CN114499839A

CN114499839A - Multi-user OAM-QKD system and method based on annular interferometer

Info

Publication number: CN114499839A
Application number: CN202111668222.7A
Authority: CN
Inventors: 郭邦红; 吴俊森; 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: CN114499839B

Abstract

The invention provides a multi-user OAM-QKD system and a method based on a ring interferometer, wherein the system comprises an Alice user sending end, an orbit angular momentum demultiplexing unit and a Bob user receiving end, the Alice user sending end comprises a signal modulation device and an orbit angular momentum multiplexing device, and the orbit angular momentum demultiplexing unit comprises a telescope component and an orbit angular momentum demultiplexing device; the Bob user receiving end comprises N Bob users, and each Bob user comprises an M-Z type phase separation device and a detection device; and the optical signals generated by the signal modulation device sequentially enter the orbital angular momentum multiplexing device, the first telescope component, the second telescope component and the orbital angular momentum demultiplexing device, are transmitted to the M-Z type phase separation device corresponding to the Bob user receiving end, finally enter the detection device for detection, and are subjected to post-processing to generate a final safe quantum key.

Description

Multi-user OAM-QKD system and method based on annular interferometer

Technical Field

The invention relates to the field of multi-user quantum communication networks and the field of free space communication, in particular to a multi-user OAM-QKD system and a multi-user OAM-QKD method based on a ring interferometer.

Background

Quantum key distribution is to ensure the security of communication by using the principle in quantum mechanics, and can establish a random and secure key between two communication parties. The quantum key distribution technology applies the physical principles of the Heisenberg uncertain relation, quantum non-cloning principle and the like, and can achieve unconditional safety in the physical theory.

Since the first quantum key distribution protocol was proposed, more and more schemes were proposed by scholars. Among them, the differential phase-shift quantum key Distribution Protocol (DPS) proposed in 2002 carries key information in the phase of a photon, and it uses the relative phase information between two pulses to encode for quantum key distribution. In the experiment, the phase control technology for the optical pulse is relatively mature, and the differential phase shift protocol has strong practicability and is easy to realize high speed, so that the method is one of the hotspots of quantum key distribution research.

The photons have a Spin Angular Momentum (SAM) which is related to the polarization characteristics of the photons, and an Orbital Angular Momentum (OAM) which is the angular momentum generated when the beam has a helical phase structure. When the light beam carries orbital angular momentum, the amplitude function of the light beam contains the azimuth phase

Where l is the topological charge number, i.e. the eigenvalue of orbital angular momentum, except for the traditional degrees of freedom: another important degree of freedom beyond wavelength, frequency, polarization, etc.

Currently, much research on free space quantum key distribution schemes is based on point-to-point user communication. In order to implement one-to-many, even many-to-many, communication schemes, routing and addressing issues must first be addressed during information transfer. Theoretically, the topological charge number l of the orbital angular momentum can reach infinity, and the orbital angular momentum state can be used as a carrier to carry quantum information, so that the purpose of multi-user transmission is achieved.

At present, the safety of orbital angular momentum has been confirmed by theory and experiment. The orbital angular momentum light beam is used as an information carrier for orbital angular momentum multiplexing, and a feasible scheme is provided for improving the capacity of a free space quantum communication system. How to effectively and nondestructively separate the orbital angular momentum of the vortex beam is a prerequisite for the application of the vortex beam in a multipath communication network.

At present, the following types of photon state identification schemes mainly exist: interference diffraction method: triangular hole diffraction method, plane wave interference method, mirror image interference method; and a rotary doppler method and a computer generated hologram grating method. The defects that the separation efficiency is low, the original quantum state is damaged, cascade connection cannot be realized, or cascade connection is extremely unstable, and the separation of the orbital angular momentum at the single photon level is extremely difficult limit the application of the orbital angular momentum photon state in orbital angular momentum multiplexing quantum communication. Therefore, further improvements to the existing quantum key distribution based on orbital angular momentum multiplexing are necessary.

The prior art discloses a patent of a multi-user orbital angular momentum wavelength division multiplexing QKD network system, which comprises an Alice control end, an orbital angular momentum wavelength division O-W type multiplexing unit and a Bob user end; the Alice control end comprises a mixed entanglement generating unit, an SAM modulating unit and a coincidence measuring unit; the O-W type multiplexing unit comprises a multiplexing module and a demultiplexing module; the Bob user side comprises n Bob users; the hybrid entanglement generation unit generates idle light and signal light carrying multi-wavelength hybrid entanglement, wherein the idle light is sent to different Bob users through the demultiplexing module, the idle light modulates OAM to load information, and the loaded information is sent to the coincidence measurement unit through the multiplexing module; the signal light is modulated and loaded with information through the SAM modulation unit and is sent to the coincidence measurement unit; the loading information sent by the multiplexing module and the SAM modulation unit is measured and decoded in the coincidence measurement unit; the quantum coding and decoding communication with large capacity is realized, the operation is convenient, the user number expansion capability is strong, each user is relatively independent in the communication, and the safety is high. However, the patent does not relate to any technical scheme on how to encode information by using the motion direction of vortex photons and use the orbital angular momentum topological charge value of the photons as multiplexing addressing information so as to realize quantum key distribution of free space.

Disclosure of Invention

The invention relates to a multi-user OAM-QKD system based on a ring interferometer, which uses the phase of photons to encode information and uses the orbital angular momentum state of the photons as multiplexing addressing information, thereby realizing the quantum key distribution of free space.

Still another object of the present invention is to provide a multi-user OAM-QKD method based on a ring interferometer.

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

a multi-user OAM-QKD based on a ring interferometer comprises an Alice user sending end, an orbital angular momentum demultiplexing unit and a Bob user receiving end, wherein:

the Alice user sending end comprises a signal modulation device and an orbital angular momentum multiplexing device, the signal modulation device comprises a light source device and a phase separation assembly, and the orbital angular momentum multiplexing device comprises a spatial light modulator and a first telescope assembly;

the orbital angular momentum demultiplexing unit comprises a second telescope component and an orbital angular momentum demultiplexing device;

the Bob user receiving end comprises N Bob users, and each Bob user comprises an M-Z type phase separation device and a detection device;

the optical signals generated by the signal modulation device sequentially enter the spatial light modulator, the first telescope component, the second telescope component and the orbital angular momentum demultiplexing device, and then the orbital angular momentum demultiplexing device outputs photons from different receiving end ports according to the values of the orbital angular momentum carried by the photons; and the signal photons output by the orbital angular momentum demultiplexing device are transmitted to an M-Z type phase separation device of a user with a corresponding port Bob, and finally enter the detection device for detection.

The signal modulation device comprises a light source device and a phase separation component; the light source device includes: the system comprises a laser light source, an attenuator and a polarization controller; the phase separation assembly includes: the beam splitter comprises a first beam splitter, a second beam splitter, a first reflector, a second reflector, a first phase modulator and a computer; the laser light source emits laser light, the laser light enters the attenuator and the polarization controller and then enters the first beam splitter. Splitting the beam in the first beam splitter into three optical paths, wherein the first optical path is as follows: a portion of the signal photons are coupled directly into the second beam splitter. The second optical path is: a portion of the signal photons enter the third beam splitter and are split again, wherein a portion of the signal photons enter the fourth beam splitter and are reflected therefrom and finally coupled into the second beam splitter. The third optical path is: a portion of the signal photons enter the first mirror from the third beam splitter, are reflected by the first mirror into the second mirror, are reflected by the second mirror into a fourth beam splitter, and are coupled from the fourth beam splitter into the second beam splitter. And the three optical paths are coupled and then enter the first phase modulator controlled by the computer, and finally are transmitted to the orbital angular momentum multiplexing device. The first to fourth beam splitters are 50:50 beam splitter.

The laser light source is a laser light source with a specific wavelength, the output laser wavelength is 1550nm, and the power is 1 mw. At the initial moment, the polarization controller outputs a polarization state set to be 45-degree polarization, and the first phase modulator controlled by the computer performs random 0 or pi phase modulation on the passing photons. At an initial time, a single-photon state in a single polarization direction is emitted from a light source device, phase separation is performed in a phase separation device, and phase modulation is performed by a first phase modulator. The light source device and the phase separation component are combined to complete the phase encoding process of the signal photons.

The orbital angular momentum multiplexing device comprises a spatial light modulator, a computer, a first convex lens and a second convex lens; after entering the spatial light modulator, the signal photons pass through the third and fourth convex lenses and enter the orbital angular momentum demultiplexing device.

The spatial light modulator is a pure phase type reflection liquid crystal spatial light modulator, is an active digital optical device based on a liquid crystal molecule electrogenerated birefringence effect, has the characteristics of low voltage, micro power consumption, miniaturization, light weight, energy conservation, high density and the like, has the advantages of high diffraction efficiency, simplicity and convenience in control, flexible transformation and the like when the orbital angular momentum of a light beam is modulated, can simultaneously modulate a plurality of different orbital angular momenta to realize multiplexing of an optical signal beam, and the generated orbital angular momentum value corresponds to the number of users at a receiving end and can be increased along with the expansion of the number of users; the first telescope component consists of two confocal convex lenses, the laser beam is a Gaussian beam and has a certain far field divergence angle, the first telescope component compresses the far field divergence angle of the laser beam, and the collimated beam is re-emitted into a free space;

the orbital angular momentum demultiplexing device comprises a fifth beam splitter, a charge coupling element, a plurality of cascaded first ring interferometers and second ring interferometers, wherein each first ring interferometer comprises an incident port, a sixth beam splitter, a third reflector, a fourth reflector, a first dove prism, a second dove prism, a wavefront corrector, an emergent reflection port and an emergent transmission port; each second ring interferometer comprises an incident port, a first polarization beam splitter, sixth to eighth reflectors, third and fourth dove prisms and an emergent port; the optical fiber module comprises a fifth reflector, a first half-wave plate, a second half-wave plate, a quarter-wave plate, a second polarization beam splitter, a reflection user side and a transmission user side.

After entering the second telescope component, the signal photons enter the first annular interferometer through the incident port, then enter a sixth beam splitter of the first annular interferometer, and form a first optical path and a second optical path at the sixth beam splitter, wherein the first optical path is as follows: a part of signal photons are reflected by the third reflector, enter the wavefront corrector, are reflected by the wavefront corrector, then enter the first dove prism after being reflected by the fourth reflector, pass through the first dove prism and return to the sixth beam splitter; the second optical path is: the other part of signal photons pass through the sixth beam splitter, are reflected by the fourth reflector, enter the wavefront corrector, are reflected by the wavefront corrector, enter the second dove prism after passing through the third reflector, pass through the second dove prism and enter the sixth beam splitter; the two beams of light are interfered by the sixth beam splitter and are emitted from the emergent reflection port and the emergent transmission port respectively after being interfered.

A part of signal photons are emitted from the exit reflection port, then enter the second ring interferometer through the first half-wave plate, and then enter the first polarization beam splitter of the second ring interferometer, and a first optical path and a second optical path are formed at the first polarization beam splitter, wherein the first optical path is as follows: part of signal photons sequentially pass through the sixth reflector and the seventh reflector, are reflected to enter the third dove prism, exit from the third dove prism, and then pass through the eighth reflector to return to the first polarization beam splitter for coupling; the second optical path is: and the other part of signal photons sequentially pass through the eighth reflector and the seventh reflector, are reflected to enter the fourth dove prism, are emitted from the fourth dove prism, pass through the sixth reflector, return to the first polarization beam splitter for coupling, then pass through the second half-wave plate and the quarter-wave plate, enter the second polarization beam splitter for splitting, and are emitted from the reflection user side and the transmission user side respectively.

Another part of signal photons are emitted from the exit transmission port, then enter the second ring interferometer in cascade through the fifth reflector and the first half-wave plate, and then enter the first polarization beam splitter of the second ring interferometer, and form a first light path and a second light path at the first polarization beam splitter, wherein the first light path is as follows: part of signal photons sequentially pass through the sixth reflector and the seventh reflector, are reflected to enter the third dove prism, exit from the third dove prism, and then pass through the eighth reflector to return to the first polarization beam splitter for coupling; the second optical path is: and the other part of signal photons sequentially pass through the eighth reflector and the seventh reflector, are reflected to enter the fourth dove prism, are emitted from the fourth dove prism, pass through the sixth reflector, return to the first polarization beam splitter for coupling, then pass through the second half-wave plate and the quarter-wave plate, enter the second polarization beam splitter for splitting, and are emitted from the reflection user side and the transmission user side respectively. Specifically, the orbital angular momentum separating device determines that the photons are output from different ports according to different orbital angular momenta carried by the photons, and the fifth beam splitter and the sixth beam splitter are 50:50 beam splitter.

The second telescope component mainly comprises two confocal convex lenses, is positioned at a signal photon receiving end, and mainly plays the roles of an optical antenna and a spatial filter to filter stray light in space. The charge coupling element is used for monitoring the intensity of laser pulses and wavefront phase distortion caused by atmospheric turbulence in real time and providing reference information for clock synchronization and phase distortion compensation. The wave-front corrector can change the optical path of the wave-front transmission of the light wave or change the refractive index of the transmission medium to change the phase structure of the wave-front of the incident light wave according to the reference information of the phase distortion, thereby achieving the purpose of compensating the wave-front phase of the light wave.

Several first ring interferometers are cascaded from front to back, while the second ring interferometer always serves as the last end of the cascade. The exit reflection port and the exit transmission port of any one of the preceding first ring interferometers can be connected with the next first ring interferometer through the entrance port of the next first ring interferometer. The exit reflection port of the first ring interferometer of any one preceding stage may also be connected to the second ring interferometer of the next stage via the first half-wave plate, and the exit transmission port may be connected to the second ring interferometer of the next stage via the fifth mirror and the first half-wave plate.

When the relative angle of two dove prisms in two optical paths of the first and second ring interferometers is alpha/2, the action of the dove prisms is equivalent to adding a beam rotator with the rotation angle alpha to one of the optical paths. The dove prism enables photons with orbital angular momentum l to generate an orbital angular momentum phase difference of l alpha on two light paths of the interferometer, and the dove prism plays a role in rotating the phase of the photon orbital angular momentum.

The effect of the first ring interferometer on the input photons is described as follows:

let the state of the photons incident on the input port of the first ring interferometer into the sixth Beam Splitter (BS) be:

|in>_BS＝|0>|1>

and |0> represents a vacuum state and |1> represents a single photon state, the same is applied below. After the action of the sixth Beam Splitter (BS), the output photon state is:

the above equation shows that the probability of the photon output from the transmission end and the reflection end of the sixth Beam Splitter (BS) is 50%, but a phase jump of 90 ° is added when the photon is output from the reflection end. After the photons are acted by the davit prism, the two light paths generate an orbital angular momentum phase difference of l alpha, and the input photon state of a sixth Beam Splitter (BS) at the output end of the first annular interferometer is as follows:

the photon state after the sixth Beam Splitter (BS) becomes:

|out>_BS'＝1/2(1-e^ilα)|0>|1>+i/2(1+e^ilα)|1>|0>

the above equation shows that the photons interfere in the sixth Beam Splitter (BS), the phase of the photons changes by l α, and when the relative angle α of the davit prism is pi:

a sixth Beam Splitter (BS) photon output state | out when the orbital angular momentum order l is odd>_BS'The following steps are changed: i0>|1>And photons are transmitted from the sixth beam splitter.

When the orbital angular momentum order l is even, the sixth Beam Splitter (BS) outputs the photon state | out>_BS'The following steps are changed: i |1>|0>The photons are reflected from the sixth beam splitter.

When a plurality of orbital angular momentum values need to be separated, a plurality of first ring interferometers need to be cascaded, wherein the relative phase of the dove prism should be adjusted to pi/2^k-1And k is expressed as a first ring interferometer cascaded in a kth stage, and a hologram with orbital angular momentum delta l ═ k (k is the kth cascaded first ring interferometer) is introduced into a specific exit port of the first ring interferometer cascaded in each stage to change the orbital angular momentum of the exiting photons so as to meet the interference condition of the next stage.

The first half-wave plate rotates the polarization state of the photon to a positive 45 ° polarization state. The fifth reflector compensates the orbital angular momentum, and photons emitted from the exit transmission port of the first annular interferometer are reflected for only odd times, so that +/-l is twisted into

And may be twisted into an initial state when passing through the fifth mirror.

The effect of the second ring interferometer on the input photons is described as follows:

let the photon state entering the first polarizing beam splitter (PBS1) from the first half-wave plate to the second ring interferometer input port be:

i H represents the horizontal polarization state, and I V represents the vertical polarization state, the same as the following. After the action of the first polarizing beam splitter (PBS1), the horizontal polarization state is transmitted into the first optical path, and the vertical polarization state is reflected into the second optical path.

After the photons are acted by the davit prism, the two optical paths generate an orbital angular momentum phase difference of l alpha, and then the output photon state of the first polarization beam splitter (PBS1) at the output end of the second ring interferometer is as follows:

after the second half-wave plate (HWP2), the output photon state is:

the photons are rotated to vertically and horizontally polarized light, respectively, in a second half-wave plate (HWP2) at plus and minus 45 ° polarization. And when the relative angle of the Da Fu prism module is pi/2 and the orbital angular momentum order l is 1:

when the topological charge number l is negative, the second half-wave plate (HWP2) photon output photon state is right-hand circular polarization.

When the topological charge number l is positive, the second half-wave plate (HWP2) photon output photon state is left-handed circular polarization.

After passing through the Quarter Wave Plate (QWP), the right-handed circular polarization is rotated into vertically polarized light, and the left-handed circular polarization is rotated into horizontally polarized light.

After passing through the second polarizing beam splitter (PBS2), the vertically polarized light is reflected and the left-hand polarized light is transmitted, and thus exits at the transmitting end of the second polarizing beam splitter (PBS2) when l is positive, and thus exits at the reflecting end of the second polarizing beam splitter (PBS2) when l is negative.

When a plurality of orbital angular momentum values need to be separated, the relative phase of the davit prism of the second ring interferometer cascaded at the last stage is adjusted to pi/2 n, wherein n is expressed as the orbital angular momentum order l emitted from the first ring interferometer is n. Photons are output from corresponding exit ports of the cascaded first ring interferometers according to different orbital angular momentums carried by the photons, are input into the second ring interferometer, are output from corresponding exit ports of the second ring interferometers, are automatically routed and addressed, have 100% of orbital angular momentum separation efficiency, and cannot damage the photon orbital angular momentum. Because whole separator is passive device, easily integrates, can separate orbit angular momentum high-efficiently fast, improves communication efficiency. The orbital angular momentum state corresponds to a corresponding user, can be expanded along with the number of users, and realizes communication of a pair of multi-quantum networks.

The Bob user receiving end comprises N Bob users, and each Bob user is respectively connected with the last stage of the multistage cascade, namely any one of the exit ports of the plurality of second ring interferometers.

Each Bob user comprises an M-Z type phase separation device and a detection device, wherein the M-Z type phase separation device comprises a second phase modulator, a seventh beam splitter, an eighth beam splitter, a ninth mirror and a tenth mirror; the detection device comprises a first detector and a second detector;

the signal photons are emitted from the second polarization beam splitter, pass through a phase modulator controlled by a computer, and are divided into two paths after entering a seventh beam splitter: a part of signal photons directly enter the eighth beam splitter to interfere; and the other part of signal photons enter the eighth beam splitter to interfere after sequentially passing through the ninth reflector and the tenth reflector, and finally the interfered emergent light is detected by the first detector and the second detector respectively.

A multi-user OAM-QKD method based on a ring interferometer comprises the following steps:

s1: initializing a system: checking an Alice user sending terminal and a Bob user receiving terminal, checking whether each component normally operates, debugging the phase modulator, the polarization controller and the spatial light modulator, and setting initial values of each device;

s2: testing a signal modulation device and a receiving end device: the light source sends a beam of light, the light sequentially enters the attenuator, the polarization controller and the phase separation assembly, and finally enters the M-Z type phase separation device and the detection device to observe whether the response of the detector is correct or not;

s3: testing an orbital angular momentum multiplexing device: the light source device emits photon signals, the photon signals are coupled to the spatial light modulator after passing through the phase separation assembly, and the photon signals are subjected to orbital angular momentum multiplexing modulation to generate a plurality of different orbital angular momentum multiplexes;

s4: testing an orbital angular momentum demultiplexing unit: demultiplexing the orbital angular momentum multiplexing light beams through the orbital angular momentum separation device, wherein the orbital angular momentum separation device selectively emits the light beams from corresponding ports according to different orbital angular momentums carried by photons;

s5: and (3) system testing: on the premise that the light source device does not emit light signals, namely the pulse number is zero, testing the noise level of the system to see whether the noise level meets the standard or not;

s6: and (3) key sending: the signal modulation device sends photons to the phase separation device, the computer controls the phase modulator to perform 0 or pi phase modulation on the photons, the computer controls the spatial light modulator to perform orbital angular momentum modulation on the photons emitted from the phase modulator, the key sending end records receiver information and random code information, and the Bob user receiving end records the response time and the response value of the detector;

s7: key screening and privacy enhancement: the Bob user receiving end compares the recorded information of the detector response with the phase information recorded by the Alice user sending end through a public channel to form a screening key; then randomly selecting some data in the screening secret key, knowing that the error rate of the DPS is 11%, obtaining the error rate of the system by calculating the screened data, judging whether interception exists or not, if the error rate is higher than a threshold value, stopping communication immediately and discarding the generated screening code, if no interception exists, then carrying out data coordination on the screening secret key again, keeping mutual information of Alice and Bob unchanged before and after information coordination, and finally carrying out data privacy enhancement through a privacy enhancement protocol;

s8: obtaining a final key: and obtaining the final security key after Bob performs privacy enhancement. The two communication parties utilize the security key and combine the one-time pad to realize quantum secret communication.

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

1. the invention uses the orbital angular momentum as an addressing channel, and the orthogonal characteristic of the orbital angular momentum enables information carried by the coaxial OV light beam to be transmitted in free space without the interference of the orbital angular momentum channel; topological charge l and azimuth of orbital angular momentum

The uncertain relation between the orbit angular momentum and the orbit angular momentum enables the information carried by the orbit angular momentum to have good safety; the orbital angular momentum can be infinitely valued, any information can be multiplexed by adjusting the photon orbital angular momentum through the spatial light modulator, each orbital angular momentum corresponds to one user side and can be expanded along with the increase of the user sides, and communication with any multiple users can be realized;

2. the invention utilizes a self-balancing annular interferometer to separate orbital angular momentum, two optical paths of the interferometer are added with two Dff prisms, the orbital angular momentum of a single photon level can be separated, stable cascade can be realized, any multiphoton orbital angular momentum can be separated, and the separation efficiency is 100%;

3. the Alice user sending end can realize one-to-many communication with the quantum network communication of the free space of the Bob user receiving end, the users are relatively independent, the number of the users can be expanded by the increase of the orbital angular momentum multiplexing, and the method has good expansibility and higher implementability;

4. the invention uses the differential phase shift quantum key distribution protocol, has proved to have unconditional safety, and under the large background that the phase modulation technology is mature, the protocol has strong practicability and is easy to realize high speed.

Drawings

FIG. 1 is a first ring interferometer configuration;

FIG. 2 is a second interferometer ring structure;

FIG. 3 is a block diagram of a demultiplexing system based on orbital angular momentum;

FIG. 4 is a schematic diagram of a cascade of multiple ring interferometers;

FIG. 5 is a schematic flow diagram of a multi-user OAM-QKD network based on a ring interferometer;

FIG. 6 is a flow chart of the method 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, the first ring interferometer includes an incident port 101, a sixth beam splitter 104, a third mirror 109, a fourth mirror 105, a first duff prism 108, a second duff prism 106, a wavefront corrector 107, an exit transmission port 102, and an exit reflection port 103;

let incident first ring interferometer input port 101 go to sixth 50: the photon state of the 50 Beam Splitter (BS)104 is:

|in>_BS＝|0>|1>

and |0> represents a vacuum state and |1> represents a single photon state, the same is applied below. After the action of the sixth Beam Splitter (BS)104, the output photon state is:

the above equation shows that the probability of the photon output from both the transmission side and the reflection side of the sixth Beam Splitter (BS)104 is 50%, but a phase jump of 90 ° is added when the photon is output from the reflection side. After the photons are acted by the davit prism, the two optical paths generate an orbital angular momentum phase difference of l alpha, and then the input photon state of the sixth Beam Splitter (BS)104 at the output end of the first annular interferometer is:

the photon state after passing through the sixth Beam Splitter (BS)104 becomes:

|out>_BS'＝1/2(1-e^ilα)|0>|1>+i/2(1+e^ilα)|1>|0>

the above equation shows that the photons interfere in the sixth Beam Splitter (BS)104, the phase of the photons changes by l α, and when the relative angle α of the davit prism is pi:

when the orbital angular momentum order l is odd, the sixth Beam Splitter (BS)104 outputs the photon output state | out>_BS'The following steps are changed: i0>|1>And the photons exit the sixth beamsplitter transmission port 102.

When the orbital angular momentum order l is even, the sixth Beam Splitter (BS)104 outputs the photon output state | out>_BS'The following steps are changed: i |1>|0>And the photons exit the sixth beam splitter reflection port 103.

As shown in fig. 2, the second ring interferometer includes an entrance port 201, a first polarizing beam splitter 203, a sixth mirror 204, a seventh mirror 206, an eighth mirror 208, a third dove prism 207, a fourth dove prism 205, and an exit port 202.

let the photon state entering the first polarizing beam splitter (PBS1)203 from the first half-wave plate to the second ring interferometer input port 201 be:

i H represents the horizontal polarization state, and I V represents the vertical polarization state, the same as the following. After being acted on by the first polarizing beam splitter (PBS1)203, the horizontal polarization state is transmitted into the first optical path, and the vertical polarization state is reflected into the second optical path.

After the photons are acted by the davit prism, the two optical paths generate an orbital angular momentum phase difference of l α, and then the output photon state of the first polarization beam splitter (PBS1)203 at the output end of the second ring interferometer is:

as shown in fig. 3, the ring interferometer structure for separating four orbital angular momentum states is a cascade structure, wherein the cascade first stage is a first ring interferometer 110, the cascade second stage is two second ring interferometers, and the cascade structure comprises a multiplexing input port 301,

second telescope groups

302 and 303, a charge coupling element 304, a fifth beam splitter 305, a first stage first ring interferometer 306, first half-

wave plates

307 and 313, second half-

wave plates

309 and 315, quarter-

wave plates

310 and 316, second

polarization beam splitters

311 and 317, a second stage separating odd OAM state photon second ring interferometer 314, a second stage separating even OAM state photon second ring interferometer 308, and

exit ports

318, 319, 320 and 321.

When the relative angle of the two light paths of the two ring interferometers is alpha/2, the effect of the dove prism is equivalent to adding a light beam rotator with a rotation angle of alpha into one of the light paths, photons with orbital angular momentum of l are incident to the first interferometer, and the second interferometer generates an orbital angular momentum phase difference of l alpha. The

first half waveplates

307, 313 rotate the polarization state of the photons to a positive 45 ° polarization state. The fifth mirror 312 compensates for orbital angular momentum, and photons emitted from the exit transmission port of the first ring interferometer are reflected only an odd number of times, so ± l are twisted into

And may be twisted into an initial state when passing through the fifth mirror.

The signal photons exiting the second ring interferometer 308, after being acted upon by the second half-wave plate (HWP2)309, output photon states:

the photons are rotated to vertically and horizontally polarized light, respectively, at plus and minus 45 ° polarization in the second half wave plate (HWP2) 309. And when the relative angle of the Da Fu prism module is pi/2 and the orbital angular momentum order l is 1:

when the orbital angular momentum order l is negative, the second half waveplate (HWP2)309 outputs photon states with right-hand circular polarization.

When the orbital angular momentum order l is positive, the second half-wave plate (HWP2)309 outputs photon states with left-hand circular polarization.

After passing through the Quarter Wave Plate (QWP)310, the right-handed circular polarization is rotated into vertically polarized light, and the left-handed circular polarization is rotated into horizontally polarized light.

After passing through the second polarizing beam splitter (PBS2)311, the vertically polarized light is reflected, and the left-hand polarized light is transmitted, and thus exits at the transmitting end of the second polarizing beam splitter (PBS2)311 when l is positive, and exits at the reflecting end of the second polarizing beam splitter (PBS2)311 when l is negative.

Assuming that photons with orbital angular momentum l + -1, + 2 are incident from the incident port 301, the relative angle of the duff prism of the first ring interferometer is pi, the relative angle of the duff prism of the second ring interferometer separating odd OAM state photons is pi/2, the relative angle of the duff prism of the second ring interferometer separating even OAM state photons is pi/4, the first stage first ring interferometer separates even and odd OAM state photons, photons with orbital angular momentum l + -1 enter the second ring interferometer 314 separating odd OAM state photons, photons with orbital angular momentum l + -2 enter the second stage second ring interferometer 308, photons with orbital angular momentum l + -2 exit from the 321 port, photons with orbital angular momentum l + -2 exit from the 320 port, photons with l + -1 exit from the 319 port, and photons with l + -1 exit from the 318 port.

As shown in fig. 4, to separate the schematic diagram of any OAM-state photonic cascading device, to implement communication with any multiple users, the first and second ring interferometers shown in fig. 1 and 2 must be cascaded in multiple stages in sequence. The first-stage cascade unit 401 shown in fig. 4 separates the topological charges l into even and odd photons, respectively, the rotation angle of the duff prism 108 in the first ring interferometer of the first stage is set to pi, and the hologram 403 changes OAM into Δ l ═ 1; the second stage has four exit ports, the rotation angle of the duff prism 108 is set to pi/2, the

holograms

406 and 409, and Δ l is 2; the third stage has 8 exit ports, and the rotation angle of the Duff prism 108 is set to pi/2^2，The third level hologram is set to Δ l — 3. The n-stage first ring interferometer is cascaded with 2ⁿAn exit port, which can be 2ⁿ⁺¹The Bob user end communicates to enable the cascade interferometer to separate any orbital angular momentum value, wherein the relative phase of the Dff prism should be adjusted to pi/2^k-1And k is expressed as a first ring interferometer cascaded in a kth stage, and a hologram with orbital angular momentum delta l ═ k (k is the kth cascaded first ring interferometer) is introduced into a specific exit port of the first ring interferometer cascaded in each stage to change the orbital angular momentum of the exiting photons so as to meet the interference condition of the next stage. And then, the photons are output from any exit port 411 of the first cascade kth stage of first ring interferometer according to different orbital angular momentum topological charge numbers l carried by the photons, one path of light passes through a half-wave plate, the other path of light passes through a reflector and the half-wave plate and is input into a second ring interferometer, the relative phase of a davit prism of the second cascade stage of second ring interferometer is adjusted to be pi/2 n, and n represents that the orbital angular momentum order l emitted from the first ring interferometer is n. The photons are output from the corresponding exit port of the second ring interferometer, the routing addressing is carried out automatically, the orbital angular momentum separation efficiency is 100%, and the photon orbital angular momentum cannot be damaged. Because whole separator is passive device, easily integrates, can separate orbit angular momentum high-efficiently fast, improves communication efficiency. The orbital angular momentum state corresponds to a corresponding user and can be expanded along with the number of the users,a pair of multiple sub-networks is implemented for communication.

As shown in fig. 5: the system is a multi-user OAM-QKD system structure based on a ring interferometer. Taking four users as an example, the method includes an Alice user sending end 539, an orbital angular momentum demultiplexing unit 540, and a Bob user receiving end 541, where:

the Alice user sending end 539 comprises a signal modulation device and an orbital angular momentum multiplexing device, wherein the signal modulation device comprises a light source device and a phase separation component; the light source device includes: a laser light source 501, an attenuator 502, a polarization controller 503; the phase separation assembly includes: first to

fourth beam splitters

504, 505, 506, 507, first and

second mirrors

508, 509, a first phase modulator 510, a computer 512; the orbital angular momentum multiplexing device in turn comprises a spatial light modulator 511 and a first

telescopic mirror assembly

514, 515.

The orbital angular momentum demultiplexing unit 540 includes a second telescope component and an orbital angular momentum demultiplexing device. The second telescope component comprises convex lenses 516, 517, the orbital angular momentum demultiplexing device comprises a fifth beam splitter 518, a charge coupled device 519, a first ring interferometer 520, a

second ring interferometer

523, 535, a fifth mirror 521, first half-

waveplates

522, 534, second half-

waveplates

524, 536, quarter-waveplates 525, 537, second

polarization beam splitters

526, 538, and respective reflection clients and transmission clients.

The Bob user receiving end 541 comprises N Bob users, each Bob user comprises an M-Z phase separation device and a detection device, and the M-Z phase separation device comprises a second phase modulator 527, a seventh beam splitter 528, an eighth beam splitter 529, a ninth mirror 530, a tenth mirror 531; the detection means comprise first and

second detectors

532, 533.

Example 2

The key distribution process of the system comprises the following steps:

the signal modulation device 539 in the Alice user sending end, after the laser light source sends laser light into the attenuator 502 and the polarization controller 503, the laser light is divided into three single photon pulses a, b, and c by the phase separation device, and then the three single photon pulses enter the first phase modulator 510 controlled by the computer, the phase modulation of 0 or pi is respectively performed on the three single photon pulses, and Alice records the value of each group of phase modulation information; the prepared quantum state is one of the following four states:

the coded signal state photons enter a spatial light modulator 511 to modulate orbital angular momentum, the spatial light modulator is controlled by a computer to modulate any orbital angular momentum state for multiplexing, each orbital angular momentum corresponds to a corresponding user at the receiving end of a Bob user, and the photons modulated by the spatial light modulator can establish quantum channels with the corresponding users for communication;

the OAM state photons enter

first telescope assemblies

514 and 515, each first telescope assembly consists of two confocal convex lenses, a laser beam is a Gaussian beam and has a certain far-field divergence angle, the first telescope assembly compresses the far-field divergence angle of the laser beam, and the collimated laser beam is re-emitted into a free space; the receiving end is an orbital angular momentum demultiplexing unit 540, and first, photons in a free space signal state are received by a second telescope assembly 516 and 517, the second telescope assembly consists of two confocal convex lenses and mainly plays the roles of an optical antenna and a spatial filter to filter stray light in space; and the OAM state photons enter the orbital angular momentum demultiplexing device through the second telescope component. The charge coupled device 519 is used for monitoring the intensity of the laser pulse and the wavefront phase distortion caused by the atmospheric turbulence in real time, and providing reference information for clock synchronization and phase distortion compensation. The wavefront corrector 107 in the first ring interferometer 520 may change the optical path of the wavefront transmission of the optical wave or change the refractive index of the transmission medium to change the phase structure of the wavefront of the incident optical wave according to the reference information of the phase distortion, thereby achieving the purpose of compensating the phase of the wavefront of the optical wave.

The process of OAM state photon automatic channel addressing is illustrated by taking two-stage cascade separation of four orbital angular momentum states as an example: the orbital angular momentum separation device 540 can realize communication between the sender Alice and four users at the corresponding receiver Bob end, assuming that the photon orbital angular momentum modulated by the spatial light modulator 511 in the orbital angular momentum multiplexing device is l ± 1 and ± 2, a photon with orbital angular momentum l ═ 1 is emitted from a transmission exit port of the polarization beam splitter and enters Bob1, a photon with l ═ 1 is emitted from a reflection exit port of the polarization beam splitter and enters Bob2, photons with the same principle l ± 2 are emitted from a 538 port and enter Bob3 and Bob4 respectively, OAM-state photons automatically address and enter corresponding users according to the carried orbital angular momentum, and OAM-state photons are efficiently addressed without destroying information encoded by signal photons; the Bob1 user end 541 is taken as an example to describe the detection process of signal photons, and 527 is taken as a phase modulator to perform phase compensation; after the three single-photon pulses enter the 50:50 beam splitter 528, the interferometer with the M-Z structure is divided into a long arm and a broken arm, the difference between the two arms needs to be as large as the difference between the two arms of Alice, and therefore a time difference exists between the three single-photon pulses. That is, the

detector

1, 532, and the

detector

2, 533, on the receiving side may respond at four times, and discard the values at the first and fourth times, and at the second and third times, when the phase difference between the two photons is 0, the detector 1 responds and is marked as 0; when the phase difference of the two photons is +/-pi, the detector 2 responds and records as 1; and the Bob user side compares the recorded information of the detector response with the phase information recorded by the Alice side through a public channel, and obtains a final secret key through screening and post-processing. The above is the whole process of quantum key distribution.

Specifically, as shown in fig. 6, the multi-user OAM-QKD method based on a ring interferometer includes the following steps:

s1, system initialization: checking an Alice user sending terminal and a Bob user receiving terminal, checking whether each component normally operates, debugging the phase modulator, the polarization controller and the spatial light modulator, and setting initial values of each device;

s2, testing a signal modulation device and a receiving end device: the light source sends a beam of light, the light sequentially enters the attenuator, the polarization controller and the phase separation assembly, and finally enters the M-Z type phase separation device and the detection device to observe whether the response of the detector is correct or not;

s3, testing an orbital angular momentum multiplexing device: the light source device emits photon signals, the photon signals are coupled to the spatial light modulator after passing through the phase separation assembly, and the photon signals are subjected to orbital angular momentum multiplexing modulation to generate a plurality of different orbital angular momentum multiplexes;

s4, testing an orbital angular momentum demultiplexing unit: demultiplexing the orbital angular momentum multiplexing light beams through the orbital angular momentum separation device, wherein the orbital angular momentum separation device selectively emits the light beams from corresponding ports according to different orbital angular momentums carried by photons;

s5, system testing: on the premise that the light source device does not emit light signals, namely the pulse number is zero, testing the noise level of the system to see whether the noise level meets the standard or not;

s6, key sending: the signal modulation device sends photons to the phase separation device, the computer controls the phase modulator to perform 0 or pi phase modulation on the photons, the computer controls the spatial light modulator to perform orbital angular momentum modulation on the photons emitted from the phase modulator, the key sending end records receiver information and random code information, and the Bob user receiving end records the response time and the response value of the detector;

s7, key screening and security enhancement: the Bob user receiving end compares the recorded information of the detector response with the phase information recorded by the Alice user sending end through a public channel to form a screening key; and then randomly selecting some data in the screening secret key, knowing that the error rate of the DPS is 11%, obtaining the error rate of the system by calculating the screened data, judging whether interception exists or not, if the error rate is higher than a threshold value, stopping communication immediately and discarding the generated screening code, if no interception exists, then carrying out data coordination on the screening secret key again, keeping mutual information of Alice and Bob unchanged before and after information coordination, and finally carrying out data privacy enhancement through a privacy enhancement protocol.

S8, obtaining a final secret key: and obtaining the final security key after Bob performs privacy enhancement. The two communication parties utilize the security key and combine the one-time pad to realize quantum secret communication.

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 multi-user OAM-QKD system based on a ring interferometer is characterized by comprising an Alice user sending end, an orbital angular momentum demultiplexing unit and a Bob user receiving end;

2. The multi-user ring interferometer-based OAM-QKD system of claim 1, wherein said signal modulation means comprises light source means and a phase separation component; the light source device includes: the system comprises a laser light source, an attenuator and a polarization controller; the phase separation assembly includes: the beam splitter comprises a first beam splitter, a second beam splitter, a first reflector, a second reflector, a first phase modulator and a computer; the laser light source emits laser light, the laser light enters the attenuator and the polarization controller and then enters the first beam splitter. Splitting the beam in the first beam splitter into three optical paths, wherein the first optical path is as follows: a portion of the signal photons are coupled directly into the second beam splitter. The second optical path is: a portion of the signal photons enter the third beam splitter and are split again, wherein a portion of the signal photons enter the fourth beam splitter and are reflected therefrom and finally coupled into the second beam splitter. The third optical path is: a portion of the signal photons enter the first mirror from the third beam splitter, are reflected by the first mirror into the second mirror, are reflected by the second mirror into a fourth beam splitter, and are coupled from the fourth beam splitter into the second beam splitter. And the three optical paths are coupled and then enter the first phase modulator controlled by the computer, and finally are transmitted to the orbital angular momentum multiplexing device.

3. The multi-user ring interferometer-based OAM-QKD system of claim 2, wherein said orbital angular momentum multiplexing device comprises a spatial light modulator, a computer, first and second convex lenses; after entering the spatial light modulator, the signal photons pass through the third and fourth convex lenses and enter the orbital angular momentum demultiplexing device.

4. The multi-user, OAM-QKD system based on a ring interferometer of claim 3, wherein said second telescope assembly includes two convex lenses that are confocal; the orbital angular momentum demultiplexing device comprises a fifth beam splitter, a charge coupling element, a plurality of cascaded first ring interferometers and second ring interferometers, wherein each first ring interferometer comprises an incident port, a sixth beam splitter, a third reflector, a fourth reflector, a first dove prism, a second dove prism, a wavefront corrector, an emergent reflection port and an emergent transmission port; each second ring interferometer comprises an incident port, a first polarization beam splitter, sixth to eighth reflectors, third and fourth dove prisms and an emergent port; the first half-wave plate, the second half-wave plate, the fourth half-wave plate, the second polarization beam splitter, the reflection user side and the transmission user side are arranged on the first reflecting mirror;

after the signal photons pass through the fifth beam splitter and the charge coupling element after passing through the second telescope component, the signal photons enter the first annular interferometer from the incident port, and then enter a sixth beam splitter of the first annular interferometer, and a first optical path and a second optical path are formed at the sixth beam splitter, wherein the first optical path is as follows: a part of signal photons are reflected by the third reflector, enter the wavefront corrector, are reflected by the wavefront corrector, then enter the first dove prism after being reflected by the fourth reflector, pass through the first dove prism and return to the sixth beam splitter; the second optical path is: the other part of signal photons pass through the sixth beam splitter, are reflected by the fourth reflector, enter the wavefront corrector, are reflected by the wavefront corrector, enter the second dove prism after passing through the third reflector, pass through the second dove prism and enter the sixth beam splitter; two beams of light are interfered by the sixth beam splitter and are respectively emitted from the emergent reflection port and the emergent transmission port after being interfered;

a part of signal photons are emitted from the exit reflection port, then enter the second ring interferometer through the first half-wave plate, and then enter the first polarization beam splitter of the second ring interferometer, and a first optical path and a second optical path are formed at the first polarization beam splitter, wherein the first optical path is as follows: part of signal photons sequentially pass through the sixth reflector and the seventh reflector, are reflected to enter the third dove prism, exit from the third dove prism, and then pass through the eighth reflector to return to the first polarization beam splitter for coupling; the second optical path is: the other part of signal photons sequentially pass through the eighth reflector and the seventh reflector, are reflected to enter the fourth dove prism, are emitted from the fourth dove prism, then pass through the sixth reflector, return to the first polarization beam splitter for coupling, then pass through the second half-wave plate and the quarter-wave plate, enter the second polarization beam splitter for splitting, and are emitted from the reflection user side and the transmission user side respectively;

another part of signal photons are emitted from the exit transmission port, then enter the second ring interferometer in cascade through the fifth reflector and the first half-wave plate, and then enter the first polarization beam splitter of the second ring interferometer, and form a first light path and a second light path at the first polarization beam splitter, wherein the first light path is as follows: part of signal photons sequentially pass through the sixth reflector and the seventh reflector, are reflected to enter the third dove prism, exit from the third dove prism, and then pass through the eighth reflector to return to the first polarization beam splitter for coupling; the second optical path is: and the other part of signal photons sequentially pass through the eighth reflector and the seventh reflector, are reflected to enter the fourth dove prism, are emitted from the fourth dove prism, pass through the sixth reflector, return to the first polarization beam splitter for coupling, then pass through the second half-wave plate and the quarter-wave plate, enter the second polarization beam splitter for splitting, and are emitted from the reflection user side and the transmission user side respectively.

5. The multi-user ring interferometer based OAM-QKD system of claim 4, wherein a number of first ring interferometers are cascaded from front to back, while a second ring interferometer always serves as the last end of the cascade; the exit reflection port and the exit transmission port of any one of the preceding first ring interferometers can be connected with the next first ring interferometer through the entrance port of the next first ring interferometer. The exit reflection port of the first ring interferometer of any one preceding stage may also be connected to the second ring interferometer of the next stage via the first half-wave plate, and the exit transmission port may be connected to the second ring interferometer of the next stage via the fifth mirror and the first half-wave plate.

6. The multi-user ring interferometer-based OAM-QKD system of claim 5, wherein said Bob user receiving end includes N Bob users, each Bob user being connected to any one of the exit ports of the last stage of the multi-stage cascade, i.e., the second plurality of ring interferometers, respectively.

7. The multi-user ring interferometer-based OAM-QKD system of claim 6, wherein each Bob user includes M-Z type phase separation means and detection means, said M-Z type phase separation means including a second phase modulator, a seventh, an eighth beam splitter, a ninth, a tenth mirror; the detection device comprises a first detector and a second detector;

the signal photons are emitted from the second polarization beam splitter, pass through a second phase modulator controlled by a computer, enter a seventh beam splitter and are divided into two paths: a part of signal photons directly enter the eighth beam splitter to interfere; and the other part of signal photons enter the eighth beam splitter to interfere after sequentially passing through the ninth reflector and the tenth reflector, and finally the interfered emergent light is detected by the first detector and the second detector respectively.

8. The ring interferometer-based multi-user OAM-QKD system of claim 7, wherein the first through sixth beam splitters are 50:50 beam splitter.

9. The multi-user OAM-QKD system based on a ring interferometer of claim 8, wherein said laser light source is a wavelength-specific laser light source, the output laser wavelength is 1550nm, the power is 1 mw; at the initial moment, the polarization controller outputs a polarization state set to be 45-degree polarization, and the first phase modulator controlled by the computer performs random 0 or pi phase modulation on the passing photons.

10. A ring interferometer based multi-user OAM-QKD method of claim 9, comprising the steps of: