CN207321266U - Quantum key distribution network system based on orbital angular momentum multiplexing - Google Patents

Abstract

The utility model discloses an Alice control end, an orbital angular momentum modulus multiplexing unit and a Bob user end, wherein: the Alice control end includes a signal modulation unit and an orbital angular momentum multiplexer unit, and the orbital angular momentum modulus multiplexing unit The user unit includes a Galileo telescope assembly and an orbital angular momentum separation device; the Bob user end includes N Bob users, and each Bob user includes a polarization controller and a detection device; the optical signal generated by the signal modulation unit enters the orbit angle The momentum multiplexing unit, the Galileo telescope assembly and the orbital angular momentum separation device are then sent to the polarization controller corresponding to the Bob user end, and finally enter the detection device for detection. The utility model realizes the one-to-many communication of the quantum network communication, and each user is relatively independent, and the number of users can be expanded by the increase of orbital angular momentum multiplexing, and has good expansibility and high implementability.

Description

Quantum key distribution network system based on orbital angular momentum multiplexing

technical field

The utility model relates to the field of free space communication and multi-user quantum communication network, in particular to a quantum key distribution network system based on orbital angular momentum multiplexing

Background technique

Quantum key distribution is the fastest-growing and most mature technology in quantum communication technology, and it has become the pioneer of practical quantum communication because of its communication security. With the deepening of research, the difficulties of quantum key distribution technology have been continuously broken through, and quantum key distribution has the prospect of large-scale application. However, in order to realize a global quantum key distribution network, it is necessary to break through the limitation of distance.

At present, due to the limitations of optical fiber loss and the imperfection of detectors, the distance of quantum key distribution using optical fiber as a channel has basically reached the limit, and the loss of light in the atmosphere is smaller than that in optical fibers, so in order to achieve long-distance The development of free-space quantum key distribution network is the only way to realize global quantum communication.

In free space communication, the atmosphere has different losses for different wavelengths of light, and the attenuation coefficient will also change with changes in meteorological conditions, so it is necessary to select a suitable light source. Secondly, light propagates approximately along a straight line in a homogeneous atmosphere, and it cannot be transmitted along an arbitrary path as it propagates in an optical fiber. In addition, the distribution of obstacles in space, as well as the air density and its gradient in the atmosphere must also be considered. The impact of changes on the light propagation path, so it is necessary to design a suitable optical path and aiming device. In addition, the spatial channel means that the channel cannot be isolated from the external environment, and the ambient background light will also enter the channel and be received by the detector, thereby causing interference to the detection process, so it is necessary to select a suitable receiving device to remove the influence of external stray light.

The birefringence effect of the atmosphere is very small, which has little effect on the polarization state of the photon transmitted in the atmosphere, so the polarization state of the photon is suitable for free space communication. In addition to the polarization state of the photon, another degree of freedom of the photon, that is, when the orbital angular momentum rotates around the propagation direction, the orbital angular momentum state remains unchanged, and the sender and receiver do not need to adjust the reference frame in real time. Therefore, orbital angular momentum has become another choice for the physical carrier of quantum information in free space.

The current research on free-space quantum key distribution schemes is basically based on point-to-point user communication. To realize one-to-many or many-to-many communication schemes, the problem of routing and addressing of quantum information transmission must be solved. Orbital angular momentum (OAM), as a physical quantity describing the characteristics of helical wavefronts in optical vortex (OV) beams, provides a new degree of freedom for the multiplexing of quantum network communications in free space. When the amplitude function of the beam contains the azimuthal phase term , the light beam carries orbital angular momentum, where l is the eigenvalue of orbital angular momentum or the so-called topological charge. In theory, the value of l is infinite, which makes it possible to use the orbital angular momentum state as the carrier of channel multiplexing to carry signals for data transmission. No matter in the classical communication system or the quantum communication system, the orbital angular momentum channel has been proved to be absolutely safe. Orbital angular momentum beams, as the information carrier of orbital angular momentum multiplexing, provide a potential method for increasing the capacity of free space quantum communication systems. How to efficiently and non-destructively separate the orbital angular momentum of vortex beams is the key to its application in multiplexing communication networks. Premise, at present, the orbital angular momentum (OAM) photon state measurement scheme mainly has the following types:

1. The common holographic grating measurement method is to use the interference pattern of the vortex beam and the plane wave to make a hologram, and then binarize it to make an amplitude grating.

2. Use the spiral phase plate (Q-plate) or spatial modulator (SLM) to convert the OAM photon state of a specific order into Gaussian light, and only allow Gaussian light to pass through in the subsequent optical path, so as to achieve a specific order Separation Measurement of OAM Photon States.

3. Use conformal transformation to convert the OAM photon state into planar light, and then use the lens to focus the planar light corresponding to different orders of OAM photon states to different spatial positions, so as to realize the separate measurement of OAM photon states.

The above schemes either have very low separation efficiency, or destroy the original quantum state, and cannot realize cascading, and there is still great difficulty in separating the single photon orbital angular momentum at the quantum level. These shortcomings limit the OAM photon state in the orbital angular momentum complex with applications in quantum communications.

Therefore, the existing quantum communication methods must be further improved.

Utility model content

The purpose of the utility model is to overcome the deficiencies of the prior art and provide a quantum key distribution network system based on orbital angular momentum multiplexing. The system uses the photon polarization state to encode information, and the photon orbital angular momentum is used as a multiplexing addressing channel to realize quantum key distribution in free space.

The technical scheme of the utility model is achieved in that it comprises an Alice control terminal, an orbital angular momentum modulus multiplexing unit and a Bob user terminal, wherein:

The Alice control terminal includes a signal modulation unit and an orbital angular momentum multiplexing unit, and the orbital angular momentum multiplexing unit includes a spatial light modulator and an inverted Galileo telescope assembly;

The orbital angular momentum mode division multiplexing unit includes a Galileo telescope assembly and an orbital angular momentum separation device;

The Bob user end includes N Bob users, and each Bob user includes a polarization controller and a detection device;

The optical signal generated by the signal modulation unit enters the spatial light modulator, the inverted Galileo telescope assembly, the Galileo telescope assembly and the orbital angular momentum separation device in sequence, and then the orbital angular momentum separation device To determine the output of photons from different ports; the signal photons output by the orbital angular momentum separation device are transmitted to the polarization controller corresponding to the Bob user end, and finally enter the detection device for detection.

Preferably, the signal modulation unit includes a specific wavelength laser source, an attenuator, a polarization modulator, a control circuit and a random number generator, and the optical signal sent by the specific wavelength laser source is attenuated by the attenuator to single-photon level The optical pulse signal, and then the single photon enters the polarization modulator for polarization modulation, and the polarization modulator is driven by the random number generator to control the control circuit to control the polarization modulator to randomly change the single photon state The polarization is adjusted to 45-degree polarization state, 135-degree polarization state, left-handed circular polarization state and right-handed circular polarization state, and finally transmitted to the orbital angular momentum multiplexing unit.

Wherein, the laser wavelength output by the specific wavelength laser light source is 1550nm, and the power is 1mw; the attenuator can attenuate the optical signal to the single photon level; the random number generator is a kind of random number used to generate a binary sequence device. At the initial moment, the optical signal generated by the laser light source enters the attenuator and is attenuated into a weakly coherent single photon pulse. At the same time, the random number generator generates a random number to drive the control circuit. The control circuit controls the polarization modulator according to the random number, and the attenuated single photon is randomly Adjusted to 45 degree polarization state, 135 degree polarization state, left-handed circular polarization state and right-handed circular polarization state, and finally transmitted to the orbital angular momentum multiplexing unit. The specific wavelength laser light source, attenuator, polarization modulator, control circuit and random number generator combine to complete the encoding process of signal photons.

The spatial light modulator is a pure phase reflective liquid crystal spatial light modulator, which is an active digital optical device based on the electro-induced birefringence effect of liquid crystal molecules, and has low voltage, low power consumption, miniaturization, light weight, With the characteristics of energy saving and high density, it has the advantages of high diffraction efficiency, simple and convenient control, and flexible transformation in terms of the orbital angular momentum of the modulated beam. It can simultaneously modulate multiple different orbital angular momentums to realize the multiplexing of optical signal beams. The generated orbital angular momentum corresponds to the number of users at the receiving end, and can increase with the expansion of the number of users; the inverted Galileo telescope assembly is composed of two confocal convex lenses, and the laser beam is a Gaussian beam with a certain far-field divergence angle. The far-field divergence angle of the compressed laser beam is compressed by the inverted Galileo telescope assembly, and the collimated beam is then emitted into free space;

The Galileo telescope assembly is mainly composed of a hyperboloid convex lens, located at the signal photon receiving end, and mainly functions as an optical antenna and a space filter to filter out stray light in space.

The orbital angular momentum separation device includes several cascaded M-Z interferometers, each M-Z interferometer includes an incident port, a first beam splitter, a first reflector, first and second Duff prism modules, and a second beam splitter device, an exit reflection port, an exit transmission port and a second mirror;

After the signal photons enter the spatial light modulator, they pass through two confocal convex lenses and a hyperbolic convex lens in sequence, and then enter the M-Z interferometer from the incident port, and then enter the first beam splitter to form the first optical path and The second optical path, wherein the first optical path path is: a part of signal photons directly enters the second reflector through the first Duff prism module; the second optical path path is: another part of signal photons enters the second reflector after passing through the first reflector The Duff prism module is then coupled with the signal photons reflected by the second mirror at the second beam splitter, and finally emitted from the outgoing reflection port and the outgoing transmission port respectively. Specifically, the orbital angular momentum separation device determines that the photons are output from different ports according to the different orbital angular momentums carried by the photons, and the first beam splitter and the second beam splitter are 50:50 beam splitters.

Preferably, the first and second Duff prism modules respectively constitute a beam rotator, and keep the polarization state of the signal photons unchanged; the first and second Duff prism modules have the same structure and include sequentially connected The Duff prism, the first quarter-wave plate and the first half-wave plate, and the first and second Duff prism modules are connected in parallel, and the first and second Duff prism modules connected in parallel are referred to as improved hereinafter Type DAF prism module.

Preferably, several M-Z interferometers are cascaded and connected sequentially from front to back, and the exit reflection port and exit transmission port of any preceding M-Z interferometer are respectively connected to the subsequent M-Z interferometer through the incident port of the M-Z interferometer.

When the relative angle of the two Duff prisms in the two optical paths of the M-Z interferometer is α/2, the function of the Duff prisms is equivalent to adding a beam rotator with a rotation angle of α to one of the optical paths. The Duff prism makes the photon with the orbital angular momentum 1 produce a phase difference of φ=lα in the two optical paths of the interferometer, and acts as a 1/4 wave plate for the photon spin angular momentum (polarization), and the photon passes through the Duff The prism will change its polarization state. It is necessary to add the first 1/4 wave plate to compensate the change of polarization, and then pass the first half wave plate to keep the polarization direction of the photon consistent with the original one. The Duff prism plays a role of rotating the phase of the photon orbital angular momentum, and plays a role of maintaining the polarization of the photon polarization state.

Specifically, the effect of the M-Z interferometer on the input photon is described as follows:

Let the photon state incident to the input port of the M-Z interferometer and enter the first beam splitter (BS1) be:

|in> _BS1 = |0>|1>

|0> represents the vacuum state, |1> represents the single photon state, the same below. After the action of the first beam splitter (BS1), the output photon state is:

The above formula shows that the probabilities of photons being output from both the transmission end and the reflection end of the first beam splitter (BS1) are 50%, but a 90-degree phase jump is added when output from the reflection end. After the photons pass through the Duff prism module, the two optical paths produce a phase difference of φ=lα, then the input photon state of the second beam splitter (BS2) at the output end of the M-Z interferometer is:

The photon state after the action of the second beam splitter (BS2) becomes:

|out> _BS2 ＝1/2(1-e ^iφ )|0>|1>+i/2(1+e ^iφ )|1>|0>

The above formula shows that the photon interferes in the second beam splitter (BS2), and the phase of the photon changes, where φ=lα, assuming that the relative angle of the Duff prism module is π:

When the orbital angular momentum order l is odd, the second beam splitter (BS2) photon output state |out> _BS2 becomes: |0>|1>, and the photon is transmitted from the beam splitter.

When the orbital angular momentum order l is even, the photon output state |out> _BS2 of the second beam splitter (BS2) becomes: i|1>|0>, and the photon is reflected from the beam splitter.

Specifically, the effect of the Duff prism on the polarization state of the photon is equivalent to a 1/4 wave plate, so a 1/4 wave plate must be introduced behind the Duff prism to compensate the polarization state of the photon, and then the polarization direction of the photon is kept the same as the original unanimous.

Taking the 45° polarization state of photons |45°> as an example, the effects of Duff prism, 1/4 wave plate and half wave plate on the polarization state of photons are described. The specific process can be described as:

The 45 degree polarization state can be described as: Left-handed circularly polarized light: 135 degree polarized light: The Duff prism acts on the 1/4 wave plate of the photon polarization state, and the Jones matrix of the 1/4 wave plate transformation is: The Jones matrix transformed by the fast axis of the half-wave plate in the X-axis direction is:

450 polarization state photons are transformed into:

450 polarized photons pass through the Duff prism polarization state into left-handed circularly polarized light, and the left-handed circularly polarized light is transformed into:

1350 polarization state photons are obtained, and finally the photon polarization state is obtained through the transformation of the half-wave plate:

After the 45-degree polarization state photon passes through the Duff prism module, the polarization state of the photon remains unchanged, and it has a polarization-maintaining effect on the encoded signal photon.

To make the cascaded MZ interferometers separate arbitrary orbital angular momentum values, the relative phase of the Duff prism should be adjusted to π/2 ^j-1 , where j is denoted as the jth cascaded MZ interferometer, and each cascaded MZ The specific exit port of the interferometer introduces a hologram with orbital angular momentum Δl=j (j is the j-th cascaded MZ interferometer) to change the orbital angular momentum of the outgoing photons to meet the next-level interference conditions. The photons are output from the corresponding output port of the cascaded MZ interferometer according to the different orbital angular momentum carried, automatic routing and addressing, the separation efficiency of the orbital angular momentum is 100%, and the orbital angular momentum of the photon will not be destroyed. The MZ interferometer adds 1/4 wave The plate and the half-wave plate can keep the polarization information encoded by the photon unchanged during the transmission process, and the entire separation device uses passive devices, which are easy to integrate, can efficiently and quickly separate the orbital angular momentum, and improve communication efficiency. The orbital angular momentum state corresponds to the corresponding user, and can be expanded with the number of users to realize one-to-many quantum network communication.

Preferably, the Bob user end includes N Bob users, and each Bob user is respectively connected to any one of the outgoing reflection port and outgoing transmission port of the last stage M-Z interferometer in the multi-stage cascaded M-Z interferometer.

Preferably, each Bob user includes a polarization controller and a detection device, and the detection device further includes a third beam splitter, a second quarter-wave plate, a second, a third half-wave plate, a first, a second polarizing beam splitter and first to fourth detectors;

The signal photons are split at the third beam splitter: a part of the signal photons pass through the second half-wave plate and then enter the first polarizing beam splitter for splitting and finally enter the first detector and the second detector for detection ; Another part of the signal photons sequentially pass through the second quarter-wave plate and the second half-wave plate and then enter the third polarization beam splitter for beam splitting and finally enter the third detector and the fourth detector for detection .

Compared with the prior art, the beneficial effects of the utility model are:

1. The utility model is a free-space quantum communication network solution based on photon polarization encoding, which breaks through the birefringence effect of optical fiber communication and the limitation of optical fiber distance, and the polarization state of free-space photons can be transmitted stably, enabling long-distance communication.

2. The utility model uses the orbital angular momentum as an addressing channel, and the orthogonal characteristic of the orbital angular momentum enables the information carried by the coaxial OV light beam to be transmitted in free space without interference from the orbital angular momentum channel; the topology of the orbital angular momentum charge l and azimuth The uncertain relationship between them makes it very safe to use the orbital angular momentum to carry information; the orbital angular momentum can be infinitely valued, and any amount of information can be reused by adjusting the photon orbital angular momentum through the spatial light modulator. Momentum corresponds to a client, can expand with the increase of clients, and can communicate with any number of users.

3. The utility model uses the M-Z interferometer to separate the orbital angular momentum, and the two optical paths of the M-Z interferometer join the first and second Duff prism modules. The M-Z interferometer can separate the orbital angular momentum at the single-photon level, and can be cascaded to separate any multi-photon orbital angular momentum, with a separation efficiency of 100%. The two optical paths can be measured without destruction after adding the first and second Duff prism modules. The orbital angular momentum of the photon can keep the polarization state of the signal photon unchanged.

4. The Alice control terminal can realize one-to-many communication with the free space quantum network communication of the Bob user terminal, and the users are relatively independent, and the number of users can be expanded by the increase of orbital angular momentum multiplexing, which has good scalability and High implementability.

Description of drawings

Figure 1 shows the structure of the Duff prism module.

Figure 2 shows the structure of the M-Z interferometer with two optical paths joined to the first and second Duff prism modules.

Fig. 3 is a two-stage cascaded M-Z interferometer structure separating four orbital angular momentum state photons.

Figure 4 shows the system structure of quantum key distribution network based on orbital angular momentum multiplexing.

Fig. 5 is a schematic diagram of a cascaded device for separating photons in arbitrary orbital angular momentum states.

Fig. 6 is a schematic diagram of the quantum key distribution process based on orbital angular momentum multiplexing.

Detailed ways

Below in conjunction with accompanying drawing, the specific embodiment of the present utility model will be further described.

Referring to FIG. 1 , it is a Duff prism module structure, including a Duff prism 101 , a first 1/4 wave plate 102 and a first half wave plate 103 . The Duff prism module rotates the phase of the photon in the orbital angular momentum (OAM) state. The Duff prism acts as a 1/4 wave plate for the polarization state of the photon. The photon will change its polarization state when it passes through the Duff prism. A 1/4 wave must be added The polarization change is compensated by a half-wave plate, and then the polarization direction of the photon is kept consistent with the original through the half-wave plate. The action process of the Duff prism module on the photon polarization state can be described by the following process:

The 45-degree polarization state photon is transformed into:

The 45-degree polarized photon becomes left-handed circularly polarized light through the polarization state of the Duff prism, and the left-handed circularly polarized light is transformed into:

135-degree polarization state photons are obtained, and finally the 45-degree photon polarization state is obtained through the transformation of the half-wave plate 103:

After the 45-degree polarization state photon passes through the first and second Duff prism modules, the polarization state of the photon remains unchanged, and has a polarization-maintaining effect on the encoded signal photon.

Referring to Figure 2, it is an M-Z interferometer structure with an improved Duff prism module 204, wherein the first and second Duff prism modules are identical in structure and connected in parallel. Each M-Z interferometer structure includes an entrance port 201, a first beam splitter 203 at the entrance port, a first mirror 202, a modified Duff prism module 204 (including the first and second Duff prism modules), and a second exit port A beam splitter 206 , an exit reflection port 205 , an exit transmission port 207 , and a second reflection mirror 208 . Wherein, the first beam splitter 203 and the second beam splitter 206 are 50:50 beam splitters.

The Duff prisms of the two optical paths of the M-Z interferometer are combined into a beam rotator. When the relative angle of the Duff prisms of the two optical paths of the M-Z interferometer is α/2, the function of the Duff prism is equivalent to adding A beam rotator with a rotation angle of α, a photon with an orbital angular momentum of 1 is incident on the M-Z interferometer to generate a phase difference of φ=lα.

The effect of the M-Z interferometer on the input photon is described as follows:

Suppose the photon state incident to the first 50:50 beam splitter (BS1) 203 at the input port of the M-Z interferometer is:

|in>BS1＝|0>|>

|0> means vacuum state, |1> means single photon state, the same below. After the action of the first beam splitter 203, the output state is:

The above formula indicates that the probabilities of photons being output from both the transmission end and the reflection end of the first beam splitter 203 are 50%, but a 90-degree phase jump is added when output from the reflection end. After the photon passes through the action of the Duff prism 101, the two optical paths produce a phase difference of φ=lα, then the input state of the second beam splitter (BS2) 206 at the output end of the M-Z interferometer is:

After the action of the second beam splitter 206, it becomes:

The above formula shows that the photon interferes in the second beam splitter (BS2) 206, and the phase of the photon changes, where φ=lα, assuming that the relative angle of the Duff prism is π:

When the orbital angular momentum order l is an odd number, the photon output state |out> _BS2 of the second beam splitter (BS2) 206 becomes: |0>|1>, and the photons are emitted from the transmission port 207 of the beam splitter.

When the orbital angular momentum order l is an even number, the photon output state |out> _BS2 of the second beam splitter (BS2) 206 becomes: i|1>|0>, and the photon is emitted from the reflection port 207 of the beam splitter.

Referring to Figure 3, it is an M-Z interferometer structure that separates four orbital angular momentum (OAM) state photons in a two-stage cascade. Including a multiplexing input port 301, a first-stage M-Z interferometer 302, a second-stage separation even-numbered OAM state photon M-Z interferometer 303, a second-stage separation odd-number OAM state photon M-Z interferometer 304, and output ports 305, 306, 307 and 308 . Assuming that photons with orbital angular momentum l=1, 2, 3 and 4 are incident from the incident port 301, the relative angle of the Duff prisms of the first-stage M-Z interferometer is π, and the relative angle of the Duff prisms of the second-stage M-Z interferometer is π /2, the first-stage M-Z interferometer separates even and odd OAM state photons, and the photons with orbital angular momentum 1=2 and 4 enter the M-Z interferometer 303 that separates even-numbered OAM state photons, and the photons with orbital angular momentum 1=1 and 3 enter M-Z interference Instrument 304, photons with different orbital angular momentum pass through the M-Z interferometer with different phase changes, the photon with orbital angular momentum l=4 is emitted from port 305, the photon with l=2 is emitted from port 306, and the photon with l=3 is emitted from port 308 The photon of l=1 is emitted from port 307.

Referring to accompanying drawing 4, it is a quantum key distribution network system structure based on orbital angular momentum multiplexing. Including Alice control terminal 438, orbital angular momentum mode division multiplexing unit 441 and Bob user terminal 443, wherein:

The Alice control terminal includes a signal modulation unit 439 and an orbital angular momentum multiplexing unit 440 . The signal modulation unit includes a specific wavelength laser light source 401, an attenuator 402, a polarization modulator 403, a control circuit 404, and a random number generator 405; the orbital angular momentum multiplexing unit includes a spatial light modulator 413 and an inverted Galileo telescope Component 414.

The orbital angular momentum mode division multiplexing unit 441 includes a Galileo telescope assembly 416 and an orbital angular momentum separation device 442 . The Galileo telescope assembly 416 includes a hyperboloid convex lens; the orbital angular momentum separation device consists of cascaded M-Z interferometers 417 , 419 and 420 . Taking four users as an example, the specific structure of the orbital angular momentum separation device 442 is the same as that of the M-Z interferometer structure of the two-stage cascaded separation of four OAM state photons in FIG. Corresponds to 304.

The Bob client includes N Bob users. Taking four users as an example, the Bob client includes users 423, 424, 425 and 426, and each Bob user includes a polarization controller 427 and a detection device 444; the detection device includes Third beam splitter 428, second 1/4 wave plate 433, second half wave plate 429, 434, first and second polarizing beam splitter 430, 435, first to fourth detectors 431, 432, 436 and 437.

The quantum key distribution process is specifically described below with reference to accompanying drawing 4:

The signal modulation unit 439 in the Alice control terminal generates a binary random sequence code according to the random number generator 405 to drive the control circuit 404 to control the polarization modulator 403 to randomly adjust the polarization state of the single photon. Its modulation process can be described as: random number "0" means diagonal base, "1" means circular base; "0" means 45-degree polarization state and left-handed circular polarization state, "1" means 135-degree polarization state and right-handed circular polarization state. The random number "00" indicates that the polarization modulator 403 modulates the single photon into the 45-degree polarization state |45°>, "01" indicates that the polarization modulator 403 modulates the single photon into the 135-degree polarization state |135°>, and "10" indicates The polarization modulator 403 modulates the single photon into a left-handed circular polarization state |L>, and "11" indicates that the polarization modulator 403 modulates the single photon into a right-handed circular polarization state |R>. Alice records the random code sequence of modulation information;

The encoded signal state photons enter the spatial light modulator 413 to modulate the orbital angular momentum. The spatial light modulator is controlled by a computer and can modulate any orbital angular momentum state for multiplexing. Each orbital angular momentum corresponds to the corresponding user of the Bob user terminal 443. The photons modulated by the spatial light modulator can establish a quantum channel with the corresponding user for communication;

OAM state photons enter the inverted Galileo telescope assembly 414, the inverted Galileo telescope assembly 414 is made up of two confocal convex lenses, the laser beam is a Gaussian beam with a certain far-field divergence angle, and the inverted Galileo telescope assembly compresses the laser beam The far-field divergence angle of the collimated light beam is then emitted into the free space; the receiving end is the orbital angular momentum demultiplexing unit 441, and the signal state photons in the free space are first received by the Galileo telescope assembly 416, and the Galileo telescope assembly 416 is mainly composed of Composed of hyperbolic convex lenses, it mainly functions as an optical antenna and a space filter to filter out stray light in space; OAM state photons enter the orbital angular momentum separation device 442 through the Galileo telescope assembly 416 .

Taking the two-stage cascade separation of four OAM states as an example to illustrate the automatic channel addressing process of OAM state photons: the specific structure of the orbital angular momentum separation device 442 and the M-Z of the two-stage cascade separation of four OAM state photons in Figure 3 The interferometer structure is the same, wherein the M-Z interferometer 417 corresponds to the interferometer 302, 419 corresponds to 303, and 420 corresponds to 304, and the orbital angular momentum separation device 442 can realize communication with the four users corresponding to the Bob end, assuming that the orbital angular momentum multiplexing unit 440 The photon orbital angular momentum modulated by the spatial light modulator 413 is l=1, 2, 3 and 4, then the photon with orbital angular momentum l=4 enters the user end 423, the photon of l=2 enters the user end 424, and the photon of l=3 The photon enters the user terminal 425, the photon of l=1 enters the user terminal 426, and the OAM state photon automatically addresses and enters the corresponding user according to the orbital angular momentum carried, which is efficient and will not destroy the information encoded by the signal photon; the Bob user terminal 425 As an example to describe the detection process of signal photons, 427 is a polarization controller, which maintains the polarization state of signal photons, and photons enter the 50:50 third beam splitter 428 to randomly select one path for transmission, assuming that the horizontal transmission path is 45°|45°>or 135°|135°> polarization state photon, 45° polarization state photon enters the half-wave plate 429 and is rotated into a horizontal polarization state |H>, enters the polarization beam splitter 430 and transmits horizontally, and the first detector 431 responds, which is recorded as " 0", the 135-degree polarization state photon enters the half-wave plate 429 and is rotated into a vertical polarization state |V>, enters the polarization beam splitter 430 and reflects vertically, and the second detector 432 responds, which is recorded as "1", if it is Circularly polarized light is transmitted horizontally through the 50:50 beam splitter 428, and the first and second detectors 431 and 432 respond randomly; assuming that photons enter the 50:50 beam splitter 428 and reflect vertically one way is left-handed |L> and right-handed| R>circularly polarized light, left-handed |L> polarized photon enters the 1/4 wave plate 433 and is rotated into 45-degree polarized light, and then rotates into a horizontally polarized state |H> through the half-wave plate 434, after entering the polarization beam splitter 435 Transmission, the response of the fourth detector 437 is recorded as "0"; right-handed |R> polarized photons enter the 1/4 wave plate 433 and are rotated into 135-degree polarized light, and then rotated into a vertically polarized state by the half-wave plate 434 |V>, reflect after entering the polarization beam splitter 435, the third detector 436 responds, and record it as "1", if the photon enters the 50:50 beam splitter 428 and reflects one path of vertically polarized light of 45 degrees and 135 degrees, then The third and fourth detectors 436 and 437 respond randomly. According to the recorded detector response information recorded by the Bob client and the random code information recorded by Alice, the information is compared through the open channel, and the final key is obtained after screening and post-processing. The above is the whole process of quantum key distribution.

Referring to Fig. 5, in order to separate the schematic diagram of photon cascade device in any OAM state, to realize communication with any multi-user, the MZ interferometer shown in Fig. 2 must be cascaded in multiple stages. The first stage cascade unit 501 shown in Fig. 5 separates even number and odd number OAM state photons, the rotation angle of the Duff prism 101 in the MZ interferometer of the first stage is set to π, and the hologram 502 changes OAM to be Δl=1; The second stage has four output ports, the rotation angle of Duff prism 101 is set to π/2, and the holograms 503 and 504Δl=2; the third level has 8 output ports, and the rotation angle of Duff prism 101 is set to π/2 ² , the third-level hologram is set to Δl=3. The n-level cascade has 2 ⁿ outgoing ports, which can communicate with 2 ⁿ Bob clients. To make the cascaded MZ interferometer separate any orbital angular momentum value, the relative phase of the Duff prism should be adjusted to π/ 2 ^j-1 , j represents the j-th cascaded MZ interferometer, each cascaded MZ interferometer’s specific output port introduces a hologram with orbital angular momentum Δl=j (j is the j-th cascaded MZ interferometer) To change the orbital angular momentum of the outgoing photon to meet the next level of interference conditions. The photons are output from the corresponding output port of the cascaded MZ interferometer according to the different orbital angular momentum carried, automatic routing and addressing, the separation efficiency of the orbital angular momentum is 100%, and the orbital angular momentum of the photon will not be destroyed. The MZ interferometer adds 1/4 wave The plate and the half-wave plate can keep the polarization information encoded by the photon unchanged during the transmission process, and the entire separation device uses passive devices, which are easy to integrate, can efficiently and quickly separate the orbital angular momentum, and improve communication efficiency.

Referring to Figure 6, the quantum key distribution method based on orbital angular momentum multiplexing includes the above-mentioned quantum key distribution network system based on orbital angular momentum multiplexing. It includes the following steps:

S1. System initialization: check Alice's control terminal and Bob's user terminal, check whether each component is operating normally, debug the spatial light modulator, and set the initial conditions of each device;

S2. Signal modulation unit test: the random number generator generates a binary random code to drive the control circuit, the control circuit controls the polarization modulator to adjust the polarization state of the photons, and checks whether the control circuit and the polarization controller are working normally.

S3. Orbital angular momentum multiplexing test: the photon signal is emitted by the weakly coherent light source, after polarization modulation by the four polarizers, it is coupled to the spatial light modulator through a beam splitter, and the photon signal is tracked Angular momentum multiplex modulation, resulting in the multiplexing of multiple different orbital angular momentum;

S4. Orbital angular momentum demultiplexing test: the orbital angular momentum multiplexing beam is demultiplexed through the orbital angular momentum separation device, and the orbital angular momentum separation device selects from the corresponding port according to the different orbital angular momentum carried by the photon shoot out

S5. System noise test: under the premise that the weakly coherent light source does not emit light signals, that is, when the number of pulses is zero, test the system noise level;

S6. Key transmission: the optical signal generated by the specific wavelength laser light source enters the attenuator and is attenuated into a weakly coherent single photon pulse. At the same time, the random number generator generates a random number to drive the control circuit. The control circuit controls the polarization modulator according to the random number, and the The attenuated single photon is randomly adjusted to a 45-degree polarization state, a 135-degree polarization state, a left-handed circular polarization state, and a right-handed circular polarization state. Alice records the random code sequence of modulation information;

S7. Key screening and coding: the Bob client records the detection event according to the detector response in the detection device, and then the Bob client responds according to the recorded detector response information and the random code information recorded by Alice Information comparison is performed through open channels to form a screening key; then some data is randomly selected in the screening key, and whether there is eavesdropping is judged by calculating the error rate of the information, and then data coordination is performed on the screening key again, before and after information coordination Alice and Bob's mutual information will remain unchanged, and finally the data will be amplified confidentially through the confidentiality amplification protocol to obtain the final security key.

Claims (7)

1. the quantum key distribution network system based on orbital angular momentum multiplexing, it is characterised in that including Alice control terminals, rail Road angular momentum mode division multiplexing unit and Bob user terminals, wherein：

The Alice control terminals include signal modulation unit and orbital angular momentum Multiplexing Unit, and the orbital angular momentum multiplexing is single Member includes spatial light modulator and is inverted Galilean telescope mirror assembly；

The orbital angular momentum mode division multiplexing unit includes Galilean telescope mirror assembly and orbital angular momentum separator；

The Bob user terminals include N number of Bob user, and each Bob user includes Polarization Controller and detection device；

The optical signal that the signal modulation unit produces sequentially enters the spatial light modulator, is inverted Galilean telescope microscope group Part, Galilean telescope mirror assembly and orbital angular momentum separator, then the orbital angular momentum separator taken according to photon The different orbital angular momentums of band determine that photon is exported from different port；The letter that the orbital angular momentum separator is exported Number photon is sent in the Polarization Controller of corresponding Bob user, is entered finally into the detection device and is detected.

2. the quantum key distribution network system as claimed in claim 1 based on orbital angular momentum multiplexing, it is characterised in that institute Stating signal modulation unit includes specific wavelength laser light source, attenuator, light polarization modulator, control circuit and randomizer, The laser light source of the specific wavelength sends the light pulse signal that optical signal decays to single photon level through the attenuator, then Single photon enters and Polarization Modulation is carried out in the light polarization modulator, and the light polarization modulator is driven by the randomizer The control circuit and then control the light polarization modulator that the polarization of single-photon state randomly is adjusted to 45 degree of polarization states, 135 Polarization state, Left-hand circular polarization state and right-hand circular polarization state are spent, is finally delivered in the orbital angular momentum Multiplexing Unit.

3. the quantum key distribution network system as claimed in claim 2 based on orbital angular momentum multiplexing, it is characterised in that institute Stating inversion Galilean telescope mirror assembly includes two confocal convex lenses, and the Galilean telescope mirror assembly includes a hyperboloid Convex lens；The orbital angular momentum separator includes the M-Z interferometers of some cascades, each M-Z interferometers include into Port is penetrated, the first beam splitter, the first speculum, the first and second dove prism modules, the second beam splitter, is emitted reflector port, It is emitted transmission port and the second speculum；

After signal photon enters spatial light modulator, two confocal convex lenses and hyperboloid convex lens are sequentially passed through, then Entered by entry port in the M-Z interferometers, then into forming the first light path and the second light path at the first beam splitter, Wherein the first optical circuit path is：A part of signal photon directly enters the second speculum by the first dove prism module；The Two optical circuit paths are：Another part signal photon after the first speculum by entering back into the second dove prism module, Ran Houyu The signal photon that second speculum reflects back couples at second beam splitter, finally reflector port and goes out from outgoing respectively Penetrate transmission port injection.

4. the quantum key distribution network system as claimed in claim 3 based on orbital angular momentum multiplexing, it is characterised in that institute Stating the first and second dove prism modules includes sequentially connected dove prism, the first quarter-wave plate and the first half-wave Piece.

5. the quantum key distribution network system as claimed in claim 3 based on orbital angular momentum multiplexing, it is characterised in that if Dry M-Z interferometers cascade is sequentially connected from front to back, and the outgoing reflector port of the M-Z interferometers of any one prime and outgoing Transmission port is connected by the entry port of M-Z interferometers and the M-Z interferometers of rear class respectively.

6. the quantum key distribution network system as claimed in claim 1 based on orbital angular momentum multiplexing, it is characterised in that institute Stating Bob user terminals includes N number of Bob user, and each Bob user does with afterbody M-Z in the M-Z interferometers of multi-stage cascade respectively The outgoing reflector port of interferometer is connected with any one port of outgoing transmission port.

7. the quantum key distribution network system as claimed in claim 6 based on orbital angular momentum multiplexing, it is characterised in that every A Bob user includes Polarization Controller and detection device, and the detection device includes the 3rd beam splitter, the second quarter-wave again Piece, second, third half-wave plate, first, second polarization beam apparatus and first to fourth detector；

Signal photon carries out branch at the 3rd beam splitter：A part of signal photon is by entering after the second half-wave plate One polarization beam apparatus punishment beam enters finally into be detected in the first detector and the second detector；Another part signal photon Pass sequentially through enter after second quarter-wave plate and the second half-wave plate the 3rd polarization beam apparatus be split it is most laggard Enter into the 3rd detector and the 4th detector and detected.