Key receiving end, quantum key distribution system and quantum key distribution method

Abstract

The application provides a key receiving end, a quantum key distribution system and a quantum key distribution method. The receiving end comprises a modulation pump source configured to generate a double-pulse pump light sequence; a frequency up-conversion source configured to perform sum frequency mixing on a quantum light signal of a received time-encoded quantum state based on the double-pulse pump light sequence to generate quantum states with mutually orthogonal polarizations on two time windows in front and back of each other, thereby obtaining a sum frequency light sequence with a changed relative phase on the two time windows; and a measurement module configured to measure the sum frequency light sequence to obtain a measurement result and generate a quantum key according to the measurement result.

Description

Technical Field

[0001] This application relates to the field of quantum communication technology, and more specifically, to a key receiver, a quantum key distribution system, and a quantum key distribution method. Background Technology

[0002] Photons possess multiple degrees of freedom, including polarization, path, orbital angular momentum, time, and frequency, enabling flexible encoding of quantum information in various scenarios. Polarization and orbital angular momentum-based encoding methods support free-space transmission, making satellite-based quantum communication schemes possible. Encoding targeting spatial degrees of freedom is suitable for large-scale photon state generation and processing via space-based optical transmission paths, while time- and frequency-based encoding is suitable for fiber optic transmission. Time-bin encoding is a technique that encodes information into non-overlapping time windows of photons for transmission and processing, primarily utilizing time-bin single-photon states and time-bin entangled states. Due to its excellent robustness to polarization fluctuations in fiber optic transmission, it has become one of the most commonly used techniques in the field of quantum communication.

[0003] Currently, quantum key distribution systems based on time-bin encoding typically use unequal-arm interferometers to measure the received time-bin superposition states. The detected photons appear in three symmetrically distributed time windows, with the central window accounting for 50% of the count, meaning the probability of successfully detecting a time-bin superposition state is always 50%. Therefore, superposition basis measurement requires post-selection of the central window, resulting in an inherent 3dB loss at the measurement end, which reduces detection efficiency. Summary of the Invention

[0004] In view of this, this application provides a key receiver, a quantum key distribution system, and a quantum key distribution method.

[0005] One aspect of this application provides a key receiver for a quantum key distribution system, comprising: a modulation pump source for generating a double-pulse pump light sequence; a frequency up-conversion source for performing sum-frequency analysis on a received time-encoded quantum optical signal based on the double-pulse pump light sequence to generate mutually orthogonal polarization quantum states in two consecutive time windows, thereby obtaining a sum-frequency light sequence with relative phase changes in the two time windows; and a measurement module for measuring the sum-frequency light sequence, obtaining measurement results, and generating a quantum key based on the measurement results.

[0006] According to an embodiment of this application, the above-mentioned modulated pump source is used to generate a sequence of orthogonally polarized double-pulse pump light.

[0007] According to an embodiment of this application, the modulation pump source includes: a first laser for generating a pulse sequence with a predetermined time interval and a target polarization angle; and an unequal-arm interferometer for interfering the pulse sequence to obtain a double-pulse pump light sequence with orthogonal polarization.

[0008] According to an embodiment of this application, the frequency upconversion source is used to perform sum-frequency conversion using a type-II phase-matched crystal and a type-I phase-matched crystal, wherein the type-II phase-matched crystal and the type-I phase-matched crystal correspond to the sum-frequency conversion of the preceding and following time windows, respectively.

[0009] According to an embodiment of this application, the frequency upconversion source includes: a beam combiner for combining the dual-pulse pump light sequence and the optical signal into a beam combiner light sequence; a first crystal and a second crystal for performing a sum-frequency effect on the beam combiner light sequence to obtain the sum-frequency light sequence, wherein the first crystal and the second crystal are second-order nonlinear crystals that satisfy type II phase matching and type I phase matching, respectively.

[0010] According to an embodiment of this application, the frequency upconversion source further includes a filter for filtering the light generated by the laser in the sum-frequency optical sequence described above.

[0011] According to an embodiment of this application, the measurement module includes: a barium borate crystal for converting the sum-frequency light sequence into target polarized light, wherein the target polarized light includes linearly polarized light or circularly polarized light; a half-wave plate for adjusting the polarization direction of the target polarized light; a first polarization correlation interferometer for generating an interference signal based on the polarization state modulation and separation of the target polarized light with adjusted polarization direction; and a first silicon-based detector for detecting the interference signal to obtain a first detection result, and processing the first detection result to obtain the quantum key.

[0012] According to an embodiment of this application, the measurement module further includes: a beam splitter for splitting the sum-frequency optical sequence to obtain a first sum-frequency optical sequence corresponding to the X-based basis and a second sum-frequency optical sequence corresponding to the Z-based basis, wherein the first sum-frequency optical sequence is input to the barium borate crystal; a second polarization correlation interferometer for generating a second interference signal based on the polarization state modulation and separation of the second sum-frequency optical sequence; and a second silicon-based detector for detecting the second interference signal to obtain a second detection result, thereby determining a detection result identical to the basis of the key sending end from the first detection result and the second detection result, and processing it to obtain the quantum key.

[0013] According to an embodiment of this application, the measurement module includes: a polarization-dependent unequal-arm interferometer for performing superposition basis measurement on the above sum-frequency optical sequence to obtain a light intensity signal; a third silicon-based detector for detecting the above light intensity signal to obtain a third detection result; wherein, two key receivers communicate with each other to perform basis selection on the measurement results of the two key receivers, thereby generating a quantum key by processing the measurement results with the same basis.

[0014] Another aspect of this application provides a quantum key distribution system, comprising: a key transmitting end for generating a time-encoded quantum optical signal and transmitting the quantum optical signal to a receiving end; and a key receiving end for detecting the received quantum optical signal to obtain a quantum key.

[0015] Another aspect of this application provides a quantum key distribution method, comprising: acquiring a quantum optical signal of a time-encoded quantum state transmitted by a key transmitter; generating a double-pulse pump light sequence using a modulation pump source; performing sum-frequency analysis on the received quantum optical signal based on the double-pulse pump light sequence using a frequency up-conversion source to generate mutually orthogonal polarization quantum states in two consecutive time windows, thereby obtaining a sum-frequency light sequence in which the relative phase changes in the two time windows; measuring the sum-frequency light sequence using a measurement module to obtain the measurement result, and generating a quantum key based on the measurement result.

[0016] According to embodiments of this application, by modulating the Time-bin quantum state using the sum-frequency effect, the polarizations of its preceding and following time windows are made mutually orthogonal, enabling superposition basis measurement without post-selection, eliminating the inherent 3dB loss, and thereby improving the detection efficiency of the key receiver. Attached Figure Description

[0017] The above and other objects, features and advantages of this application will become clearer from the following description of embodiments with reference to the accompanying drawings, in which:

[0018] Figure 1 A schematic diagram of a quantum key distribution system according to an embodiment of this application is shown;

[0019] Figure 2 A structural block diagram of a quantum key distribution system according to a first embodiment of this application is shown;

[0020] Figure 3 A schematic diagram of the structure of a quantum key distribution system according to a second embodiment of this application is shown;

[0021] Figure 4 A schematic diagram of the structure of a quantum key distribution system according to a third embodiment of this application is shown;

[0022] Figure 5 A structural block diagram of a quantum key distribution system according to a fourth embodiment of this application is shown;

[0023] Figure 6 A schematic diagram of the structure of a quantum key distribution system according to a fifth embodiment of this application is shown;

[0024] Figure 7 A schematic diagram of the structure of a quantum key distribution system according to a sixth embodiment of this application is shown; and

[0025] Figure 8 A schematic flowchart of a quantum key distribution method according to an embodiment of this application is shown. Detailed Implementation

[0026] The embodiments of this application will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of this application. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of this application for ease of explanation. However, it will be apparent that one or more embodiments may be implemented without these specific details. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concepts of this application.

[0027] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0028] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0029] When using expressions such as "at least one of A, B and C", they should generally be interpreted in accordance with the meaning that is commonly understood by those skilled in the art (e.g., "a system having at least one of A, B and C" should include, but is not limited to, a system having A alone, a system having B alone, a system having C alone, a system having A and B, a system having A and C, a system having B and C, and / or a system having A, B and C, etc.).

[0030] In the embodiments of this application, the collection, updating, analysis, processing, use, transmission, provision, disclosure, and storage of data (e.g., including but not limited to user personal information) comply with relevant laws and regulations, are used for legitimate purposes, and do not violate public order and good morals. In particular, necessary measures have been taken to prevent unauthorized access to user personal information data and to safeguard user personal information security and network security.

[0031] Embodiments of this application provide a key receiver, a quantum key distribution system, and a quantum key distribution method. The receiver includes a modulation pump source for generating a double-pulse pump light sequence; a frequency up-conversion source for performing sum-frequency analysis on the received time-encoded quantum optical signal based on the double-pulse pump light sequence to generate mutually orthogonal polarization quantum states in two consecutive time windows, thereby obtaining a sum-frequency light sequence with relative phase changes in the two time windows; and a measurement module for measuring the sum-frequency light sequence, obtaining the measurement result, and generating a quantum key based on the measurement result.

[0032] Figure 1 A schematic diagram of a quantum key distribution system according to an embodiment of this application is shown.

[0033] like Figure 1 As shown, the quantum key distribution system includes: a key transmitter 100, used to generate a time-coded quantum optical signal and send the quantum optical signal to a receiver; and a key receiver 200, used to detect the received quantum optical signal to obtain the quantum key.

[0034] According to an embodiment of this application, the key transmitter 100 is responsible for preparing a quantum state in the time-bin degree of freedom and sending it to the key receiver 200, and the key receiver 200 is responsible for measuring the quantum state of the quantum optical signal, thereby obtaining the quantum key. The specific structures of the key transmitter 100 and the key receiver 200 are described below.

[0035] According to an embodiment of this application, a key receiver for a quantum key distribution system includes: a modulation pump source for generating a double-pulse pump light sequence; a frequency up-conversion source for performing sum-frequency analysis on the received time-encoded quantum optical signal based on the double-pulse pump light sequence to generate mutually orthogonal polarization quantum states in two consecutive time windows, thereby obtaining a sum-frequency light sequence with relative phase changes in the two time windows; and a measurement module for measuring the sum-frequency light sequence, obtaining measurement results, and generating a quantum key based on the measurement results.

[0036] According to an embodiment of this application, a modulated pump source emits a sequence of dual-pulse pump light, which is synchronized with the received quantum light signal.

[0037] According to embodiments of this application, a modulated dual-pulse pump light sequence and a quantum light signal are jointly incident on a frequency upconversion source, undergoing a sum-frequency process. During this process, mutually orthogonal polarization quantum states can be generated in two consecutive time windows. By adjusting the relative phase of the dual-pulse pump light sequence, the relative phase of the quantum states in the two time windows after sum-frequency generation can be changed.

[0038] According to an embodiment of this application, the measurement module measures the optical signal in the sum-frequency optical sequence to obtain the measurement result, and performs post-processing such as error correction and privacy amplification on the measurement result to obtain the quantum key.

[0039] According to embodiments of this application, a modulated pump source is used to generate a sequence of orthogonally polarized dual-pulse pump light.

[0040] According to an embodiment of this application, a modulated pump source emits a sequence of orthogonally polarized double-pulse pump light, wherein the relative phase between the two pulses in the double-pulse pump light sequence can be flexibly modulated as needed. This double-pulse pump light sequence can be obtained by directly modulating the laser, or the light pulses can be modulated after laser emission; no limitation is imposed here.

[0041] According to an embodiment of this application, the key transmitting end includes: a second laser for generating a coherent first pulse sequence with time intervals; and a quantum state preparation module for modulating the intensity and phase of the first pulse sequence to obtain a single photon with a target quantum state, wherein the quantum optical signal includes a single photon.

[0042] According to embodiments of this application, the second laser can be a 1550nm mode-locked pulsed laser or other types of lasers.

[0043] In some embodiments, the quantum state preparation module can consist of two quarter-wave plates, a half-wave plate, an α-BBO crystal, and a polarization beam splitter. The α-BBO crystal (α-phase barium borate crystal) is a high-performance birefringent crystal. In this embodiment, the second laser generates a coherent pulse sequence. The waveplate group (i.e., the quarter-wave plate, half-wave plate, and quarter-wave plate placed sequentially) adjusts the pulses to a suitable polarization state, so that each pulse, after passing through the α-BBO crystal, generates two orthogonally polarized pulses with different time bins due to birefringence. The polarization beam splitter projects the two pulses onto the same polarization, thereby generating a quantum optical signal of a time-bin quantum state. By changing the optical axis direction of the waveplate group, four quantum states from two mutually unbiased bases (Z-based and X-based) can be obtained. The key transmitter can randomly select a base and send the quantum state by randomly rotating the waveplate group.

[0044] In some embodiments, the quantum state preparation module may consist of a polarization controller, an α-BBO crystal, and a polarization beam splitter. A second laser generates a coherent sequence of pulses with a time interval of T. The polarization controller adjusts the pulses to a suitable polarization state, so that after each pulse passes through the α-BBO crystal, birefringence produces two orthogonally polarized pulses with different time bins. The polarization beam splitter projects the two pulses onto the same polarization, generating a time-bin quantum state. , Represents the pre-pulse. Represents the afterpulse. , This is the phase added by the polarization controller. By adjusting the polarization controller, the relative phase of the time-bin superposition states can be changed. 0, and This yields four quantum states from two sets of mutually unbiased bases (X-based and Y-based). The key transmitter can randomly select bases to send quantum states by randomly adjusting the polarization controller.

[0045] According to an embodiment of this application, the modulation pump source includes: a first laser for generating a pulse sequence with a predetermined time interval and a target polarization angle; and an unequal-arm interferometer for interfering the pulse sequence to obtain a double-pulse pump light sequence with orthogonal polarization.

[0046] According to an embodiment of this application, a first laser can generate a pulse sequence with a time interval of T and a 45° slant polarization.

[0047] In some embodiments, the 1900nm mode-locked pulsed laser in the first laser outputs a pulse sequence with a time interval of T, which is then converted into a 45° linearly polarized beam by a polarization beam splitter and a half-wave plate.

[0048] According to embodiments of this application, an unequal-arm interferometer may include two polarization beam splitters, two mirrors, and a phase shifter. In some embodiments, the unequal-arm interferometer may delay the V component of a pulse sequence relative to the H component. The specific pulse sequence is split into V and H components by a polarization beam splitter. The V component enters the phase shifter through the first mirror, thereby adjusting the phase of the V component so that the relative phase between the two components is 0 or H. The V component passes through another mirror and enters the second polarization beam splitter for beam combining, thereby generating a sequence of orthogonally polarized dual-pulse pump light.

[0049] According to embodiments of this application, when selecting X-based and Y-based encoding, the relative phase of the pump light pulses before and after is adjusted to 0 or... It enables the active selection of measurement bases.

[0050] According to an embodiment of this application, a frequency upconversion source is used to perform sum-frequency conversion using a type-II phase-matched and a type-I phase-matched crystal, wherein the type-II phase-matched and type-I phase-matched crystals correspond to the sum-frequency conversion of the preceding and following time windows, respectively.

[0051] According to an embodiment of this application, the frequency upconversion source includes: a beam combiner for combining a dual-pulse pump light sequence and a quantum light signal into a beam combiner light sequence; a first crystal and a second crystal for performing sum-frequency analysis on the beam combiner light sequence based on a second-order nonlinear sum-frequency effect to obtain a sum-frequency light sequence, wherein the first crystal and the second crystal are second-order nonlinear crystals that satisfy type II phase matching and type I phase matching, respectively.

[0052] According to an embodiment of this application, the frequency upconversion source further includes a filter for filtering the light generated by the laser in the modulation pump source in the sum-frequency light sequence.

[0053] According to embodiments of this application, a beam combiner can combine a dual-pulse pump light sequence and a quantum light signal into a combined light sequence, which is then subjected to frequency summation by a first crystal and a second crystal to obtain a frequency summation light sequence. The first crystal and the second crystal can be a type-I periodically polarized lithium niobate crystal and a type-II periodically polarized lithium niobate crystal, respectively.

[0054] In some embodiments, a dichroic mirror can be used as a beam combiner. The dichroic mirror reflects the dual-pulse pump light sequence while simultaneously transmitting the quantum light signal, allowing both to propagate collinearly. The trigger signals of the lasers in the key transmitter and the first laser are adjusted to synchronize the preceding and following pulses of the dual-pulse pump light sequence and the quantum light signal. The quantum light signal is controlled to be V-polarized. In the preceding time window, the quantum light signal and the dual-pulse pump light sequence are orthogonally polarized, generating V-polarized and frequency pulses in a type-II periodically polarized lithium niobate crystal; in the following time window, the quantum light signal and the dual-pulse pump light sequence are polarized in the same way, generating H-polarized and frequency pulses in a type-I periodically polarized lithium niobate crystal.

[0055] In some embodiments, a short-pass filter can be used to filter out residual unconverted components, i.e., the light generated by the first laser. Through the action of the frequency up-conversion source, the photons in the converted sum-frequency light sequence are... With polarization state correspond, and correspond.

[0056] According to an embodiment of this application, the measurement module includes: a barium borate crystal for converting a sum-frequency light sequence into target polarized light, wherein the target polarized light includes linearly polarized light or circularly polarized light; a half-wave plate for adjusting the polarization direction of the target polarized light; a first polarization correlation interferometer for controlling and separating the polarization state of the target polarized light based on the adjusted polarization direction to generate an interference signal; and a first silicon-based detector for detecting the interference signal to obtain a first detection result, and processing the first detection result to obtain a quantum key.

[0057] According to embodiments of this application, when the basis of the quantum optical signal prepared by the key transmitter is the same as the basis selected for measurement, the relative phase of the converted pulses is 0 or... In a barium borate (α-BBO) crystal recombination and frequency-optical sequence, the orthogonally polarized pulses are converted into linearly polarized light. This light is then rotated to horizontal or vertical polarization by a half-wave plate and passes through a first polarization-correlated interferometer, which has only one counting window. A first silicon-based detector is used to detect the interference signal output from the first polarization-correlated interferometer, obtaining a first detection result. This first detection result is then processed to obtain a quantum key, which may include error correction and privacy amplification.

[0058] According to embodiments of this application, when different bases are selected for preparation and measurement, the relative phase of the converted pulses is... or- In an α-BBO crystal, the recombined orthogonally polarized pulses become circularly polarized light. This light remains circularly polarized after passing through a half-wave plate, and then passes through a first polarization-correlated interferometer, resulting in two counting windows. A first silicon-based detector is used to detect the interference signal output from the first polarization-correlated interferometer, obtaining a first detection result. This first detection result is then processed to obtain the quantum key; this processing may include error correction and privacy amplification.

[0059] According to an embodiment of this application, following the BB84 protocol, the sender selects two sets of random numbers for basis selection and quantum state generation, respectively. After frequency up-conversion modulation, the receiver selects a set of random numbers to determine whether to perform X-basis or Y-basis measurement. After completing one round of the above operations, the receiver discloses the basis selection and measurement results, retains the measurement results under the same basis as the sender, and obtains the final key after data post-processing.

[0060] According to an embodiment of this application, the measurement module further includes: a beam splitter for splitting the sum-frequency optical sequence to obtain a first sum-frequency optical sequence corresponding to the X-based basis and a second sum-frequency optical sequence corresponding to the Z-based basis, wherein the first sum-frequency optical sequence is input to a barium borate crystal; a second polarization correlation interferometer for controlling and separating the polarization state based on the second sum-frequency optical sequence to generate a second interference signal; and a second silicon-based detector for detecting the second interference signal to obtain a second detection result, so as to determine the detection result that is the same as the basis of the key sending end from the first detection result and the second detection result, and process it to obtain a quantum key.

[0061] According to an embodiment of this application, the sum-frequency optical sequence is transmitted and reflected by a beam splitter to form two optical paths, namely a first sum-frequency optical sequence and a second sum-frequency optical sequence. The two optical paths are used for X-basis and Z-basis measurements, respectively. If the key transmitter sends an X-basis quantum state, the first sum-frequency optical sequence transmitted by the beam splitter is reconstructed by an α−BBO crystal to convert the orthogonally polarized pulses into linearly polarized light. It is then rotated to horizontal or vertical polarization by a half-wave plate and passes through a polarization correlation interferometer, resulting in only one counting window. However, the second sum-frequency optical sequence reflected by the beam splitter will have two counting windows after passing through a second polarization correlation interferometer.

[0062] According to an embodiment of this application, if the key transmitter sends a Z-based quantum state, the first sum-frequency light sequence transmitted by the beam splitter is rotated to diagonal or anti-diagonal polarization by an α-BBO crystal and a half-wave plate. After passing through a first polarization correlation interferometer, there are two counting windows. However, the second sum-frequency light sequence reflected by the beam splitter only has one counting window after passing through a second polarization correlation interferometer. After detection by a second silicon-based detector and subsequent data post-processing, four discrete counting windows appear: the first two correspond to X-based measurements, and the last two correspond to Z-based measurements. Only when the same basis is prepared as the measurement, a definite measurement result can be obtained.

[0063] According to an embodiment of this application, following the BB84 protocol, the key sending end selects two sets of random numbers for basis selection and quantum state generation, respectively. After modulation by a frequency up-conversion source, the key receiving end selects a set of random numbers to determine whether to perform X-basis or Z-basis measurement. After completing one round of the above operations, the key receiving end discloses the basis selection results and the first and second detection results, retains the measurement results under the same basis as the sender, and obtains the final key after data post-processing. Data post-processing can refer to error correction and privacy amplification.

[0064] According to an embodiment of this application, when there are multiple key receivers (e.g., two) and different key receivers share a modulation pump source, the key transmitter includes: a third laser for generating a coherent second pulse sequence with time intervals; and an entanglement module for performing frequency doubling and spontaneous parametric conversion on the second pulse sequence to obtain time-bin encoded entangled photon pairs, wherein the quantum optical signal includes entangled photon pairs.

[0065] According to an embodiment of this application, the third laser can be a mode-locked laser in the communication band.

[0066] In some embodiments, the entanglement module comprises an optical amplifier, a frequency-doubling crystal, an unequal-arm Mach-Zehnder interferometer, and a spontaneous parametric down-conversion crystal. A third laser generates a long sequence of coherent pulses with a time interval of T, which is then amplified by the optical amplifier to ensure sufficient pump power for the subsequent spontaneous parametric down-conversion process. The frequency-doubling crystal is used to frequency-double the amplified pulse sequence in the communication band, thereby generating entangled photon pairs in the communication band through the spontaneous parametric down-conversion process of the spontaneous parametric down-conversion crystal. The unequal-arm Mach-Zehnder interferometer consists of two 50:50 optical beamsplitters and two mirrors, which convert a single pulse into a pulse with a delay by utilizing the optical path difference between the long and short arms. The two pulses before and after, It must be less than T, but also much greater than the photon coherence time, to ensure that each pulse in the frequency doubling pulse sequence can generate a time-bin superposition state. , Represents the pre-pulse. Represents the afterpulse. , This refers to the phase added to the back pulse by the long arm of the interferometer. Using the frequency-doubled pump light, two low-frequency photons are simultaneously generated in the spontaneous parametric downconversion crystal, producing a time-bin entangled state. This embodiment can employ a zero-type quasi-phase-matched spontaneous parametric down-conversion crystal to ensure that the quantum properties of the generated entangled photon pairs are completely identical. Therefore, the modulation pump sources in different key receivers can use the same configuration to modulate the received quantum states.

[0067] In some embodiments, the entanglement module comprises an erbium-doped fiber amplifier, an unequal-arm Mach-Zehnder interferometer, a polarization controller, a circulator, a zero-type periodically polarized lithium niobate waveguide, a mirror, and a dense wavelength division multiplexer. A third laser generates a long sequence of coherent pulses with a time interval of T, which is then amplified using the erbium-doped fiber amplifier. The unequal-arm Mach-Zehnder interferometer consists of two 50:50 fiber beamsplitters, which convert a single pulse into a time-delayed pulse by using the optical path difference between the long and short arms. The two pulses before and after generate a Time-bin superposition state. , Represents the pre-pulse. Represents the afterpulse. , This is the phase added to the back pulse by the long arm of the interferometer. The polarization controller adjusts the optical pulse to a suitable polarization state, generating a 775nm frequency-harmonic pulse through a circulator and a periodically polarized lithium niobate waveguide. This pulse is reflected back to the periodically polarized lithium niobate waveguide by a mirror, where it undergoes a spontaneous parametric down-conversion process to generate a 1550nm photon pair, thus creating a Time-bin entangled state. The dense wavelength division multiplexer then distributes the entangled photon pairs to two quantum channels for transmission, and then uses fiber optic couplers to couple the signal light into free space for transmission to two key receivers.

[0068] In some embodiments, when there are multiple key receivers (e.g., two) and different key receivers share a modulation pump source, the measurement module in each key receiver includes: a polarization-dependent unequal-arm interferometer for performing superposition basis measurements on the sum-frequency optical sequence to obtain a light intensity signal; and a third silicon-based detector for detecting the light intensity signal to obtain a third detection result; wherein, the multiple key receivers communicate with each other to perform basis selection on the measurement results of the multiple key receivers, thereby generating a quantum key by processing the measurement results with the same basis.

[0069] In some embodiments, when the two key receivers perform superposition basis measurements, two polarization-dependent unequal-arm interferometers respectively receive the double-pulse and frequency-frequency light sequences transmitted by their respective frequency up-conversion sources at the two key receivers. The arm length difference in the polarization-dependent unequal-arm interferometers needs to precisely correspond to the pulse delays before and after in the double-pulse and frequency-frequency light sequences. This allows for the recombination of pulses before and after the dual-pulse and frequency-frequency optical sequence output. As a result, the two third silicon-based detectors can only detect the light intensity signal within the same time window each time, thus obtaining the corresponding third detection result.

[0070] In some embodiments, time-based measurements can be performed simply by removing the polarization-dependent unequal-arm interferometer and directly measuring the photon arrival time. The obtained data is then post-processed, and the two key receivers retain the measurement results when using the same basis through classical communication, thus obtaining the final quantum key.

[0071] According to embodiments of this application, second-order nonlinearity and frequency effects are used to modulate the time-bin quantum state, making the polarizations of its preceding and following time windows mutually orthogonal. This allows for superposition basis measurement without post-selection, eliminating the inherent 3dB loss. Furthermore, when selecting X-based and Y-based encoding, the relative phase of the preceding and following pump light pulses is adjusted to 0 or... Active selection of the measurement base can be achieved. Finally, the frequency of the modulated communication band entangled photon pair is upconverted to the sensitive band of the silicon-based detector. Since the detection efficiency of the silicon-based single-photon detector at room temperature is higher than that of the communication band single-photon detector (such as the InGaAs single-photon avalanche diode detector), the use of silicon-based single-photon detectors in the measurement module can effectively improve the detection efficiency.

[0072] To more clearly illustrate the specific structure and signal flow of the key receiver, the following... Figures 2 to 4 The embodiments shown are described in detail, in which the key sending end has a quantum state preparation module.

[0073] Figure 2 A structural block diagram of a quantum key distribution system according to a first embodiment of this application is shown.

[0074] In the first specific embodiment, such as Figure 2 The quantum key distribution system shown includes a key transmitter comprising a second laser and a quantum state preparation module. The second laser generates coherent pulse signals, which can be obtained through continuous light chopping, an unequal-arm interferometer, the walk-off effect in a birefringent crystal, or a mode-locked laser; the specific generation method is not limited. Subsequently, the quantum state preparation module modulates the intensity and relative phase of the coherent pulses to prepare the quantum states required for the quantum key distribution protocol. Finally, the key transmitter sends the quantum optical signal to the key receiver via a quantum channel.

[0075] The key receiver consists of a modulation module and a measurement module. The modulation module includes a modulation pump source and a frequency upconversion source. The modulation pump source emits a sequence of orthogonally polarized double-pulse pump light, which is synchronized with the received quantum light (i.e., the quantum light signal). The relative phase between the two pulses can be flexibly modulated as needed. This pump sequence can be obtained by directly modulating the laser, or by modulating the light pulses after the first laser is emitted; no restriction is imposed here. The modulated pump pulses and the signal light are incident together on the frequency upconversion source composed of a combination of nonlinear crystals, resulting in a sum-frequency process that generates mutually orthogonal polarized quantum states in two time windows. By adjusting the relative phase of the pump, the relative phase of the quantum states in the two time windows after sum-frequency generation can be changed.

[0076] The measurement module includes a random basis selection module, a quantum state detection module, and data post-processing. The random basis selection module randomly selects the required measurement basis according to the protocol requirements. This module can be implemented by modulating the pump pulse in the modulation module, or passively using a beam splitter; no specific restrictions are imposed, as long as random basis selection can be achieved.

[0077] If time-bin superposition state measurement is chosen, such as X-based or Y-based measurement, the preceding and following orthogonally polarized pulses can be reconstructed into a polarization superposition state, and then the polarization state of the photon can be measured. For Z-based measurement, the photon arrival time is measured directly.

[0078] Figure 3 A schematic diagram of the structure of a quantum key distribution system according to a second embodiment of this application is shown.

[0079] In the second specific embodiment, such as Figure 3 The quantum key distribution system shown has a 1550nm mode-locked pulsed laser as the second laser in the key transmitter. The quantum state preparation module includes a quarter-wave plate, a half-wave plate, an α-BBO crystal, and a polarization beam splitter. The mode-locked pulsed laser generates a coherent pulse sequence. The waveplate group adjusts the pulses to appropriate polarization states, so that each pulse, after passing through the α-BBO crystal, produces two orthogonally polarized pulses with different time bins due to birefringence. The polarization beam splitter projects the two pulses onto the same polarization, generating time-bin quantum states. By changing the optical axis direction of the waveplate group, four quantum states from two sets of mutually unbiased bases (Z-based and X-based) can be obtained. The key receiver can randomly select a basis and send quantum states by randomly rotating the waveplate group.

[0080] The key receiver includes a modulation module and a measurement module. The modulation module comprises a modulation pump source and a frequency up-conversion source. The modulation pump source consists of a mode-locked pulsed laser (i.e., the first laser) and a polarization-dependent unequal-arm interferometer. The laser emits a pulse sequence with a time interval of T and 45° angled polarization. The unequal-arm interferometer generates a sequence of orthogonally polarized double-pulse pump light, synchronized with the received quantum light signal. The frequency up-conversion source consists of a beam combiner and two second-order nonlinear crystals (i.e., the first crystal and the second crystal). The two crystals satisfy type II and type I phase matching, respectively, corresponding to the sum-frequency processes of the preceding and following time windows. For the time-bin superposition state, it, along with the pump light, generates a sequence of orthogonally polarized double-pulse sum-frequency light through the frequency up-conversion source. The preceding pulse is V-polarized, and the following pulse is H-polarized, which is then transmitted to the measurement module via a quantum channel.

[0081] The measurement module in the key receiver includes a 50:50 beamsplitter, an α-BBO crystal, a half-wave plate, a polarization correlation interferometer, a silicon-based single-photon detector, and data post-processing. X-based and Z-based measurements are performed on the transmission and reflection paths after the beamsplitter, respectively. If the quantum state preparation module sends an X-based quantum state, the α-BBO crystal at the beamsplitter's transmission end reassembles the orthogonally polarized pulses into linearly polarized light. This light is then rotated to horizontal or vertical polarization by the half-wave plate, resulting in only one counting window after passing through the polarization correlation interferometer, while two counting windows appear at the beamsplitter's reflection end. If the preparation module sends a Z-based quantum state, the α-BBO crystal and half-wave plate at the beamsplitter's transmission end rotate the pulses to diagonal or anti-diagonal polarization, resulting in two counting windows after passing through the polarization correlation interferometer, while only one counting window appears at the beamsplitter's reflection end. Finally, during data post-processing, four discrete counting windows appear: the first two corresponding to X-based measurements and the last two to Z-based measurements. A definite measurement result is only obtained when the preparation and measurement use the same basis. According to the BB84 protocol, the sender selects two sets of random numbers for basis selection and quantum state generation, respectively. After frequency up-conversion modulation, the receiver selects a set of random numbers to determine whether to perform X-basis or Z-basis measurement. After completing one round of the above operations, the receiver publishes the basis selection and measurement results, retains the measurement results under the same basis as the sender, and obtains the final quantum key after data post-processing.

[0082] Figure 4 A schematic diagram of the structure of a quantum key distribution system according to a third embodiment of this application is shown.

[0083] In the third specific embodiment, such as Figure 4 The quantum key distribution system shown has a 1550nm mode-locked pulsed laser as the second laser in the key transmitter. The quantum state preparation module includes a polarization controller, an α-BBO crystal, and a polarization beam splitter. The second laser generates a coherent pulse sequence with a time interval of T. The polarization controller adjusts the pulses to a suitable polarization state, so that each pulse, after passing through the α-BBO crystal, generates two orthogonally polarized pulses with different time bins due to birefringence. The polarization beam splitter projects the two pulses onto the same polarization, generating a time-bin quantum state. , Represents the pre-pulse. Represents the afterpulse. , This is the phase added by the polarization controller. By adjusting the polarization controller, the relative phase of the time-bin superposition states can be changed. 0, and This yields four quantum states from two sets of mutually unbiased bases (X-based and Y-based). The key transmitter can randomly select bases to send quantum states by randomly adjusting the polarization controller.

[0084] The modulation module in the key receiver includes a modulation pump source and a frequency up-conversion source. The first laser in the modulation pump source is a 1900nm mode-locked pulsed laser, whose output pulse sequence with a time interval of T is converted to 45° linear polarization by polarization beamsplitter 1 and a half-wave plate. An unequal-arm interferometer composed of polarization beamsplitter 2 and 3, along with two mirrors, delays the V component of the pump pulse relative to the H component. The phase shifter adjusts the relative phase of the two components to 0 or... The X-based or Y-based measurement is selected. A sequence of orthogonally polarized double-pulse pump light is generated and sent to the frequency upconversion source. A dichroic mirror in the frequency upconversion source reflects the pump light (i.e., the double-pulse pump light sequence) and simultaneously transmits the signal light (i.e., the quantum light signal), causing them to propagate collinearly. The trigger signals of the first and second lasers are adjusted to synchronize the preceding and following pulses of the signal light and pump light. The signal light pulse sequence is controlled to be V-polarized. In the first time window, the signal light and pump light are orthogonally polarized, generating V-polarized sum-frequency pulses in a type-II periodically polarized lithium niobate crystal; in the second time window, the signal light and pump light are identically polarized, generating H-polarized sum-frequency pulses in a type-I periodically polarized lithium niobate crystal. The center wavelength of the converted sum-frequency light is 854 nm, and residual unconverted components are filtered out by a short-pass filter. Through the action of the frequency upconversion source, the converted photons... With polarization state correspond, and correspond.

[0085] The measurement module in the key receiver includes: an α-BBO crystal, a half-wave plate, a first polarization correlation interferometer, a silicon-based single-photon avalanche diode detector (i.e., a first silicon-based detector), and data post-processing. When the same basis is selected for fabrication and measurement, the relative phase of the converted pulses is 0 or 0. In an α−BBO crystal, the orthogonally polarized pulses are reconstituted into linearly polarized light, which is then rotated to horizontal or vertical polarization by a half-wave plate. After passing through a first polarization correlation interferometer, only one counting window is available. When different bases are chosen for preparation and measurement, the relative phase of the converted pulses is... or- In the α-BBO crystal, the recombined orthogonally polarized pulses become circularly polarized light. After passing through a half-wave plate, the light remains circularly polarized. After passing through the first polarization-correlated interferometer, two counting windows appear. According to the BB84 protocol, the key transmitter selects two sets of random numbers for basis selection and quantum state generation, respectively. After frequency up-conversion modulation, the key receiver selects a set of random numbers to determine whether to perform X-basis or Y-basis measurement. After completing one round of the above operations, the key receiver publishes the basis selection and measurement results, retaining the measurement results under the same basis as the key transmitter. After data post-processing, the final quantum key is obtained.

[0086] To more clearly illustrate the specific structure and signal flow of the key receiver, the following... Figures 5 to 7 The embodiments shown are described in detail, in which the key sender has an entanglement module.

[0087] Figure 5 A structural block diagram of a quantum key distribution system according to a fourth embodiment of this application is shown.

[0088] In the fourth specific embodiment, such as Figure 5 The quantum key distribution system shown comprises a key transmitter and two key receivers, namely receiver 1 and receiver 2 in the diagram. The key transmitter is responsible for preparing entangled states in the time-bin degrees of freedom and sending them to the two key receivers. The two key receivers are responsible for measuring the quantum state of photons.

[0089] The key transmitting end includes a pump pulse module (i.e., the third laser) and an entanglement module. The pump pulse module generates coherent pulse signals, which can be obtained through continuous light chopping, an unequal-arm interferometer, the walk-off effect in a birefringent crystal, or a mode-locked laser; the specific generation method is not limited. The generated pulse signal pumps the entanglement module, producing time-bin entangled photon pairs in the communication band. The arrival times of each pair of entangled photons are indistinguishable, but the arrival times of the two photons are identical. Subsequently, the two entangled photons are transmitted to receiver 1 and receiver 2 respectively via quantum channels. The entanglement module can employ a spontaneous parametric down-conversion source or a spontaneous four-wave mixer source.

[0090] Receiver 1 and Receiver 2 have identical structures, consisting of a modulation module and a measurement module. The modulation module includes a modulation pump source and a frequency up-conversion source. The modulation pump source emits a sequence of orthogonally polarized double-pulse pump light, synchronized with the received quantum light. The relative phase between the double pulses can be flexibly modulated as needed. This pump sequence can be obtained by directly modulating the laser, or by modulating the light pulses after laser emission; no restriction is imposed here. The modulated pump pulses and the signal light are simultaneously incident on the frequency conversion module, which is composed of a combination of nonlinear crystals, resulting in a sum-frequency process that generates mutually orthogonal polarized quantum states in two time windows. By adjusting the relative phase of the pump, the relative phase of the quantum states in the two time windows after sum-frequency generation can be changed.

[0091] The measurement module includes a random basis selection module, a quantum state detection module, and data post-processing. The random basis selection module randomly selects the required measurement basis according to the protocol requirements. If a time-bin superposition state measurement is selected, such as X-basis or Y-basis measurement, the preceding and following orthogonally polarized pulses can be reconstructed into a polarization superposition state, and then the polarization state of the photon can be measured. The quantum state detection module accumulates counts over a period of time before performing data post-processing.

[0092] Receiver 1 and Receiver 2 communicate using classical methods, retain the measurement results under the same base selection, and generate the final key.

[0093] According to embodiments of this application, the frequency of the modulated communication band entangled photon pair is upconverted to the sensitive band of the silicon-based detector. Since the detection efficiency of the silicon-based single-photon detector at room temperature is higher than that of the communication band single-photon detector (such as the InGaAs single-photon avalanche diode detector), the use of the silicon-based single-photon detector in the measurement module can effectively improve the detection efficiency.

[0094] Figure 6 A schematic diagram of the structure of a quantum key distribution system according to a fifth embodiment of this application is shown.

[0095] In the fifth specific embodiment, such as Figure 6 The quantum key distribution system shown has a third laser in the key transmitter that is a mode-locked laser in the communication band. The entanglement module includes an optical amplifier, a frequency-doubling crystal, an unequal-arm Mach-Zehnder interferometer, and a spontaneous parametric down-conversion crystal. The mode-locked laser in the communication band generates a long sequence of coherent pulses with a time interval of T, which is then amplified by the optical amplifier to ensure sufficient pump power for the subsequent spontaneous parametric down-conversion process. The frequency-doubling crystal is used to frequency-double the amplified communication band pulse sequence, thereby generating entangled photon pairs in the communication band through the spontaneous parametric down-conversion process. The unequal-arm Mach-Zehnder interferometer consists of two 50:50 optical beamsplitters and two mirrors, which convert a single pulse into a pulse with a time delay by utilizing the optical path difference between the long and short arms. The two pulses before and after. Here It must be less than T, but also much greater than the photon coherence time, to ensure that each pulse in the frequency doubling pulse sequence can generate a time-bin superposition state. , Represents the pre-pulse. Represents the afterpulse. , This refers to the phase added to the back pulse by the long arm of the interferometer. Using the frequency-doubled pump light, two low-frequency photons are simultaneously generated in the spontaneous parametric downconversion crystal, producing a time-bin entangled state. In this embodiment, a zero-type quasi-phase-matched spontaneous parametric down-conversion crystal is used to ensure that the quantum properties of the generated entangled photon pairs are completely identical. Therefore, modulation modules 1 and 2 can use the same configuration to modulate the received quantum state.

[0096] Both key receivers' modulation modules include a modulation pump source and a frequency up-conversion source. The modulation pump source consists of a mode-locked pulsed laser (i.e., the first laser) and a polarization-dependent unequal-arm interferometer (i.e., the unequal-arm interferometer). The first laser emits a pulse sequence with a time interval of T and a 45° angled polarization. After passing through the unequal-arm interferometer, a series of orthogonally polarized intervals are generated. A dual-pulse pump light sequence is generated and synchronized with the received dual-pulse signal light sequence. The frequency up-conversion source consists of a beam combiner and two second-order nonlinear crystals (i.e., the first crystal and the second crystal). The two crystals satisfy type II phase matching and type I phase matching, respectively, corresponding to the sum-frequency processes of the preceding and following time windows. The signal light and pump light generate a series of dual-pulse sum-frequency light sequences through the frequency up-conversion source, with the first pulse being V-polarized and the second pulse being H-polarized. The time-bin entanglement characteristics of the signal light are preserved and then sent to the measurement module for interferometric measurement.

[0097] Both key receivers include a measurement module comprising a polarization-correlated unequal-arm interferometer, a single-photon detector, and data post-processing. Here, the polarization-correlated interferometer is used for superposition basis measurements, and its arm length difference needs to precisely correspond to the delays of preceding and following pulses. This allows for the recombination of preceding and following pulses, resulting in a superposition of H and V states at the interferometer exit. Consequently, the detector only experiences a single counting time window. Time-based measurements can be performed simply by removing the polarization-dependent interferometer. After accumulating counts over a period, data post-processing is performed. Finally, classical communication is used to retain the measurement results under the same basis selection, generating the final quantum key.

[0098] It is important to note that the repetition frequency of the pump light and the signal light pulsed laser, as well as the pulse delays before and after, must be completely identical to ensure that the centers of the pulses are aligned when the modulated pump light and signal light arrive at the frequency upconversion source. Since the signal light received by the frequency upconversion source is at the single-photon level, the generated sum-frequency light is also at the single-photon level and is easily overwhelmed by noise light of other wavelengths. The pump light is very strong and will generate frequency-doubled light in the crystal. If the pump light and signal light wavelengths are close, the frequency-doubled light will overwhelm the sum-frequency quantum light, affecting detection. Therefore, the center wavelength of the modulated pump source needs to be far from the center wavelength of the signal light. The pulse width of the pump pulse should be as large as possible compared to the signal pulse to ensure that the pulse waveforms before and after conversion are basically consistent. The two crystals in the frequency upconversion source need to be of the same type, and the nonlinear coefficient matrix of this crystal must contain matrix elements corresponding to the two types of phase-matched elements. By designing the lengths of the two crystals, the conversion efficiency of the pulses before and after conversion can be made the same. If there is a group velocity mismatch in the sum-frequency generation process of the pulsed light, the waveform of the sum-frequency light will be distorted compared to the signal light after passing through a longer crystal. Therefore, the crystal in the frequency upconversion source cannot be too long to reduce the impact of group velocity mismatch on the quantum state after modulation.

[0099] Figure 7 A schematic diagram of the structure of a quantum key distribution system according to a sixth embodiment of this application is shown.

[0100] In the sixth specific embodiment, such as Figure 7 The quantum key distribution system shown includes a key transmitter and two key receivers. The two key receivers share a modulation module, which can be set in either key receiver.

[0101] The third laser in the key transmitter is a 1550nm mode-locked laser. The entanglement module includes: an erbium-doped fiber amplifier, an unequal-arm Mach-Zehnder interferometer, a polarization controller, a circulator, a zero-type periodically polarized lithium niobate waveguide, a mirror, and a dense wavelength division multiplexer. The third laser generates a long sequence of coherent pulses with a time interval of T, which is then amplified using the erbium-doped fiber amplifier. The unequal-arm Mach-Zehnder interferometer consists of two 50:50 fiber beamsplitters, which convert a single pulse into a delayed pulse by using the optical path difference between the long and short arms. The two pulses before and after generate a Time-bin superposition state. , Represents the pre-pulse. Represents the afterpulse. , This refers to the phase added to the back pulse by the long arm of the interferometer. The polarization controller adjusts the optical pulse to a suitable polarization state, generating a 775nm frequency-harmonic pulse through a periodically polarized lithium niobate waveguide. This pulse is reflected back to the periodically polarized lithium niobate waveguide by a mirror, where it undergoes a spontaneous parametric down-conversion process to generate a 1550nm photon pair, thus creating a Time-bin entangled state. The dense wavelength division multiplexer then distributes the entangled photon pairs to two quantum channels for transmission, and then uses an optical fiber coupler to couple the signal light into free space.

[0102] In the two key receivers, the first laser of the modulation pump source is a 1900nm mode-locked pulsed laser, whose output pulse sequence with a time interval of T is converted into a 45° linearly polarized pulse by polarization beamsplitter 1 and a half-wave plate. The unequal-arm interferometer composed of polarization beamsplitters 2 and 3 and two mirrors delays the V component of the pump pulse relative to the H component. A sequence of orthogonally polarized dual-pulse pump light is generated and sent to the frequency upconversion sources in two key receivers via a 50:50 beam splitter. A dichroic mirror in the frequency upconversion source reflects the pump light while simultaneously transmitting the signal light, causing them to propagate collinearly. The trigger signals of the two lasers are adjusted to synchronize the preceding and following pulses of the signal light with those of the pump light. The signal light pulse sequence is controlled to be V-polarized. In the first time window, the signal light and pump light are orthogonally polarized, generating V-polarized sum-frequency pulses in a type-II periodically polarized lithium niobate crystal; in the second time window, the signal light and pump light are polarized in the same direction, generating H-polarized sum-frequency pulses in a type-I periodically polarized lithium niobate crystal. The center wavelength of the upconverted sum-frequency light is 854 nm, and residual unconverted components are filtered out by a short-pass filter. Through the action of the frequency upconversion source, the entangled photon pairs... With polarization state correspond, and correspond.

[0103] The measurement modules 1 and 2 in the two key receivers include: a polarization-dependent unequal-arm interferometer, a silicon-based single-photon avalanche diode detector, and data post-processing. During superposition basis measurements, the two interferometers receive dual-pulse and frequency-frequency optical sequences from two frequency up-conversion sources, respectively. The arm length difference needs to precisely correspond to the delay of the preceding and following pulses. This allows for the recombination of pulses before and after the interferometer exit, ensuring that both detectors can only detect photons within the same time window each time. Time-based measurements can be performed simply by removing the polarization-dependent interferometer and directly measuring the photon arrival time. Post-processing of the obtained data, along with classical communication to retain the measurement results under the same basis selection, yields the final key.

[0104] The key distribution method based on frequency upconversion technology provided in this application achieves efficient conversion of signal light polarization state and frequency, and active selection of the measurement basis through active modulation and time synchronization of pump pulses and second-order nonlinear sum-frequency effects. The measurement scheme using a polarization-correlated interferometer and a silicon-based single-photon detector eliminates the 3dB loss in the original Time-bin encoded superposition basis measurement, while simultaneously improving photon detection efficiency. The frequency upconversion polarization modulation scheme provided by this invention can significantly improve the communication efficiency of Time-bin encoded quantum key distribution systems, which has important scientific significance and practical value for promoting the practical application of Time-bin encoding.

[0105] Figure 8 A schematic flowchart of a quantum key distribution method according to an embodiment of this application is shown.

[0106] like Figure 8 As shown, the quantum key distribution method includes operations S801 to S804.

[0107] In operation S801, the time-encoded quantum optical signal of the quantum state sent by the key transmitter is acquired.

[0108] In operation of S802, a dual-pulse pump light sequence is generated using a modulated pump source.

[0109] In operation of S803, a frequency upconversion source is used to sum-frequency the received quantum optical signal based on a dual-pulse pump optical sequence to generate mutually orthogonal quantum states in two consecutive time windows, thereby obtaining a sum-frequency optical sequence with a relative phase change in the two time windows.

[0110] When operating the S804, the measurement module is used to measure the sum-frequency optical sequence, the measurement results are obtained, and a quantum key is generated based on the measurement results.

[0111] The quantum key distribution method part in the embodiments of this application corresponds to the quantum key distribution system part in the embodiments of this application. The description of the quantum key distribution method part is specifically referred to in the quantum key distribution system part, and will not be repeated here.

[0112] The embodiments of this application have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of this application. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. This application does not depart from its scope, and those skilled in the art can make various substitutions and modifications, all of which should fall within the scope of this application.

Claims

1. A key receiver for a quantum key distribution system, characterized in that, include: Modulated pump source for generating dual-pulse pump light sequence; A frequency upconversion source is used to sum-frequency the received time-encoded quantum optical signal based on a dual-pulse pump light sequence to generate mutually orthogonal polarization quantum states in two consecutive time windows, thereby obtaining a sum-frequency light sequence with relative phase changes in the two time windows; The measurement module is used to measure the sum-frequency optical sequence, obtain the measurement result, and generate a quantum key based on the measurement result.

2. The key receiving end according to claim 1, characterized in that, The modulated pump source is used to generate a sequence of orthogonally polarized double-pulse pump light.

3. The key receiving end according to claim 2, characterized in that, The modulation pump source includes: A first laser is used to generate a pulse sequence with a predetermined time interval at a target polarization angle; An unequal-arm interferometer is used to perform interference processing on the pulse sequence to obtain an orthogonally polarized double-pulse pump light sequence.

4. The key receiving end according to claim 1, characterized in that, The frequency upconversion source is used to perform sum-frequency conversion through type II phase-matched and type I phase-matched crystals, wherein type II phase-matching and type I phase-matching correspond to the sum-frequency conversion of the preceding and following time windows, respectively.

5. The key receiving end according to claim 4, characterized in that, The frequency upconversion source includes: A beam combiner is used to combine the dual-pulse pump light sequence and the quantum light signal into a combined light sequence; A first crystal and a second crystal are used to perform sum-frequency analysis on the combined light sequence based on the second-order nonlinear sum-frequency effect to obtain the sum-frequency light sequence, wherein the first crystal and the second crystal are second-order nonlinear crystals that satisfy type II phase matching and type I phase matching, respectively. This also includes: A filter is used to filter the light generated by the laser in the modulation pump source in the sum-frequency optical sequence.

6. The key receiving end according to claim 1, characterized in that, The measurement module includes: Barium borate crystals are used to convert the sum-frequency light sequence into target polarized light, wherein the target polarized light includes linearly polarized light or circularly polarized light; A half-wave plate is used to adjust the polarization direction of the target polarized light; The first polarization correlation interferometer is used to generate interference signals by controlling and separating the polarization state of target polarized light based on adjusting the polarization direction; A first silicon-based detector is used to detect the interference signal and obtain a first detection result, and to process the first detection result to obtain the quantum key.

7. The key receiving end according to claim 6, characterized in that, Also includes: A beam splitter is used to split the sum-frequency optical sequence to obtain a first sum-frequency optical sequence corresponding to the X-based basis and a second sum-frequency optical sequence corresponding to the Z-based basis, wherein the first sum-frequency optical sequence is input to the barium borate crystal; A second polarization correlation interferometer is used to modulate and separate the polarization states based on the second sum-frequency optical sequence to generate a second interference signal; A second silicon-based detector is used to detect the second interference signal and obtain a second detection result, so as to determine the same detection result as the basis of the key sending end from the first detection result and the second detection result, and process it to obtain the quantum key.

8. The key receiving end according to claim 1, characterized in that, The measurement module includes: A polarization-dependent unequal-arm interferometer is used to perform superposition basis measurements on the sum-frequency optical sequence to obtain the light intensity signal; A third silicon-based detector detects the light intensity signal and obtains a third detection result; In this process, multiple key receivers communicate with each other to perform basis selection on the measurement results from multiple key receivers, thereby generating quantum keys by processing the measurement results with the same basis.

9. A quantum key distribution system, characterized in that, include: A key transmitting end is used to generate a time-encoded quantum optical signal and transmit the quantum optical signal to a receiving end; The key receiver as described in any one of claims 1 to 8 is used to detect the received quantum optical signal to obtain a quantum key.

10. A quantum key distribution method, characterized in that, The method includes: Acquire the quantum optical signal of the time-encoded quantum state sent by the key transmitter; A dual-pulse pump light sequence is generated using a modulated pump source; The received quantum optical signal is sum-frequencyd using a frequency upconversion source based on a dual-pulse pump light sequence to generate mutually orthogonal quantum states in two consecutive time windows, thereby obtaining a sum-frequency light sequence with relative phase changes in the two time windows. The sum-frequency optical sequence is measured using a measurement module to obtain the measurement results, and a quantum key is generated based on the measurement results.

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