CN114499838A

CN114499838A - Central symmetry QKD annular multi-user system and key distribution method thereof

Info

Publication number: CN114499838A
Application number: CN202111668161.4A
Authority: CN
Inventors: 郭邦红; 赵韬; 胡敏
Original assignee: South China Normal University
Current assignee: Guangdong Yukopod Technology Development Co ltd
Priority date: 2021-12-30
Filing date: 2021-12-30
Publication date: 2022-05-13
Anticipated expiration: 2041-12-30
Also published as: CN114499838B

Abstract

The invention provides a central symmetric QKD annular multi-user system and a key distribution method thereof, wherein the system is realized based on a Sagnac ring, the network structure is simpler, and the technology is mature; meanwhile, each user stabilizes the polarization state of light through a polarization controller, so that the system has better stability; the two multi-user sides can expand the number of network users according to the actual situation, and the quantum key distribution of the users and a plurality of receiving users at the receiving end can be controlled simultaneously through the wavelength division multiplexing/demultiplexing device; each multi-user end shares one multi-wavelength laser generating device, and shares one group of detectors through the arrangement of the adjustable light delay lines, so that the network structure is simplified, and the network cost is saved; the multi-wavelength laser generating device selectively generates multi-wavelength optical pulses with certain wavelength intervals, so that signal crosstalk caused by four-wave mixing effect can be reduced, and the error rate is reduced.

Description

Central symmetry QKD annular multi-user system and key distribution method thereof

Technical Field

The invention relates to the technical field of quantum secret communication and optical communication, in particular to a central symmetric QKD annular multi-user system and a key distribution method thereof.

Background

Quantum secure communication is different from classical communication, and does not depend on the complexity of mathematical computation to increase the eavesdropping difficulty of an eavesdropper in a limited time, but finds the existence of eavesdropping on the basis of the basic principle and the characteristics of quantum mechanics, so that the encrypted information of both sides of legal communication has unconditional security in theory. With the development of information technology and the popularization of internet application, quantum secure communication has become a research hotspot of the interdisciplinary science of quantum physics and information science at home and abroad.

The research directions of quantum secure communication mainly include: quantum key distribution, quantum secret sharing, quantum invisible transfer and quantum relay. Among them, Quantum Key Distribution (QKD) has been drawing attention since its introduction as an application field in which quantum secure communication is developed most rapidly and has the highest degree of engineering. Quantum key distribution refers to a key distribution method in which both communication parties negotiate a key through a quantum channel by using a quantum state as a carrier of information. Since the first quantum key distribution protocol, the BB84 protocol, was proposed, many experimental protocols have demonstrated the correctness and feasibility of QKD. At present, the QKD technology is relatively mature and gradually goes to practical application.

At this stage, point-to-point QKD systems have been more sophisticated. To meet the wider communication demand, large-scale networking of QKD is an urgent issue to be solved. Since 2004, the international related organizations or organizations have been deployed successively to build many typical multi-node quantum secure communication networks, such as DARPA quantum communication network in the united states, SECOQC quantum communication network in the european union, tokyo quantum key distribution network in japan, seoul quantum communication network in korea, and so on. China's quantum communication network is also in the forefront of the world, four-user quantum communication network is successfully built in 2007, and the metropolitan quantum communication network covering dozens of nodes can be realized at present.

The first multi-user quantum communication network experiment was completed by Townsend in 1997, which demonstrated a passive star QKD network based on a beam splitter. Thereafter, a loop type QKD network based on a Sagnac loop, a switchable QKD network based on an optical switch, and a WDM-based wavelength division multiplexing network have been proposed. The networks can realize the distribution of the multi-user quantum key, have simpler structure and have certain defects. The passive star network has a low photon utilization rate, and as the number of users increases, the uncertainty of receiving photon signals by each user also increases. Other multi-user QKD schemes improve, but one optical pulse can only be used by one user at a time, and in fact, the scheme is still a one-to-one multi-user QKD scheme, and only one user is on the control end, which easily causes network breakdown. These problems not only limit the expansion of the number of users, but also cause problems such as low utilization of light pulses. In addition, in the scheme, both the transmitting side and the receiving side are fixed, and quantum key distribution in one direction can be only carried out.

Disclosure of Invention

The invention provides a central symmetric QKD annular multi-user system which can simultaneously realize quantum key distribution between a sending end user and a plurality of users.

It is a further object of the present invention to provide a key distribution method for the above-mentioned centrosymmetric QKD ring-type multi-user system.

In order to achieve the technical effects, the technical scheme of the invention is as follows:

a central symmetric QKD annular multi-user system comprises an Alice multi-user end, a Bob multi-user end and an optical fiber link; the optical fiber link comprises an Alice multi-user terminal Sagnac ring-based optical fiber link, a Bob multi-user terminal Sagnac ring-based optical fiber link and a public optical fiber link; the Alice multi-user terminal is connected with the Bob multi-user terminal through a public optical fiber link;

the Alice multi-user end comprises n Alice users, an optical path gating device A, Alice end coupling unit and an Alice multi-user end phase modulator A; the n Alice users are sequentially accessed into an optical fiber link of the Alice multi-user terminal based on the Sagnac ring; the light path gating device A and the Alice end coupling unit are sequentially connected into a public optical fiber link and are connected with an optical fiber link of the Alice multi-user end based on a Sagnac ring, wherein n is more than or equal to 2 and is an integer;

each Alice user of the Alice multi-user end comprises an Alice intensity modulator, an Alice polarization controller, a first wavelength division multiplexing/demultiplexing device, an adjustable light delay line, an Alice phase modulator, a second wavelength division multiplexing/demultiplexing device and a variable optical attenuator which are sequentially connected, and the Alice multi-user end is connected into an optical fiber link of the Alice multi-user end on the Sagnac ring; the Alice tunable optical delay line is connected with an Alice first wavelength division multiplexing/demultiplexing device, and the Alice phase modulator is connected with an Alice second wavelength division multiplexing/demultiplexing device; the adjustable optical delay line and the phase modulator of each Alice user are arranged on a pulse link with corresponding wavelength between the first wavelength division multiplexing/demultiplexing device and the second wavelength division multiplexing/demultiplexing device;

the Alice end coupling unit comprises a multi-wavelength laser generating device A, a third detector, a fourth detector, a third circulator and a fourth circulator; the multi-wavelength laser generating device A, the coupler A and the third detector are respectively connected to one end of the third circulator; the fourth detector is connected to one end of the fourth circulator; the multi-wavelength laser generating device A is used for simultaneously generating a plurality of optical pulses with proper wavelength intervals so as to reduce the influence of a four-wave mixing effect on signal transmission;

the Bob multi-user end comprises n Bob users, an optical path gating device B, Bob end coupling unit and a Bob multi-user end phase modulator B; the n Bob users are sequentially accessed into a fiber link of a Bob multi-user terminal based on a Sagnac ring; the optical path gating device B and the Bob end coupling unit are sequentially connected into a public optical fiber link and are connected with an optical fiber link of a Bob multi-user end based on a Sagnac ring;

each Bob user at the Bob multi-user end comprises a Bob intensity modulator, a Bob multi-user end polarization controller, a first wavelength division multiplexing/demultiplexing device, an adjustable optical delay line, a Bob phase modulator, a second wavelength division multiplexing/demultiplexing device and a variable optical attenuator which are sequentially connected, and the Bob multi-user end is connected into an optical fiber link of a Sagnac ring; the Bob tunable optical delay line is connected with a first wavelength division multiplexing/demultiplexing device of Bob, and the Bob phase modulator is connected with a second wavelength division multiplexing/demultiplexing device of Bob; the adjustable optical delay line and the phase modulator of each Bob user are arranged on a pulse link with corresponding wavelength between the first wavelength division multiplexing/demultiplexing device and the second wavelength division multiplexing/demultiplexing device;

the Bob end coupling unit comprises a multi-wavelength laser generating device B, a first detector, a second detector, a first circulator and a second circulator; the multi-wavelength laser generating device B, the coupler B and the first detector are respectively connected to one end of the first circulator; the second detector is connected to one end of the second circulator; the multi-wavelength laser generating device B is used for simultaneously generating a plurality of optical pulses with proper wavelength intervals so as to reduce the influence of four-wave mixing effect on signal transmission.

Furthermore, the third circulator and the fourth circulator are both three-port circulators, and each three-port circulator comprises a first port, a second port and a third port, wherein a signal input by the first port is only output by the second port, and a signal input by the second port is only output by the third port; a first port of the third circulator is connected with the multi-wavelength laser generation device A, a second port of the third circulator is connected with a first input end of the coupler A, and a third port of the third circulator is connected with the third detector; a first port of the fourth circulator is connected to a common optical fiber link, a second port of the fourth circulator is connected with a second input end of the coupler A, and a third port of the fourth circulator is connected with the fourth detector; the first circulator and the second circulator are both three-port circulators and comprise a first port, a second port and a third port, signals input by the first port are only output at the second port, and signals input by the second port are only output at the third port; a first port of the first circulator is connected with the multi-wavelength laser generation device B, a second port of the first circulator is connected with a first input end of a coupler B, and a third port of the first circulator is connected with the first detector; and a first port of the second circulator is connected with a public optical fiber link, a second port of the second circulator is connected with a second input end of the coupler B, and a third port of the second circulator is connected with the second detector.

Further, the third detector and the fourth detector detect interference photon pulse signals output by the optical fiber link of the Alice multi-user terminal based on the Sagnac ring through the coupler A; the Alice multi-user terminal phase modulator A is connected to one side of an optical fiber link of a Bob multi-user terminal based on a Sagnac ring, and modulates the phase of each Alice user optical pulse from the Alice multi-user terminal; the optical path gating device A is used for gating the output direction of the optical pulse flowing into the optical path gating device A; the first detector and the second detector detect interference photon pulse signals output by the Bob multi-user terminal Sagnac ring-based optical fiber link through a coupler B; the Bob multi-user terminal phase modulator B is accessed to one side of an optical fiber link of the Alice multi-user terminal based on the Sagnac ring, and modulates the phase of each Bob user light pulse from the Alice multi-user terminal; the optical path gating device B is used for gating the output direction of the optical pulse flowing into the optical path gating device B.

Further, if the Alice multi-user terminal serves as the control terminal of the entire system, the Bob multi-user terminal serves as the receiving terminal of the entire system:

when the m-th user Alice m of the n Alice users and the n Bob users of the Bob multi-user end simultaneously carry out quantum key distribution, m is more than or equal to 1 and less than or equal to n, and the multi-wavelength laser generation device A simultaneously generates multiple wavelengths lambda₁、λ₂、…、λ_nThe optical pulse passes through the third circulator a and the coupler a and flows into the optical path gating device a, the optical path gating device a allows the optical pulse to enter only from one end of the optical fiber link of the Alice multi-user end based on the Sagnac loop, and the optical pulse sequentially passes through the Alice users and the Bob multi-user end phase modulators B in the counterclockwise direction; when the optical pulse passes through the Alice m user, the Alice m intensity modulator performs trap state modulation, the Alice m polarization controller is used for stabilizing the optical polarization state, the Alice m first wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength optical pulse to a corresponding wavelength chain, the Alice m adjustable optical delay line does not modulate delay, the Alice m phase modulator allows the multi-wavelength optical pulse to pass through without interference, the Alice m second wavelength division multiplexing/demultiplexing device multiplexes the multi-wavelength optical pulse to an optical fiber link, and the Alice m variable optical attenuator attenuates the optical pulse to a proper average photon number mu per pulse to obtain a photon pulse at a single photon level; each device in other n-1 Alice users not participating in communication and the phase modulator B of the Bob multi-user end allow the multi-wavelength optical pulse to pass through without interference; the multi-wavelength photon pulse flows out from the other end of the optical fiber link of the Alice multi-user terminal based on the Sagnac ring, flows out from the first port through the optical path gating device A, does not flow back into the coupler A, then enters the public optical fiber link, flows into the optical path gating device B through the circulator and the coupler B, and the optical path gating device B allows the photon pulse to flow outThe Bob multi-user end enters at both ends of the Sagnac loop's fiber link.

Further, the photon pulse is divided into two beams by the coupler B according to the proportion of 50:50, and the two beams enter a clockwise CW-A optical fiber link and a counterclockwise CCW-A optical fiber link in the Bob multi-user optical fiber link based on the Sagnac ring respectively to form a clockwise photon pulse and a counterclockwise photon pulse;

clockwise CW-a photon pulse: the clockwise photon pulse sequentially passes through the Alice multi-user-side phase modulator A and each Bob user in a forward direction; the Alice multi-user-end phase modulator A modulates the clockwise photon pulse to generate an additional phase

When the clockwise photon pulse passes through each Bob user in sequence, the variable optical attenuator of each Bob user allows the clockwise photon pulse to pass through without interference, the second wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength photon pulse to a corresponding wavelength path, the phase modulator allows the clockwise photon pulse of the corresponding wavelength to pass through without interference, and the adjustable optical delay line of each Bob user only modulates the photon pulse of the corresponding wavelength to a proper delay value T₁、T₂……T_nThen, each wavelength clockwise photon pulse enters a first wavelength division multiplexing/demultiplexing device to be multiplexed into a multi-wavelength clockwise photon pulse, a polarization controller stabilizes the light polarization state of the photon pulse, an intensity modulator does not modulate the photon pulse, and finally the multi-wavelength clockwise photon pulse returns to the coupler B through the light path gating device B to be output;

CCW-a photon pulse in the counter-clockwise direction: the anticlockwise photon pulses sequentially reversely pass through each Bob user and the Alice multi-user-side phase modulator A; specifically, when the counterclockwise photon pulse sequentially passes through each Bob user, each Bob user intensity modulator does not modulate the photon pulse, the polarization controller stabilizes the optical polarization state of the photon pulse, the first wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength photon pulse to the corresponding wavelength path, and the adjustable optical delay line of each Bob user only modulates the photon pulse with the corresponding wavelength to a proper delay valueT₁、T₂……T_nPhase modulators modulating photonic pulses to produce additional phases

Then, photon pulses with various wavelengths counterclockwise enter a second wavelength division multiplexing/demultiplexing device for multiplexing, and the variable optical attenuator allows the photon pulses to pass through without interference; the Alice multi-user-end phase modulator A allows the counter-clockwise photon pulse to pass through without interference, and finally the counter-clockwise photon pulse returns to the coupler B through the optical path gating device B and interferes with the clockwise photon pulse arriving at the same time; .

Further, the first detector and the second detector are based on a phase difference

Responding to the interference result of the clockwise photon pulse and the anticlockwise photon pulse;

the adjustable light delay line of each Bob user only modulates the photon pulse with the corresponding wavelength to a proper delay value;

because each Bob user shares the photon interference signal detector group B, when the adjustable light delay line of each Bob user modulates the photon pulse with the corresponding wavelength, the adjustable light delay line is modulated to be proper and different delay values so as to ensure that the clockwise photon pulse and the anticlockwise photon pulse with each wavelength do not interfere at the coupler B at the same time; meanwhile, the adjustable optical delay line of each Bob user is respectively used for controlling that the CW-A photon pulse in the clockwise direction and the CCW-A photon pulse in the anticlockwise direction do not exist in the Alice multi-user-side phase modulator A at the same time.

Further, if the Bob multi-user terminal serves as a control terminal of the whole system, the Alice multi-user terminal serves as a receiving terminal of the whole system:

when the kth user Bob k of the n Bob users and the n Alice users of the Alice multi-user end simultaneously carry out quantum key distribution, k is more than or equal to 1 and less than or equal to n, and the multi-wavelength laser generation device B simultaneously generates multiple wavelengths lambda₁、λ₂、…、λ_nThe light pulse passes through the first circulator,The coupler B flows into the optical path gating device B, the optical path gating device B allows optical pulses to enter only from one end of the optical fiber link of the Bob multi-user terminal based on the Sagnac ring, and the optical pulses sequentially pass through the phase modulators A of each Bob user and the Alice multi-user terminal in the counterclockwise direction; when the optical pulse passes through the Bob k user, the Bob k intensity modulator performs decoy state modulation, the Bob k polarization controller is used for stabilizing the optical polarization state, a first Bob k wavelength division multiplexing/demultiplexing device is used for demultiplexing multi-wavelength optical pulses onto a corresponding wavelength chain, Bob k adjustable optical delay lines do not modulate delay, the Bob k phase modulator allows the multi-wavelength optical pulses to pass through without interference, a second Bob k wavelength division multiplexing/demultiplexing device multiplexes the multi-wavelength optical pulses into an optical fiber link, and a Bob k variable optical attenuator attenuates the optical pulses to a proper average photon number mu per pulse to obtain photon pulses at a single photon level; each device in other n-1 Bob users not participating in communication and the Alice multi-user-side phase modulator A allow the multi-wavelength optical pulse to pass through without interference; the multi-wavelength photon pulse flows out from the other end of the optical fiber link of the Bob multi-user terminal based on the Sagnac ring, flows out from the first port through the optical path gating device B, does not flow back into the coupler B, then enters the public optical fiber link, flows into the optical path gating device A through the fourth circulator and the coupler A, and the optical path gating device A allows the photon pulse to enter from the two ends of the optical fiber link of the Alice multi-user terminal based on the Sagnac ring.

Further, the photon pulse is divided into two beams by the coupler A according to the proportion of 50:50, and the two beams respectively enter a clockwise CW-B optical fiber link and a counterclockwise CCW-B optical fiber link in the Alice multi-user-end Sagnac ring-based optical fiber link to form a clockwise photon pulse and a counterclockwise photon pulse;

clockwise CW-B photon pulse: the clockwise photon pulse sequentially passes through the Bob multi-user terminal phase modulator B and each Alice user in a forward direction; the Bob multi-user-side phase modulator B modulates the clockwise photon pulse to generate an additional phase

The order ofWhen the hour hand photon pulse passes through each Alice user in sequence, the variable optical attenuator of each Alice user allows the pulse to pass through without interference, the second wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength photon pulse to the corresponding wavelength path, the phase modulator allows the clockwise photon pulse of the corresponding wavelength to pass through without interference, and the adjustable optical delay line of each Alice user only modulates the photon pulse of the corresponding wavelength to a proper delay value T₁、T₂……T_nThen, each wavelength clockwise photon pulse enters a first wavelength division multiplexing/demultiplexing device to be multiplexed into a multi-wavelength clockwise photon pulse, a polarization controller stabilizes the light polarization state of the photon pulse, an intensity modulator does not modulate the photon pulse, and finally the multi-wavelength clockwise photon pulse returns to the coupler A through the optical path gating device A to be output;

CCW-B photon pulse in counter-clockwise direction: the anticlockwise photon pulse sequentially reversely passes through the phase modulators B of the Alice users and the Bob multi-user end; specifically, when the counterclockwise photon pulse sequentially passes through each Alice user, each Alice user intensity modulator does not modulate the photon pulse, the polarization controller stabilizes the optical polarization state of the photon pulse, the first wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength photon pulse to the corresponding wavelength path, and the adjustable optical delay line of each Bob user only modulates the corresponding wavelength photon pulse to the appropriate delay value T₁、T₂……T_nPhase modulators modulate photonic pulses to produce additional phases

Then, each wavelength anticlockwise photon pulse enters a second wavelength division multiplexing/demultiplexing device for multiplexing, and the variable optical attenuator allows the photon pulse to pass through without interference; the Bob multi-user-side phase modulator B allows the counter-clockwise photon pulse to pass through without interference, and finally the counter-clockwise photon pulse returns to the coupler A through the optical path gating device A and interferes with the clockwise photon pulse arriving at the same time;

the adjustable light delay line of each Alice user only modulates the photon pulse with the corresponding wavelength to a proper delay value;

because each Alice user shares the photon interference signal detector group A, when the adjustable light delay line of each Alice user modulates the photon pulse with the corresponding wavelength, the adjustable light delay line should be modulated to be proper and different delay values so as to ensure that the clockwise photon pulse and the anticlockwise photon pulse with each wavelength do not interfere at the coupler A at the same time; meanwhile, the adjustable light delay line of each Alice user is respectively used for controlling that the CW-B photon pulse in the clockwise direction and the CCW-B photon pulse in the anticlockwise direction do not exist in the Bob multi-user-end phase modulator B at the same time.

A key distribution method of a central symmetric QKD ring-type multi-user system comprises the following steps:

s1: establishing the identity of both communication parties: confirming that one end of an Alice multi-user end or a Bob multi-user end is a control end and the other end is a multi-user receiving end through a public channel; after a multi-user control end is determined, a certain user of the control end is determined as a control user according to communication needs, and the control user can perform quantum key distribution with a plurality of users of another multi-user receiving end at the same time;

s2: generating a multi-wavelength optical pulse: confirming a control user of the multi-user control end and a multi-user receiving end through a public channel again, and if mutual confirmation is carried out, simultaneously generating optical pulses with multiple wavelengths (lambda 1, lambda 2, … and lambda n) by the multi-wavelength laser generating device of the multi-user control end; proper wavelength interval is kept between the wavelengths so as to minimize signal crosstalk generated by four-wave mixing effect;

s3: setting the length of a delay line of each user at a multi-user receiving end: according to the actual length of the optical fiber link, the length of a delay line of each user at the multi-user receiving end is adjusted, so that the fact that a CW photon pulse in the clockwise direction and a CCW photon pulse in the anticlockwise direction in the optical fiber link based on the Sagnac ring at the multi-user receiving end do not exist in the multi-user receiving end phase modulator at the same time is ensured, and the fact that the clockwise photon pulse and the anticlockwise photon pulse of the corresponding wavelength of each user received at the multi-user receiving end do not interfere at a coupler at the same time is ensured;

s4: modulating multi-wavelength light pulses: the generated multi-wavelength light pulse is subjected to random intensity modulation by controlling an intensity modulator of a user to become a signal state, a decoy state or a vacuum state; attenuating the multi-wavelength optical pulse to a proper average photon number per pulse through a variable optical attenuator to obtain a photon pulse for quantum key distribution;

s5: photon pulse interference: the multi-wavelength photon pulse is divided into a CW photon pulse in a clockwise direction and a CCW photon pulse in a counterclockwise direction through a coupler of a multi-user receiving end; the clockwise multi-wavelength photon pulse is modulated by a control end phase modulator to generate an additional phase

The counter-clockwise multi-wavelength photon pulse is demultiplexed to the corresponding wavelength path by the first wavelength division multiplexing/demultiplexing device of each receiving user, and the phase modulator of each receiving user modulates the counter-clockwise photon pulse of the corresponding wavelength to generate an additional phase

Finally, two paths of photon pulses interfere at a coupler of a multi-user receiving end;

s6: interference signal detection and screening into codes: after a first detector and a second detector (a third detector and a fourth detector) detect an interference result of a first receiving user, sequentially detecting interference results of a second receiving user and a third receiving user … … nth receiving user according to the length of a delay line of each receiving user, recording code values by each receiving user according to response results of the first detector and the second detector (the third detector and the fourth detector), carrying out base pairing with a control user, discarding different data, and reserving the same data to obtain a screening key; judging whether the error rate exceeds a set threshold value or not through corresponding error rate calculation, if the error rate does not exceed the set threshold value, controlling the user and each receiving user to perform data post-processing processes including data coordination, confidentiality enhancement and the like, finally obtaining the same security key, and completing the quantum key distribution process of the control user and each receiving user; if the error rate exceeds the set threshold, the communication is terminated and the process is restarted.

Compared with the prior art, the technical scheme of the invention has the beneficial effects that:

the invention is realized based on the Sagnac loop, the network structure is simpler, the technology is mature, and the implementation is easy; because the clockwise optical path and the anticlockwise optical path of the receiving end based on the Sagnac ring structure are completely consistent, the phase jitter can be greatly reduced; meanwhile, each user stabilizes the polarization state of light through a polarization controller, so that the system has better stability; the two multi-user sides can expand the number of network users according to the actual situation, and the quantum key distribution of the users and a plurality of receiving users at the receiving end can be controlled simultaneously through the wavelength division multiplexing/demultiplexing device; each multi-user end shares one multi-wavelength laser generating device, and shares one group of detectors through the arrangement of the adjustable light delay lines, so that the network structure is simplified, and the network cost is saved; the multi-wavelength laser generating device selectively generates multi-wavelength optical pulses with certain wavelength intervals, so that signal crosstalk caused by four-wave mixing effect can be reduced, and the error rate is reduced; by using the optical path gating device, a bidirectional quantum key distribution method of a multi-user system based on a Sagnac ring is provided.

Drawings

FIG. 1 is a block diagram of the overall structure of the present invention;

FIG. 2 is a schematic structural diagram of each Alice user of the Alice multi-user terminal according to the present invention;

FIG. 3 is a schematic structural diagram of each Bob user of Bob multi-user terminal in the invention;

fig. 4 is a schematic structural diagram of an Alice multi-ue coupling unit according to the present invention;

FIG. 5 is a schematic structural diagram of a Bob multi-subscriber-side coupling unit according to the present invention;

FIG. 6 is a structural framework diagram of an embodiment of the present invention;

FIG. 7 is a flow chart of the present invention.

Detailed Description

The drawings are for illustrative purposes only and are not to be construed as limiting the patent;

for the purpose of better illustrating the embodiments, certain features of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product;

it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.

The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.

Example 1

As shown in fig. 1, a central symmetric QKD ring-type multi-user system includes Alice multi-user end 1, Bob multi-user end 2, and an optical fiber link; the optical fiber links comprise an Alice multi-user terminal Sagnac ring-based optical fiber link 104, a Bob multi-user terminal Sagnac ring-based optical fiber link 204 and a public optical fiber link 3; the Alice multi-user terminal 1 is connected with the Bob multi-user terminal 2 through a public optical fiber link 3;

the Alice multi-user terminal 1 comprises n (n is more than or equal to 2 and is an integer) Alice users, an optical path gating device A100, an Alice terminal coupling unit and an Alice multi-user terminal phase modulator A103;

as shown in fig. 2 and fig. 6, each Alice user includes an Alice intensity modulator, an Alice polarization controller, a first wavelength division multiplexing/demultiplexing device, a tunable optical delay line, an Alice phase modulator, a second wavelength division multiplexing/demultiplexing device, and a variable optical attenuator, which are connected in sequence, and is connected to an Alice multi-user terminal Sagnac loop-based optical fiber link 104; the Alice tunable optical delay line is connected with an Alice first wavelength division multiplexing/demultiplexing device, and the Alice phase modulator is connected with an Alice second wavelength division multiplexing/demultiplexing device; the adjustable optical delay line and the phase modulator of each Alice user are arranged on a pulse link with corresponding wavelength between the first wavelength division multiplexing/demultiplexing device and the second wavelength division multiplexing/demultiplexing device; for example, the Alice 1 user includes an Alice 1 intensity modulator 111, an Alice 1 polarization controller 112, an Alice 1 first wavelength division multiplexing/demultiplexing device 113, and an Alice 1 tunable optical delay line114. An Alice 1 phase modulator 115, an Alice 1 second wavelength division multiplexing/demultiplexing device 116 and an Alice 1 variable optical attenuator 117, wherein the Alice 1 tunable optical delay line 114 and the Alice 1 phase modulator 115 of the Alice 1 user are arranged between the Alice 1 first wavelength division multiplexing/demultiplexing device 113 and the Alice 1 second wavelength division multiplexing/demultiplexing device 116 with a wavelength λ₁On the pulse link of (2); the Alice 2 user comprises an Alice 2 intensity modulator 121, an Alice 2 polarization controller 122, an Alice 2 first wavelength division multiplexing/demultiplexing device 123, an Alice 2 tunable optical delay line 124, an Alice 2 phase modulator 125, an Alice 2 second wavelength division multiplexing/demultiplexing device 126 and an Alice 2 variable optical attenuator 127, wherein the Alice 2 tunable optical delay line 124 and the Alice 2 phase modulator 125 of the Alice 2 user are arranged between the Alice 2 first wavelength division multiplexing/demultiplexing device 123 and the Alice 2 second wavelength division multiplexing/demultiplexing device 126 at a wavelength of lambda₂… … on the pulse link, and so on;

as shown in fig. 4 and 6, the Alice end-coupling unit includes a multi-wavelength laser generating device a102, a third detector 119, a fourth detector 129, a third circulator 118, and a fourth circulator 128; the multi-wavelength laser generating device a102, the coupler a101 and the third detector 119 are respectively connected to one end of a third circulator 118; the fourth detector 129 is connected to one end of the fourth circulator 128;

the multi-wavelength laser generating device A102 is used for simultaneously generating a plurality of optical pulses (lambda) with proper wavelength intervals₁、λ₂、…、λ_n) To reduce the influence of the four-wave mixing effect on signal transmission;

the third circulator 118 and the fourth circulator 128 are each a three-port circulator, and each three-port circulator includes a first port, a second port, and a third port, where a signal input by the first port is output only at the second port, and a signal input by the second port is output only at the third port; a first port of the third circulator 118 is connected to the multi-wavelength laser generating device a102, a second port is connected to a first input terminal of the coupler a101, and a third port is connected to the third detector 119; a first port of the fourth circulator 128 is connected to the common optical fiber link 3, a second port is connected to a second input end of the coupler a101, and a third port is connected to the fourth detector 129; a

The third detector 119 and the fourth detector 129 are used for detecting an interference photon pulse signal output by the Alice multi-user terminal Sagnac loop-based optical fiber link 104 through the coupler a 101;

the Alice multi-user terminal phase modulator A103 is accessed to one side of an optical fiber link 204 of a Bob multi-user terminal based on a Sagnac ring and is used for modulating the phase of each Alice user optical pulse from the Alice multi-user terminal;

the optical path gating device A100 is used for gating the output direction of the optical pulse flowing into the optical path gating device A;

similarly, as shown in fig. 3, fig. 5 and fig. 6, each Bob user structure of the Bob multi-user terminal 2 and the connection manner of the devices included in the Bob multi-user terminal 2 are completely consistent with those of the Alice multi-user terminal 1;

in order to better describe the bidirectional quantum key distribution method of the multi-user system, the following two cases are developed:

1. if the Alice multi-user terminal serves as a control terminal of the whole system, the Bob multi-user terminal serves as a receiving terminal of the whole system:

specifically, suppose that an Alice 1 user of n Alice users and n Bob users of the Bob multi-user end 2 perform quantum key distribution simultaneously, where the Alice 1 user is a control user and each of the n Bob users is a receiving user; taking the quantum key distribution process of the Alice 1 user and the n Bob users as an example, the transmission process of the optical pulse is described in detail, and the transmission process of the optical pulse in the quantum key distribution process of any other Alice m (m is more than or equal to 1 and less than or equal to n) user and the n Bob users is similar to the following process;

as shown in FIG. 6, the multi-wavelength laser generating device A102 generates a plurality of wavelengths (λ) at the same time₁、λ₂、…、λ_n) The optical pulse flows into the optical path gating device a100 through the third circulator 118 and the coupler a101, the optical path gating device a100 gates the first port and the third port, only the optical pulse is allowed to enter from the upper end of the optical fiber link 104 of the Alice multi-user terminal based on the Sagnac loop, and the optical pulse sequentially flows along the upper end of the optical fiber link 104 of the Alice multi-user terminalPassing through the phase modulators B203 of various Alice users and Bob multi-user terminals in a counterclockwise direction; when the optical pulse passes through the Alice 1 user, the Alice 1 intensity modulator 111 performs decoy state modulation, the Alice 1 polarization controller 112 is used for stabilizing the optical polarization state, the Alice 1 first wavelength division multiplexing/demultiplexing device 113 is used for demultiplexing the multi-wavelength optical pulse onto the corresponding wavelength path, the Alice 1 tunable optical delay line 114 does not modulate the delay, the Alice 1 phase modulator 115 allows the multi-wavelength optical pulse to pass through without interference, the Alice 1 second wavelength division multiplexing/demultiplexing device 116 multiplexes the multi-wavelength optical pulse into the optical fiber link, and the Alice 1 variable optical attenuator 117 attenuates the optical pulse to an appropriate average photon number μ per pulse to obtain a photon pulse at a single photon level; each device of the (n-1) Alice users not participating in the communication and the Bob multi-user end phase modulator B203 allow the multi-wavelength optical pulse to pass through without interference; the multi-wavelength photon pulse flows out from the lower end of the optical fiber link 104 of the Alice multi-user terminal based on the Sagnac ring, flows out from the first port through the third port of the optical path gating device A100, but does not flow back into the coupler A101 from the second port, then enters a common optical fiber link and flows into the optical path gating device B200 through the second circulator 228 and the coupler B201, and the optical path gating device B200 gates the second port and the third port to allow photon pulses to enter from two ends of the optical fiber link 204 of the Bob multi-user terminal based on the Sagnac ring;

the photon pulse is divided into two beams by the coupler B201 in a ratio of 50:50, and the two beams respectively enter a CW-A optical fiber link in a clockwise direction and a CCW-A optical fiber link in a counterclockwise direction in the optical fiber link 204 of the Bob multi-user terminal based on the Sagnac ring to form a clockwise photon pulse and a counterclockwise photon pulse;

clockwise CW-a photon pulse: the clockwise photon pulse sequentially passes through the Alice multi-user-side phase modulator A103 and each Bob user in a forward direction; specifically, the Alice multi-user-side phase modulator a103 modulates the clockwise photon pulse to generate an additional phase

As the clockwise photon pulse passes through each Bob user in turn,the variable optical attenuator of each Bob user allows the variable optical attenuator to pass through without interference, the second wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength photon pulse to the corresponding wavelength path, the phase modulator allows the clockwise photon pulse of the corresponding wavelength to pass through without interference, and the adjustable optical delay line of each Bob user only modulates the photon pulse of the corresponding wavelength to a proper delay value (T)₁、T₂……T_n) Then, each wavelength clockwise photon pulse enters a first wavelength division multiplexing/demultiplexing device to be multiplexed into a multi-wavelength clockwise photon pulse, a polarization controller stabilizes the light polarization state of the photon pulse, an intensity modulator does not modulate the photon pulse, and finally the multi-wavelength clockwise photon pulse returns to the coupler B201 through the optical path gating device B200 to be output;

CCW-a photon pulse in the counter-clockwise direction: the anticlockwise photon pulses sequentially reversely pass through the phase modulators A103 of each Bob user and the Alice multi-user end; specifically, when the counterclockwise photon pulse sequentially passes through each Bob user, each Bob user intensity modulator does not modulate the photon pulse, the polarization controller stabilizes the optical polarization state of the photon pulse, the first wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength photon pulse to the corresponding wavelength path, and the adjustable optical delay line of each Bob user only modulates the photon pulse with the corresponding wavelength to a proper delay value (T)₁、T₂……T_n) Phase modulators modulating photonic pulses to produce additional phases

Then, each wavelength anticlockwise photon pulse enters a second wavelength division multiplexing/demultiplexing device for multiplexing, and the variable optical attenuator allows the photon pulse to pass through without interference; the Alice multi-user-side phase modulator A allows the counter-clockwise photon pulse to pass through without interference, and finally the counter-clockwise photon pulse returns to the coupler B201 through the optical path gating device B200 and interferes with the clockwise photon pulse arriving at the same time;

the first detector 219 and the second detector 229 are based on a phase difference

the tunable optical delay line of each Bob user modulates only the photon pulse of the corresponding wavelength to an appropriate delay value, e.g., Bob 1 user modulates only the wavelength λ₁Clockwise/counter-clockwise photon pulse to delay value T₁Bob 2 user modulates only wavelength λ₂Clockwise/counter-clockwise photon pulse to delay value T₂… … Bob n user modulates only the wavelength λ_nClockwise/counter-clockwise photon pulse to delay value T_n；

2. Similarly, if the Bob multi-user terminal serves as the control terminal of the whole system, the Alice multi-user terminal serves as the receiving terminal of the whole system:

specifically, assume that a Bob 1 user of n Bob users and n Alice users of the Alice multi-user end 1 perform quantum key distribution simultaneously, where the Bob 1 user is a control user and each of the n Alice users is a receiving user; taking the quantum key distribution process of the Bob 1 user and the n Alice users as an example, the transmission process of the optical pulse is described in detail, and the transmission process of the optical pulse in the quantum key distribution process of any other Bob k (k is more than or equal to 1 and less than or equal to n) user and the n Alice users is similar to the following process;

as shown in FIG. 6, the multi-wavelength laser generating device B202 generates a plurality of wavelengths (λ) simultaneously₁、λ₂、…、λ_n) The optical pulse passes through the first circulator 218 and the coupler B201 and flows into the optical path gating device B200, the optical path gating device B200 gates the first port and the third port, only the optical pulse is allowed to enter from the lower end of the optical fiber link 204 of the Bob multi-subscriber terminal based on the Sagnac loop, and the optical pulse passes through the phase modulators a103 of each Bob subscriber and Alice multi-subscriber terminal in sequence in the counterclockwise direction; when the optical pulse passes through the Bob 1 user, the Bob 1 intensity modulator 211 performs decoy state modulation, the Bob 1 polarization controller 212 is used for stabilizing the optical polarization state, the Bob 1 first wavelength division multiplexing/demultiplexing device 213 is used for demultiplexing the multi-wavelength optical pulse to the corresponding wavelength path, the Bob 1 tunable optical delay line 214 does not modulate the delay, and the Bob 1 phase modulator 215 allows the optical pulse to pass through the corresponding wavelength path without interferenceThe Bob 1 second wavelength division multiplexing/demultiplexing device 216 multiplexes the multi-wavelength optical pulses into the optical fiber link, and the Bob 1 variable optical attenuator 217 attenuates the optical pulses to an appropriate average number of photons per pulse μ to obtain photon pulses at a single photon level; each device of the (n-1) Bob users not participating in the communication and the Alice multi-user end phase modulator a103 allow the multi-wavelength optical pulse to pass through without interference; the multi-wavelength photon pulse flows out from the upper end of the optical fiber link 204 of the Bob multi-subscriber based Sagnac ring, flows out from the first port through the third port of the optical path gating device B200, but does not flow back into the coupler B201 from the second port, then enters the common optical fiber link and flows into the optical path gating device A100 through the fourth circulator 128 and the coupler A101, and the optical path gating device A100 gates the second port and the third port to allow photon pulses to enter from two ends of the optical fiber link 104 of the Alice multi-subscriber based Sagnac ring;

the photon pulse is divided into two beams by the coupler A101 according to the proportion of 50:50, and the two beams respectively enter a CW-B optical fiber link in the clockwise direction and a CCW-B optical fiber link in the counterclockwise direction in the optical fiber link 104 of the Alice multi-user terminal based on the Sagnac ring to form a clockwise photon pulse and a counterclockwise photon pulse;

clockwise CW-B photon pulse: the clockwise photon pulse sequentially passes through the Bob multi-user terminal phase modulator B203 and each Alice user in a forward direction; specifically, the Bob multi-user-side phase modulator B203 modulates the clockwise photon pulse to generate the additional phase

When the clockwise photon pulse passes through each Alice user in sequence, the variable optical attenuator of each Alice user allows the clockwise photon pulse to pass through without interference, the second wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength photon pulse to a corresponding wavelength path, the phase modulator allows the clockwise photon pulse of the corresponding wavelength to pass through without interference, and the adjustable optical delay line of each Alice user only modulates the photon pulse of the corresponding wavelength to a proper delay value (T)₁、T₂……T_n) Then each wavelength is pulsed clockwise with photonsThe multi-wavelength clockwise photon pulse enters a first wavelength division multiplexing/demultiplexing device for multiplexing into a multi-wavelength clockwise photon pulse, a polarization controller stabilizes the light polarization state of the photon pulse, an intensity modulator does not modulate the photon pulse, and finally the multi-wavelength clockwise photon pulse returns to the coupler A101 through the light path gating device A100 for output;

CCW-B photon pulse in counter-clockwise direction: the anticlockwise photon pulses sequentially reversely pass through the phase modulators B203 of the Alice users and the Bob multi-user end; specifically, when the counterclockwise photon pulse sequentially passes through each Alice user, each Alice user intensity modulator does not modulate the photon pulse, the polarization controller stabilizes the optical polarization state of the photon pulse, the first wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength photon pulse onto the corresponding wavelength path, and the adjustable optical delay line of each Alice user only modulates the photon pulse with the corresponding wavelength onto a proper delay value (T) by the adjustable optical delay line₁、T₂……T_n) Phase modulators modulating photonic pulses to produce additional phases

Then, each wavelength anticlockwise photon pulse enters a second wavelength division multiplexing/demultiplexing device for multiplexing, and the variable optical attenuator allows the photon pulse to pass through without interference; the Bob multi-user-side phase modulator B allows the counter-clockwise photon pulse to pass through without interference, and finally the counter-clockwise photon pulse returns to the coupler a101 through the optical path gating device a100 to interfere with the clockwise photon pulse arriving at the same time;

the third detector 119 and the fourth detector 129 are based on the phase difference

preferably, the tunable optical delay line of each Alice user modulates only the photon pulse of the corresponding wavelength to an appropriate delay value, e.g., Alice 1 user modulates only the wavelength λ₁Clockwise/counter-clockwise photon pulse to delay value T₁Alice 2 user modulates only wavelength λ₂Clockwise/counterclockwise photon pulseRush-to-delay value T₂… … Alice n user modulates only the wavelength λ_nClockwise/counter-clockwise photon pulse to delay value T_n；

The above is the transmission process of the optical pulse in two cases.

Specifically, for both of the above cases, the first detector 219 and the second detector 229 (the third detector 119 and the fourth detector 129) are based on the phase difference

In response to the interference of the clockwise photon pulse and the counter-clockwise photon pulse. A receiving end user records code values according to response results of the first detector 219 and the second detector 229 (the third detector 119 and the fourth detector 129) in sequence, performs base pairing with a control user, discards different data, and retains the same data to obtain a screening key; and then controlling the user and each receiving user to perform data post-processing processes including data coordination, privacy enhancement and the like, finally obtaining the same security key, and finishing the quantum key distribution process of the controlling user and each receiving user.

According to the BB84 protocol of quantum key distribution phase coding, the light intensity of two photon pulses at the first detector (third detector) is determined by the following formula:

or

Wherein, I₁Is the output intensity at the first detector (third detector), I₀Is the input light intensity.

Controlling user modulation phase

Pi or

Receiving user modulation phase

Or

Wherein the phase takes 0 and pi as a set of orthogonal bases, and the phase takes

And

as another set of orthogonal substrates; when controlling the modulation phase of the user

Or

When, it means that the transmission code value is 0; when controlling the modulation phase of the user

Or

When the value is 1, the transmission code value is indicated. TABLE 1A. about

And

the possible corresponding detection results of the various phase values are listed.

(Note:

or

In the actual communication is

Or also

The decision is made according to whether Alice multi-user terminal is used as a control terminal or Bob multi-user terminal is used as a control terminal. )

Table 1: corresponding table of different phase values and detector response results

As can be seen from the above table, when the phase difference is large

Only the first detector (third detector) responds, the code can be formed, and the receiving user receiving code value is 0; when the phase difference is between

Only the second (fourth) detector responds, the code can be formed, and the receiving user code receiving value is 1; when the phase difference is between

Or

Both the first and second detectors (or the third and fourth detectors) may respond and fail to code, which is a result of the different selection of modulation phases for the controlling and receiving users.

Example 2

As shown in fig. 7, the present invention provides a bidirectional quantum key distribution method for a multi-user system based on a Sagnac loop, which is applied to a centrosymmetric QKD loop type multi-user system as described above, and the method includes the following steps:

s1, establishing the identity of both communication parties: confirming that one end of an Alice multi-user end or a Bob multi-user end is a control end and the other end is a multi-user receiving end through a public channel; after a multi-user control end is determined, a certain user of the control end is determined as a control user according to communication needs, and the control user can perform quantum key distribution with a plurality of users of another multi-user receiving end at the same time;

s2, generating multi-wavelength light pulse: confirming the control user of the multi-user control end and the multi-user receiving end through the common channel again, and if mutual confirmation is carried out, simultaneously generating multiple wavelengths (lambda) by the multi-wavelength laser generating device of the multi-user control end₁、λ₂、…、λ_n) The light pulse of (2); proper wavelength interval is kept between the wavelengths so as to minimize signal crosstalk generated by four-wave mixing effect;

s3, setting the length of the delay line of each user at the multi-user receiving end: according to the actual length of the optical fiber link, the length of a delay line of each user at the multi-user receiving end is adjusted, so that the fact that a CW photon pulse in the clockwise direction and a CCW photon pulse in the anticlockwise direction in the optical fiber link based on the Sagnac ring at the multi-user receiving end do not exist in the multi-user receiving end phase modulator at the same time is ensured, and the fact that the clockwise photon pulse and the anticlockwise photon pulse of the corresponding wavelength of each user received at the multi-user receiving end do not interfere at a coupler at the same time is ensured;

s4, modulating the multi-wavelength light pulse: the generated multi-wavelength light pulse is subjected to random intensity modulation by controlling an intensity modulator of a user to become a signal state, a decoy state or a vacuum state; attenuating the multi-wavelength optical pulse to a proper average photon number per pulse through a variable optical attenuator to obtain a photon pulse for quantum key distribution;

s5, photon pulse interference: the multi-wavelength photon pulse is divided into a CW photon pulse in a clockwise direction and a CCW photon pulse in a counterclockwise direction through a coupler of a multi-user receiving end; the clockwise multi-wavelength photon pulse is modulated by a control end phase modulator to generate an additional phase

s6, detecting and screening interference signals into codes: after a first detector and a second detector (a third detector and a fourth detector) detect an interference result of a first receiving user, sequentially detecting interference results of a second receiving user and a third receiving user … … nth receiving user according to the length of a delay line of each receiving user, recording code values by each receiving user according to response results of the first detector and the second detector (the third detector and the fourth detector), carrying out base pairing with a control user, discarding different data, and reserving the same data to obtain a screening key; judging whether the error rate exceeds a set threshold value or not through corresponding error rate calculation, if the error rate does not exceed the set threshold value, controlling the user and each receiving user to perform data post-processing processes including data coordination, confidentiality enhancement and the like, finally obtaining the same security key, and completing the quantum key distribution process of the control user and each receiving user; if the error rate exceeds the set threshold, the communication is terminated and the process is restarted.

The principle and the structure of the invention are integrated, the invention can realize that a control user and a plurality of receiving users simultaneously carry out quantum key distribution, and provides a bidirectional quantum key distribution method of a multi-user system based on a Sagnac ring, and the network structure is simple and easy to realize; based on the structural characteristics of the Sagnac ring and the use of the polarization controller, the system has better stability.

The same or similar reference numerals correspond to the same or similar parts;

the positional relationships depicted in the drawings are for illustrative purposes only and are not to be construed as limiting the present patent;

it should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A central symmetric QKD annular multi-user system is characterized by comprising an Alice multi-user end, a Bob multi-user end and an optical fiber link; the optical fiber link comprises an Alice multi-user terminal Sagnac ring-based optical fiber link, a Bob multi-user terminal Sagnac ring-based optical fiber link and a public optical fiber link; the Alice multi-user terminal is connected with the Bob multi-user terminal through a public optical fiber link;

the Bob end coupling unit comprises a multi-wavelength laser generation device B, a first detector, a second detector, a first circulator and a second circulator; the multi-wavelength laser generating device B, the coupler B and the first detector are respectively connected to one end of the first circulator; the second detector is connected to one end of the second circulator; the multi-wavelength laser generating device B is used for simultaneously generating a plurality of optical pulses with proper wavelength intervals so as to reduce the influence of four-wave mixing effect on signal transmission.

2. The centrosymmetric QKD ring-type multi-user system according to claim 1, wherein the third circulator and the fourth circulator are three-port circulators, each of which comprises a first port, a second port and a third port, the signal input from the first port is output only at the second port, and the signal input from the second port is output only at the third port; a first port of the third circulator is connected with the multi-wavelength laser generation device A, a second port of the third circulator is connected with a first input end of the coupler A, and a third port of the third circulator is connected with the third detector; a first port of the fourth circulator is connected to a common optical fiber link, a second port of the fourth circulator is connected with a second input end of the coupler A, and a third port of the fourth circulator is connected with the fourth detector; the first circulator and the second circulator are both three-port circulators and comprise a first port, a second port and a third port, signals input by the first port are only output at the second port, and signals input by the second port are only output at the third port; a first port of the first circulator is connected with the multi-wavelength laser generation device B, a second port of the first circulator is connected with a first input end of a coupler B, and a third port of the first circulator is connected with the first detector; and a first port of the second circulator is connected to a public optical fiber link, a second port of the second circulator is connected with a second input end of the coupler B, and a third port of the second circulator is connected with the second detector.

3. The centrosymmetric QKD loop type multiuser system according to claim 2, wherein the third and fourth detectors detect interfering photon pulse signals output by the Alice multiuser-end Sagnac loop-based fiber optic link via coupler a; the Alice multi-user terminal phase modulator A is connected to one side of an optical fiber link of a Bob multi-user terminal based on a Sagnac ring, and modulates the phase of each Alice user optical pulse from the Alice multi-user terminal; the optical path gating device A is used for gating the output direction of the optical pulse flowing into the optical path gating device A; the first detector and the second detector detect interference photon pulse signals output by the Bob multi-user terminal Sagnac ring-based optical fiber link through a coupler B; the Bob multi-user terminal phase modulator B is accessed to one side of an optical fiber link of the Alice multi-user terminal based on the Sagnac ring, and modulates the phase of each Bob user light pulse from the Alice multi-user terminal; the optical path gating device B is used for gating the output direction of the optical pulse flowing into the optical path gating device B.

4. The centrosymmetric QKD ring-type multi-user system according to claim 3, wherein if Alice multi-user end is used as the control end of the whole system, Bob multi-user end is used as the receiving end of the whole system:

when the m-th user Alice m of the n Alice users and the n Bob users of the Bob multi-user end simultaneously carry out quantum key distribution, m is more than or equal to 1 and less than or equal to n, and the multi-wavelength laser generation device A simultaneously generates multiple wavelengths lambda₁、λ₂、…、λ_nThe optical pulse passes through the third circulator a and the coupler a and flows into the optical path gating device a, the optical path gating device a allows the optical pulse to enter only from one end of the optical fiber link of the Alice multi-user end based on the Sagnac loop, and the optical pulse sequentially passes through the Alice users and the Bob multi-user end phase modulators B in the counterclockwise direction; when the optical pulses pass through the Alice m user, the Alice m intensity modulator performs decoy state modulation, the Alice m polarization controller is used for stabilizing the optical polarization state, the Alice m first wavelength division multiplexing/demultiplexing device is used for demultiplexing multi-wavelength optical pulses onto corresponding wavelength lines, the Alice m tunable optical delay lines do not modulate delay, the Alice m phase modulator allows the multi-wavelength optical pulses to pass through without interference, the Alice m second wavelength division multiplexing/demultiplexing device multiplexes the multi-wavelength optical pulses into an optical fiber link, and the Alice m variable optical attenuator attenuates the optical pulses to a proper average photon number mu per pulse so as to obtain single-photon-level photon pulses; each device in other n-1 Alice users not participating in communication and the phase modulator B of the Bob multi-user end allow the multi-wavelength optical pulse to pass through without interference; the multi-wavelength photon pulse flows out from the other end of the optical fiber link of the Alice multi-user terminal based on the Sagnac ring, flows out from the first port through the optical path gating device A, does not flow back into the coupler A, and then enters a public optical fiber linkAnd the optical signal flows into an optical path gating device B through a circulator and a coupler B, wherein the optical path gating device B allows photon pulses to enter from two ends of a fiber link of the Bob multi-user terminal based on the Sagnac ring.

5. The centrosymmetric QKD loop type multiuser system according to claim 4, wherein the photon pulse is split into two beams by coupler B in a 50:50 ratio, entering the clockwise CW-a fiber link and the counterclockwise CCW-a fiber link of the Bob multiuser-loop based fiber link, respectively, to form a clockwise photon pulse and a counterclockwise photon pulse;

CCW-a photon pulse in the counter-clockwise direction: the anticlockwise photon pulses sequentially reversely pass through each Bob user and the Alice multi-user-side phase modulator A; specifically, each Bob user intensity modulator does not modulate the photon pulse as the counter-clockwise photon pulse passes through each Bob user in turnThe polarization controller stabilizes the light polarization state of the photon pulse, the first wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength photon pulse to the corresponding wavelength path, and the adjustable light delay line of each Bob user only modulates the corresponding wavelength photon pulse to a proper delay value T₁、T₂……T_nPhase modulators modulating photonic pulses to produce additional phases

Then, each wavelength anticlockwise photon pulse enters a second wavelength division multiplexing/demultiplexing device for multiplexing, and the variable optical attenuator allows the photon pulse to pass through without interference; the Alice multi-user-side phase modulator A allows the counter-clockwise photon pulse to pass through without interference, and finally the counter-clockwise photon pulse returns to the coupler B through the optical path gating device B and interferes with the clockwise photon pulse arriving at the same time.

6. The centrosymmetric QKD loop-type multi-user system of claim 5, wherein the first and second detectors are phase-difference dependent

7. The centrosymmetric QKD ring-type multi-user system according to claim 6, wherein if said Bob multi-user end is used as the control end of the whole system, said Alice multi-user end is used as the receiving end of the whole system:

when the kth user Bob k of the n Bob users and the n Alice users of the Alice multi-user end simultaneously carry out quantum key distribution, k is more than or equal to 1 and less than or equal to n, and the multi-wavelength laser generation device B simultaneously generates multiple wavelengths lambda₁、λ₂、…、λ_nThe optical pulse passes through the first circulator and the coupler B and flows into the optical path gating device B, the optical path gating device B allows the optical pulse to enter only from one end of the optical fiber link of the Bob multi-user terminal based on the Sagnac ring, and the optical pulse sequentially passes through each Bob user and Alice multi-user terminal phase modulator A along the counterclockwise direction; when the optical pulse passes through the Bob k user, the Bob k intensity modulator performs decoy state modulation, the Bob k polarization controller is used for stabilizing the optical polarization state, a first Bob k wavelength division multiplexing/demultiplexing device is used for demultiplexing multi-wavelength optical pulses onto a corresponding wavelength chain, Bob k adjustable optical delay lines do not modulate delay, the Bob k phase modulator allows the multi-wavelength optical pulses to pass through without interference, a second Bob k wavelength division multiplexing/demultiplexing device multiplexes the multi-wavelength optical pulses into an optical fiber link, and a Bob k variable optical attenuator attenuates the optical pulses to a proper average photon number mu per pulse to obtain photon pulses at a single photon level; each device in other n-1 Bob users not participating in communication and the Alice multi-user-side phase modulator A allow the multi-wavelength optical pulse to pass through without interference; the multi-wavelength photon pulse flows out from the other end of the optical fiber link of the Bob multi-user terminal based on the Sagnac ring, flows out from the first port through the optical path gating device B, does not flow back into the coupler B, then enters the public optical fiber link, flows into the optical path gating device A through the fourth circulator and the coupler A, and the optical path gating device A allows the photon pulse to enter from the two ends of the optical fiber link of the Alice multi-user terminal based on the Sagnac ring.

8. The centrosymmetric QKD loop-type multiuser system of claim 7, wherein the photon pulse is split into two beams by coupler a at a ratio of 50:50, entering the clockwise CW-B fiber link and the counterclockwise CCW-B fiber link of the Alice multiuser-end Sagnac loop-based fiber link, respectively, to form a clockwise photon pulse and a counterclockwise photon pulse;

When the clockwise photon pulse passes through all the Alice users in sequence, the variable optical attenuator of each Alice user allows the clockwise photon pulse to pass through without interference, the second wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength photon pulse to a corresponding wavelength path, the phase modulator allows the clockwise photon pulse of the corresponding wavelength to pass through without interference, and the adjustable optical delay line of each Alice user only modulates the photon pulse of the corresponding wavelength to a proper delay value T₁、T₂……T_nThen, each wavelength clockwise photon pulse enters a first wavelength division multiplexing/demultiplexing device to be multiplexed into a multi-wavelength clockwise photon pulse, a polarization controller stabilizes the light polarization state of the photon pulse, an intensity modulator does not modulate the photon pulse, and finally the multi-wavelength clockwise photon pulse returns to the coupler A through the optical path gating device A to be output;

CCW-B photon pulse in counter-clockwise direction: the anticlockwise photon pulse sequentially reversely passes through the phase modulators B of the Alice users and the Bob multi-user end; specifically, when the counterclockwise photon pulse sequentially passes through each Alice user, each Alice user intensity modulator does not modulate the photon pulse, the polarization controller stabilizes the optical polarization state of the photon pulse, the first wavelength division multiplexing/demultiplexing device is used for demultiplexing the multi-wavelength photon pulse to the corresponding wavelength path, and the adjustable optical delay line of each Bob user only modulates the corresponding wavelength photon pulse to the appropriate delay value T₁、T₂……T_nPhase modulators modulating photonic pulses to produce additional phases

Then, each wavelength anticlockwise photon pulse enters a second wavelength division multiplexing/demultiplexing device for multiplexing, and the variable optical attenuator allows the photon pulse to pass through without interference; the Bob multi-user-side phase modulator B allows the counter-clockwise photon pulse to pass through without interference, and finally the counter-clockwise photon pulse returns to the coupler a through the optical path gating device a to interfere with the clockwise photon pulse arriving at the same time.

9. The centrosymmetric QKD loop-type multi-user system of claim 8, wherein the first and second detectors are phase-difference dependent

10. A key distribution method for a centrally symmetric QKD ring-type multi-user system according to claim 9, characterized by the steps of:

The counter-clockwise multi-wavelength photon pulse is demultiplexed to the corresponding wavelength path by the first wavelength division multiplexing/demultiplexing device of each receiving user, and the phase modulator of each receiving user modulates the counter-clockwise photon pulse with the corresponding wavelength to generate an additional phase