A multi-user quantum key distribution full-connection network system based on a multi-functional integrated optical quantum chip

Abstract

The present invention discloses a multi-user quantum key distribution full-connection network system based on a multi-functional integrated optical quantum chip, including: a multi-degree-of-freedom integrated high-dimensional entangled photon pair source module at the network service provider side, a mode configuration module based on mode division multiplexing technology, a mode demultiplexing module, an encoding / decoding module, and a photon detection module at the user side. The present invention can further improve the key rate and also facilitate the construction of a quantum communication network based on the existing optical fiber communication network.

Description

Technical Field

[0001] The present invention relates to the field of quantum information technology, and in particular to a multi-user quantum key distribution full-connection network system based on a multi-functional integrated optical quantum chip. Background Art

[0002] As a new generation of communication technology, Quantum Key Distribution (QKD) utilizes fundamental principles of quantum mechanics such as the "Heisenberg uncertainty principle" and the "quantum no-cloning theorem" to ensure "unconditional security" in theory during communication, and already has a certain ability to be deployed in actual application scenarios. It can achieve short-distance communication through optical fibers, long-distance communication through quantum repeaters, or ultra-long-distance communication covering the globe through satellite relays. However, most current implementations and protocols are limited to two communication parties, which to a certain extent restricts the large-scale practical application of quantum key distribution.

[0003] Quantum communication networks aim to connect numerous users, and their development is crucial for secure communication and information transmission among multiple users. Currently, the main types of quantum communication network solutions studied and implemented are as follows: (1) Networks based on quantum repeaters use quantum memories and entanglement swapping to expand and construct quantum communication networks. However, the storage duration and storage efficiency of existing quantum memories still need to be improved technically; (2) Networks based on trusted relay nodes require all relay nodes on the link connecting the terminal user nodes in the network to participate in the key distribution process. This solution requires all relay nodes to be trusted and equipped with receiving and transmitting devices simultaneously, which threatens the security of the entire network and also increases resource and maintenance costs; (3) Point-to-multipoint active / passive routing networks based on passive beam splitters, active optical switches, and time / frequency multiplexing, etc. This solution allows multiple users to share light sources or receiving devices, but only allows certain users to exchange keys at a time; (4) Full-connection quantum networks based on high-dimensional or multi-body entanglement, where multiple users share entanglement resources and are connected to any other user in the network simultaneously through entanglement distribution. The expansion of this solution requires a relatively high system dimension and convenient configuration. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a multi-user quantum key distribution full-connection network system based on a multi-functional integrated optical quantum chip, which can further improve the key rate and also facilitate the construction of a quantum communication network based on the existing optical fiber communication network.

[0005] To solve the above technical problems, the present invention provides a multi-user quantum key distribution full-connection network system based on a multi-functional integrated optical quantum chip, including: a multi-degree-of-freedom integrated high-dimensional entangled photon pair source module at the network service provider end, a mode configuration module based on mode division multiplexing technology, a mode demultiplexing module, an encoding / decoding module, and a photon detection module at the user end; the mode configuration module based on mode division multiplexing technology at the network service provider end is used to distribute the correlated modes in the entangled photon pair source to different users through a certain combination method, and this mode configuration needs to satisfy that at least one pair of modes that are correlated or entangled in a certain degree of freedom are shared between any pair of users; the mode demultiplexing module at the user end is used to demultiplex the received modes, and each demultiplexed mode passes through an encoding / decoding module and a group of photon detection modules respectively; the encoding / decoding module is used to encode or decode the photons of different modes received by the user; the photon detection module is used to detect the photons encoded / decoded at the user end.

[0006] Preferably, the multi-degree-of-freedom integrated high-dimensional entangled photon pair source module includes a laser, an electro-optical modulator, an optical filter, a polarization controller, and a pump functional integrated optical quantum chip; the output of the laser is connected through an optical fiber or free-space coupled and sequentially passes through an electro-optical modulator, an optical filter, a polarization controller, etc. After specifically modulating the intensity, phase, frequency, and polarization properties of the laser, the nonlinear medium on the pump functional integrated optical quantum chip is pumped, and photon pairs with entanglement in certain degrees of freedom of the photons with a dimension M≥2 are generated through optical nonlinear effects. Its quantum state is written as:

[0007]

[0008] where m and m′ are a pair of correlated modes of a certain degree of freedom of the photon, and the degrees of freedom include time, frequency, phase, spin, path, polarization, orbital angular momentum; s mm′ is the amplitude, satisfying ∑|s mm′ | 2 =1; M is the dimension of the degree of freedom. According to different network topologies, for a full-connection network topology that does not use passive routing, M needs to satisfy 2M≥N(N - 1). For a network topology that uses a 1-to-k passive router to build N users into k subnets to achieve full connection, M needs to satisfy or 2M≥k 2 +k, to ensure that at least one pair of correlated modes are shared between any two users of this light source, so that at least one pair of modes that are correlated or entangled in a certain degree of freedom are shared between any two users in the communication network, thereby constructing a simultaneously full-connection quantum communication network.

[0009] Preferably, the nonlinear medium includes magnesium fluoride, silicon, silicon dioxide, silicon nitride, aluminum nitride, highly doped quartz glass, and lithium niobate.

[0010] Preferably, the entanglement methods include path entanglement, polarization entanglement, energy-time entanglement, and time-bin entanglement.

[0011] Preferably, in the mode configuration module based on mode division multiplexing technology, to achieve a full-connection network topology without using passive routing for pairwise communication among N users, the number of modes required is or pairs of modes; then the first step is to demultiplex the N(N - 1) modes in a certain degree of freedom of the multi-degree-of-freedom integrated high-dimensional entangled photon pair source module through a multi-channel mode division multiplexer, that is, to achieve mode demultiplexing from 1 to N(N - 1). To ensure that at least one pair of modes is shared pairwise among N users, each user needs to receive (N - 1) modes and share a pair of associated modes (m and m′) with any other user; the corresponding second step is to use N multi-channel mode division multiplexers to multiplex (N - 1) modes in a specific combination into one channel, that is, to achieve multiplexing from (N - 1) to 1; to achieve a full-connection network topology with passive routing for pairwise communication among N users, the consumption of passive routing mode resources of 1 to k is used.

[0012] Preferably, to achieve a full-connection network topology with passive routing for pairwise communication among N users, the consumption of passive routing mode resources of 1 to k includes two schemes: The first scheme divides N users into k sub-networks, and the users within each sub-network are fully connected through modes, and 1 to k passive routings are required. The k sub-networks are fully connected through modes, and 1 to k passive routings are required. That is, a total of 1 to k passive routings and modes are required; then in the mode configuration module based on mode division multiplexing technology, the first step is to demultiplex the modes in a certain degree of freedom of the multi-degree-of-freedom integrated high-dimensional entangled photon pair source module through a multi-channel mode division multiplexer, that is, to achieve mode demultiplexing from 1 to ; the second step is to use a 1 to k passive routing to divide each mode into k; the third step is to use N multi-channel mode division multiplexers to multiplex modes in a specific combination into one channel modes for communication among users within each sub-network, and (k - 1) modes for communication between users in different sub-networks), that is, to achieve multiplexing from to 1, to ensure that at least one pair of associated modes (m and m′) is shared between any two users among N users;

[0013] The second solution is to divide N users into k subnets. The k subnets are fully connected to each other through k(k - 1) modes and use a 1-to-k passive router to distribute to all users within the subnet, and the users within each subnet share the same pair of modes and use the same 1-to-k passive router to distribute to all users within the subnet to achieve the connection between users within the subnet. This network topology requires a total of k 2 + k modes and k passive routers; then the first step in the mode configuration module based on mode division multiplexing technology is to demultiplex the k 2 + k modes under a certain degree of freedom of the multi-degree-of-freedom integrated high-dimensional entangled photon pair source module through a multi-channel mode division multiplexer, that is, to achieve mode demultiplexing from 1 to k 2 + k; the second step is to divide each mode into k using a 1-to-k passive router; the third step is to use N multi-channel mode division multiplexers to multiplex (k + 1) modes in a specific combination into one channel (2k modes are used for communication between users within the subnet, and (k - 1) modes are used for communication between users in different subnets), that is, to achieve multiplexing from (k + 1) to 1, so as to ensure that any two users among the N users share at least one pair of associated modes (m and m′). The combination method needs to ensure that any pair of users share at least one pair of modes that are associated or entangled in a certain degree of freedom, so that quantum correlations can be established between every pair of users in the N-user quantum communication full-connection network.

[0014] Preferably, the encoding / decoding module adopts different encoding / decoding methods such as path encoding, polarization encoding, energy-time encoding, time-bin encoding, and phase encoding.

[0015] Preferably, the photon detection module adopts an avalanche diode single-photon detector, a superconducting nanowire single-photon detector, and a photon number / frequency-resolving single-photon detector.

[0016] Preferably, the network service provider distributes different mode combinations to different users through an optical fiber communication network. Since each user shares at least one pair of modes that are associated or entangled in a certain degree of freedom with the other N - 1 users, that is, quantum correlations are established between every pair of users, a full-connection network of N users is thus constructed.

[0017] Preferably, users use the entanglement-based or MDI-based entanglement time-reversal quantum key distribution protocol such as E91 and BBM92 for key distribution. The users respectively measure the allocated photons using specific basis vectors, and then remove the error codes in the original key and the information that may be leaked to eavesdroppers through data post-processing methods such as comparison, error correction, and privacy amplification, and a consistent secure key string can be obtained, thus realizing quantum key distribution among users. Without changing the fiber connection and system configuration, users can use this method to perform quantum key distribution with any other user, thereby constructing an N-user fully connected quantum key distribution network.

[0018] The beneficial effects of the present invention are as follows: At the network architecture level, the fully connected network architecture adopted by the present invention utilizes the quantum correlation between entangled photon pairs and only uses N(N - 1) modes (without using passive routing), or k 2 + k modes (using a 1-to-k passive router) to establish a full connection among N users;

[0019] At the resource consumption level, the present invention only requires the network provider to have a set of multi-degree-of-freedom high-dimensional entangled photon pair sources and mode configuration modules, and the integrated photonics chip based on CMOS manufacturing technology can conveniently, stably, and mass-produce such entangled light sources. Each user only needs to have a set of mode demultiplexing modules, encoding / decoding modules, and photon detection modules;

[0020] In terms of scalability, increasing or decreasing the number of users in the network only requires increasing the number of modes and adjusting the mode configuration scheme, without the need to provide additional entangled light source resources, while traditional point-to-point communication networks require a set of independent light sources, two sets of encoding / decoding modules, and photon detection modules for each pair of users;

[0021] The present invention connects each user to an entangled light source through a single optical fiber, so it can be well compatible with mode division multiplexing technologies commonly used in traditional optical fiber communication networks such as time division multiplexing (TDM), frequency division multiplexing (FDM), code division multiplexing (CDM), and wavelength division multiplexing (WDM). This can not only further improve the key rate but also facilitate the construction of a quantum communication network based on the existing optical fiber communication network. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the architecture of the multi-user quantum key distribution fully connected network system of the present invention.

[0023] Figure 2 It is an actual case diagram of a four-user fully connected quantum communication network of the present invention.

[0024] Figure 3For a four-user fully-connected quantum communication network instance based on the present invention, it is the classical transmission spectrum of the silicon nitride microring cavity on the functional integrated optical quantum chip, as well as the free spectral range and quality factor diagrams obtained from the transmission spectrum.

[0025] Figure 4 For a four-user fully-connected quantum communication network instance based on the present invention, it is the frequency comb-like single-photon spectrum generated by the spontaneous four-wave mixing effect in the silicon nitride microring cavity on the functional integrated optical quantum chip under the excitation of the pump light, and the variation diagram of the coincidence count of 6 pairs of photons selected therefrom with respect to the on-chip pump power.

[0026] Figure 5 For a four-user fully-connected quantum communication network instance based on the present invention, when the silicon nitride microring cavity on the functional integrated optical quantum chip undergoes the spontaneous four-wave mixing process under the excitation of the pump light, it is the variation of the accidental coincidence count (not generated by the spontaneous four-wave mixing process) of 6 pairs of photons with respect to the on-chip pump power, as well as the variation diagram of the signal-to-noise ratio of the light source calculated from the true coincidence count and the accidental coincidence count with respect to the on-chip pump power.

[0027] Figure 6 For a four-user fully-connected quantum communication network instance based on the present invention, it is the quantum correlation layer, physical topology layer, and wavelength configuration scheme diagram.

[0028] Figure 7 For a four-user fully-connected quantum communication network instance based on the present invention, when verifying the entanglement between four users, it is the variation diagram of three characteristic peaks of the Franson interference with respect to the phase (φ s +φ i ).

[0029] Figure 8 For a four-user fully-connected quantum communication network instance based on the present invention, it is the actual effect diagram of using the temporal degree of freedom of photons to distinguish different user combinations.

[0030] Figure 9 For the BBM92 communication protocol, when using the present invention to implement a multi-user fully-connected quantum communication network, it is the process diagram of realizing quantum key distribution and secure communication between any two users.

[0031] Figure 10 For a four-user fully-connected quantum communication network instance based on the present invention, when implementing a quantum key distribution network using the BBM92 protocol, it is the actual contrast ratio, quantum bit error rate, and final secure key rate diagrams when realizing key distribution among four users. Detailed implementation manners

[0032] Such as Figure 1As shown in the figure, a full - connected network system for multi - user quantum key distribution based on a multi - functional integrated optical quantum chip includes: a multi - degree - of - freedom integrated high - dimensional entangled photon pair source module at the network service provider end, a mode configuration module based on mode - division multiplexing technology, a mode demultiplexing module, an encoding / decoding module, and a photon detection module at the user end; the mode configuration module based on mode - division multiplexing technology at the network service provider end is used to distribute the correlated modes in the entangled photon pair source to different users through a certain combination method, and this mode configuration needs to satisfy that at least one pair of correlated or entangled modes in a certain degree of freedom are shared between any pair of users; the mode demultiplexing module at the user end is used to demultiplex at least N - 1 received modes, and each demultiplexed mode passes through an encoding / decoding module and a group of photon detection modules respectively; the encoding / decoding module is used to encode or decode photons of different modes received by users; the photon detection module is used to detect the photons encoded / decoded at the user end.

[0033] As Figure 2 shown, in the above - mentioned entangled light source module ( Figure 2 (a) in it), a coherent light source emits single - frequency laser light of a certain wavelength as pump light, which is coupled into a single - mode optical fiber. The fiber polarization controller is used to adjust the polarization of the pump light so that it matches a specific mode (here the TE00 mode) of the optical waveguide on the integrated photonics chip. The pre - filtering module consists of an adjustable band - pass filter or a wavelength - division multiplexer, and is used to suppress the spontaneous emission noise of the laser to the single - photon level to avoid affecting the subsequent generated broadband quantum light source. The pump light is coupled into the chip through a tapered fiber and coupled from the chip to the single - mode optical fiber. The post - filtering module consists of an adjustable band - pass filter or a wavelength - division multiplexer, and is used to suppress the residual pump light to the single - photon quantum level to avoid strong pump light damaging the single - photon detector at the user end.

[0034] In this example, the core layer medium of the optical waveguide in the functional integrated optical quantum chip is silicon nitride, the cladding layer medium is silicon dioxide, the width of the optical waveguide is 1600 nanometers, and the height is 800 nanometers. The main structure on the functional integrated optical quantum chip in this example is a single - bus micro - ring resonator ( Figure 2 (b) in it), and its specific structure is a straight waveguide of a certain length and a circular waveguide with a diameter of 460 microns at a certain distance from the straight waveguide. The light in the straight waveguide is coupled into the circular waveguide in the form of an evanescent wave and coupled from the circular waveguide to the straight waveguide in the same way. The micro - ring resonator in this example has a high quality factor, thus generating a very strong field enhancement effect in the cavity. The actually used micro - ring resonator in this example has as Figure 3The shown transmission spectrum shows that the average free spectral range (i.e., the interval between adjacent resonance modes) of this microring resonator is 97.8 GHz, which is very close to the channel interval of the standard 100 GHz dense wavelength division multiplexer (DWDM), and provides 128 resonance modes in the range of 1500 nm to 1600 nm, which can provide us with a rich selection of photon pairs in the telecommunication band.

[0035] Specifically, in this example, we use λ p = 1549.32 nm - corresponding to the central wavelength of the CH35 channel specified by the International Telecommunication Union (ITU) - as the pump light. The single-photon spectrum actually generated by the silicon nitride microring resonator in this example is as shown in Figure 4 (a). It can be seen that the spectrum covers the entire communication C-band (the shown spectral width is limited by our measurement instrument, rather than the spectrum itself). We label the pump light located in the ITU CH35 channel as λ0. To avoid the influence of the post-filtering part on the two wavelength channels adjacent to the pump light, we do not use the two channels CH34 and CH36, and label the photons generated in the wavelength channels CH37, CH38,..., CH42 as λ1, λ2,..., λ6 in sequence, and label the photons generated in the wavelength channels CH33, CH32,..., CH28 as λ -1 , λ -2 ,..., λ -6 in sequence. Without confusion and other instructions, the wavelength channel CHxx (xx is the channel number specified by the International Telecommunication Union) mentioned later in the text refers to the photons generated in this channel by the silicon nitride microring resonator under the action of the pump light. To obtain data such as the spectral brightness and signal-to-noise ratio of the broadband entangled light source module in this example, we perform coincidence counting measurements on the six pairs of channels CH37-CH33, CH38-CH32, CH39-CH31, CH40-CH30, CH41-CH29, and CH42-CH28 selected. The coincidence counts of each channel pair under different pump powers are as shown in Figure 4 (b), and the accidental coincidence and signal-to-noise ratio are as shown in Figure 5 . By fitting the measured results, we can obtain the coherence time, bandwidth, and pair generation rate (PGR) of different channel pairs of this light source module, as shown in Table 1. The above results show that this light source module has characteristics such as a wide spectral range, narrow-band comb linewidth, high signal-to-noise ratio, and high pair generation rate, demonstrating its application prospects in constructing multi-user quantum communication networks, connecting quantum memories in the communication band, etc.

[0036] Table 1 Coherence time, bandwidth, and photon pair yield of different channel pairs of the light source module

[0037]

[0038] In the wavelength channel configuration module of this example ( Figure 2 (c) in), the wavelength demultiplexing part realizes the frequency demultiplexing of the broadband light source through a multi-channel arrayed waveguide grating (AWG), and the wavelength multiplexing part realizes the frequency multiplexing of specific wavelength channels through a multi-channel arrayed waveguide grating. Specifically, in this example, a full-connected 4-user quantum communication network requires a total of 4×(4 - 1) = 12 wavelength channels, and each user needs to be assigned (4 - 1) = 3 channels. That is, a multi-channel waveguide array grating is used to demultiplex 12 wavelength channels from the broadband light source, and then 4 multi-channel waveguide array gratings are used to multiplex every 3 specific wavelength channels into one optical fiber and assign them to one user. Thus, each user is assigned 3 wavelength channels and shares a pair of entangled photons with each other user.

[0039] Figure 6 respectively show the quantum correlation layer, the physical topology layer, and the wavelength channel allocation scheme for each user of the full-connected 4-user quantum communication network in this example. According to Figure 6 the wavelength allocation scheme shown, Alice obtains CH37, CH38, CH39; Bob obtains CH33, CH40, CH41; Chloe obtains CH30, CH32, CH42; Dave obtains CH28, CH29, CH31. Thus, Alice and Bob share the pair of correlated channels CH37 - CH33, Alice and Chloe share the pair of correlated channels CH38 - CH32, Alice and Dave share the pair of correlated channels CH39 - CH32, Bob and Chloe share the pair of correlated channels CH40 - CH30, Bob and Dave share the pair of correlated channels CH41 - CH29, and Chloe and Dave share the pair of correlated channels CH42 - CH28. The network multiplexing provider assigns each wavelength channel to each user according to the above wavelength configuration scheme, and then each user can encode information on the received photons, thus constructing a full-connected 4-user quantum communication network.

[0040] Each user can use Figure 2The device shown in (c) in [reference] examines the feasibility of this network architecture. This device includes an unequal-arm interferometer and a multi-channel single-photon detector. Specifically, the unequal-arm interferometer is a Franson interferometer, which consists of an optical fiber beam splitter, an optical fiber phase shifter, and an optical fiber Faraday mirror. The time difference ΔT between the long arm and the short arm of the interferometer needs to meet the following two conditions: 1) ΔT is much larger than the coherence time of the single photons generated by the light source part to avoid single-photon interference of the photons returning from the long arm and the short arm on the optical fiber beam splitter; 2) ΔT is much smaller than the coherence time of the laser emitted by the laser to ensure the coherence of the photons generated before and after. Each output of the interferometer is also equipped with a single-photon detection channel for single-count measurement of the photons received by the user and comparison with the measurement results of other users, which is equivalent to performing coincidence detection. The photons received by the user can either pass through the short arm of the interferometer or the long arm of the interferometer. This will show three coincidence peaks in the coincidence counting histogram between two users: the left (right) coincidence peak corresponds to the photons of one user passing through the short (long) arm of the interferometer and the photons of the other user passing through the long (short) arm; the middle coincidence peak corresponds to the photons of both users passing through the short arm of the interferometer or both passing through the long arm of the interferometer. By post-selection, the middle coincidence peak is extracted, and thus a two-body entangled state can be established between two users. where φ s (φ i ) is the relative phase determined by the phase shifters of the interferometers in the hands of each user. Theoretically, when changing the phase (φ s +φ i ) in the hands of the users, the coincidence counting between the two users will be proportional to (1 + cos(φ s +φ i ))).

[0041] The actual test results are as Figure 7 shown. Figure 7 In (a), (b), (c), (d), (e), (f) in [reference], the changes of the three characteristic peaks are respectively shown when changing the sum of their phases (φ s +φ i ) between Alice and Bob, Alice and Chloe, Alice and Dave, Bob and Chloe, Bob and Dave, Chloe and Dave. It can be found that the count of the middle peak changes with the phase (φ s +φ i) varies sinusoidally, while the counts of the two side peaks hardly change with the phase. Further fitting of the data can obtain the actual interference contrast. The interference contrasts between six pairs of users are 92.70±0.70%, 90.50±0.90%, 85.56±1.37%, 91.89±0.94%, 91.25±0.97%, and 89.48±1.10% respectively, all of which are greater than the classical bound of 70.70% required by violating the Bell inequality in the CHSH form, that is, it is proved that a two-body entangled state has been established between all six pairs of users.

[0042] It should be noted that in order to distinguish different user combinations, we utilized the time degree of freedom of photons. In actual operation, we introduced long optical fibers with different lengths for different wavelength channels between wavelength demultiplexing and wavelength multiplexing. The effect is that when six pairs of users compare their data respectively (i.e., calculate the coincidence counts), the coincidence peaks will appear at different positions on the time axis. The actual effect is as Figure 8 shown, which respectively shows the relative positions of the three characteristic peaks between six pairs of users when the phase sum (φ s +φ i ) = 0 and (φ s +φ i ) = π of the interferometers in the users' hands. That is, it verifies that we have distinguished different user pairs by utilizing the time degree of freedom of photons.

[0043] This example uses the BBM92 protocol to construct a quantum key distribution network to distribute keys between different users. The BBM92 protocol requires users to share a pair of entangled photon pairs and randomly use two sets of non-orthogonal bases for measurement. In this example, users use phase 0 and π as the measurement Z basis, and phases and as the measurement X basis. We encode the phase 0 of the Z basis and the phase of the X basis as the classical bit value 0, and encode the phase π of the Z basis and the phase of the X basis as the classical bit value 1. As Figure 9 shown, the quantum key distribution between two users (taking Alice and Bob as an example) in our example under the BBM92 protocol is achieved through the following steps:

[0044] Step 1, Reception. Alice and Bob respectively receive one of a pair of entangled photons from the entanglement source at the network service provider side. They randomly select a measurement basis (Z basis or X basis) to measure each photon, obtain a measurement result, and convert their respective results into classical bit values.

[0045] Step 2: Screening. Alice and Bob communicate through a classical channel and publicly disclose the measurement bases they use for each received photon. This process ensures that the eavesdropper cannot obtain any crucial information. By comparing the measurement bases, Alice and Bob discard the photons for which they used different measurement bases from the original key. On average, the screening process leaves approximately half of the bits available for generating the final key.

[0046] Step 3: Bit Flipping. Bob flips his bit string so that he and Alice obtain an identical, random key shared between them. We refer to this part of the key as the raw key.

[0047] Step 4: Security Check. Alice and Bob need to check the security of their raw key, i.e., compare a random subset of bits in their raw key over a public classical channel to estimate the quantum bit error rate (QBER).

[0048] Step 5: Error Correction and Privacy Amplification. After estimating the QBER, the raw key needs to go through two more processing steps to obtain the final secure key. In a practical real-life scenario (without an eavesdropper), there are some small imperfections that lead to a non-zero error rate. Therefore, it is necessary to perform classical error correction on the raw key to eliminate these errors. Finally, the eavesdropper Eve may adopt a strategy where she only measures a certain percentage of the quantum bits to keep the QBER measured by Alice and Bob below an acceptable threshold. Thus, Alice and Bob also need to perform a classical privacy amplification protocol on their error-corrected key to reduce the potential information that Eve may obtain about the key to a sufficiently small value.

[0049] Through the above steps, Alice and Bob have a string of secure and random keys, enabling them to communicate securely between themselves using the one-time pad method.

[0050] Specifically, taking the communication between Alice and Bob as an example for the user, both Alice and Bob have a device for randomly selecting measurement bases, as shown in (e) of Figure 2 . This measurement device includes a 50:50 optical beam splitter and two unequal-arm interferometers respectively set in a group of non-orthogonal bases. The 50:50 optical beam splitter enables the photons in the hands of Alice and Bob to enter the two unequal-arm interferometers with a 50% equal probability, thus performing a random selection of the measurement bases. Note that, compared with the interferometer in (d) of Figure 2 , we added a circulator to the incident port of the interferometer, turning it into an interferometer with two-port output. The outputs of the two ports are related to the phase (φ s +φ i)Complementary, that is, the output of one port is proportional to (1 + V cos(φ s + φ i )) and the output of the other port is proportional to (1 - V cos(φ s + φ i ))。Therefore, in order to set the two interferometers of Alice and Bob to the Z - basis and X - basis respectively, we only need to set the relative phases of the two interferometers of Alice to φ A = 0 and set the relative phases of the two interferometers of Bob to φ B = 0 and respectively, where the relative phases of the interferometers of Alice and Bob are determined by their interference results.

[0051] During the communication between Alice and Bob, the two parties share a pair of entangled photon pairs (corresponding to the channel pair CH37–CH33). Therefore, Alice measures the photons in the CH37 channel in the Z - basis (0 and π) with a probability of 50% and in the X - basis ( and ) with a probability of 50%, and Bob measures the photons in the CH33 channel in the Z - basis (0 and π) with a probability of 50% and in the X - basis ( and ) with a probability of 50%. By comparing the data between Alice and Bob and using general post - processing methods, we can remove the error codes in the original key and the information that may be leaked to Eve, thus obtaining a consistent secure key string. Specifically, after performing error correction and privacy amplification, the length of the final secure key string is given by the formula SKR≥3 sift [1 - f(δ b )H2(δ b ) - H2(δ p )], where n sift is the sifted key rate of successful basis for the two users, δ b and δ p are the bit error rate and phase error rate respectively, f(x) is the error - correction efficiency (which is a function of the error rate, generally taking f(x)=1.2), and H2(x)= - x log2x-(1 - x) log2(1 - x) is the binary entropy function. Due to the symmetry of the Z - measurement basis and X - measurement basis, we have δ b = δ p ≡ E λ , where E λ is the total quantum bit error rate (QBER, quantum bit error rate), and its relationship with the contrast satisfies Therefore, the user's measurement basis and data are compared to obtain information such as the screening key and bit error rate. Then, through error correction and privacy amplification, the final secure key string can be obtained. In this example, the quantum key distribution process among six pairs of users was tested, and data such as the contrast ratio, quantum bit error rate, and secure key rate are as Figure 10 shown. Table 2 summarizes the total screening key (within 2000 seconds), contrast ratio, quantum bit error rate, and secure key rate of the quantum key distribution process among six pairs of users. It can be seen that the quantum bit error rate among the six pairs of users ranges between 2.57% and 3.09%, and the average key rate ranges between 180 Hz and 251 Hz. The results show that in this example, an integrated photonics chip integrated with Si3N4 microring resonators is used to generate an energy-time entangled state in the form of a frequency comb. Through wavelength division multiplexing technology, 12 wavelength channels are allocated to 4 users to establish quantum correlations between any two users. Finally, a four-user fully connected quantum communication network is successfully constructed through the BBM92 protocol, which verifies the feasibility of this technical solution.

[0052] Table 2 Total screening key, contrast ratio, quantum bit error rate, and secure key rate of the quantum key distribution process among six pairs of users

[0053]

Claims

1. A multi-user quantum key distribution full-connection network system based on a multi-functional integrated optical quantum chip, characterized in that, Including: A multi-degree-of-freedom integrated high-dimensional entangled photon pair source module, a mode configuration module based on mode division multiplexing technology at the network service provider side, a mode demultiplexing module, an encoding / decoding module, and a photon detection module at the user side; the mode configuration module based on mode division multiplexing technology at the network service provider side is used to distribute the correlated modes in the entangled photon pair source to different users through a certain combination method, and this mode configuration needs to satisfy that at least one pair of modes that are correlated or entangled in a certain degree of freedom are shared between any pair of users; the mode demultiplexing module at the user side is used to demultiplex the received modes, and each demultiplexed mode passes through an encoding / decoding module and a group of photon detection modules respectively; the encoding / decoding module is used to encode or decode the photons of different modes received by the user; the photon detection module is used to detect the photons encoded / decoded at the user side; the multi-degree-of-freedom integrated high-dimensional entangled photon pair source module includes a laser, an electro-optic modulator, an optical filter, a polarization controller, and a pump functional integrated optical quantum chip; the output of the laser is connected through an optical fiber or free space coupling and sequentially passes through the electro-optic modulator, the optical filter, and the polarization controller, and after specifically modulating the intensity, phase, frequency, and polarization properties of the laser, it pumps the nonlinear medium on the pump functional integrated optical quantum chip, and through the optical nonlinear effect, photon pairs that are entangled with a dimension M≥2 in some degrees of freedom of the photons are generated, and their quantum state is written as: where m and m′ are a pair of correlated modes of a certain degree of freedom of a photon, and the degrees of freedom include time, frequency, phase, spin, path, polarization, orbital angular momentum; s mm′ is the amplitude, satisfying ∑|s mm′ | 2 = 1; M is the dimension of the degree of freedom. According to different network topologies, for a fully connected network topology without using passive routing, M needs to satisfy 2M ≥ N(N - 1). For a network topology that uses 1-to-k passive routing to build N users into k subnets to achieve full connection, M needs to satisfy or 2M ≥ k 2 + k, to ensure that the light source shares at least one pair of correlated modes between any two users, so that there is at least one pair of modes that are correlated or entangled in a certain degree of freedom between any two users in the communication network, thereby constructing a simultaneously fully connected quantum communication network.

2. The multi-user quantum key distribution full-connection network system based on the multi-functional integrated optical quantum chip according to claim 1, characterized in that, The nonlinear medium includes magnesium fluoride, silicon, silicon dioxide, silicon nitride, aluminum nitride, highly doped quartz glass, and lithium niobate.

3. The multi-user quantum key distribution full-connection network system based on the multi-functional integrated optical quantum chip according to claim 1, characterized in that, The entanglement methods include path entanglement, polarization entanglement, energy-time entanglement, and time-bin entanglement.

4. The multi-user quantum key distribution full-connection network system based on the multi-functional integrated optical quantum chip according to claim 1, characterized in that, In the mode configuration module based on mode division multiplexing technology, to achieve a full - connected network topology without using passive routing for pairwise communication among N users, the number of modes required is or pairs of modes. Then the first step is to demultiplex the N(N - 1) modes under a certain degree of freedom of the multi - degree - of - freedom integrated high - dimensional entangled photon pair source module through a multi - channel mode division multiplexer, that is, to achieve mode demultiplexing from 1 to N(N - 1). To ensure that at least one pair of modes is shared pairwise among N users, each user needs to receive (N - 1) modes and share a pair of associated modes (m and m′) with any other user. The corresponding second step is to use N multi - channel mode division multiplexers to multiplex (N - 1) modes in a specific combination into one channel, that is, to achieve multiplexing from (N - 1) to 1. To achieve a full - connected network topology with passive routing for pairwise communication among N users, the consumption of passive routing mode resources of 1 - to - k is used.

5. The multi-user quantum key distribution full-connection network system based on the multi-functional integrated optical quantum chip according to claim 4, characterized in that, To implement a fully connected network topology using passive routing for pairwise communication among N users, the resource consumption of using a 1-to-k passive routing mode includes two scenarios: In the first scenario, N users are divided into k subnets. The users within each subnet are fully connected through modes, requiring 1-to-k passive routers. The k subnets are fully connected through modes, requiring 1-to-k passive routers. That is, a total of 1-to-k passive routers and modes are required. Then, in the mode configuration module based on mode division multiplexing technology, the first step is to demultiplex the modes at a certain degree of freedom of the multi-degree-of-freedom integrated high-dimensional entangled photon pair source module through a multi-channel mode division multiplexer, that is, to achieve demultiplexing from 1 to ; the second step is to divide each mode into k using a 1-to-k passive router; the third step is to multiplex the modes in a specific combination manner to one channel using N multi-channel mode division multiplexers, modes are used for communication among users within the subnet, and (k - 1) modes are used for communication among users in different subnets, that is, to achieve multiplexing from to 1 to ensure that at least one pair of associated modes (m and m′) is shared between any two users among the N users; The second solution is to divide N users into k sub-networks. The k sub-networks are fully connected to each other through k(k - 1) modes and use 1-to-k passive routing to distribute to all users within the sub-network, and users within each sub-network share the same pair of modes and use the same 1-to-k passive routing to distribute to all users within the sub-network to achieve the connection between users within the sub-network. This network topology requires a total of k 2 + k modes and k passive routings; then the first step in the mode configuration module based on mode division multiplexing technology is to demultiplex the k 2 + k modes under a certain degree of freedom of the multi-degree-of-freedom integrated high-dimensional entangled photon pair source module through a multi-channel mode division multiplexer, that is, to achieve 1-to-k 2 + k mode demultiplexing; the second step is to divide each mode into k using 1-to-k passive routing; the third step is to use N multi-channel mode division multiplexers to multiplex (k + 1) modes in a specific combination into one channel, 2k modes are used for communication between users within each sub-network, and (k - 1) modes are used for communication between users in different sub-networks, that is, to achieve (k + 1)-to-1 multiplexing, so as to ensure that at least one pair of associated modes (m and m′) is shared between any two users among the N users. The combination method needs to ensure that at least one pair of modes that are associated or entangled in a certain degree of freedom is shared between any pair of users, that is, to establish a quantum correlation between each pair of users in the N-user quantum communication fully connected network.

6. The multi-user quantum key distribution full-connection network system based on the multi-functional integrated optical quantum chip according to claim 1, wherein The encoding / decoding module adopts different encoding / decoding methods such as path encoding, polarization encoding, energy-time encoding, time-bin encoding, and phase encoding.

7. The multi-user quantum key distribution full-connection network system based on the multi-functional integrated optical quantum chip according to claim 1, characterized in that The photon detection module adopts an avalanche diode single photon detector, a superconducting nanowire single photon detector, and a photon number / frequency resolution single photon detector.

8. The multi-user quantum key distribution full-connection network system based on the multi-functional integrated optical quantum chip according to claim 1, characterized in that The network service provider distributes different mode combinations to different users through an optical fiber communication network. Since each user shares at least one pair of modes that are correlated or entangled in a certain degree of freedom with the other N-1 users, that is, quantum correlations are established between each pair of users, a full-connected network of N users is thus constructed.

9. The multi-user quantum key distribution full-connection network system based on the multi-functional integrated optical quantum chip according to claim 1, wherein, The users use the E91, BBM92 entanglement-based or MDI entanglement time-reversal-based quantum key distribution protocol to distribute keys. The users measure the allocated photons using specific basis vectors respectively, and then by comparing, error correcting, and privacy amplifying the data post-processing method, the error codes and information that may be leaked to eavesdroppers in the original key are removed, and a consistent secure key string can be obtained, thus realizing quantum key distribution between users. Without changing the optical fiber connection and system configuration, the users use this method to perform quantum key distribution with any other user, thus constructing a full-connected quantum key distribution network of N users.

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