Quantum communication system and method based on high-dimensional entangled light source

Abstract

The invention provides a quantum communication system and method. The method comprises the steps that a first communication end and a second communication end generate a string of coherent dual-mode compression states based on a high-dimensional entanglement source respectively, photons in one mode in the dual-mode compression states are sent to an intermediate node, and photons in the other mode are measured locally; jointly measuring the photons from the first communication end and the second communication end by the intermediate node; the first communication end and the second communication end respectively determine time windows with detector responses of the first communication end, the second communication end and the intermediate node as effective windows based on detector response information of the first communication end and the second communication end, detector response information obtained from the opposite side and detector response information obtained from the intermediate node; and pairing detection results of a first effective window and a different second effective window in the effective windows so as to realize entanglement between local photons of the first communication end and the second communication end in a specified mode represented by the paired effective windows.

Description

Technical Field

[0001] The present application relates to the field of quantum communication, and specifically to a quantum communication system and method based on a high-dimensional entangled light source. Background Art

[0002] The rise of quantum information technology has greatly promoted the development of modern science and technology. It will bring capabilities that traditional technologies cannot achieve, including unconditionally secure communication, more accurate measurement perception, and super computing power, etc. After decades of intensive research, the field of quantum information has made significant progress, among which core technologies such as quantum communication have gradually entered the practical stage. As a technology for transmitting quantum states between users who are far apart, quantum communication can serve as the infrastructure for many distributed quantum information tasks. However, due to the inevitable loss in the channel, photons will decay exponentially with distance after being transmitted through the channel. The no-cloning theorem in quantum mechanics determines that people cannot amplify or copy the encoded quantum signal, so only a small number of photons can be successfully detected after long-distance transmission. For example, the sender sends quantum signals every second. After a photon is transmitted through a 1000km optical fiber channel, only Photons are transmitted every 300 years, equivalent to receiving only one photon every 300 years. This result indicates that channel loss will make long-distance quantum communication unrealistic. Researchers currently hope to overcome this problem by developing quantum repeaters to achieve segmented transmission. However, due to the limitations of quantum memory development, quantum relay technology is not yet practical. Summary of the Invention

[0003] In view of this, the present application provides a quantum communication system and method based on a high-dimensional entangled light source.

[0004] According to the first aspect of the present application, a quantum communication method is provided, comprising: a first communication end and a second communication end each generating a series of coherent two-mode compressed states based on a high-dimensional entangled source, sending photons on one mode in the two-mode compressed state to an intermediate node, and locally measuring photons on the other mode in the two-mode compressed state; the intermediate node jointly measuring the photons from the first communication end and the photons from the second communication end; the first communication end and the second communication end each determining, based on their own detector response information, the detector response information obtained from the other party, and the detector response information obtained from the intermediate node, a time window in which the first communication end, the second communication end, and the intermediate node all have detector responses as an effective window, and pairing the detection result of the first effective window in the effective window with the detection result of a different second effective window, so as to achieve entanglement between the local photons of the first communication end and the second communication end in the specified mode represented by the paired effective window.

[0005] According to an embodiment, local measurement can be performed by causing a photon in the other mode of the two-mode squeezed state to undergo HOM interference with a locally generated auxiliary photon. The auxiliary photon can be obtained by attenuating the auxiliary light. Alternatively, a high-dimensional entangled source can utilize a pump light pulse sequence to pump a nonlinear crystal to generate a series of coherent two-mode squeezed states. The repetition frequencies of the pump light pulse sequences and the auxiliary light of the first and second communication terminals can be synchronized.

[0006] According to an embodiment, the pump light pulse sequence may include a series of coherent pulses, and adjacent pulses may have a fixed phase difference. The photons sent by the first communication end and the second communication end to the intermediate node may have a consistent spectrum.

[0007] According to an embodiment, the photon on the other mode in the dual-mode squeezed state and the auxiliary photon may have a consistent wavelength and a consistent spectrum.

[0008] According to an embodiment, the method may further include: in response to the detector responses in the first effective window and the second effective window both coming from the same detector, indicating that the projection to the quantum state On, where the quantum state is the quantum state prepared locally in the first and second effective windows, and Represent the first effective window and the second effective window respectively, where m and n are natural numbers and n>m; if the detector responses in the first effective window and the second effective window come from different detectors, then the projection to the quantum state is indicated. superior.

[0009] According to a second aspect of the present application, a quantum communication system is provided, comprising: an intermediate node; a first communication terminal and a second communication terminal, each configured to generate a coherent two-mode squeezed state using a high-dimensional entangled source, send photons in one mode of the two-mode squeezed state to the intermediate node, and locally measure photons in the other mode of the two-mode squeezed state. The intermediate node is configured to jointly measure the photons from the first communication terminal and the photons from the second communication terminal. The first communication terminal and the second communication terminal are each configured to: determine, based on their own detector response information, detector response information obtained from the other terminal, and detector response information obtained from the intermediate node, a time window in which the first communication terminal, the second communication terminal, and the intermediate node all have detector responses as an effective window; and pair the detection result of the first effective window with the detection result of a different second effective window within the effective window to achieve entanglement between the local photons of the first communication terminal and the second communication terminal in a specified mode represented by the paired effective window.

[0010] According to an embodiment, a high-dimensional entangled source can be configured to pump a nonlinear crystal using a sequence of pump light pulses to generate a series of two-mode squeezed states. The first and second communication terminals can each be configured to cause a photon in another mode of the two-mode squeezed state to undergo HOM interference with an auxiliary photon, allowing for local measurement. The auxiliary photon can be obtained by attenuating the auxiliary light. The pump light pulse sequences and the repetition frequencies of the auxiliary light at the first and second communication terminals can be synchronized.

[0011] According to an embodiment, the auxiliary photon may have a wavelength and a spectrum consistent with those of a photon on another mode in the two-mode squeezed state.

[0012] According to an embodiment, the pump light pulse sequence may include a series of coherent pulses, and adjacent pulses have a fixed phase difference. The photons sent by the first communication end and the second communication end to the intermediate node may have a consistent spectrum.

[0013] According to an embodiment, the first communication end and the second communication end may generate respective pump light pulse sequences and auxiliary light based on the same seed light.

[0014] According to an embodiment, a high-dimensional entangled source may include: a nonlinear crystal configured to generate a series of coherent two-mode squeezed states based on a sequence of pump light pulses; and a separation device configured to separate photons on one mode from photons on another mode.

[0015] According to an embodiment, the nonlinear crystal may generate photon pairs based on spontaneous parametric down conversion, and the separation device may include a polarization beam splitter. Alternatively, the nonlinear crystal may generate photon pairs based on spontaneous four-wave mixing, and the separation device may include a dense wavelength division multiplexer.

[0016] According to an embodiment, the quantum communication system may further include: a modulation module configured to modulate pulses in the pump light pulse sequence to a desired optical field state.

[0017] According to an embodiment, the first communication end and the second communication end may each include: a beam splitter configured to receive auxiliary photons and photons on another mode in the above-mentioned dual-mode squeezed state; and a first detector and a second detector respectively connected to different output ports of the beam splitter.

[0018] According to an embodiment, the first communication end and the second communication end are each configured to: in response to the detector responses in the first effective window and the second effective window both coming from the same detector in the first detector and the second detector, indicate that the quantum state projected onto On, where the quantum state are the locally prepared quantum states in the first and second effective windows, and Represent the first effective window and the second effective window respectively, where m and n are natural numbers and n>m; the detector responses in the first effective window and the second effective window come from different detectors in the first detector and the second detector respectively, indicating that the projection to the quantum state superior.

[0019] According to the embodiments of the present application, entanglement exchange based on this pairing can greatly reduce the impact of channel loss, thereby greatly improving the efficiency of long-distance quantum communication. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above contents and other objects, features and advantages of the present application will become more apparent through the following description of the embodiments of the present application with reference to the accompanying drawings, in which:

[0021] Figure 1 A schematic diagram of a quantum communication system according to an embodiment of the present application is shown;

[0022] Figure 2 A schematic diagram of a high-dimensional entangled source according to an embodiment of the present application is shown;

[0023] Figure 3 shows a schematic diagram of a measurement module according to an embodiment of the present application;

[0024] Figure 4 A schematic diagram of a communication terminal according to an embodiment of the present application is shown;

[0025] Figure 5 A schematic diagram showing a communication terminal according to another embodiment of the present application; and

[0026] Figure 6 A schematic diagram of a quantum communication method according to an embodiment of the present application is shown. DETAILED DESCRIPTION

[0027] Hereinafter, embodiments of the present application will 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 the present application. In the detailed description below, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present application. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessarily confusing the concepts of the present application.

[0028] The terms used herein are only for describing specific embodiments and are not intended to limit this application. The terms "comprise," "include," etc. used herein indicate the presence of the features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0029] 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 should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0030] When expressions such as "at least one of A, B, and C, etc." are used, they should generally be interpreted in accordance with the meaning commonly understood by those skilled in the art (for example, "a system having at least one of A, B, and C" should include but is not limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.).

[0031] Figure 1 A schematic diagram of a quantum communication system according to an embodiment of the present application is shown.

[0032] like Figure 1 As shown, the quantum communication system 100 according to this embodiment may include a first communication terminal 1000 , a second communication terminal 2000 and an intermediate node 3000 .

[0033] The first communication terminal 1000 and the second communication terminal 2000 may be devices that wish to communicate with each other and may be located at a relatively long distance from each other. For example, the first communication terminal 1000 may send information to the second communication terminal 2000, and vice versa. The intermediate node 3000 may facilitate communication between the first communication terminal 1000 and the second communication terminal 2000. For example, the intermediate node 3000 may facilitate entanglement exchange.

[0034] According to an embodiment, the first communication terminal 1000 and the second communication terminal 2000 can each generate a series of coherent two-mode squeezed states based on a high-dimensional entangled source, send photons on one mode of the two-mode squeezed state to the intermediate node 3000, and locally measure photons on the other mode. The intermediate node 3000 can jointly measure the photons from the first communication terminal 1000 and the second communication terminal 2000. The first communication terminal 1000 and the second communication terminal 2000 can each determine, based on their own detector response information, the detector response information obtained from the other party, and the detector response information obtained from the intermediate node 3000, a time window in which the first communication terminal 1000, the second communication terminal 2000, and the intermediate node 3000 all have detector responses as an effective window, and pair the detection results of the first effective window with the detection results of a different second effective window within the effective window to achieve entanglement between the local photons of the first communication terminal 1000 and the second communication terminal 2000 in the specified mode represented by the paired effective window.

[0035] According to the embodiments of the present application, post-selective pairing of measurement results maximizes the use of effective detection events, thereby reducing the impact of channel loss and significantly improving communication rates over long distances. Furthermore, quantum relay-based solutions are currently difficult to implement due to the difficulties in implementing quantum storage. However, according to the embodiments of the present application, quantum storage is not required, significantly reducing implementation difficulties.

[0036] Here, the so-called "high-dimensional" refers to more than two dimensions, for example, three or more dimensions. According to embodiments of the present application, a high-dimensional entangled light source can be realized based on nonlinear effects such as spontaneous parametric down-conversion or spontaneous four-wave mixing.

[0037] The high-dimensional entangled source can use a pump light pulse sequence to pump a nonlinear crystal to produce a series of coherent two-mode compressed states. The pump light pulse sequence can include a series of coherent pulses, and adjacent pulses can have a fixed phase difference. In addition, the first communication terminal 1000 and the second communication terminal 2000 can each cause the photons on the other mode in the dual-mode compressed state to undergo HOM interference with the auxiliary photons to perform local measurements. The auxiliary photons can be obtained by attenuating the auxiliary light. The auxiliary light can have the same repetition frequency as the pump light pulse sequence, and the auxiliary photons can have the same wavelength and spectrum as the photons on the other mode in the dual-mode compressed state.

[0038] According to an embodiment, the pump light pulse sequences of the first communication terminal 1000 and the second communication terminal 2000 can have the same repetition frequency, and the photons sent to the intermediate node 3000 can have a consistent spectrum. The repetition frequencies of the pump light pulse sequences and auxiliary light of the first communication terminal 1000 and the second communication terminal 2000 can be synchronized. For example, the first communication terminal 1000 and the second communication terminal 2000 can generate their respective pump light pulse sequences and auxiliary light based on the same seed light.

[0039] The first communication terminal 1000 may include a high-dimensional entangled source 1001, a measurement module 1003, and a data storage and post-processing module 1005. Although three modules are shown for illustrative purposes, this does not mean that the first communication terminal 1000 cannot include other modules, such as a classical communication module. Furthermore, several of the modules shown may be combined into the same module, or a particular module may be split into multiple modules.

[0040] High-dimensional entangled source 1001 can generate a series of coherent two-mode squeezed states. Photons in one mode of the two-mode squeezed state (referred to as "signal photons") can be sent to intermediate node 3000. Meanwhile, photons in the other mode of the two-mode squeezed state (referred to as "local photons") can be retained locally, for example, and sent to measurement module 1003 for measurement.

[0041] Figure 2 A schematic diagram of a high-dimensional entangled source according to an embodiment of the present application is shown.

[0042] like Figure 2 As shown, the high-dimensional entangled light source may include a nonlinear crystal 202 and a separation device 204.

[0043] The nonlinear crystal 202 can generate a series of coherent two-mode compressed states based on a sequence of pump light pulses. The sequence of pump light pulses can include a series of coherent pulses, and adjacent pulses can have a fixed phase difference. Here, the quantum state generated by each pulse individually can be a two-mode compressed state. Due to the coherence between the pump pulses, when the brightness of the light source is low, the quantum state generated by the entire pulse sequence can be regarded as a high-dimensional entangled state. In terms of particle number representation, the quantum state generated by each pulse can be a pure state, and the quantum states generated by different pulses can be combined into a new particle number pure state. According to an embodiment of the present application, a light source with higher brightness can be used, and the brightness can be set so that the new quantum state formed by the combination of the two-mode compressed states at any two positions has a lower average number of photons. For example, due to the coherence between the pump pulses, different time windows can be represented as a superposition state, and when there is only one photon, the overall quantum state can be represented as a high-dimensional entangled state. ,in Indicates the time window, represents the phase difference between adjacent pump pulses, a represents the photon entering the local measurement module (i.e., local photon), and b represents the photon sent to the intermediate node (i.e., signal photon).

[0044] According to an embodiment of the present application, the nonlinear crystal 202 may include a barium borate crystal, a periodically poled lithium niobate crystal, a potassium titanyl phosphate crystal, or the like.

[0045] The separation device 204 can separate photons in one mode of the dual-mode squeezed state from photons in the other mode. The appropriate separation device 204 can be selected based on the characteristics of the dual-mode squeezed state generated by the nonlinear crystal 202, or the characteristics of the nonlinear process on which it depends. For example, if the nonlinear crystal 202 generates photon pairs based on spontaneous parametric down-conversion, the separation device 204 can include a polarization beam splitter to spatially separate photons in the two polarization modes. For another example, if the nonlinear crystal 202 generates photon pairs based on spontaneous four-wave mixing, the separation device 204 can include a dense wavelength division multiplexer (DWDM) to spatially separate the two modes of the dual-mode squeezed state.

[0046] Although an example implementation of a high-dimensional entangled light source is shown here, the specific implementation method of the high-dimensional entangled light source is not limited, as long as it is a photon pair light source that can produce coherent superposition of different coding modes and the particle number state is also a superposition state.

[0047] Return Reference Figure 1 The measurement module 1003 can perform local measurement by generating HOM interference between local photons and auxiliary photons.

[0048] Figure 3 A schematic diagram of a measurement module according to an embodiment of the present application is shown.

[0049] like Figure 3 As shown, the measurement module may include a beam splitter 306. For example, the beam splitter 306 may be a 50:50 beam splitter (i.e., a semi-transmissive, semi-reflective beam splitter). A first photon and a second photon (e.g., a local photon and an auxiliary photon) may be incident on the beam splitter 306 and may undergo HOM interference. Here, the optical path delay may be adjusted so that the first photon and the second photon arrive at the beam splitter 306 simultaneously. The beam splitter 306 may include two output ports, which are connected to a first detector 308 and a second detector 310, respectively. Based on the characteristics of HOM interference, one of the first detector 308 and the second detector 310 may produce a detector response in response to the photon incident on the beam splitter 306.

[0050] Auxiliary photons can be generated using auxiliary light. For example, auxiliary light generated by a laser can be attenuated by an attenuator to a level substantially equivalent to that of local photons to produce auxiliary photons. The repetition frequency of the auxiliary light can be the same as the repetition frequency of the pump light pulse sequence. Furthermore, the wavelength of the auxiliary photons can be consistent with the wavelength of the quantum light and can have a consistent (substantially identical or similar) spectrum. Of course, the present application is not limited to this. For example, when the auxiliary light or auxiliary photons have different wavelengths, they can be converted to a wavelength consistent with the quantum light through wavelength conversion.

[0051] Return Reference Figure 1 The data storage and post-processing module 1005 can store the measurement results of the measurement module 1003. For example, the measurement results can include the time when the detector response occurs and the detector (the first detector or the second detector) at which the detector response occurs.

[0052] The second communication terminal 2000 may include a high-dimensional entangled source 2001, a measurement module 2003, and a data storage and post-processing module 2005. The above description of the first communication terminal 1000 is also applicable to the second communication terminal 2000, and thus will not be repeated.

[0053] The pump light pulse sequences used by the high-dimensional entangled sources of the first communication terminal 1000 and the second communication terminal 2000 can have the same repetition frequency, and the photons sent to the intermediate node 3000 can have a consistent spectrum.

[0054] The intermediate node 3000 may include a measurement module 3001 and a data post-processing module 3003 .

[0055] The measurement module 3001 may have the above combination Figure 3 In the described structure, in the case of intermediate node 3000, the first photon and the second photon can be photons originating from first communication terminal 1000 and second communication terminal 2000, respectively. However, the implementation of measurement module 3001 is not limited to this, and can be any method capable of Bell state detection in any two modes. For example, when the photons transmitted by the communicating parties are indistinguishable, measurement can be achieved directly using an optical beam splitter. When the photons transmitted by the communicating parties are distinguishable, measurement can be achieved using wavelength conversion followed by a beam splitter.

[0056] By adjusting the optical path delay, photons from the first communication terminal 1000 and the second communication terminal 2000 can arrive at the measurement module 3001 simultaneously, thereby causing two-photon interference, thereby erasing the information that the photons originated from the first communication terminal 1000 and the second communication terminal 2000. When the photons from the first communication terminal 1000 and the second communication terminal 2000 arrive, the measurement module 3001 can perform a joint measurement of the photons, and the data post-processing module 3003 records the measurement results for each pulse position. The measurement results of any two positions can be combined, which is equivalent to performing Bell state detection in two modes. The measurement results can include the time when the detector response occurs.

[0057] For example, if is less than the coherence time of the pulse sequence, then and The two measurement results in the two time windows are combined to be considered as Bell state detection of the two time patterns. For example, when any detector in the measurement module is in the time window and (where i and j are natural numbers and j>i) all respond, then it is considered as a projection to When one of the detectors The other detector responds in the window When the window responds, it is considered as projected onto In actual operation, the probability of a photon existing in each pulse is significantly less than 0.5, and every two pulse positions can be combined into a time-bin quantum state.

[0058] After one of the probes of the measurement module 3001 responds, the response status can be notified to the first communication terminal 1000 and the second communication terminal 2000. When both the first communication terminal 1000 and the second communication terminal 2000 have probe responses in the time window, the detection result is retained. Following this rule, that is, when the first communication terminal 1000, the second communication terminal 2000 and the intermediate node 3000 all have probe responses in the same time window, the time window is regarded as a valid time window. In the first communication terminal 1000, the second communication terminal 2000 or the intermediate node 3000, if the first communication terminal 1000, the second communication terminal 2000 or the intermediate node 3000 has probe responses in the time window, the detection result is retained. Hedi (where m and n are natural numbers and ) time windows are valid, the detection results of these two time windows can be paired. After pairing, it is equivalent to performing and Bell state measurement on the mode (projected onto or For the first communication terminal 1000 and the second communication terminal 2000, it can be regarded as and If the quantum state prepared locally by the first communication terminal 1000 in these two valid time windows is , then when and When the responses in the time window come from the same detector, it means that the projection to On, among them, represents the probability distribution factor, Represents the relative phase factor; if the responses come from different detectors, it represents the projection to The same is true for the second communication terminal 2000, which will not be described in detail here. Finally, the first communication terminal 1000 and the second communication terminal 2000 can classify and process the data according to the measurement results to achieve a specific quantum information task.

[0059] Classical channels, such as various wired communication media such as optical cables or wireless communication media such as cellular networks, are still provided between the first communication end 1000 and the second communication end 2000, between the first communication end 1000 and the intermediate node 3000, and between the second communication end 2000 and the intermediate node 3000. The first communication end 1000 can send the detector response information in its measurement results to the second communication end 2000 via the classical channel. The second communication end 2000 can send the detector response information in its measurement results to the first communication end 1000 via the classical channel. The intermediate node can send the detector response information in its measurement results to the first communication end 1000 and the second communication end 2000 via the classical channel. The detector response information can indicate the time window in which the detector response occurs.

[0060] The detection at the intermediate node 3000 is the measurement result after the channel loss. According to the embodiment of the present application, the local measurement results of the first communication terminal 1000 and the second communication terminal 2000 can play the role of measurement and marking, ensuring that the measurement event at the intermediate node 3000 is caused by the photons sent by both communicating parties rather than by only one party. When the pairable pulse sequence is long enough, this post-selection pairing method can ensure that the effective detection event decreases more slowly with the channel loss, from the original e^(-αL) to e^(-αL / 2). Therefore, the quantum communication scheme of the present application will have a better communication rate in long-distance quantum communication.

[0061] In the measurement modules of the first communication terminal 1000, the second communication terminal 2000, and the intermediate node 3000, the first photon and the second photon incident on the beam splitter can arrive simultaneously. The repetition frequency of all lasers in the system can be consistent. The specific method for achieving repetition frequency synchronization is not limited here, as long as it can ensure that the arrival times of the photons in the measurement modules of the first communication terminal 1000, the second communication terminal 2000, and the intermediate node 3000 do not change with time and are detected in the same time window. The pulses originate from corresponding pulses.

[0062] Figure 4 A schematic diagram of a communication terminal according to an embodiment of the present application is shown.

[0063] The high-dimensional entangled source 4001 may include a nonlinear crystal 4011 and a polarization beam splitter 4013. Figure 1 and 2 The description of high-dimensional entangled sources also applies here. Nonlinear crystal 4011 can generate photon pairs under the action of pump light based on a nonlinear process such as spontaneous parametric down-conversion. In this example, nonlinear crystal 4011 can be a type-II phase-matched nonlinear crystal, so the photons in the two polarization modes can be spatially separated using polarization beam splitter 4013. One of the photons separated by polarization beam splitter 4013 (i.e., the signal photon) can be sent to an intermediate node (e.g., intermediate node 3000 described above) via a quantum channel, and the other (i.e., the local photon) can be sent to the local measurement module 4003.

[0064] The measurement module 4003 may include a 50:50 optical beam splitter 4031, a first detector 4033 and a second detector 4035. Figure 1 and 3 The description of the measurement module also applies here. Figure 1 and 3As described above, the optical beam splitter 4031 can receive local photons from the high-dimensional entangled source 4011 and auxiliary photons from the auxiliary source 4009. By adjusting the delay of the optical path, these two photons can arrive at the optical beam splitter 4031 simultaneously, causing HOM interference. The response information of the first detector 4033 and the second detector 4035 can be recorded by the data storage and post-processing module 4005 for subsequent selection and use.

[0065] Pump source 4007 can be used to generate pump light. To this end, pump source 4007 can include a laser 4071, which is used to generate a coherent pulse sequence. As described above, adjacent pulses in the sequence can have a fixed phase difference. According to embodiments of the present application, the coherent pulse sequence can be generated using a mode-locked laser or a cascaded interferometer.

[0066] In addition to laser 4071, pump source 4007 can be configured based on the pump light requirements of high-dimensional entangled source 4001. For example, pump source 4007 can include a modulation module 4073 for modulating the optical pulses emitted by laser 4071 to the desired optical field state. For example, the phase of each pulse can be modulated to ensure that the phase difference meets the matching requirements. In addition, pump source 4007 can include an optical amplifier 4075 for amplifying the pulsed light to ensure that the subsequent high-dimensional entangled source 4001 can obtain sufficient pump power.

[0067] In addition, depending on wavelength requirements, the pump source 4007 may also include a frequency doubling module 4077. For example, when the high-dimensional entangled source 4001 generates photon pairs based on the spontaneous parametric down-conversion process as described above, the wavelength of the pump light needs to be smaller than the wavelength of the communication band (the high-frequency pump light is converted into a pair of low-frequency photons, namely, a signal photon and a local photon, through the spontaneous parametric down-conversion process, with the signal photon having a wavelength in the communication band). When the wavelength of the laser light generated by the laser 4071 is in the communication band, the frequency doubling module 4077 can frequency-multiply the pulse sequence in the communication band to pump the nonlinear crystal 4011 in the high-dimensional entangled source 4001. Alternatively, the laser 4071 can directly use a laser near 775 nm, in which case the frequency doubling module 4077 is unnecessary.

[0068] Auxiliary source 4009 can be used to generate auxiliary photons. For example, auxiliary source 4009 can include a laser 4091, such as a mode-locked laser. Laser 4091 can have the same repetition frequency as laser 4071 in pump source 4007, and its wavelength can be consistent with the wavelength of the local photons and have a similar spectral shape. Similar to pump source 4007, auxiliary source 4009 can also include a modulation module for adjusting the intensity and phase of pulses at different positions. In addition, auxiliary source 4009 can include an attenuator 4093 for attenuating the laser light generated by laser 4091 to the same level as the local photons (to obtain "auxiliary photons"). As described above, the auxiliary photons can arrive at beam splitter 4031 simultaneously with the local photons from high-dimensional entangled source 4001, and undergo HOM interference.

[0069] In this embodiment, the pump source and the auxiliary source use different lasers, and the communication ends that communicate with each other can use their own lasers, and the repetition frequencies of these lasers can be synchronized.

[0070] Figure 5 A schematic diagram of a communication terminal according to another embodiment of the present application is shown.

[0071] like Figure 5 As shown, the communication terminal 5000 may include a high-dimensional entangled source 5001, a measurement module 5003, a data storage and post-processing module 5005, and a pump and auxiliary source 5009. The following mainly describes the differences from the above embodiment, and duplicate descriptions are omitted.

[0072] The high-dimensional entangled source 5001 may include a nonlinear crystal 5011 and a dense wavelength division multiplexer (DWDM) 5013. In this example, the nonlinear crystal 5011 can generate a sequence of dual-mode squeezed states based on spontaneous four-wave mixing. Because the pump pulse sequence has good coherence, these dual-mode squeezed states can form a new superposition state. When the number of photons is small, this becomes a high-dimensional time-bin entangled source. According to embodiments of the present application, a higher-brightness high-dimensional entangled source can be used, so that the quantum state formed by combining any two dual-mode squeezed states can be considered a two-dimensional entangled state. The DWDM 5013 can spatially separate the two modes of the dual-mode squeezed state, one of which (i.e., the signal photon) is sent to an intermediate node (e.g., the aforementioned intermediate node 3000) via a quantum channel, and the other (i.e., the local photon) is sent to the measurement module 5003.

[0073] The measurement module 5003 may include a 50:50 optical beam splitter 5031, a first detector 5033, and a second detector 5035. Regarding the measurement module 5003 and the data storage and post-processing module 5005, please refer to the above combined Figures 1 to 4 Description.

[0074] In this embodiment, the pump light and the auxiliary light are not generated by separate light sources. Instead, the pump and auxiliary source 5009 can generate the pump light and the auxiliary light based on the same source. For example, the pump and auxiliary source 5009 can receive a sequence of optical pulses from the laser 520, which serves as a seed laser, and generate the pump light and the auxiliary light based on the received seed light.

[0075] Here, the laser 520 can be a local laser of the communication terminal 5000. In this case, as described above, Figure 4 As described above, the laser 520 can be synchronized with the repetition frequency of other lasers in the system (for example, a laser in another communication terminal). According to other embodiments, the laser 520 can be a seed laser shared with other communication terminals, so that both communicating parties can operate based on the same seed light, ensuring that both parties have a strictly consistent repetition frequency and extremely low time jitter. This shared seed laser can be deployed in an intermediate node, so that the communication terminals connected to the intermediate node can use the same seed light. When the laser 520 is deployed remotely, it can be a mode-locked laser with a wavelength (for example, 1550nm) in the communication band.

[0076] The pump and auxiliary source 5009 can be configured according to the requirements for pump light and auxiliary light.

[0077] For example, the pump and auxiliary source 5009 may include an optical amplifier 530 for amplifying the intensity of the seed light from the laser 520. For example, the laser 520 may be located remotely (e.g., in an intermediate node), and therefore the received seed light has a relatively low intensity. The pump and auxiliary source 5009 may include a beam splitter 540 for splitting the amplified seed light into two beams, one of which serves as pump light and the other as auxiliary light. The pump light can pump the nonlinear crystal 5011 in the high-dimensional entangled source 5001, for example, to generate a dual-mode squeezed state sequence based on spontaneous four-wave mixing. The auxiliary light can be used to generate auxiliary photons to assist in the measurement of the photons remaining locally. The corresponding components in the pump and auxiliary source 5009 can be configured according to the requirements for the auxiliary photons. For example, in the case of spontaneous four-wave mixing, the auxiliary light from the beam splitter 540 can have a different wavelength from the local photons. The pump and auxiliary source 5009 can include an optical parametric oscillator 5091 for converting the wavelength of the auxiliary light to match the wavelength of the local photons. Similarly, the pump and auxiliary source 5009 can include a modulation module 5093 for adjusting the auxiliary light to a desired optical field state, and an attenuator 5095 for attenuating the auxiliary light to a single-photon level to generate auxiliary photons for transmission to the measurement module 5003.

[0078] Combination of the above Figure 4 and 5Two example implementations of communication terminals are described. The two communication terminals communicating with each other can use the same configuration (for example, both use Figure 4 Configuration or Figure 5 configuration), or different configurations can be used (for example, one party uses Figure 4 The configuration of the other side Figure 5 configuration).

[0079] Figure 6 A schematic diagram of a quantum communication method according to an embodiment of the present application is shown.

[0080] like Figure 6 As shown, the quantum communication method according to the embodiment can be performed by two communication terminals (for example, the first communication terminal 1000 and the second communication terminal 2000 described above) and an intermediate node (for example, the intermediate node 3000 described above).

[0081] The first communication terminal 1000 and the second communication terminal 2000 can each generate a series of coherent two-mode squeezed states based on a high-dimensional entangled source in steps S101 and S201, respectively. As described above, the operations of the first communication terminal 1000 and the second communication terminal 2000 to generate the two-mode squeezed states can be synchronized. The first communication terminal 1000 and the second communication terminal 2000 can send photons in one mode of the two-mode squeezed state (i.e., signal photons) to the intermediate node 3000. The intermediate node 3000 can jointly measure the photons from the first communication terminal 1000 and the second communication terminal 2000 in step S301 and send the detection information to the first communication terminal 1000 and the second communication terminal 2000. In addition, the first communication terminal 1000 and the second communication terminal 2000 can perform local measurements on photons in the other mode of the two-mode squeezed state (i.e., local photons) in steps S103 and S203, respectively, and can exchange the detection information. The first communication end 1000 and the second communication end 2000 can determine the valid window according to the detection information in steps S105 and S205 respectively, and perform post-selection pairing on the valid window. Figures 1 to 5 Description.

[0082] According to the embodiments of the present application, by utilizing quantum states and particle number states that are coherently superimposed in a temporal pattern, and by post-processing the detector's successful response time, it is possible to ensure that the signal has lower attenuation due to channel loss during long-distance transmission. In related technologies, quantum memory is usually required to achieve this effect, but the implementation of quantum memory is very difficult. According to the solution based on a high-dimensional entangled light source of the present application, a similar effect can be achieved without the need for quantum memory, thereby greatly reducing the difficulty of actual implementation and greatly improving the practicality of long-distance quantum communication.

[0083] Those skilled in the art will appreciate that the features described in the various embodiments of this application may be combined and / or coupled in various ways, even if such combinations or couplings are not explicitly described in this application. In particular, the features described in the various embodiments of this application may be combined and / or coupled in various ways without departing from the spirit and teachings of this application. All such combinations and / or couplings fall within the scope of this application.

[0084] The embodiments of the present application have been described above. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application. Although each embodiment has been described separately above, this does not mean that the measures in each embodiment cannot be advantageously used in combination. Without departing from the scope of the present application, those skilled in the art may make various substitutions and modifications, and these substitutions and modifications should all fall within the scope of the present application.

Claims

1. A quantum communication method, comprising: The first communication end and the second communication end each generate a series of coherent two-mode squeezed states based on a high-dimensional entangled source, send photons on one mode of the two-mode squeezed state to the intermediate node, and locally measure photons on the other mode of the two-mode squeezed state; The intermediate node performs a joint measurement on the photons from the first communication end and the photons from the second communication end; The first communication end and the second communication end each determine, based on their own detector response information, the detector response information obtained from the other party, and the detector response information obtained from the intermediate node, a time window in which the first communication end, the second communication end, and the intermediate node all have detector responses as a valid window, and pair the detection result of the first valid window among the valid windows with the detection result of a different second valid window to achieve entanglement between the local photons of the first communication end and the second communication end in a specified pattern represented by the paired valid windows.

2. The quantum communication method according to claim 1, wherein: The locally measuring a photon on another mode in the dual-mode squeezed state comprises: causing a photon on another mode in the two-mode squeezed state to undergo HOM interference with a locally generated auxiliary photon, The auxiliary photons are obtained by attenuating the auxiliary light. The high-dimensional entangled source utilizes a pump light pulse sequence to pump a nonlinear crystal to generate the series of coherent two-mode squeezed states. The pump light pulse sequences and the repetition frequencies of the auxiliary light of the first communication end and the second communication end are synchronized.

3. The quantum communication method according to claim 2, wherein: The pump light pulse sequence includes a series of coherent pulses, and there is a fixed phase difference between adjacent pulses. The photons sent by the first communication end and the second communication end to the intermediate node have the same spectrum.

4. The quantum communication method according to claim 2, wherein: The photon on the other mode in the two-mode squeezed state has the same wavelength and the same spectrum as the auxiliary photon.

5. The quantum communication method according to claim 1, further comprising: In response to the detector responses in the first effective window and the second effective window both coming from the same detector, the indication is projected onto the quantum state On, where the quantum state are quantum states prepared locally in the first effective window and the second effective window, and denote the first effective window and the second effective window respectively, wherein m and n are natural numbers and n>m, In response to the detector responses in the first effective window and the second effective window coming from different detectors, the indication is projected onto the quantum state superior.

6. A quantum communication system comprising: Intermediate nodes; The first communication end and the second communication end are each configured to generate a series of coherent two-mode compressed states through a high-dimensional entangled source, send photons on one mode of the two-mode compressed state to the intermediate node, and locally measure photons on the other mode of the two-mode compressed state, The intermediate node is configured to perform a joint measurement on the photons from the first communication end and the photons from the second communication end, The first communication terminal and the second communication terminal are each configured as: Determine, based on its own probe response information, probe response information obtained from the other party, and probe response information obtained from the intermediate node, a time window in which the first communication end, the second communication end, and the intermediate node all have probe responses as a valid window; The detection result of the first effective window among the effective windows is paired with the detection result of a different second effective window to achieve entanglement between the local photons of the first communication end and the second communication end in the specified mode represented by the paired effective windows.

7. The quantum communication system according to claim 6, wherein: The high-dimensional entangled source is configured to pump a nonlinear crystal using a sequence of pump light pulses to generate the series of coherent two-mode squeezed states. The first communication end and the second communication end are each configured to cause a photon on another mode in the dual-mode squeezed state to undergo HOM interference with an auxiliary photon to perform local measurement. The auxiliary photons are obtained by attenuating the auxiliary light, and the pump light pulse sequences and the repetition frequencies of the auxiliary light of the first communication end and the second communication end are synchronized.

8. The quantum communication system according to claim 7, wherein: The pump light pulse sequence includes a series of coherent pulses, and there is a fixed phase difference between adjacent pulses. The photons sent by the first communication end and the second communication end to the intermediate node have a consistent spectrum.

9. The quantum communication system according to claim 7, wherein: The photon on the other mode in the two-mode squeezed state has the same wavelength and the same spectrum as the auxiliary photon.

10. The quantum communication system according to claim 6, wherein: The first communication end and the second communication end generate respective pump light pulse sequences and auxiliary light based on the same seed light.

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