirectional polarization interferometer are respectively connected to the input port and the output port of the LD and PBS1 through a 45° polarization rotation structure; The input port and two output ports of BS1 correspond to the fourth port of the bidirectional polarization interferometer, the first polarization analysis module and the input port of the variable attenuator VOA respectively; Another output port of PBS1 is connected to PD1; The output port of the VOA is connected to the optical fiber channel as the quantum state output port of the transmitting end; LD is used to generate horizontally polarized light pulses with a period of T and random phases; The arm length delay difference of the bidirectional polarization interferometer in the transmission direction from the third port to the fourth port is an integer multiple of T, and is twice the arm length delay difference in the transmission direction from the first port to the second port; The first polarization analysis module is used to perform polarization analysis on the optical signal emitted from the fourth port of the bidirectional polarization interferometer and split by BS1; The receiving end includes a polarization controller PC, a second beam splitter BS2 and a second polarization analysis module; The polarization controller is used to perform polarization compensation on the received quantum state; BS2 is used to split the quantum state after polarization compensation to generate two quantum state components; The second polarization analysis module is used to perform polarization measurement on the two quantum state components respectively.

2. The quantum key distribution system according to claim 1, characterized in that: The bidirectional polarization interferometer polarizes and splits the optical signal incident to its first port into two polarization components that are transmitted along the short arm and the long arm with a time delay difference of T, and emits them from its second port; and polarizes and splits the optical signal incident to its third port into two polarization components that are transmitted along the short arm and the long arm with a time delay difference of 2T, and emits them from its fourth port.

3. The quantum key distribution system according to claim 1 or 2, characterized in that: The bidirectional polarization interferometer includes a second polarization beam splitter PBS2, a third polarization beam splitter PBS3, a fourth polarization beam splitter PBS4 and a fifth polarization beam splitter PBS5; Two output ports of PBS2 are connected to one input port of PBS3 and one input port of PBS4 respectively; The two output ports of PBS4 are connected to the two input ports of PBS5 respectively; The output port of PBS5 is connected to another input port of PBS3; PBS2 and PBS3 constitute a first unequal-arm Mach-Zehnder interferometer; PBS4 and PBS5 constitute a second unequal-arm Mach-Zehnder interferometer; The second unequal-arm Mach-Zehnder interferometer is located on the long arm of the first unequal-arm Mach-Zehnder interferometer, and the arm length delay difference between the two is T; and the optical signal transmitted along the slow axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer enters the short arm of the second unequal-arm Mach-Zehnder interferometer, and the optical signal transmitted along the fast axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer enters the long arm of the second unequal-arm Mach-Zehnder interferometer.

4. The quantum key distribution system according to claim 1 or 2, characterized in that: The bidirectional polarization interferometer includes a second polarization beam splitter PBS2, a third polarization beam splitter PBS3 and a sixth polarization beam splitter PBS6; Two output ports of PBS2 are connected to one input port of PBS3 and one input port of PBS6 respectively; One output port of PBS6 is directly connected to another input port thereof through an optical fiber, forming a ring structure; Another output port of PBS6 is connected to another input port of PBS3; PBS2 and PBS3 form a first unequal-arm Mach-Zehnder interferometer with an arm length delay difference of T; The ring structure is located on the long arm of the first unequal-arm Mach-Zehnder interferometer, and the introduced delay is T; and the optical signal transmitted along the slow axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer does not enter the ring structure, and the optical signal transmitted along the fast axis of the long-arm polarization-maintaining optical fiber of the first unequal-arm Mach-Zehnder interferometer enters the ring structure.

5. The quantum key distribution system according to claim 3, characterized in that: The first polarization analysis module includes a third beam splitter BS3, a first polarizer POL1, a second polarizer POL2, a first quarter wave plate QWP1, a second photodetector PD2 and a third photodetector PD3. The angle between the polarization transmission direction of POL1 and the horizontal polarization direction is 45°; The polarization transmission direction of POL2 is parallel to the horizontal polarization direction; The main axis direction of QWP1 is at an angle of 45° to the horizontal polarization direction; The input port of BS3 is connected to an output port of BS1; An output port of BS3 is connected to POL1 and PD2 in sequence; Another output port of BS3 is connected to QWP1, POL2 and PD3 in sequence.

6. The quantum key distribution system according to claim 3, characterized in that: The first polarization analysis module includes a second quarter wave plate QWP2, a seventh polarization beam splitter PBS7 and a fourth photodetector PD4. The main axis direction of QWP2 is at an angle of 45° to the horizontal polarization direction; The input port of PBS7 is connected to an output port of BS1 through QWP2; The output port of PBS7 is connected to PD4.

7. The quantum key distribution system according to claim 5, characterized in that: The second polarization analysis module includes an eighth polarization beam splitter PBS8, a ninth polarization beam splitter PBS9, a first single photon detector SPD1, a second single photon detector SPD2, a third single photon detector SPD3, and a fourth single photon detector SPD4. One output port of BS2 is connected to the input port of PBS8; the other output port is connected to the input port of PBS9 through a 45° polarization rotation structure; The two output ports of PBS8 and the two output ports of PBS9 are connected to SPD1 to SPD4 respectively.

8. The quantum key distribution system according to claim 6, characterized in that: The second polarization analysis module includes a tenth polarization beam splitter PBS10, a first single photon detector SPD1, and a second single photon detector SPD2. The two output ports of BS2 are connected to the two input ports of PBS10 through optical fibers with a delay difference of T / 2; a 45° polarization rotation structure is provided on the longer optical fiber; The two output ports of PBS10 are connected to SPD1 and SPD2 respectively.

9. The quantum key distribution system according to claim 1, characterized in that: The beam splitting ratio of BS1 is 90:10, and the output port with greater intensity is connected to the first polarization analysis module.

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