Quantum relay system and quantum network

Abstract

The application provides a quantum relay system and a quantum network, which can be applied to the technical field of quantum information. The quantum relay system comprises a first quantum entanglement establishment device, a second quantum entanglement establishment device and a quantum entanglement exchange device; based on photon pairs generated at different times with probability, single-photon interference is carried out in the first quantum entanglement establishment device, and a second photon corresponding to successful prediction of the first single-photon interference is stored in a time-space multimode multiplexing mode to establish time-state entanglement between the first node and the second node; based on photon pairs generated at different times with probability, single-photon interference is carried out in the second quantum entanglement establishment device, and a second photon corresponding to successful prediction of the second single-photon interference is stored in a time-space multimode multiplexing mode to establish time-state entanglement between the third node and the fourth node; and based on polarization encoding entanglement exchange, entanglement between the first node and the fourth node at a long distance is realized.

Description

Technical Field

[0001] At least one embodiment of this application relates to the field of quantum information technology, and more specifically to a quantum relay system and a quantum network. Background Technology

[0002] The no-cloning theorem guarantees the security of quantum communication. According to this theorem, quantum signals cannot be directly amplified using classical signal amplification methods. During photon transmission in optical fibers, the attenuation loss increases exponentially with transmission distance. Therefore, the transmission loss characteristics of optical fiber channels limit the realization of long-distance quantum communication over hundreds of kilometers or more in a repeater-free architecture. Furthermore, existing quantum repeater systems struggle to simultaneously achieve high-speed, high-fidelity long-distance entangled transmission, hindering the practical application of these systems. Summary of the Invention

[0003] According to a first aspect of this application, a quantum repeater system is provided, the quantum repeater system comprising: a first quantum entanglement establishment device configured to: perform first single-photon interference on a first photon in a first photon pair emitted by a first node and a second node at multiple different times; based on the successful prediction of the first single-photon interference, synchronously read out a second photon in a first spatial mode and a second spatial mode different from the first spatial mode from a second node storing the second photon in the first photon pair in a time-space multimode multiplexing manner; convert the read-out second photon into a first polarization entangled photon pair; and a second quantum entanglement establishment device configured to: perform first single-photon interference on a first photon in a first photon pair emitted by a third node and a fourth node at multiple different times. A second single-photon interference is performed on the first photon in a second photon pair emitted at multiple different times; based on the successful prediction of the second single-photon interference, the second photon in the third spatial mode and the second photon in the fourth spatial mode, which is different from the third spatial mode, are simultaneously read out from the third node that stores the second photon in the second photon pair in a time-space multimode multiplexing manner; the read-out second photon is converted into a second polarization entangled photon pair; a quantum entanglement swapping device is configured to perform Bell state measurement on the first polarization entangled photon pair and the second polarization entangled photon pair to realize quantum entanglement between the second photons in the photon pairs stored in the first node and the fourth node in a time-space multimode multiplexing manner.

[0004] According to an embodiment of this application, the first quantum entanglement establishment device is further configured to: establish quantum entanglement between the second photons in the photon pairs stored in the first node and the second node in a time-space multimode multiplexing manner based on the successful prediction of the first single-photon interference at different times; the second quantum entanglement establishment device is further configured to: establish quantum entanglement between the second photons in the photon pairs stored in the third node and the fourth node in a time-space multimode multiplexing manner based on the successful prediction of the second single-photon interference at different times.

[0005] According to an embodiment of this application, when it is determined that the number of successfully predicted spatial modes in the second node is greater than the number of successfully predicted spatial modes in the third node, the first quantum entanglement establishment device is further configured to: synchronously read out the second photon from any time mode in the first spatial mode and any time mode in the second spatial mode from the second node; convert the synchronously read-out second photon into a first polarization entangled photon pair; the second quantum entanglement establishment device is further configured to: sort the spatial modes with a time mode number greater than 1 for spatial modes with a time mode number greater than 1, and synchronously read out the second photon from the i-th spatial mode from the third node. The last time pattern and the first time pattern in the (i+1)th spatial pattern, where i is greater than or equal to 1 and less than the number of spatial patterns with a time pattern count greater than 1; for spatial patterns with a time pattern count of 1, the time pattern of the spatial pattern with a time pattern count of 1 and the unpaired time patterns in the spatial patterns with a time pattern count greater than 1 are synchronously read from the third node; if there are no unpaired time patterns in the spatial patterns with a time pattern count greater than 1, the time patterns in the third spatial pattern and the time patterns in the fourth spatial pattern are synchronously read from the third node; the synchronously read second photon is converted into a second polarization entangled photon pair.

[0006] According to an embodiment of this application, the second node is configured to: probabilistically generate a first photon pair with the first node at multiple different times, and transmit the first photon in the generated first photon pair to the first quantum entanglement establishment device; and determine the temporal and spatial patterns of the second photon in the first photon pair in response to a successful prediction of the first single-photon interference. The third node is configured to: probabilistically generate a second photon pair with the fourth node at multiple different times, and transmit the first photon in the generated second photon pair to the second quantum entanglement establishment device; and determine the temporal and spatial patterns of the second photon in the second photon pair in response to a successful prediction of the second single-photon interference.

[0007] According to embodiments of this application, each of the first node, the second node, the third node, and the fourth node includes: a probabilistic light source configured to probabilistically generate photon pairs at multiple different times and emit a first photon from the photon pair; and a quantum memory configured to receive a second photon from the photon pair and, in response to a successful prediction of single-photon interference, determine the temporal and spatial patterns of the second photon from the photon pair.

[0008] According to an embodiment of this application, the aforementioned quantum entanglement swapping device includes: a polarization beamsplitter configured to receive the first polarization entangled photon pair from the second node at a first receiving end of the polarization beamsplitter, and to receive the second polarization entangled photon pair from the third node at a second receiving end of the polarization beamsplitter; a first polarization beam-splitting cubic prism configured at a first output end of the polarization beamsplitter; a second polarization beam-splitting cubic prism configured at a second output end of the polarization beamsplitter; a first Bell state detector configured to detect the two output ends of the first polarization beam-splitting cubic prism; and a second Bell state detector configured to detect the two output ends of the second polarization beam-splitting cubic prism.

[0009] According to an embodiment of this application, the first quantum entanglement establishment device includes: a first non-polarized beam splitter configured to receive a first photon from a first photon pair from a first node at a first receiving end of the first non-polarized beam splitter, and to receive a first photon from a first photon pair from a second node at a second receiving end of the first non-polarized beam splitter; and a first detector configured to perform single-photon detection at the output end of the first non-polarized beam splitter.

[0010] According to an embodiment of this application, the second quantum entanglement establishment device includes: a second unpolarized beam splitter configured to receive a first photon from a second photon pair from the third node at a first receiving end of the second unpolarized beam splitter, and to receive a first photon from a second photon pair from the fourth node at a second receiving end of the second unpolarized beam splitter; and a second detector configured to perform single-photon detection at the output end of the second unpolarized beam splitter.

[0011] Another aspect of this application provides a quantum network, which includes multiple user nodes, multiple first relay stations, and multiple second relay stations. The first relay station includes: a first quantum entanglement establishment device configured to: perform first single-photon interference on first photons in a first photon pair emitted by user nodes and second nodes at multiple different times; based on the successful prediction of the first single-photon interference, synchronously read out a second photon in a first spatial mode and a second spatial mode different from the first spatial mode from the second node storing the second photon in the first photon pair in a time-space multimode multiplexing manner; convert the read-out second photon into a first polarization entangled photon pair; and a quantum entanglement exchange device configured to perform Bell state measurement on the first polarization entangled photon pair and the second polarization entangled photon pair, thereby realizing the above... The second relay station includes a second quantum entanglement establishment device configured to: perform second single-photon interference on the first photon pairs emitted at multiple different times by the third node of the first relay station and the third node of the first relay station entangled with other user nodes; based on the successful prediction of the second single-photon interference, synchronously read out the second photons in a third spatial mode and a fourth spatial mode different from the second spatial mode from the third node of the first relay station storing the second photons in the second photon pairs in the time-space multimode multiplexing; and convert the read-out second photons into a second polarization entangled photon pair.

[0012] According to an embodiment of this application, the first relay station and the second relay station are connected by a balloon chain located in the stratosphere as an optical path relay.

[0013] According to embodiments of this application, based on photon pairs generated probabilistically at different times, single-photon interference is performed in a first quantum entanglement establishment device. This is combined with storing the second photon corresponding to the successful prediction of the first single-photon interference using a time-space multimode multiplexing method, establishing temporal entanglement between a first node and a second node. Similarly, based on photon pairs generated probabilistically at different times, single-photon interference is performed in a second quantum entanglement establishment device. This is combined with storing the second photon corresponding to the successful prediction of the second single-photon interference using a time-space multimode multiplexing method, establishing temporal entanglement between a third node and a fourth node. Entanglement between the first and fourth nodes at a relatively long distance is achieved through polarization-encoded entanglement swapping. The quantum repeater system of this application, based on probabilistic sources and single-photon interference, can improve the entanglement transmission rate. Combined with polarization-encoded entanglement swapping, it can improve the success rate and entanglement transmission fidelity of quantum communication. Attached Figure Description

[0014] The above-mentioned contents, other objects, features and advantages of this application will become clearer from the following description of embodiments of this application with reference to the accompanying drawings.

[0015] Figure 1 A schematic diagram of a quantum repeater system according to an embodiment of this application is shown.

[0016] Figure 2 A schematic diagram of a time-space multimodal multiplexing method according to an embodiment of this application is shown.

[0017] Figure 3 A schematic diagram of a quantum repeater system according to an embodiment of this application is shown.

[0018] Figure 4 A schematic diagram of a time-space multimodal multiplexing storage method according to an embodiment of this application is shown.

[0019] Figure 5 A schematic diagram of ball chain pairing according to an embodiment of this application is shown.

[0020] Figure 6 A schematic diagram of a quantum network according to an embodiment of this application is shown.

[0021] Figure 7 A schematic diagram of a quantum network based on fiber optic links is shown. Detailed Implementation

[0022] 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.

[0023] 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 above-described features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0024] 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.

[0025] 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.).

[0026] Quantum repeater methods based on quantum memories can divide long-distance transmission links into multiple segments. Entangled pairs are generated at both ends of each segment, and entanglement is established between corresponding nodes through entanglement swapping. Two quantum memories mounted on the same node perform entanglement swapping operations, achieving entanglement establishment between adjacent links. Repeating this operation enables the construction of entanglement between nodes at both ends of a long-distance link.

[0027] Based on different entanglement swapping methods and physical systems, existing technologies have led to a variety of specific implementation schemes for quantum repeaters.

[0028] The DLCZ scheme is a classic quantum repeater scheme that uses single-photon interference to achieve entanglement swapping. Proposed in 2001 by Academician Duan Luming and his collaborators, it stands for Duan-Lukin-Cirac-Zoller. This scheme relies on the collective excitation of atomic ensembles, constructing quantum entanglement between long-range atomic ensembles through photon emission and detection. During entanglement generation, a single successful prediction only requires the photon to withstand losses on one side of the link from both ends to the middle, resulting in a high prediction rate. However, this scheme has inherent technical constraints. It requires ensuring absolute phase stability of the link from both ends to the middle; photon transmission and detection losses can induce multi-photon erroneous states; and detector dark noise can cause empty-state storage problems, affecting the entanglement transmission rate and entanglement fidelity.

[0029] One approach to entanglement swapping using two-photon interference combines a probabilistic quantum light source with an absorption quantum memory. The probabilistic quantum light source generates entangled photon pairs, one photon is stored in the quantum memory, and the other photon is transmitted to complete the two-photon interference, thus achieving entanglement generation and swapping. This approach only requires ensuring the relative phase stability of the channels from both ends to the middle, but it must withstand the transmission losses of both links, which reduces the entanglement transmission rate. Furthermore, the multi-photon error states generated by the probabilistic quantum light source reduce entanglement fidelity.

[0030] This application proposes a quantum repeater system that combines the entanglement generation rate advantage of single-photon interference with the phase stability advantage of two-photon interference. This system can effectively solve the problems of empty state storage generated in single-photon interference and multi-photon error states generated in both types of interference, thereby further improving the speed and robustness of quantum repeaters.

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

[0032] like Figure 1 As shown, the quantum relay system of this application embodiment includes a first quantum entanglement establishment device 11, a second quantum entanglement establishment device 12, and a quantum entanglement exchange device 13.

[0033] The first quantum entanglement establishment device 11 is configured to: perform first single-photon interference on the first photon in a first photon pair emitted by the first node 21 and the second node 22 at multiple different times; based on the successful prediction of the first single-photon interference, synchronously read out the second photon in the first spatial mode and the second photon in the second spatial mode different from the first spatial mode from the second node that stores the second photon in the first photon pair in a time-space multimode multiplexing manner; and convert the read-out second photon into a first polarization entangled photon pair.

[0034] The second quantum entanglement establishment device 12 is configured to: perform second single-photon interference on the first photon in the second photon pair emitted by the third node 23 and the fourth node 24 at multiple different times; based on the successful prediction of the second single-photon interference, synchronously read out the second photon in the third spatial mode and the second photon in the fourth spatial mode, which is different from the third spatial mode, from the third node that stores the second photon in the second photon pair in a time-space multimode multiplexing manner; and convert the read-out second photon into a second polarization entangled photon pair.

[0035] The quantum entanglement swapping device 13 is configured to perform Bell state measurements on the first polarization entangled photon pair and the second polarization entangled photon pair, thereby realizing quantum entanglement between the second photon in the photon pair stored in the first node 21 and the fourth node 24 in a time-space multimode multiplexing manner.

[0036] The quantum repeater system according to the embodiment combines quantum storage and entanglement swapping operations to progressively extend short-distance quantum entanglement, thereby overcoming the problem of photon loss in the transmission channel, completing long-distance quantum entanglement distribution, and supporting long-distance quantum communication.

[0037] Nodes 21, 22, 23, and 24 can be quantum optical devices capable of photon generation, modulation, and emission. They can generate photon pairs discretely and probabilistically in time, and there may be empty pulse events without photons. For example, such photon pair sources can be spontaneous parametric down-conversion sources. Spontaneous parametric down-conversion is a random quantum process and cannot achieve deterministic photon pair output. Therefore, within any discrete time window, there is only a certain probability of generating photon pairs. Thus, at any given moment, a photon pair may be successfully generated, or it may not be generated at all. Each node transmits the first photon from its generated photon pair to the quantum entanglement establishment device via optical fiber or free space.

[0038] For example, the first node 21 at the first moment The first photon pair is produced (first photon) Second photon ), at the second moment The first photon pair is produced (first photon) Second photon ), at the 9th moment The first photon pair is produced (first photon) Second photon The subscript indicates the corresponding generation time, not the photon pair number. The second node 22 occurs at time 1. The first photon pair is produced (first photon) Second photon ), at the second moment The first photon pair is produced (first photon) Second photon ), at the 9th moment The first photon pair is produced (first photon) Second photon Although here it is represented as the first photon pair (first photon) Second photon However, it might be an empty pulse. Because it's uncertain whether it was successfully generated, for the sake of consistency, it's named the first photon pair (first photon). Second photon ).here 1, 2, and 9 are merely examples.

[0039] Nodes 21, 22, 23, and 24 can also employ a time-space multi-mode storage configuration. Time-space multi-mode multiplexing is a two-dimensional resource scheduling method. Spatial multiplexing utilizes independent physical storage areas (i.e., different spatial modes) within a node as parallel channels to achieve parallel access to photons. Time multiplexing divides the same spatial mode into different time slots (i.e., different time modes) for time-division storage of photons.

[0040] Figure 2 A schematic diagram of a time-space multimodal multiplexing method according to an embodiment of this application is shown.

[0041] like Figure 2 As shown in the example, each node can include multiple spatial patterns, and each spatial pattern can be divided into multiple temporal patterns. For example, a node can include 6 spatial patterns (121-126), and each spatial pattern can include 8 temporal patterns (t1-t8, t9-t). 16 t17 -t 24 t 25 -t 32 t 33 -t 40 t 41 -t 48 ).exist Figure 2 In the diagram, each square corresponds to a unique storage location for a spatiotemporal multimode multiplexing method, and the gray-filled squares represent the storage location corresponding to a successful prediction of the first single-photon interferometry. It should be noted that... Figure 2 This is merely a schematic diagram. The arrangement, quantity, and relative size of the storage locations shown in the diagram are only for illustrative purposes to explain the working principle of this application's embodiments and do not represent the physical arrangement of the memory inside the node, nor should they be construed as limiting the actual arrangement of the storage locations. Furthermore, in the following description, for convenience, it is assumed that each node has the same temporal-spatial multi-mode multiplexing storage mode; more specifically, it has the same number of spatial modes, and each spatial mode has the same number of temporal modes. However, this is merely an example, and this application is not limited thereto; at least some nodes may have different temporal-spatial multi-mode multiplexing storage modes.

[0042] At the first moment The first node 21 will send photons The photon is transmitted to the first quantum entanglement establishment device 11, and the second node 22 transmits the photon. Simultaneously, the first node 21 transmits the photon to the first quantum entanglement establishment device 11. Storing photons using a time-space multimode multiplexing method allows for the determination of photons. The time mode is The spatial mode is the first spatial mode 121; the second node 22 will send photons. Storing photons using a time-space multimode multiplexing method allows for the determination of photons. The time mode is The spatial mode is the first spatial mode 121.

[0043] At the second moment The first node 21 will send photons The photon is transmitted to the first quantum entanglement establishment device 11, and the second node 22 transmits the photon. Simultaneously, the first node 21 transmits the photon to the first quantum entanglement establishment device 11. Storing photons using a time-space multimode multiplexing method allows for the determination of photons. The time mode is The spatial mode is the first spatial mode 121; the second node 22 will send photons. Storing photons using a time-space multimode multiplexing method allows for the determination of photons. The time mode is The spatial mode is the first spatial mode 121.

[0044] At the 9th moment The first node 21 will send photons The photon is transmitted to the first quantum entanglement establishment device 11, and the second node 22 transmits the photon. Simultaneously, the first node 21 transmits the photon to the first quantum entanglement establishment device 11. Storing photons using a time-space multimode multiplexing method allows for the determination of photons. The time mode is The spatial mode is the second spatial mode 122; the second node 22 will send photons Storing photons using a time-space multimode multiplexing method allows for the determination of photons. The time mode is The spatial pattern is the second spatial pattern 122.

[0045] The successful prediction of the first single-photon interferometry indicates that the first quantum entanglement-establishing device has successfully detected a photon.

[0046] Based on the first moment The successful prediction of the first single-photon interferometry within the time window can be understood as the generation of actual photon pairs at the first and / or second nodes. This temporarily confirms that photon pairs have been generated at the second node, with the second node using a time pattern as the basis for prediction. The spatial mode 121 stores photons. .like Figure 2 As shown, the time mode is The first spatial pattern, 121, is filled with gray squares, indicating a successful prediction of the first single-photon interference, and 1211 is used as the unique identifier of the temporal and spatial multiplexing pattern of this square.

[0047] Based on the second moment The failure to predict the first single-photon interference within the time window can be understood as the absence of actual photon pairs at both the first and second nodes. This confirms that no photon pairs were generated at the second node, and the time pattern at the second node is... The spatial mode is the first spatial mode 121, and there is no photon at that position. .

[0048] Based on the 9th time point The successful prediction of the first single-photon interferometry within the time window can be understood as the generation of actual photon pairs at the first and / or second nodes. This temporarily confirms that photon pairs have been generated at the second node, with the second node using a time pattern as the basis for prediction. The second spatial mode, 122, stores photons. .like Figure 2 As shown, the time mode is The second spatial pattern, 122, is filled with gray squares, indicating a successful prediction of the first single-photon interference, and 1221 is used as the unique identifier of the temporal and spatial multiplexing pattern of this square.

[0049] Synchronously read out the photons of the first spatial mode, such as the first spatial mode 121, from the second node. Photons of a second spatial mode, such as the second spatial mode 122, which is different from the first spatial mode (in this example, the first spatial mode 121). Photons and photons They are converted into the first polarization entangled photon pair.

[0050] Similarly, the second photon in the third spatial mode and the fourth spatial mode (different from the third spatial mode) are synchronously read out from the third node; the read-out second photon is converted into a second polarization entangled photon pair. The quantum entanglement swapping device 13 performs Bell state measurements on the first polarization entangled photon pair and the second polarization entangled photon pair to realize quantum entanglement between the second photons in the photon pairs stored in the first node 21 and the fourth node 24 in a time-space multimode multiplexing manner.

[0051] Temporal entanglement is established using a first quantum entanglement establishment device and a second quantum entanglement establishment device based on temporal-spatial parallel multiplexing: the successful prediction of the first single-photon interferometry, and the storage of the second photon in the first and second nodes in a temporal-spatial multimode multiplexing manner, indicate that the second photon in the corresponding first and second nodes has successfully established temporal entanglement; the successful prediction of the second single-photon interferometry, and the storage of the second photon in the third and fourth nodes in a temporal-spatial multimode multiplexing manner, indicate that the second photon in the corresponding third and fourth nodes has successfully established temporal entanglement.

[0052] Synchronous readout based on temporal entanglement and conversion from temporal entanglement to polarization entanglement are achieved using a first and a second quantum entanglement establishment device: Based on the successful prediction of the first single-photon interferometry, two second photons in different temporal and spatial modes are synchronously read out from the second node and converted into a first polarization entangled photon pair; based on the successful prediction of the second single-photon interferometry, two second photons in different temporal and spatial modes are synchronously read out from the third node and converted into a second polarization entangled photon pair. Through temporal and spatial multiplexing, photons in different temporal modes can be synchronously read out from different spatial modes.

[0053] Entanglement based on polarization entanglement is achieved by using a quantum entanglement exchange device: the quantum entanglement exchange device performs Bell state measurements on the first polarization entangled photon pair and the second polarization entangled photon pair, so that quantum entanglement is generated between the second photon of the corresponding first node and the second photon in the fourth node, which have never interacted, thereby realizing quantum relay.

[0054] According to embodiments of this application, based on photon pairs generated probabilistically at different times, single-photon interference is performed in a first quantum entanglement establishment device. This is combined with storing the second photon corresponding to the successful prediction of the first single-photon interference using a time-space multimode multiplexing method, establishing temporal entanglement between a first node and a second node. Similarly, based on photon pairs generated probabilistically at different times, single-photon interference is performed in a second quantum entanglement establishment device. This is combined with storing the second photon corresponding to the successful prediction of the second single-photon interference using a time-space multimode multiplexing method, establishing temporal entanglement between a third node and a fourth node. Entanglement between the first and fourth nodes at a relatively long distance is achieved through polarization-encoded entanglement swapping. The quantum repeater system of this application, based on probabilistic sources and single-photon interference, can improve the entanglement transmission rate. Combined with polarization-encoded entanglement swapping, it can improve the success rate and entanglement transmission fidelity of quantum communication.

[0055] According to embodiments of this application, a second node is configured to: probabilistically generate a first photon pair with a first node at multiple different times, and emit the first photon from the generated first photon pair to a first quantum entanglement establishment device; and determine the temporal and spatial patterns of the second photon in the first photon pair in response to a successful prediction of the first single-photon interference. Similarly, a third node is configured to: probabilistically generate a second photon pair with a fourth node at multiple different times, and emit the first photon from the generated second photon pair to a second quantum entanglement establishment device; and determine the temporal and spatial patterns of the second photon in the second photon pair in response to a successful prediction of the second single-photon interference.

[0056] According to embodiments of this application, the first node, the second node, the third node, and the fourth node each include: a probabilistic light source configured to probabilistically generate photon pairs at multiple different times and emit a first photon in the photon pair; a quantum memory configured to receive a second photon in the photon pair and, in response to a successful prediction of single-photon interference, determine the temporal and spatial patterns of the second photon in the photon pair.

[0057] According to an embodiment of this application, the first quantum entanglement establishment device is further configured to establish quantum entanglement between second photons in photon pairs stored in a time-space multimode multiplexing manner in the first node and the second node based on the successful prediction of the first single-photon interference at different times; the second quantum entanglement establishment device is further configured to establish quantum entanglement between second photons in photon pairs stored in a time-space multimode multiplexing manner in the third node and the fourth node based on the successful prediction of the second single-photon interference at different times.

[0058] Based on the first moment The first successful prediction of single-photon interferometry established a photon number entanglement state in the first time mode between the second photons stored in the first and second nodes respectively in the first time mode. :

[0059] (1);

[0060] in, Indicates the first time mode marker; This indicates that a photon exists in the first node in the first-time mode; This indicates that there are no photons in the first node in the first-time mode; This indicates that a photon exists in the second node in the first-time mode; This indicates that there are no photons in the second node in the first time mode; This indicates that in the first time mode, photons are present in the first node and no photons are present in the second node; This indicates that photons exist in the second node in the first time mode, but not in the first node. The relative phase factor represents the time-varying evolution between the paths of the first photon emitted by the first node at the first moment and the first photon emitted by the second node. The phase difference is the difference between the first photon emitted by the first node and the first photon emitted by the second node at the first moment.

[0061] For a set of orthogonal photon number entangled basis vectors in the first time mode, "Distinguish the phase signs of coherent superposition terms; This is the normalization coefficient.

[0062] Based on a different time period The second moment The first successful prediction of single-photon interference establishes a photon number entanglement state in the second time mode between the second photons stored in the first and second nodes in the second time mode.

[0063] Based on the second moment The first successful prediction of single-photon interferometry established a number-entangled state of photons in the second time mode between the second photons stored in the first and second nodes respectively in the second time mode. :

[0064] (2);

[0065] in, Indicates the second time mode marker; The relative phase factor representing the evolution of the paths of the first photon emitted by the first node and the first photon emitted by the second node over time at the second moment. The phase difference between the first photon emitted by the first node and the first photon emitted by the second node at the second time point.

[0066] If in the first moment Second moment Both nodes successfully predicted the first single-photon interferometry, establishing photon-entangled states between the second photons stored in the first time mode and the second photons stored in the second time mode at the first and second nodes, respectively. It can be represented as:

[0067] (3);

[0068] in, Represents the tensor product; Indicates the first moment The detected photons originated from the first node; Indicates the second moment The detected photons originated from the first node; Indicates the first moment The detected photons originated from the second node; Indicates the second moment The detected photons came from the second node.

[0069] This represents the temporally entangled state established between the second photons stored in the first and second nodes respectively in the first time mode and the second photons stored in the second time mode respectively. Phase term. Compared to two-photon interference, the phase term is more robust.

[0070] According to embodiments of this application, the first node and the second node respectively emit their respective first photon pairs at different times using probabilistic light sources. The second photon in each first photon pair is stored in its respective quantum memory. The first photon in each first photon pair is remotely transmitted. After determining that a valid detection signal has been obtained, the first quantum entanglement establishment device can establish phase-insensitive temporal entanglement by reasonably pairing different modes of the quantum memory of the second node. Based on temporal and spatial parallel multiplexing of temporal entanglement establishment, temporal entanglement can be established between quantum memories that are multiplexed in temporal and spatial multimodes.

[0071] Figure 3 A schematic diagram of a quantum repeater system according to an embodiment of this application is shown.

[0072] like Figure 3 As shown, according to an embodiment of this application, the first quantum entanglement establishment device 11 includes a first non-polarization beam splitter 11_1 and a first detector 11_2.

[0073] The first unpolarized beamsplitter 11_1 is configured to receive a first photon from a first photon pair from a first node 21 at its first receiving end, and to receive a first photon from a first photon pair from a second node 22 at its second receiving end. The first detector 11_2 is configured to perform single-photon detection at the output of the first unpolarized beamsplitter 11_1.

[0074] The first detector 11_2 may include a first single-photon detector. Second single-photon detector The first single-photon detector Second single-photon detector A detector with indistinguishable photon counts. The first single-photon detector. Second single-photon detector They are respectively configured at the two output terminals of the first non-polarization beam splitter 11_1, and at the first single-photon detector Second single-photon detector There is only one single-photon response event, which is recorded as a valid detection signal, indicating the successful prediction of the first single-photon interferometry.

[0075] In one example, at time 1 Photon and photons Single-photon interference is performed within the first quantum entanglement establishment device 11. (Photon) and photons The beams enter the first unpolarized beamsplitter 11_1 from its first receiving end and its second receiving end, respectively, for beam combining and single-photon interference. At the first single-photon detector... Second single-photon detector There is one and only one single-photon response event, corresponding to the photon in the first node. Photons in the second node Successfully established between them in time mode The number of entangled photons under the following conditions.

[0076] At the second moment Photon and photons Single-photon interference is performed within the first quantum entanglement establishment device 11. (Photon) and photons The beams enter the first unpolarized beamsplitter 11_1 from its first receiving end and its second receiving end, respectively, for beam combining and single-photon interference. This is based on the first single-photon detector. Second single-photon detector Prediction unsuccessful; photon in the corresponding first node confirmed. Photons in the second node Not available.

[0077] At the 9th moment Photon and photons Single-photon interference is performed within the first quantum entanglement establishment device 11. (Photon) and photons The beams enter the first unpolarized beamsplitter 11_1 from its first receiving end and its second receiving end, respectively, for beam combining and single-photon interference. At the first single-photon detector... Second single-photon detector There is one and only one single-photon response event, corresponding to the photon in the first node. Photons in the second node Successfully established between them in time mode The number of entangled photons under the following conditions.

[0078] Therefore, based on the first time point at different times and the 9th moment Time mode is Photons in the first node Photons in the second node Time mode is Photons in the first node Photons in the second node It is possible to successfully establish a time-entangled state.

[0079] This indicates that photons exist at both the first and second nodes at the corresponding time. For a single-photon interference process, This is multiphoton noise, and the quantum repeater system of this application embodiment can naturally eliminate some of the multiphoton noise.

[0080] like Figure 3 As shown, according to an embodiment of this application, the second quantum entanglement establishment device 12 includes a second non-polarization beam splitter 12_1 and a second detector 12_2.

[0081] The second unpolarized beamsplitter 12_1 is configured to receive the first photon from the second photon pair from the third node 23 at its first receiving end, and to receive the first photon from the second photon pair from the fourth node 24 at its second receiving end. The second detector 12_2 is configured to perform single-photon detection at the output of the second unpolarized beamsplitter 12_1.

[0082] The second detector 12_2 may include a third single-photon detector. and the fourth single-photon detector The third single-photon detector and the fourth single-photon detector This is a detector with indistinguishable photon counts. The third single-photon detector. and the fourth single-photon detector They are respectively configured at the two output terminals of the second non-polarizing beam splitter 12_1, and at the third single-photon detector and the fourth single-photon detector Only one single-photon response event occurs, which is recorded as a valid detection signal, indicating a successful prediction of the second single-photon interferometry.

[0083] Based on the successful prediction of second single-photon interference at different times, the second photons in the photon pairs stored in the third and fourth nodes using time-space multimode multiplexing can successfully establish a time-entangled state.

[0084] Based on the successful predictions of the first single-photon interferometry at different times, the spatial and temporal modes corresponding to the successful predictions in the second node can be determined. Based on the successful predictions of the second single-photon interferometry at different times, the spatial and temporal modes corresponding to the successful predictions in the third node can be determined.

[0085] Before entanglement swapping, the second photons of the spatial and temporal modes corresponding to multiple successful predictions need to be paired up in pairs. Two second photons need to be read out synchronously (converted into polarization entangled photon pairs) so as to interfere with the synchronous readout of two second photons (converted into polarization entangled photon pairs) of other nodes.

[0086] The second photon stored in different time modes within the same spatial mode of a quantum memory cannot be read out independently, while the second photon stored in different spatial modes of a quantum memory can be read out independently. Different time modes within the same spatial mode cannot be paired for synchronous reading, but different spatial modes can be paired.

[0087] In one example, the first quantum entanglement establishment device can synchronously read out the second photon from any time mode in the first spatial mode and any time mode in the second spatial mode from the second node; and convert the synchronously read-out second photon into a first polarization entangled photon pair; the second quantum entanglement establishment device can synchronously read out the second photon from any time mode in the third spatial mode and any time mode in the fourth spatial mode from the third node; and convert the synchronously read-out second photon into a second polarization entangled photon pair.

[0088] For example, both the second and third nodes can use a pairwise spatial pairing scheme. The second photon in the photon pair stored by the second node using temporal-spatial multimode multiplexing can be in the form of... Figure 2 As shown. Based on the successful prediction of the first single-photon interferometry at multiple different times, the first time mode of the first spatial mode 121 is marked as 1211, the fourth time mode of the first spatial mode 121 is marked as 1212, and the sixth time mode of the first spatial mode 121 is marked as 1213; the first time mode of the second spatial mode 122 is marked as 1221, and the second time mode of the second spatial mode 122 is marked as 1222; the third time mode of the third spatial mode 123 is marked as 1231, and the fifth time mode of the third spatial mode 123 is marked as 1232; the first time mode of the fourth spatial mode 124 is marked as 1241, and the second time mode of the fourth spatial mode 124 is marked as 1242; the first time mode of the fifth spatial mode 125 is marked as 1251; and the fifth time mode of the sixth spatial mode 126 is marked as 1261.

[0089] The second photon in any time mode of the first spatial mode and any time mode of the second spatial mode can be synchronously read from the second node. For example, taking the first spatial mode 121 as the first spatial mode and the second spatial mode 122 as the second spatial mode, the second photons labeled 1211 and 1221 can be synchronously read from the second node and converted into a first polarization entangled photon pair. Alternatively, the second photons labeled 1213 and 1222 can be synchronously read from the second node and converted into a first polarization entangled photon pair.

[0090] In one example, the second photon in the photon pair stored by the third node in a time-space multimode multiplexing manner can be in the form of: Figure 2 As shown. Second photons from any time pattern in the third spatial mode and any time pattern in the fourth spatial mode are synchronously read from the third node. For example, using the first spatial mode 121 as the third spatial mode and the second spatial mode 122 as the fourth spatial mode, second photons labeled 1211 and 1221 can be synchronously read from the third node and converted into a second polarization entangled photon pair. Alternatively, second photons labeled 1213 and 1222 can be synchronously read from the third node and converted into a second polarization entangled photon pair.

[0091] Using only one successfully predicted temporal model for pairing within each spatial model may waste other successfully predicted temporal models. Pairing is performed on the side with more successfully predicted spatial models (assuming it's the second node), while more pairing schemes are used on the side with fewer successfully predicted spatial models (assuming it's the third node). This application provides a ball-chain pairing scheme to maximize the utilization of multiple successfully predicted models.

[0092] According to an embodiment of this application, when it is determined that the number of successfully predicted spatial modes in the second node is greater than the number of successfully predicted spatial modes in the third node, the first quantum entanglement establishment device is further configured to: synchronously read out the second photon from any time mode in the first spatial mode and any time mode in the second spatial mode from the second node; convert the synchronously read-out second photon into a first polarization entangled photon pair; the second quantum entanglement establishment device is further configured to: sort the spatial modes with a time mode number greater than 1 for spatial modes with a time mode number greater than 1, and synchronously read out the last photon from the i-th spatial mode from the third node. The first time pattern in the i+1th spatial pattern and the time pattern in the i-th time pattern, where i is greater than or equal to 1 and less than the number of spatial patterns greater than 1; for the spatial pattern with a time pattern of 1, the time pattern of the spatial pattern with a time pattern of 1 and the unpaired time patterns in the spatial patterns with a time pattern greater than 1 are synchronously read from the third node; if there are no unpaired time patterns in the spatial patterns with a time pattern greater than 1, the time patterns in the third spatial pattern and the time patterns in the fourth spatial pattern are synchronously read from the third node; the synchronously read second photon is converted into a second polarization entangled photon pair.

[0093] Figure 4 A schematic diagram of a time-space multimodal multiplexing storage method according to an embodiment of this application is shown.

[0094] Figure 5 A schematic diagram of ball chain pairing according to an embodiment of this application is shown.

[0095] In one example, the second photon in the photon pair stored by the third node in a time-space multimode multiplexing manner can be in the form of: Figure 2 As shown. Figure 4 This is another schematic diagram illustrating the time-space multi-mode multiplexing storage method. Taking the diagram in the upper left corner as an example, the circle represents the first successfully predicted spatial mode 121, and the three straight lines connected to the circle represent the three successfully predicted temporal modes corresponding to the first spatial mode 121, labeled as 1211, 1212, and 1213 respectively.

[0096] Combination Figure 2 and Figure 4 For spatial patterns with more than one time pattern, sort them. Spatial patterns with more than one time pattern include the first spatial pattern 121, the second spatial pattern 122, the third spatial pattern 123, and the fourth spatial pattern 124. i is greater than or equal to 1 and less than 4. Simultaneously read the last time pattern from the i-th spatial pattern and the first time pattern from the (i+1)-th spatial pattern from the third node.

[0097] Following the order of the first spatial mode 121, the second spatial mode 122, the third spatial mode 123, and the fourth spatial mode 124, the second photons marked 1213 and 1221 are read out synchronously, the second photons marked 1222 and 1231 are read out synchronously, and the second photons marked 1232 and 1241 are read out synchronously.

[0098] Combination Figure 2 , Figure 4 , Figure 5 The spatial modes with one time mode include the fifth spatial mode 125 and the sixth spatial mode 126. The second photon labeled 1251 is read out synchronously with the second photon labeled 1211, 1212, or 1242. For example, the second photon labeled 1251 is read out synchronously with the second photon labeled 1211. Additionally, the second photon labeled 1261 is read out synchronously with the second photon labeled 1212 or 1242. For example, the second photon labeled 1261 is read out synchronously with the second photon labeled 1242.

[0099] In the third node, the two paired second photons belong to different spatial modes, can be independently converted to orthogonal polarization, and can be read out synchronously. For the third node, there may be more than one paired time mode within the same spatial mode. For example, second photons labeled 1213 and 1221, and second photons labeled 1222 and 1231, cannot be read out simultaneously (because they both involve the second spatial mode 122). There needs to be a time interval between the readouts of the two photon pairs.

[0100] When the third node reads the second photons labeled 1213 and 1221, the second node can simultaneously read from two different spatial modes; when the third node reads the second photons labeled 1222 and 1231, the second node can still simultaneously read from two different spatial modes. Because the second node uses pairings of different spatial modes, there is no limitation on readout time.

[0101] According to embodiments of this application, the third node employs a ball-chain pairing scheme, which maximizes the utilization of the number of temporal patterns corresponding to multiple successful predictions, thereby improving the success rate of the quantum entanglement exchange device. Combined with the pairing scheme of the second node's spatial patterns, the second and third nodes can simultaneously read out the two second photons from their respective pairing schemes and convert them into polarization-entangled pairs, which are then sent to the quantum entanglement exchange device. The conversion to polarization-entangled pairs is achieved based on temporal entanglement pairing and synchronous readout, improving the success rate and robustness of the quantum repeater system.

[0102] According to embodiments of this application, the first quantum entanglement establishment device is further configured to convert a synchronously read-out second photon into a first polarization entangled photon pair with orthogonal polarization. In one example, the second photon at a first moment can be converted into a horizontally polarized state. The photon at the second moment converts the second photon at the second moment into a vertically polarized state. The photons. Converting the temporal entanglement between the second photons stored in the first and second nodes in the first and second time modes, respectively, into polarization entanglement, the polarization entangled state between the corresponding second photons in the first and second nodes can be represented as: . Indicates the horizontal polarization state. This indicates the vertical polarization state. This indicates that the photon from the first node is in a horizontal polarization state; This indicates that the photon from the first node is in a vertical polarization state; This indicates that the photon from the second node is in a horizontal polarization state; This indicates that the photon from the second node is in a vertically polarized state.

[0103] like Figure 3 As shown, according to an embodiment of this application, the quantum entanglement swapping device 13 includes a polarization beam splitter 13_1, a first polarization beam splitter cubic prism 13_2, a second polarization beam splitter cubic prism 13_3, a first Bell state detector 13_4, and a second Bell state detector 13_5.

[0104] Polarization beamsplitter 13_1 is configured to receive a first polarization-entangled photon pair from a second node 22 at its first receiving end and a second polarization-entangled photon pair from a third node 23 at its second receiving end. A first polarization-splitting cubic prism 13_2 is configured at the first output end of polarization beamsplitter 13_1. A second polarization-splitting cubic prism 13_3 is configured at the second output end of polarization beamsplitter 13_1. A first Bell state detector 13_4 is configured to detect both output ends of the first polarization-splitting cubic prism 13_2. A second Bell state detector 13_5 is configured to detect both output ends of the second polarization-splitting cubic prism 13_3.

[0105] The polarization beam splitter 13_1 can be a DA polarization beam splitter (Diagonal-Anti-diagonal polarizing beam splitter), used to separate the diagonal polarization component and the anti-diagonal polarization component of the incident photon to different output ports. Anti-angle polarization state , and A set of orthogonal photon polarization bases is formed. The first and second polarization-entangled photon pairs enter the polarization beamsplitter 13_1 from the first and second receiving ends, respectively, and are combined and interfered with. The first and second polarization-splitting cubic prisms 13_2 and 13_3 can be HV polarization beamsplitters (Horizontal-Vertical Polarizing Beam Splitter, HV-PBS). The first polarization-splitting cubic prism 13_2 can receive photons from the first output end of the polarization beamsplitter 13_1 and transmit or reflect them. The second polarization-splitting cubic prism 13_3 can receive photons from the second output end of the polarization beamsplitter 13_1 and transmit or reflect them.

[0106] The first Bell state detector 13_4 may include a fifth single-photon detector. The sixth single-photon detector The second Bell state detector 13_5 may include a seventh single-photon detector. and the eighth single-photon detector The fifth single-photon detector The sixth single-photon detector The seventh single-photon detector and the eighth single-photon detector It can be a detector with indistinguishable photon numbers. The fifth single-photon detector. and the sixth single-photon detector These are respectively located at the two output ends of the first polarizing beam-splitting cubic prism 13_2. Seventh single-photon detector. and the eighth single-photon detector They are respectively set at the two output ends of the second polarizing beam splitter cube prism 13_3.

[0107] In one example, the second and third nodes synchronously emit the first polarization entangled photon pair and the second polarization entangled photon pair at the third time point, respectively. Within the time window of the third time point, the fifth single-photon detector... and the sixth single-photon detector Only one of them will experience a single-photon response event, and the seventh single-photon detector... and the eighth single-photon detector If there is only one single-photon response event, it is recorded as a valid detection signal, indicating a successful prediction of the Bell state measurement.

[0108] When the photon paths corresponding to the first polarizing beam-splitting cubic prism 13_2 and the second polarizing beam-splitting cubic prism 13_3 are both transmission paths or both reflection paths, the collapsed quantum state can be represented as follows: . This indicates that the photon from the second node is in a horizontal polarization state; This indicates that the photon from the second node is in a vertical polarization state; This indicates that the photon from the third node is in a horizontal polarization state; This indicates that the photon from the third node is in a vertical polarization state. In one example, the first and second polarization-entangled photon pairs are combined and interfered with within the polarization beam splitter 13_1. The photon output from the first output end is transmitted (or reflected) from the first polarization beam splitter prism 13_2, and the photon output from the second output end is transmitted (or reflected) from the second polarization beam splitter prism 13_3. In the fifth single-photon detector... and the eighth single-photon detector In the event of a single-photon response event, the fifth single-photon detector and the eighth single-photon detector The quantum state of the detected photon collapses.

[0109] When the photon paths corresponding to the first polarizing beam-splitting cubic prism 13_2 and the second polarizing beam-splitting cubic prism 13_3 are one transmission path and the other reflection path, the collapsed quantum state can be represented as follows: In one example, the first and second polarization-entangled photon pairs are combined and interfered with within the polarization beam splitter 13_1. Photons output from the first output end are transmitted through the first polarization beam splitter prism 13_2, and photons output from the second output end are reflected through the second polarization beam splitter prism 13_3. In the fifth single-photon detector... and the seventh single-photon detector In the event of a single-photon response event, the fifth single-photon detector and the seventh single-photon detector The quantum state of the detected photon collapses.

[0110] In one example, the second photon read out synchronously from the second node could be from the first moment of a different spatial mode within the second node. Second moment The second photon. When photons are present in both time modes read out synchronously from the second node, the pairing of two second photons can be represented as... This is a mispairing, and the corresponding first polarization entangled photon pair can be represented as... That is, the first polarization entangled photon pair output from the second node contains two photons with orthogonal polarizations. When passing through polarization beam splitter 13_1, it can only exit from one end.

[0111] if If both photons are successfully detected by the two single-photon detectors behind the first polarization beam-splitting cubic prism 13_2 or the second polarization beam-splitting cubic prism 13_3, then regardless of the photon state in the second polarization entangled photon pair output by the third node, a successful prediction will not be obtained. If one of the detected losses results in only one photon being detected, then when the third node synchronously outputs the second polarization entangled photon pair... If the second polarization entangled photon pair emitted from the same port of polarization beamsplitter 13_1, or synchronously output from the third node, is also mispaired and outputs from another port of polarization beamsplitter 13_1 and is detected simultaneously by two single-photon detectors, or if the second polarization entangled photon pair synchronously output from the third node has no photon output, a successful prediction will not be obtained. Therefore, the quantum repeater system of this application embodiment can naturally eliminate some mispairings.

[0112] According to embodiments of this application, noise can be filtered out through polarization-encoded entanglement swapping, overcoming to some extent the disadvantages of probabilistic sources and single-photon interference pairing in entanglement fidelity. By combining a series of polarization beamsplitters, photon pairs from the second and third nodes collapse to specific Bell states after passing through the beamsplitters, achieving entanglement swapping between the first and second polarization-entangled photon pairs. Polarization-encoded entanglement swapping and noise filtering can eliminate some incorrect pairings, for example... , In addition, multiphoton noise from probabilistic sources can be reduced to improve the fidelity of quantum repeater systems.

[0113] The quantum repeater system of this application employs a global quantum repeater protocol with time-polarization hybrid coding and multimode multiplexing. It can achieve time entanglement establishment based on time-space parallel multiplexing, synchronous readout and pairing based on time entanglement to polarization entanglement conversion, entanglement swapping and noise filtering based on polarization coding, and the construction of a few-node global quantum network based on a stratospheric balloon chain backbone channel. The quantum repeater system of this application exhibits phase robustness and combines advantages in entanglement transmission rate and entanglement fidelity.

[0114] This application also proposes a quantum network.

[0115] Figure 6 A schematic diagram of a quantum network according to an embodiment of this application is shown.

[0116] like Figure 6 As shown, the quantum network in this embodiment of the application includes multiple user nodes 601, multiple first relay stations 610, and multiple second relay stations 620;

[0117] The first relay station 610 includes a first quantum entanglement establishment device 11 and a quantum entanglement exchange device 13. The second relay station 620 includes a second quantum entanglement establishment device 12.

[0118] The first quantum entanglement establishment device 11 is configured to perform a first single-photon interference on the first photon in a first photon pair emitted by user node 601 and second node 22 at multiple different times; based on the successful prediction of the first single-photon interference, the second photon in the first spatial mode and the second photon in the second spatial mode different from the first spatial mode are synchronously read out from the second node 22 which stores the second photon in the first photon pair in a time-space multimode multiplexing manner; and the read-out second photon is converted into a first polarization entangled photon pair.

[0119] The second quantum entanglement establishment device 12 is configured to perform second single-photon interference on the first photon in a second photon pair emitted at multiple different times by the third node 23 in the first relay station 610 and the third node in the first relay station that has established entanglement with other user nodes; based on the successful prediction of the second single-photon interference, the second photon in the third spatial mode and the second photon in the fourth spatial mode, which is different from the third spatial mode, are synchronously read out from the third node 23 in the first relay station 610 that stores the second photon in the second photon pair in a time-space multimode multiplexing manner; and the read-out second photon is converted into a second polarization entangled photon pair.

[0120] The quantum entanglement exchange device 13 is configured to perform Bell state measurements on the first polarization entangled photon pair and the second polarization entangled photon pair, thereby realizing quantum entanglement between the second photon in the photon pair stored in the third node of the first relay station that has established entanglement with other user nodes in a time-space multimode multiplexing manner.

[0121] According to an embodiment of this application, a balloon chain located in the stratosphere is used as an optical path relay between the first relay station 610 and the second relay station 620.

[0122] In one embodiment, the balloon chain located in the stratosphere 700 can be an upper-level hot air balloon chain located in the stratosphere at an altitude of approximately 20 km above the ground.

[0123] like Figure 6As shown, the third node 23 of different first relay stations 610 can emit the first photon from their respective second photon pairs from the ground (e.g., laser emission). The corresponding first photon enters the high-altitude hot air balloon chain via the uplink, is horizontally transmitted within the high-altitude hot air balloon chain, and then reaches the second quantum entanglement establishment device 12 in the second relay station 620 located on the ground via the downlink for interference. The first relay station 610 and the second relay station 620 can include photon transceivers, which can be adaptive optics systems. By optimizing the transmission beam waist position, they can effectively reduce the diffraction loss of the free space channel (high-altitude hot air balloon chain) and the collection and single-mode coupling losses caused by atmospheric turbulence disturbances.

[0124] Compared to satellite links, using high-altitude hot air balloon links takes advantage of the thin atmosphere and low optical path disturbance in the stratosphere, which can reduce the uplink and downlink distances and corresponding losses, and still achieve theoretical efficiency superior to near-Earth satellites at distances of tens of thousands of kilometers.

[0125] For the quantum network of this embodiment, the first target user node and the second target user node, which are to be pre-connected, can establish a connection through three links. The first link is an optical fiber link between the first target user node and the second node 22 of the first relay station 610. The second link is a free-space link between the third node 23 of the first relay station 610 and the third node of the first relay station connected to the second target user node. The third link is an optical fiber link between the second target user node and the second node of the first relay station connected to the second target user node.

[0126] The quantum network of this embodiment includes two layers of relays. The first layer of relays includes the establishment of entanglement for the three links mentioned above. The second layer of relays includes entanglement exchange between the second node 22 and the third node 23 in the first relay station 610, and entanglement exchange between the second node and the third node in the first relay station connected to the second target user node. Based on the two layers of relays, entanglement is established between the first target user node and the second target user node, thereby realizing quantum communication between the first target user node and the second target user node.

[0127] Based on the calculation results showing that entanglement rate and entanglement fidelity decrease exponentially with the number of relay layers, a trade-off needs to be made between entanglement rate and fidelity and the number of relay layers in long-range quantum networks. A few-node global quantum network based on the stratospheric balloon chain backbone channel represents a global quantum network spanning the globe, established using a time-polarization hybrid coding quantum relay protocol combined with the stratospheric balloon chain backbone channel.

[0128] Figure 7 A schematic diagram of a quantum network based on fiber optic links is shown.

[0129] like Figure 7As shown, a quantum network based on fiber optic links can include multiple nodes and multiple quantum repeaters. The multiple nodes can include a first remote node X and a second remote node Z located far apart, as well as multiple intermediate nodes between the first remote node X and the second remote node Z. The first remote node X and the second remote node Z can achieve quantum communication through multiple intermediate nodes and corresponding quantum repeaters. For example, the first remote node X establishes entanglement with intermediate node Y1 on the first X side, intermediate node Y2 on the second X side establishes entanglement with intermediate node Y3 on the third X side, and intermediate node Y1 on the first X side exchanges entanglement with intermediate node Y2 on the second X side, thereby establishing entanglement between the first remote node X and intermediate node Y3 on the third X side. Similarly, the second remote node Z establishes entanglement with intermediate node W1 on the first Z side, intermediate node W2 on the second Z side establishes entanglement with intermediate node W3 on the third Z side, and intermediate node W1 on the first Z side exchanges entanglement with intermediate node W2 on the second Z side, thereby establishing entanglement between the second remote node Z and intermediate node W3 on the third Z side. Quantum communication between the first remote node X and the second remote node Z is gradually realized through the establishment and exchange of entanglement among multiple intermediate nodes.

[0130] Compared to fiber optic links with exponential loss, using high-altitude hot air balloon chains can reduce the number of segments and layers in quantum networks.

[0131] According to the global quantum repeater protocol of the quantum network in this application embodiment, the ultra-low loss stratospheric balloon chain backbone channel is used to replace the exponential loss optical fiber channel. Over ultra-long distances on a global scale, the number of segment nodes and layers of the link can be effectively reduced, thereby fundamentally overcoming the problem that the rate and fidelity of probabilistic light source entanglement distribution decrease sharply with the number of layers. This allows the global quantum repeater protocol of the quantum repeater system in this application embodiment to maintain a sub-hertz entanglement transmission rate and more than 90% entanglement fidelity over a long distance of 20,000 km.

[0132] Those skilled in the art will understand that the features described in the various embodiments of this application can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this application. In particular, the features described in the various embodiments of this application can be combined and / or combined in various ways without departing from the spirit and teachings of this application. All such combinations and / or combinations fall within the scope of this application.

[0133] 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. Without departing from the scope of this application, 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 quantum repeater system, characterized in that, The quantum repeater system includes: A first quantum entanglement establishment device is configured to: perform first single-photon interference on a first photon in a first photon pair emitted by a first node and a second node at multiple different times; based on the successful prediction of the first single-photon interference, synchronously read out a first spatial mode and a second photon in a second spatial mode different from the first spatial mode from a second node that stores the second photon in the first photon pair in a time-space multimode multiplexing manner; and convert the read-out second photon into a first polarization entangled photon pair. The second quantum entanglement establishment device is configured to: perform second single-photon interference on the first photon in a second photon pair emitted by a third node and a fourth node at multiple different times; based on the successful prediction of the second single-photon interference, synchronously read out the second photon in a third spatial mode and a fourth spatial mode different from the third spatial mode from the third node that stores the second photon in the second photon pair in a time-space multimode multiplexing manner; and convert the read-out second photon into a second polarization entangled photon pair. A quantum entanglement swapping device is configured to perform Bell state measurements on a first polarization entangled photon pair and a second polarization entangled photon pair, thereby realizing quantum entanglement between the second photon in the photon pair stored in the first node and the fourth node in a time-space multimode multiplexing manner.

2. The quantum repeater system according to claim 1, characterized in that: The first quantum entanglement establishment device is further configured to: establish quantum entanglement between the second photon in the photon pair stored in the first node and the second node in a time-space multimode multiplexing manner based on the successful prediction of the first single-photon interference at different times; The second quantum entanglement establishment device is further configured to: establish quantum entanglement between the second photons in the photon pairs stored in the third and fourth nodes in a time-space multimode multiplexing manner based on the successful prediction of the second single-photon interference at different times.

3. The quantum repeater system according to claim 1, characterized in that, If the number of successfully predicted spatial patterns in the second node is greater than the number of successfully predicted spatial patterns in the third node... The first quantum entanglement establishment device is further configured to: synchronously read out a second photon from any time pattern in the first spatial mode and any time pattern in the second spatial mode from the second node; and convert the synchronously read-out second photon into a first polarization entangled photon pair; The second quantum entanglement establishment device is also configured as follows: For spatial patterns with a time pattern count greater than 1, sort the spatial patterns with a time pattern count greater than 1, and synchronously read the last time pattern in the i-th spatial pattern and the first time pattern in the (i+1)-th spatial pattern from the third node, where i is greater than or equal to 1 and less than the number of spatial patterns with a time pattern count greater than 1. For a spatial pattern with a time pattern count of 1, the time pattern of the spatial pattern with a time pattern count of 1 and the unpaired time patterns in the spatial patterns with a time pattern count greater than 1 are synchronously read from the third node; if there are no unpaired time patterns in the spatial patterns with a time pattern count greater than 1, the time patterns in the third spatial pattern and the time patterns in the fourth spatial pattern are synchronously read from the third node. The second photon read out synchronously is converted into a second polarization entangled photon pair.

4. The quantum repeater system according to claim 1, characterized in that: The second node is configured to: probabilistically generate a first photon pair with the first node at multiple different times, and emit the first photon in the generated first photon pair to the first quantum entanglement establishment device; and determine the temporal and spatial patterns of the second photon in the first photon pair in response to a successful prediction of the first single-photon interference. The third node is configured to: probabilistically generate a second photon pair with the fourth node at multiple different times, and emit the first photon in the generated second photon pair to the second quantum entanglement establishment device; and determine the temporal and spatial patterns of the second photon in the second photon pair in response to a successful prediction of the second single-photon interference.

5. The quantum repeater system according to claim 4, characterized in that, The first node, the second node, the third node, and the fourth node each include: A probabilistic light source is configured to probabilistically generate photon pairs at multiple different times and emit the first photon in the photon pair; A quantum memory is configured to receive the second photon in the photon pair and, in response to a successful prediction of single-photon interference, determine the temporal and spatial patterns of the second photon in the photon pair.

6. The quantum repeater system according to claim 1, characterized in that, The quantum entanglement swapping device includes: A polarization beamsplitter is configured to receive a first polarization entangled photon pair from a second node at a first receiving end of the polarization beamsplitter, and to receive a second polarization entangled photon pair from a third node at a second receiving end of the polarization beamsplitter. A first polarizing beam splitter cubic prism is disposed at the first output end of the polarizing beam splitter; The second polarizing beam splitter cubic prism is configured at the second output end of the polarizing beam splitter. The first Bell state detector is configured to detect the two output ends of the first polarization beam-splitting cubic prism; The second Bell state detector is configured to detect the two outputs of the second polarization beam-splitting cubic prism.

7. The quantum repeater system according to claim 1, characterized in that, The first quantum entanglement establishment device includes: A first unpolarized beamsplitter is configured to receive a first photon from a first photon pair from the first node at a first receiving end of the first unpolarized beamsplitter, and to receive a first photon from a first photon pair from the second node at a second receiving end of the first unpolarized beamsplitter. The first detector is configured to perform single-photon detection at the output of the first unpolarized beam splitter.

8. The quantum repeater system according to claim 1, characterized in that, The second quantum entanglement establishment device includes: The second unpolarized beamsplitter is configured to receive a first photon from a second photon pair from the third node at a first receiving end of the second unpolarized beamsplitter, and to receive a first photon from a second photon pair from the fourth node at a second receiving end of the second unpolarized beamsplitter. The second detector is configured to perform single-photon detection at the output of the second unpolarized beam splitter.

9. A quantum network, characterized in that, The quantum network includes multiple user nodes, multiple first relay stations, and multiple second relay stations; The first relay station includes: A first quantum entanglement establishment device is configured to: perform a first single-photon interference on a first photon in a first photon pair emitted by a user node and a second node at multiple different times; based on the successful prediction of the first single-photon interference, synchronously read out a first spatial mode and a second photon in a second spatial mode different from the first spatial mode from a second node that stores the second photon in the first photon pair in a time-space multimode multiplexing manner; and convert the read-out second photon into a first polarization entangled photon pair. A quantum entanglement exchange device is configured to perform Bell state measurements on the first polarization entangled photon pair and the second polarization entangled photon pair to realize quantum entanglement between the second photon in the photon pair stored in a time-space multimode multiplexing manner in the third node of the first relay station that has established entanglement with other user nodes; The second relay station includes: The second quantum entanglement establishment device is configured to: perform second single-photon interference on the first photon in a second photon pair emitted at multiple different times by the third node in the first relay station and the third node in the first relay station that has established entanglement with other user nodes; based on the successful prediction of the second single-photon interference, synchronously read out the second photon in a third spatial mode and a fourth spatial mode different from the second photon in the first relay station that stores the second photon in the second photon pair in a time-space multimode multiplexing manner; and convert the read-out second photon into a second polarization entangled photon pair.

10. The quantum network according to claim 9, characterized in that, The first relay station and the second relay station use a balloon chain located in the stratosphere as an optical path relay.

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