A polarization-independent phase encoder-decoder and quantum key distribution system

Abstract

A polarization-independent phase encoder-decoder, two interfaces of a circulator are respectively connected to a first single-photon detector and a first polarization beam splitter, the light outlet of the first polarization beam splitter is respectively connected to the light inlet of a third polarization beam splitter through the fast axis and the slow axis of a polarization-maintaining optical fiber, the light outlet of the third polarization beam splitter is respectively connected to the light inlet of a second polarization beam splitter through the fast axis and the slow axis of a polarization-maintaining optical fiber, a second phase modulator is connected between one light outlet and the inlet of the second polarization beam splitter, one light outlet of the first polarization beam splitter is connected to a second single-photon detector, and the present invention also provides a quantum key distribution system. Compared with the prior art, the present invention has low requirements on the transmitting end, a simple receiving end structure, and the detectors all contain H and V components, and the interference result will not be affected by the change of the incident polarization state, so it is polarization-independent, and can resist the influence of channel polarization disturbance on the system, reduce the bit error rate, and improve the key generation rate.

Description

Technical Field

[0001] The present invention relates to the field of quantum polarization coding technology, and in particular to a polarization-independent phase encoder-decoder and a quantum key distribution system. Background Art

[0002] Fiber-based quantum key distribution systems generally use single-mode optical fibers as transmission channels. However, due to the inherent birefringence effect of optical fiber channels, the polarization state of photons will change during transmission and will change with changes in the external environment, making the polarization state of photons unpredictable when they enter the receiving end. Therefore, the traditional quantum key distribution system based on the dual-unequal-arm Mach-Zehnder interferometer ring scheme has poor stability and is easily affected by environmental interference.

[0003] In order to improve the stability and practicality of quantum key distribution systems, researchers have proposed two types of solutions. One type is active polarization compensation, which adds a polarization compensation module to the receiving end and performs polarization tracking and compensation through feedback control. This type of solution will increase the complexity of the system, consume time and resources, and have a high bit error rate; the other type is passive compensation for polarization state, such as the plug-and-play round-trip quantum key distribution system, which uses the Faraday mirror to rotate the polarization state of the incident light by 90 degrees to offset the effect of the optical fiber channel on the polarization state of the photon, thereby ensuring the stability of the system. However, due to its round-trip structure, this solution has security risks and is vulnerable to Trojan attacks. In addition, the operating frequency of the system is limited, and the Raman scattering effect of the optical fiber will also increase the system noise. Another solution is to add a depolarizer at the transmitting end to randomize the polarization state before the photon enters the optical fiber channel, which can eliminate the effect of optical fiber birefringence and the influence of environmental disturbances on the polarization state. Adding a polarization beam splitter at the receiving end for polarization can obtain a stable interference result. However, this solution will double the loss and reduce the efficiency of the system by half. Summary of the invention

[0004] The purpose of the present invention is to provide a polarization-independent phase codec and a quantum key distribution system to solve the technical defects of some quantum key distribution in the prior art, such as the potential security risks of being vulnerable to attacks, the high system complexity, the high bit error rate, and the low efficiency of generating keys.

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

[0006] A polarization-independent phase encoding and decoding device comprises a circulator, a first polarization beam splitter, a second polarization beam splitter, a third polarization-maintaining beam splitter, a second phase modulator, a first single-photon detector and a second single-photon detector, wherein two interfaces of the circulator are respectively connected to the first single-photon detector and the first polarization beam splitter, a light outlet of the first polarization beam splitter is respectively connected to a light inlet of the third polarization-maintaining beam splitter through a fast axis and a slow axis of a polarization-maintaining optical fiber, a polarization rotator is provided on the slow axis connected between the first polarization beam splitter and the third polarization-maintaining beam splitter, a light outlet of the third polarization-maintaining beam splitter is respectively connected to a light inlet of the second polarization beam splitter through a fast axis and a slow axis of the polarization-maintaining optical fiber, a second phase modulator is connected between one light outlet and an inlet of the second polarization beam splitter, and one light outlet of the first polarization beam splitter is connected to the second single-photon detector.

[0007] The present invention also provides a quantum key distribution system, including a transmitting end and a receiving end, the receiving end includes a phase codec, the transmitting end includes a laser, a first intensity modulator, a first polarization-maintaining beam splitter, a second polarization-maintaining beam splitter, a first phase modulator and an optical attenuator, the laser is connected to the intensity modulator and the first polarization-maintaining beam splitter in sequence, the first polarization-maintaining beam splitter is connected to the light input port of the second polarization-maintaining beam splitter through the fast axis and the slow axis of the polarization-maintaining optical fiber respectively, a first phase modulator is arranged on the slow axis between the first polarization-maintaining beam splitter and the second polarization-maintaining beam splitter, the light output port of the second polarization-maintaining beam splitter is connected to one end of the optical attenuator, and the other end of the optical attenuator is connected to the circulator of the receiving end through the optical fiber.

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

[0009] The polarization-independent phase codec and quantum key distribution system of the present invention have low requirements on the transmitting end and a simple receiving end structure. The detectors contain H and V components, and the interference result will not be affected by the change of the incident polarization state, so it is polarization-independent. Therefore, it can resist the influence of channel polarization disturbance on the system, reduce the bit error rate, and improve the key generation rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 A quantum key distribution system of the present invention;

[0011] Figure 2 It is the polarization-independent phase encoder / decoder of the present invention.

[0012] In the figure: transmitting end 100, laser 101, intensity modulator 102, first polarization maintaining beam splitter 103, second polarization maintaining beam splitter 104, first phase modulator 105, optical attenuator 106, receiving end 200, circulator 201, first polarization beam splitter 202, second polarization beam splitter 203, third polarization maintaining beam splitter 204, second phase modulator 205, first single photon detector 206, second single photon detector 207, polarization rotator 208. DETAILED DESCRIPTION

[0013] The present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0014] like Figure 1 As shown, the BOb part is a polarization-independent phase encoder / decoder, including a circulator 201, a first polarization beam splitter 202, a second polarization beam splitter 203, a third polarization-maintaining beam splitter 204, a second phase modulator 205, a first single-photon detector 206, and a second single-photon detector 207. The two interfaces of the circulator 201 are respectively connected to the first single-photon detector 206 and the first polarization beam splitter 202. The light outlets of the first polarization beam splitter 202 are respectively connected to the fast axis and the slow axis of the polarization-maintaining optical fiber. A polarization rotator 208 is provided on the slow axis connected to the light input port of the third polarization maintaining beam splitter 204 and connected between the first polarization beam splitter 202 and the third polarization maintaining beam splitter 204; a light output port of the third polarization maintaining beam splitter 204 is connected to the light input port of the second polarization beam splitter 203 through the fast axis and the slow axis of the polarization maintaining optical fiber respectively; a second phase modulator 205 is connected between one light output port and the light input port of the second polarization beam splitter 203; and one light output port of the first polarization beam splitter 202 is connected to a second single-photon detector 207.

[0015] like Figure 1 As shown, the present invention also provides a quantum key distribution system, including a transmitting end 100 and a receiving end 200, the receiving end 200 includes a phase codec, the transmitting end includes a laser 101, an intensity modulator 102, a first polarization-maintaining beam splitter 103, a second polarization-maintaining beam splitter 104, a first phase modulator 105 and an optical attenuator 106, the laser 101 is connected to the intensity modulator 102 and the first polarization-maintaining beam splitter 103 in sequence, the first polarization-maintaining beam splitter 103 is connected to the light input port of the second polarization-maintaining beam splitter 104 through the fast axis and the slow axis of the polarization-maintaining optical fiber respectively, the first phase modulator 105 is arranged on the slow axis between the first polarization-maintaining beam splitter 103 and the second polarization-maintaining beam splitter 104, the light output port of the second polarization-maintaining beam splitter 104 is connected to one end of the optical attenuator 106, and the other end of the optical attenuator 106 is connected to the circulator 201 of the receiving end through an optical fiber.

[0016] The principle of the phase codec of the present invention is as follows:

[0017] like Figure 2 As shown, the time interval between the two pulses (P1, P2) emitted from the transmitter is T, and the two pulses have the same polarization. After passing through the channel, the polarization state of the photon changes randomly, so when entering the receiving end, the polarization states of the two pulses are arbitrary, but they are subject to the same disturbance, so they maintain the same polarization.

[0018] After P1 and P2 enter the receiving end, they enter port 2 from port 1 of the circulator and then enter port 4 of the polarization beam splitter PBS1. At this time, the two can be decomposed into horizontal polarization and vertical polarization, which are H1, V1 and H2, V2 respectively. The V1 and V2 components are emitted from port 5 of PBS1 and propagate along the slow axis of the polarization-maintaining fiber, which is represented by the superscript s, that is, and Then, after passing through a 90-degree polarization rotation structure, it propagates along the fast axis (f) of the polarization-maintaining fiber, that is, and Then, the two pulses enter the c port of the polarization-maintaining beam splitter BS. At this time, each pulse will be divided into two beams, which will be emitted from the a port and the b port. The pulse emitted from the a port is and The pulse emitted from port b is and and Enter from port 8 of PBS2, verify that it propagates counterclockwise in the Sagnac loop formed by PBS2, passes through port 11 of PBS2, PMB, and port 10 of PBS2, and then exits from port 8 and returns to port a of BS. At this time, the polarization directions of the two pulses are rotated 90 degrees and propagate along the slow axis of the polarization-maintaining fiber, that is, and akin, and Along the path port b - port 9 - port 11 - PMB - port 10 - port 9 - port b, it returns to BS again. The polarization directions of the two are also rotated by 90 degrees, becoming and In this process and Passing through short arm l1 twice, and The difference in length of the two long arms l2 is 2(l2-l1), and the time taken is equal to T, so the pulse and will overlap at the BS and cause interference. The interference result emitted from the BS port c is The output from port d is Let the phase difference between the transmitter and receiver modulation be Ignoring the non-interference peaks, the interference results have the following relationship

[0019]

[0020]

[0021] Using a similar analysis process, we can get the interference results of H1 and H2 components after they interfere at BS, which are emitted from ports c and d:

[0022]

[0023]

[0024] The additional phase π is caused by the H component and the V component being incident from two different ports of the BS. Therefore, the pulse emitted from the BS port c is and The pulse emitted from BS port d is and in and After the polarization rotation structure, it becomes and Enter port 5 of PBS1. will be emitted from port 4 of PBS1 and become Then it enters port 3 from circulator port 2 and finally reaches SPD1; Transmitted directly from PBS1 into port 7 and finally to SPD2. After entering port 6 of PBS1, it exits from port 7 and becomes Enter SPD2; Then it is directly transmitted from port 6 of PBS1 to port 4, and then enters SPD1 through the circulator. At this time, it can be seen that the component entering SPD1 is The two have the same interference result, that is, the interference maximum or minimum is obtained at the same time. Similarly, the component entering SPD2 is The two also have the same interference results. Moreover, each detector contains H and V components, and the interference results will not be affected by changes in the incident polarization state, so it is polarization-independent.

[0025] It should be noted that in order to avoid interference when the V component enters the BS from port c and the H component enters the BS from port d, the lengths of the polarization-maintaining fibers l3 and l4 need to be adjusted so that the time difference between the two components reaching the BS is greater than the coherence time and less than the effective gate width of the single-photon detector.

[0026] The polarization-independent phase codec and quantum key distribution system of the present invention have low requirements on the transmitting end and a simple receiving end structure. The detectors contain H and V components, and the interference result will not be affected by the change of the incident polarization state, so it is polarization-independent. Therefore, it can resist the influence of channel polarization disturbance on the system, reduce the bit error rate, and improve the key generation rate.

Claims

1. A polarization-independent phase codec, characterized in that: It includes a circulator, a first polarization beam splitter, a second polarization beam splitter, a third polarization-maintaining beam splitter, a second phase modulator, a first single-photon detector, and a second single-photon detector. The two interfaces of the circulator are respectively connected to the first single-photon detector and the first polarization beam splitter. The light outlet of the first polarization beam splitter is respectively connected to the light inlet of the third polarization-maintaining beam splitter through the fast axis and the slow axis of the polarization-maintaining optical fiber. The slow axis connected between the first polarization beam splitter and the third polarization-maintaining beam splitter is provided with a polarization rotator. The light outlet of the third polarization-maintaining beam splitter is respectively connected to the light inlet of the second polarization beam splitter through the fast axis and the slow axis of the polarization-maintaining optical fiber. A second phase modulator is connected between one light outlet and the light inlet of the second polarization beam splitter. One light outlet of the first polarization beam splitter is connected to the second single-photon detector.

2. A quantum key distribution system, comprising a transmitting end and a receiving end, characterized in that: The receiving end is the phase codec as claimed in claim 1, and the transmitting end includes a laser, an intensity modulator, a first polarization-maintaining beam splitter, a second polarization-maintaining beam splitter, a first phase modulator and an optical attenuator. The laser is connected to the intensity modulator and the first polarization-maintaining beam splitter in sequence. The first polarization-maintaining beam splitter is connected to the light input port of the second polarization-maintaining beam splitter through the fast axis and the slow axis of the polarization-maintaining optical fiber respectively. The first phase modulator is arranged on the slow axis between the first polarization-maintaining beam splitter and the second polarization-maintaining beam splitter. The light output port of the second polarization-maintaining beam splitter is connected to one end of the optical attenuator, and the other end of the optical attenuator is connected to the circulator of the receiving end through the optical fiber.

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