Quantum repeater system and method for single-copy entanglement purification

Abstract

This disclosure provides a single-copy entanglement purification quantum repeater system and method, relating to the field of quantum communication technology. The quantum repeater system includes a repeater node, a first super-entangled source, a second super-entangled source, a first node, and a second node. The repeater node is used to synchronously send pulse signals to the first and second super-entangled sources. The first super-entangled source is used to generate a first super-entangled quantum pair in response to the pulse signal. The second super-entangled source is used to generate a second super-entangled quantum pair in response to the pulse signal. The first node is used to perform entanglement purification with the repeater node by respectively entanglement purification of a first idler photon and a first signal photon in the first super-entangled quantum pair. The second node is used to perform entanglement purification with the repeater node by respectively entanglement purification of a second idler photon and a second signal photon in the second super-entangled quantum pair, so that the repeater node can achieve quantum repeater between the first and second nodes based on the purified first signal photon and the purified second signal photon.

Description

Technical Field

[0001] At least one embodiment of this disclosure relates to the field of quantum communication technology, and more specifically to a quantum repeater system and method for single-copy entanglement purification. Background Technology

[0002] After years of development, quantum communication technology has become quite mature. However, due to channel loss and system noise, the maximum transmission distance of quantum communication remains limited. In this context, quantum repeaters, as the only currently available scalable and distance-unlimited quantum communication technology, have made the research of entanglement-based quantum repeaters increasingly important. However, during continuous quantum repeating, depolarization noise accumulates in channel transmission, Bell state measurement, and other processes, gradually reducing the purity of the original entangled state and ultimately rendering it unusable for normal communication. Therefore, entanglement purification has become one of the indispensable key technologies for realizing quantum repeaters.

[0003] In related technologies, the entanglement purification scheme used in quantum repeaters requires two pairs of photons to complete one entanglement purification, which is very inefficient and almost impossible to apply in scenarios with high channel loss. Summary of the Invention

[0004] In view of the above problems, this disclosure provides a quantum repeater system and method for improving resistance to depolarization noise and increasing the communication rate of the quantum repeater system by single-copy entanglement purification.

[0005] According to a first aspect of this disclosure, a single-copy entanglement purification quantum repeater system is provided, the quantum repeater system comprising a repeater node, a first super-entangled source, a second super-entangled source, a first node, and a second node; the repeater node is configured to synchronously transmit pulse signals to the first super-entangled source and the second super-entangled source; the first super-entangled source is configured to generate a first super-entangled quantum pair in response to the pulse signal; the second super-entangled source is configured to generate a second super-entangled quantum pair in response to the pulse signal; the first node is configured to perform entanglement purification with the repeater node by respectively performing entanglement purification on a first idler photon and a first signal photon in the first super-entangled quantum pair; the second node is configured to perform entanglement purification with the repeater node by respectively performing entanglement purification on a second idler photon and a second signal photon in the second super-entangled quantum pair, so that the repeater node realizes quantum repeater between the first node and the second node based on the purified first signal photon and the purified second signal photon.

[0006] According to embodiments of this disclosure, the quantum repeater system includes: a plurality of controlled NOT gate devices for performing controlled NOT gate operations on single photons, such that the photon state characteristics of the single photon in a first degree of freedom are preserved, and the photon state characteristics in a second degree of freedom are eliminated; wherein, the repeater node includes a first controlled NOT gate device and a second controlled NOT gate device, the first node includes a third controlled NOT gate device, and the second node includes a fourth controlled NOT gate device; wherein, the first controlled NOT gate device is connected to the first super-entangled source to receive the first signal photon; the second controlled NOT gate device is connected to the second super-entangled source to receive the second signal photon; the third controlled NOT gate device is connected to the first super-entangled source to receive the first idler photon; and the fourth controlled NOT gate device is connected to the second super-entangled source to receive the second idler photon.

[0007] According to an embodiment of this disclosure, the relay node includes a Bell state measurement device for performing Bell state measurements on the purified first signal photon and the purified second signal photon in the first degree of freedom to obtain Bell state measurement results.

[0008] According to an embodiment of this disclosure, the first node includes: a first quantum state measurement device, used to perform quantum state measurement on the purified first idler photon in the first degree of freedom, and obtain the first quantum state measurement result.

[0009] According to an embodiment of this disclosure, the second node includes a second quantum state measurement device, used to perform quantum state measurement on the purified second idler photon in the first degree of freedom to obtain the second quantum state measurement result.

[0010] According to embodiments of this disclosure, the first node and the second node are used to realize entanglement swapping based on the Bell state measurement results, and to complete quantum communication based on the first quantum state measurement results and the second quantum state measurement results.

[0011] According to an embodiment of this disclosure, when the first degree of freedom is polarization degree of freedom and the second degree of freedom is timestamp degree of freedom, the control NOT gate device includes: an unequal-arm Mach-Zehnder interferometer, used to perform control NOT gate operation on the single photon, and to adjust the photon state characteristics of the single photon in the timestamp degree of freedom according to the photon state characteristics of the single photon in the polarization degree of freedom, so as to obtain a purified single photon with consistent photon state in the timestamp degree of freedom and unchanged photon state in the polarization degree of freedom.

[0012] According to an embodiment of this disclosure, when the first degree of freedom is a timestamp degree of freedom and the second degree of freedom is a polarization degree of freedom, the control NOT gate device includes: an electrically controlled polarization controller, used to perform a control NOT gate operation on the single photon, and adjust the photon state characteristics of the single photon in the polarization degree of freedom according to the photon state characteristics of the single photon in the timestamp degree of freedom, to obtain a purified single photon with consistent photon states in the polarization degree of freedom and unchanged photon states in the timestamp degree of freedom; and a polarizer, used to perform post-selection on the polarization degree of freedom of the purified single photon, and output a purified single photon with horizontal polarization or a purified single photon with vertical polarization.

[0013] According to an embodiment of this disclosure, the relay node further includes: a synchronization signal generator, used to synchronously send the pulse signal to the first super-entangled source and the second super-entangled source.

[0014] According to another aspect of this disclosure, a single-copy entanglement purification quantum repeater method is provided, applied to the aforementioned quantum repeater system. The quantum repeater method includes: synchronously sending pulse signals to a first hyperentangled source and a second hyperentangled source using a repeater node; generating a first hyperentangled quantum pair using the first hyperentangled source in response to the pulse signals; generating a second hyperentangled quantum pair using the second hyperentangled source in response to the pulse signals; entanglement purification of a first idler photon and a first signal photon in the first hyperentangled quantum pair using a first node and the repeater node, respectively; and entanglement purification of a second idler photon and a second signal photon in the second hyperentangled quantum pair using a second node and the repeater node, respectively, so that the repeater node achieves quantum repeater between the first node and the second node based on the purified first signal photon and the purified second signal photon.

[0015] According to embodiments of this disclosure, based on the principle of quantum entanglement purification in hyper-entanglement, the first node and relay node improve the entanglement degree of the first signal photon and the first idler photon in the first hyper-entangled quantum pair through entanglement purification. Similarly, the second node and relay node improve the entanglement degree of the second signal photon and the second idler photon in the second hyper-entangled quantum pair through entanglement purification. This better protects the quantum state, effectively enhances the resistance to channel depolarization noise during quantum relay, and strengthens the security and reliability of quantum communication. Compared to the technical solution using two physical bits for entanglement purification, the single-copy entanglement purification quantum relay system of this disclosure entangles and purifies the first idler photon and the first signal photon separately, and the second idler photon and the second signal photon separately. Through single-copy entanglement purification, the efficiency of entanglement purification is effectively improved, enabling efficient and reliable support for quantum communication tasks. The communication efficiency meets practical needs and contributes to the practical application of long-distance quantum communication. Attached Figure Description

[0016] The foregoing contents, as well as other objects, features, and advantages of this disclosure, will become clearer from the following description of embodiments with reference to the accompanying drawings, in which:

[0017] Figure 1 This schematic diagram illustrates the principle of a quantum repeater system with single-copy entanglement purification according to an embodiment of the present disclosure;

[0018] Figure 2 This schematic diagram illustrates the principle of a single-copy entanglement purification quantum repeater system according to an embodiment of the present disclosure;

[0019] Figure 3 This schematically illustrates a quantum repeater system with single-copy entanglement purification according to another embodiment of the present disclosure;

[0020] Figure 4 The schematic diagram illustrates the principle of controlling the NOT gate operation of an unequal-arm Mach-Zehnder interferometer according to an embodiment of the present disclosure;

[0021] Figure 5 A schematic diagram illustrating the principle of a single-copy entanglement purification quantum repeater system according to another embodiment of the present disclosure is shown; and

[0022] Figure 6 The illustration schematically shows a multi-user node principle of a quantum repeater system with single-copy entanglement purification according to an embodiment of the present disclosure. Detailed Implementation

[0023] The embodiments of the present disclosure 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 the disclosure. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of the present disclosure for ease of explanation. However, it will be apparent that one or more embodiments may be practiced without these specific details. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concepts of the present disclosure.

[0024] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit this disclosure. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0025] 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.

[0026] 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.).

[0027] Figure 1 A schematic diagram illustrating the principle of a quantum repeater system with single-copy entanglement purification according to an embodiment of the present disclosure is shown.

[0028] like Figure 1 As shown, the quantum relay system for single-copy entanglement purification includes a relay node, a first super-entangled source, a second super-entangled source, a first node, and a second node.

[0029] According to embodiments of this disclosure, a relay node is used to synchronously transmit pulse signals to a first hyperentangled source and a second hyperentangled source. The first hyperentangled source is used to generate a first hyperentangled quantum pair in response to the pulse signal. The second hyperentangled source is used to generate a second hyperentangled quantum pair in response to the pulse signal. The first node is used to work with the relay node to entangle and purify the first idler photon and the first signal photon in the first hyperentangled quantum pair, respectively. The second node is used to work with the relay node to entangle and purify the second idler photon and the second signal photon in the second hyperentangled quantum pair, respectively, so that the relay node can achieve quantum relay between the first and second nodes based on the purified first signal photon and the purified second signal photon.

[0030] A hyperentangled state refers to a quantum state in a quantum system that is simultaneously entangled in multiple degrees of freedom (e.g., polarization, spatial mode, orbital angular momentum, frequency, timestamp, etc.). In one example, the two quantum states of the first hyperentangled quantum pair can be quantum states that are simultaneously entangled in the polarization degree of freedom and the orbital angular momentum degree of freedom.

[0031] According to embodiments of this disclosure, based on the principle of quantum entanglement purification in hyper-entanglement, the first node and relay node improve the entanglement degree of the first signal photon and the first idler photon in the first hyper-entangled quantum pair through entanglement purification. Similarly, the second node and relay node improve the entanglement degree of the second signal photon and the second idler photon in the second hyper-entangled quantum pair through entanglement purification. This better protects the quantum state, effectively enhances the resistance to channel depolarization noise during quantum relay, and strengthens the security and reliability of quantum communication. Compared to the technical solution using two physical bits for entanglement purification, the single-copy entanglement purification quantum relay system of this disclosure entangles and purifies the first idler photon and the first signal photon separately, and the second idler photon and the second signal photon separately. Through single-copy entanglement purification, the efficiency of entanglement purification is effectively improved, enabling efficient and reliable support for quantum communication tasks. The communication efficiency meets practical needs and contributes to the practical application of long-distance quantum communication.

[0032] Figure 2 The schematic diagram illustrates the principle of a single-copy entanglement purification quantum repeater system according to an embodiment of the present disclosure.

[0033] like Figure 2 As shown, the quantum repeater system with single-copy entanglement purification includes multiple control NOT gate devices.

[0034] According to embodiments of this disclosure, a controlled NOT gate device is used to perform a controlled NOT gate operation on a single photon, such that the photon state characteristics of the single photon in the first degree of freedom are preserved, while the photon state characteristics in the second degree of freedom are eliminated.

[0035] like Figure 2 As shown, the relay node includes a first control NOT gate device and a second control NOT gate device; the first node includes a third control NOT gate device; and the second node includes a fourth control NOT gate device. The first control NOT gate device is connected to a first super-entangled source to receive a first signal photon; the second control NOT gate device is connected to a second super-entangled source to receive a second signal photon; the third control NOT gate device is connected to the first super-entangled source to receive a first idler photon; and the fourth control NOT gate device is connected to the second super-entangled source to receive a second idler photon.

[0036] According to embodiments of this disclosure, a first and a third control NOT gate device respectively entangle and purify a first signal photon and a first idler photon, such that the photonic state characteristics of the first signal photon and the first idler photon in the first degree of freedom are preserved, while the photonic state characteristics in the second degree of freedom are eliminated, thereby improving the quantum state entanglement purity of the first signal photon and the first idler photon in the first degree of freedom. A second and a fourth control NOT gate device respectively entangle and purify a second signal photon and a second idler photon, such that the photonic state characteristics of the second signal photon and the second idler photon in the first degree of freedom are preserved, while the photonic state characteristics in the second degree of freedom are eliminated, thereby improving the quantum state entanglement purity of the second signal photon and the second idler photon in the first degree of freedom.

[0037] like Figure 2 As shown, the relay node includes a Bell state measurement device.

[0038] According to embodiments of this disclosure, a Bell state measurement device is used to perform Bell state measurements on a purified first signal photon and a purified second signal photon in a first degree of freedom to obtain Bell state measurement results.

[0039] According to embodiments of this disclosure, Bell state projection measurements are performed in the first degree of freedom according to a quantum communication protocol in the first degree of freedom. In a quantum entanglement distribution network, Bell state measurements are used to establish entanglement between non-adjacent nodes. Two independent qubits can be entangled through quantum entanglement swapping.

[0040] like Figure 2 As shown, the first node includes a first quantum state measurement device.

[0041] According to an embodiment of this disclosure, a first quantum state measurement device is used to perform quantum state measurement on a purified first idler photon in a first degree of freedom to obtain a first quantum state measurement result.

[0042] like Figure 2 As shown, the second node includes a second quantum state measurement device.

[0043] According to an embodiment of this disclosure, the second quantum state measurement device is used to perform quantum state measurement on the purified second idler photon in the first degree of freedom to obtain the second quantum state measurement result.

[0044] According to embodiments of this disclosure, the first node and the second node are used to realize entanglement swapping based on Bell state measurement results, and to complete quantum communication based on the first quantum state measurement results and the second quantum state measurement results.

[0045] According to embodiments of this disclosure, in a communication link, the first signal photon, the second signal photon, the first idler photon, and the second idler photon are affected by depolarization noise, which reduces the entanglement fidelity of the photon states. The first node and the relay node respectively perform single-copy entanglement purification on the first idler photon and the first signal photon using NOT gates. The resulting purified first signal photon and purified first idler photon are entangled only in the first degree of freedom, thus improving fidelity. The second node and the relay node respectively perform single-copy entanglement purification on the second idler photon and the second signal photon using NOT gates. The resulting purified second signal photon and purified second idler photon are entangled only in the first degree of freedom, further improving fidelity.

[0046] According to embodiments of this disclosure, relay nodes perform Bell state measurements on purified first and second signal photons and communicate the Bell state measurement results to the first and second nodes via classical communication. A Bell state is the maximally entangled state of two qubits, and there are four orthogonal Bell states. Bell state measurement projects the joint state of the two qubits onto one of these four Bell states, thus determining their entanglement relationship. Based on the Bell state measurement results, the first and second nodes perform local unitary transformations on their respective idler photons to obtain one of the four Bell states agreed upon before communication, establishing a defined entanglement relationship and completing the entanglement exchange process. A local unitary transformation is an operation performed locally (e.g., on a single qubit) of a quantum system and does not affect the overall entanglement properties of the system. The first and second nodes perform quantum state measurements on their respective entangled pairs, converting the entangled qubits into associated classical information, thus completing the quantum communication process.

[0047] In one example, classic communication methods include fiber optic communication, wireless communication, and satellite communication.

[0048] Figure 3 The schematic diagram illustrates the principle of a quantum repeater system with single-copy entanglement purification according to another embodiment of the present disclosure.

[0049] like Figure 3 As shown, the first node can be represented as the Alice end, the second node as the Bob end, and the relay node as the Charlie end. The Charlie end synchronously transmits pulse signals through the first servo signal and the second servo signal to excite the first and second super-entangled sources, respectively. The first super-entangled source transmits a first signal photon to the Charlie end through the first signal channel and a first idler photon to the Alice end through the first idler channel. The second super-entangled source transmits a second signal photon to the Charlie end through the second signal channel and a second idler photon to the Bob end through the second idler channel.

[0050] like Figure 3 As shown, according to an embodiment of this disclosure, when the first degree of freedom is the polarization degree of freedom and the second degree of freedom is the timestamp degree of freedom, the control NOT gate device can be an unequal-arm Mach-Zehnder interferometer.

[0051] According to embodiments of this disclosure, an unequal-arm Mach-Zehnder interferometer is used to perform controlled NOT gate operations on single photons. Based on the photon state characteristics of the single photon in the polarization degree of freedom, the photon state characteristics of the single photon in the timestamp degree of freedom are adjusted to obtain purified single photons with consistent photon states in the timestamp degree of freedom and unchanged photon states in the polarization degree of freedom.

[0052] Figure 4 The schematic diagram illustrates the principle of controlling the NOT gate operation of an unequal-arm Mach-Zehnder interferometer according to an embodiment of the present disclosure.

[0053] like Figure 4 As shown, in one example, the unequal-arm Mach-Zehnder interferometer includes two polarization beam splitters (PBS).

[0054] Entanglement purification refers to the process of extracting a higher-purity entangled state from a lower-purity quantum entangled state. Based on the principle of entanglement purification, a controlled NOT gate is a two-qubit quantum logic gate, where one qubit acts as the control bit and the other as the target bit. When the control bit is in a specific state (usually...), the gate... When the target bit is in a state of NOT gate operation, its state will be flipped; if the control bit is in a state of NOT gate operation, its state will be flipped. If the state remains unchanged, then the target bit remains unchanged.

[0055] like Figure 4 As shown, in one example, the polarization degree of freedom is defined as the control bit, and the timestamp degree of freedom is defined as the target bit. The quantum state of a single photon (e.g., the first signal photon) includes... , , , ,definition It is in a horizontal polarization state (H). It is in the vertical polarization state (V). The late photon state (L) is the one that arrives late on the timestamp. This refers to the earliest arriving photon state (E) on the timestamp. Before performing single-copy entanglement purification, the incident polarized photon state needs to be calibrated with the unequal-arm Mach-Zehnder interferometer using a polarization reference system. The horizontal polarization state is set to follow the short arm, and the vertical polarization state follows the long arm; additionally, the arm length difference of the unequal-arm Mach-Zehnder interferometer needs to be set to be the same as the time interval of the pulse signal. When the incident polarized photon state is horizontally polarized, the timestamp bit following the short arm is used as the reference, i.e., defined... The state remains the same after passing through the unequal-arm Mach-Zehnder interferometer. ,So The photon state at the time-stamp degree of freedom remains unchanged after the state is incident on the unequal-arm Mach-Zehnder interferometer, i.e., it is still the same. When the incident polarized photon is in a vertically polarized state, The state will experience an additional delay of one arm length difference, causing the photon state at the timestamp degree of freedom to flip, changing to... state; and The state will undergo an additional delay change. State. Among them, This indicates that the first signal photon, arriving later, is timestamped later after passing through the long arm. In the embodiments of this application, due to the original vertically polarized later photon... The CNOT operation was not successfully implemented; therefore, a partial-CNOT (p-CNOT) operation was performed, meaning only a portion of the degrees of freedom were successfully entangled using CNOT. Finally, a post-selection operation is needed to filter out successful events and discard failed ones. It's important to emphasize that although the success rate of the p-CNOT operation is less than 100%, the post-selection operation ensures that the final entanglement purification effect remains unaffected.

[0056] Table 1 illustrates the transformation process of the control NOT gate operation of the unequal-arm Mach-Zehnder interferometer according to an embodiment of the present disclosure.

[0057] Table 1

[0058]

[0059] According to embodiments of this disclosure, a controlled NOT gate operation with polarization degrees of freedom as control bits and timestamp degrees of freedom as target bits is achieved by measuring the late photon state of the short arm or the early photon state of the long arm.

[0060] like Figure 3As shown, in one example, the unequal-arm Mach-Zehnder interferometer of the first control NOT gate device includes polarization beamsplitter 1 and polarization beamsplitter 2. Similarly, the second control NOT gate device includes polarization beamsplitter 3 and polarization beamsplitter 4; the third control NOT gate device includes polarization beamsplitter 5 and polarization beamsplitter 6; and the fourth control NOT gate device includes polarization beamsplitter 7 and polarization beamsplitter 8.

[0061] like Figure 3 As shown, according to an embodiment of this disclosure, the Bell state measurement device is a polarization Bell state measurement device. The first quantum state measurement device and the second quantum state measurement device are polarization quantum state measurement devices. After performing a control NOT gate operation, Bell state measurements in polarization degrees of freedom are required for the purified first signal photon and the purified second signal photon, and quantum state measurements in polarization degrees of freedom are performed for the purified first idler photon and the purified second idler photon, respectively.

[0062] Figure 5 The schematic diagram illustrates the principle of a single-copy entangled purification quantum repeater system according to another embodiment of the present disclosure.

[0063] like Figure 5 As shown, the first node can be represented as the Alice end, the second node as the Bob end, and the relay node as the Charlie end. The Charlie end synchronously transmits pulse signals through the first servo signal and the second servo signal to excite the first and second super-entangled sources, respectively. The first super-entangled source transmits a first signal photon to the Charlie end through the first signal channel and a first idler photon to the Alice end through the first idler channel. The second super-entangled source transmits a second signal photon to the Charlie end through the second signal channel and a second idler photon to the Bob end through the second idler channel.

[0064] like Figure 5 As shown, according to an embodiment of this disclosure, when the first degree of freedom is the timestamp degree of freedom and the second degree of freedom is the polarization degree of freedom, the control NOT gate device may include an electrically controlled polarization controller and a polarizer.

[0065] According to embodiments of this disclosure, an electrically controlled polarization controller is used to perform a control NOT gate operation on a single photon, adjusting the photon state characteristics of the single photon in the polarization degree of freedom based on the photon state characteristics of the single photon in the timestamp degree of freedom, to obtain a purified single photon with consistent photon states in the polarization degree of freedom and unchanged photon states in the timestamp degree of freedom. A polarizer is used to perform post-selection of the polarization degree of freedom of the purified single photon, outputting a purified single photon with horizontal polarization or a purified single photon with vertical polarization.

[0066] According to embodiments of this disclosure, the timestamp degree of freedom is defined as the control bit, and the polarization degree of freedom is defined as the target bit. Based on the arrival time of the timestamp photon state of an incident single photon (e.g., a first signal photon), a control voltage is applied to determine whether the polarization photon state of the incident timestamp photon state is flipped. In one example, the electrically controlled polarization controller is configured not to flip the polarization photon state of an early-arriving single photon (e.g., the first signal photon), but to flip the polarization photon state of a later-arriving single photon (e.g., the first signal photon).

[0067] Table 2 illustrates the transformation process of the control NOT gate operation of the electrically controlled polarization controller according to another embodiment of the present disclosure.

[0068] Table 2

[0069]

[0070] According to embodiments of this disclosure, after a control NOT gate operation, a polarizer is used to perform post-selection of a single photon (e.g., a first signal photon) in terms of polarization degrees of freedom, allowing only horizontally or vertically polarized single photons (e.g., the first signal photon) to pass through.

[0071] like Figure 5 As shown, in one example, the first control NOT gate device includes a polarization beam splitter 1 and a polarization beam splitter 2. Similarly, the second control NOT gate device includes an electrically controlled polarization controller 2 and a polarizer 2; the third control NOT gate device includes an electrically controlled polarization controller 3 and a polarizer 3; and the fourth control NOT gate device includes an electrically controlled polarization controller 4 and a polarizer 4.

[0072] like Figure 5 As shown, according to an embodiment of this disclosure, the Bell state measurement device is a timestamped Bell state measurement device. The first quantum state measurement device and the second quantum state measurement device are timestamped quantum state measurement devices. After performing the control NOT gate operation, Bell state measurements in the timestamped degree of freedom are required for the purified first signal photon and the purified second signal photon, and quantum state measurements in the timestamped degree of freedom are performed for the purified first idler photon and the purified second idler photon, respectively.

[0073] According to embodiments of this disclosure, the relay node further includes a synchronization signal generator. The synchronization signal generator is used to synchronously transmit pulse signals to the first and second hyperentangled sources.

[0074] like Figure 3 As shown, the synchronization signal generating device may also include a mode-locked pulse seed laser and a first beam splitter.

[0075] According to embodiments of this disclosure, the synchronization signal generator at the Charlie end can be a mode-locked pulse seed laser. The mode-locked pulse seed laser is used to generate pulse signals with coherent characteristics between consecutive pulses. The pulse signals are transmitted to the first servo channel and the second servo channel on both sides via a first beam splitter.

[0076] According to embodiments of this disclosure, the synchronization signal generating device may also be a pulse seed laser and an interferometer. The pulse seed laser is used to generate a pulse seed, and the interferometer is used to transform the pulse seed into a pulse signal in which the preceding and following pulses have coherent characteristics.

[0077] like Figure 5 As shown, the synchronization signal generating device may also include a pulse seed laser, a circulator, and a second beam splitter. The interferometer may be a Faraday-Michelson interferometer. The pulse seed laser may be a gain-switching type laser.

[0078] According to embodiments of this disclosure, the first super-entangled source includes a first polarization entanglement unit. The second super-entangled source includes a second polarization entanglement unit.

[0079] According to embodiments of this disclosure, a first polarization entanglement unit is used to generate a first photon pair entangled in polarization degrees of freedom in response to a pulse signal, and, based on the coherence characteristics of the pulse signal, to make the first photon pair entangled in the timestamp degree of freedom, thereby generating a first super-entangled quantum pair entangled in polarization-timestamp degrees of freedom. A second polarization entanglement unit is used to generate a second photon pair entangled in polarization degrees of freedom in response to a pulse signal, and, based on the coherence characteristics of the pulse signal, to make the second photon pair entangled in the timestamp degree of freedom, thereby generating a second super-entangled quantum pair entangled in polarization-timestamp degrees of freedom.

[0080] It should be noted that the first polarization entanglement unit and the second polarization entanglement unit are not limited to specific structures. They can use the Sagnac ring structure based on PPKTP (periodically polarized potassium titanate phosphate), or any other method that can achieve polarization entanglement.

[0081] The single-copy entanglement purification device of this disclosure can not only achieve high-efficiency entanglement purification, but also be compatible with fiber optic channels and is suitable for practical deployment of quantum repeaters.

[0082] This disclosure also provides a single-copy entanglement purification quantum relay method, which is applied to the relay nodes of a single-copy entanglement purification quantum relay system.

[0083] The single-copy entanglement purification quantum repeater method includes: synchronously sending pulse signals to a first super-entangled source and a second super-entangled source using a repeater node; generating a first super-entangled quantum pair using the first super-entangled source in response to the pulse signal; generating a second super-entangled quantum pair using the second super-entangled source in response to the pulse signal; entanglement purification of the first idler photon and the first signal photon in the first super-entangled quantum pair using a first node and a repeater node, respectively; and entanglement purification of the second idler photon and the second signal photon in the second super-entangled quantum pair using a second node and a repeater node, respectively, so that the repeater node can realize quantum repeater between the first node and the second node based on the purified first signal photon and the purified second signal photon.

[0084] Figure 6 The illustration schematically shows a multi-user node principle of a quantum repeater system with single-copy entanglement purification according to an embodiment of the present disclosure.

[0085] like Figure 6 As shown, in one example, the relay node Charlie includes a Bell state measurement device and multiple control NOT gates. The relay node synchronously transmits a pulse signal with a wavelength of approximately 1550 nm to super-entangled source 1 and super-entangled source 2 via a seed light distribution link. Super-entangled source 1 and super-entangled source 2 preprocess the pulse signal, including dispersion compensation, optical amplification, and frequency doubling, to obtain pump light of approximately 775 nm. Under the action of the pump light, super-entangled source 1 and super-entangled source 2 simultaneously generate super-entangled quantum pairs entangled in two degrees of freedom. The wavelength of the super-entangled quantum pairs is approximately 1550 nm. Super-entangled source 1 and super-entangled source 2 respectively transmit one photon from super-entangled quantum pair 1 and super-entangled quantum pair 2 back to the relay node's control NOT gates 10 and 20 via the backbone link, and transmit the other photon to the two user ends, Alice and Bob, via the access link. For example, the Alice terminal includes a quantum state measurement device 11 and a control NOT gate device 11, and the Bob terminal includes a quantum state measurement device 21 and a control NOT gate device 21. Alternatively, the Alice terminal includes a quantum state measurement device 12 and a control NOT gate device 12, and the Bob terminal includes a quantum state measurement device 21 and a control NOT gate device 21. Or, the Alice terminal includes a quantum state measurement device 13 and a control NOT gate device 13, and the Bob terminal includes a quantum state measurement device 22 and a control NOT gate device 22.

[0086] like Figure 6As shown, in one example, the relay node Charlie synchronously sends a pulse signal to super-entangled sources 2 and 3 via a seed light distribution link. Super-entangled sources 2 and 3 generate super-entangled quantum pairs in response to the pulse signal. Super-entangled sources 2 and 3 each send one photon from their respective super-entangled quantum pair back to the relay node's control NOT gate device 20 and control NOT gate device 30 via a backbone link, and send the other photon to two users, Alice and Bob, via an access link. For example, the Alice end includes a quantum state measurement device 21 and a control NOT gate device 21, and the Bob end includes a quantum state measurement device 31 and a control NOT gate device 31. Alternatively, the Alice end includes a quantum state measurement device 22 and a control NOT gate device 22, and the Bob end includes a quantum state measurement device 31 and a control NOT gate device 31.

[0087] Those skilled in the art will understand that the features described in the various embodiments of this disclosure can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this disclosure. In particular, the features described in the various embodiments of this disclosure can be combined and / or combined in various ways without departing from the spirit and teachings of this disclosure. All such combinations and / or combinations fall within the scope of this disclosure.

[0088] The embodiments of this disclosure have been described above. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of this disclosure. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of this disclosure, and all such substitutions and modifications should fall within the scope of this disclosure.

Claims

1. A quantum repeater system for single-copy entanglement purification, characterized in that, The quantum relay system includes a relay node, a first super-entangled source, a second super-entangled source, a first node, and a second node; The relay node is used to synchronously send pulse signals to the first hyperentangled source and the second hyperentangled source; The first hyperentangled source is used to generate a first hyperentangled quantum pair in response to the pulse signal; The second hyperentangled source is used to generate a second hyperentangled quantum pair in response to the pulse signal; The first node is used to entangle and purify the first idler photon and the first signal photon in the first super-entangled quantum pair with the relay node, respectively. The second node is used to entangle and purify the second idler photon and the second signal photon in the second super-entangled quantum pair with the relay node, so that the relay node can realize quantum relay between the first node and the second node based on the purified first signal photon and the purified second signal photon.

2. The quantum repeater system according to claim 1, characterized in that, The quantum repeater system includes: Multiple control NOT gate devices are used to perform control NOT gate operations on a single photon, so that the photon state characteristics of the single photon in the first degree of freedom are preserved and the photon state characteristics in the second degree of freedom are eliminated; The relay node includes a first control NOT gate device and a second control NOT gate device, the first node includes a third control NOT gate device, and the second node includes a fourth control NOT gate device. Wherein, the first control NOT gate device is connected to the first super-entangled source to receive the first signal photon; the second control NOT gate device is connected to the second super-entangled source to receive the second signal photon; the third control NOT gate device is connected to the first super-entangled source to receive the first idler photon; and the fourth control NOT gate device is connected to the second super-entangled source to receive the second idler photon.

3. The quantum repeater system according to claim 2, characterized in that, The relay nodes include: The Bell state measurement device is used to perform Bell state measurements on the purified first signal photon and the purified second signal photon in the first degree of freedom, and to obtain the Bell state measurement results.

4. The quantum repeater system according to claim 3, characterized in that, The first node includes: The first quantum state measurement device is used to perform quantum state measurement on the purified first idler photon in the first degree of freedom, and obtain the first quantum state measurement result.

5. The quantum repeater system according to claim 4, characterized in that, The second node includes: The second quantum state measurement device is used to perform quantum state measurement on the purified second idler photon in the first degree of freedom to obtain the second quantum state measurement result.

6. The quantum repeater system according to claim 5, characterized in that: The first node and the second node are used to realize entanglement swapping based on the Bell state measurement results, and to complete quantum communication based on the first quantum state measurement results and the second quantum state measurement results.

7. The quantum repeater system according to claim 2, characterized in that, When the first degree of freedom is the polarization degree of freedom and the second degree of freedom is the timestamp degree of freedom, the control NOT gate device includes: An unequal-arm Mach-Zehnder interferometer is used to perform controlled NOT gate operations on the single photon, and to adjust the photon state characteristics of the single photon in the time stamp degree of freedom according to the photon state characteristics of the single photon in the polarization degree of freedom, so as to obtain a purified single photon with consistent photon state in the time stamp degree of freedom and unchanged photon state in the polarization degree of freedom.

8. The quantum repeater system according to claim 2, characterized in that, When the first degree of freedom is the timestamp degree of freedom and the second degree of freedom is the polarization degree of freedom, the control NOT gate device includes: An electrically controlled polarization controller is used to perform a control NOT gate operation on the single photon, adjusting the photon state characteristics of the single photon in the polarization degree of freedom according to the photon state characteristics of the single photon in the timestamp degree of freedom, to obtain a purified single photon with consistent photon states in the polarization degree of freedom and unchanged photon states in the timestamp degree of freedom; and A polarizer is used to perform post-selection of the polarization degree of freedom of the purified single photon, and output purified single photons with horizontal polarization degree of freedom or vertical polarization degree of freedom.

9. The quantum repeater system according to claim 1, characterized in that, The relay node also includes: A synchronization signal generating device is used to synchronously send the pulse signal to the first super-entangled source and the second super-entangled source.

10. A single-copy entanglement purification method for quantum repeaters, applied to the quantum repeater system as described in any one of claims 1-9, characterized in that, The quantum repeater method includes: Pulse signals are synchronously transmitted to the first and second hyperentangled sources using relay nodes; The first hyperentangled quantum pair is generated by responding to the pulse signal using the first hyperentangled source. The second hyperentangled quantum pair is generated using the second hyperentangled source in response to the pulse signal; The first node and the relay node are used to entangle and purify the first idler photon and the first signal photon in the first super-entangled quantum pair, respectively. By entanglement purification of the second idler photon and the second signal photon in the second super-entangled quantum pair between the second node and the relay node, the relay node can realize quantum relay between the first node and the second node based on the purified first signal photon and the purified second signal photon.

Images