Quantum relay system and method for single-copy entanglement purification

Abstract

The invention provides a quantum relay system and method for single copy entanglement purification, and relates to the technical field of quantum communication, the quantum relay system comprises a relay node, a first hyper-entanglement source, a second hyper-entanglement source, a first node and a second node; the relay node is used for synchronously sending pulse signals to the first hyper-entanglement source and the second hyper-entanglement source; a first hyper-entanglement source for generating a first hyper-entanglement quantum pair in response to the pulse signal; a second hyper-entanglement source for generating a second hyper-entanglement quantum pair in response to the pulse signal; the first node is used for carrying out entanglement purification on the first idler frequency photon and the first signal photon in the first hyper-entanglement quantum pair together with the relay node; and the second node is used for carrying out entanglement purification on the second idle frequency photon and the second signal photon in the second hyper-entanglement quantum pair together with the relay node, so that the relay node realizes quantum relay between the first node and the second node based on the purified first signal photon and the purified second signal photon.

Description

Technical Field

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

[0002] After years of development, quantum communication technology has become very mature. However, due to the existence of channel loss and system noise, the maximum transmission distance of quantum communication is still limited. In this case, as the only quantum communication technology that can achieve scalable and distance-independent quantum communication currently, it has become increasingly important to study entanglement-based quantum relays. However, during the continuous process of quantum relaying, depolarization noise will accumulate in links such as channel transmission and Bell state measurement, gradually reducing the purity of the original entangled state and ultimately making it unusable for normal communication. Therefore, quantum entanglement purification has become one of the key technologies indispensable for realizing quantum relays.

[0003] In related technologies, the entanglement purification scheme used in quantum relays requires two pairs of photons to complete one entanglement purification, with very low efficiency and being almost inapplicable in scenarios with high channel loss. Summary of the Invention

[0004] In view of the above problems, the present disclosure provides a quantum relay system and method for single-copy entanglement purification that improve the ability to resist depolarization noise and the communication rate of the quantum relay system.

[0005] According to a first aspect of the present disclosure, there is provided a quantum relay system for single-copy entanglement purification. The quantum relay system includes a relay node, a first super-entanglement source, a second super-entanglement source, a first node, and a second node. The relay node is configured to synchronously send pulse signals to the first super-entanglement source and the second super-entanglement source. The first super-entanglement source is configured to generate a first super-entangled quantum pair in response to the pulse signal. The second super-entanglement 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 on the first idler photon and the first signal photon in the first super-entangled quantum pair with the relay node. The second node is configured to perform entanglement purification on 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 realizes quantum relay between the first node and the second node based on the purified first signal photon and the purified second signal photon.

[0006] According to an embodiment of the present disclosure, the above-mentioned quantum relay system includes: a plurality of controlled-NOT gate devices for performing a controlled-NOT operation on single photons, so that the photon state characteristics of the above-mentioned single photons in the first degree of freedom are retained and the photon state characteristics in the second degree of freedom are eliminated; wherein, the above-mentioned relay node includes a first controlled-NOT gate device and a second controlled-NOT gate device, the above-mentioned first node includes a third controlled-NOT gate device, and the above-mentioned second node includes a fourth controlled-NOT gate device; wherein, the above-mentioned first controlled-NOT gate device is connected to the above-mentioned first hyperentangled source to receive the above-mentioned first signal photon; the above-mentioned second controlled-NOT gate device is connected to the above-mentioned second hyperentangled source to receive the above-mentioned second signal photon; the above-mentioned third controlled-NOT gate device is connected to the above-mentioned first hyperentangled source to receive the above-mentioned first idler photon; the above-mentioned fourth controlled-NOT gate device is connected to the above-mentioned second hyperentangled source to receive the above-mentioned second idler photon.

[0007] According to an embodiment of the present disclosure, the above-mentioned relay node includes: a Bell state measurement device for performing a Bell state measurement on the purified first signal photon and the purified second signal photon in the above-mentioned first degree of freedom to obtain a Bell state measurement result.

[0008] According to an embodiment of the present disclosure, the above-mentioned first node includes: a first quantum state measurement device for performing a quantum state measurement on the purified first idler photon in the above-mentioned first degree of freedom to obtain a first quantum state measurement result.

[0009] According to an embodiment of the present disclosure, the above-mentioned second node includes: a second quantum state measurement device for performing a quantum state measurement on the purified second idler photon in the above-mentioned first degree of freedom to obtain a second quantum state measurement result.

[0010] According to an embodiment of the present disclosure, the above-mentioned first node and the above-mentioned second node are used to implement entanglement swapping according to the above-mentioned Bell state measurement result, and complete quantum communication according to the above-mentioned first quantum state measurement result and the above-mentioned second quantum state measurement result.

[0011] According to an embodiment of the present disclosure, when the above-mentioned first degree of freedom is the polarization degree of freedom and the above-mentioned second degree of freedom is the time stamp degree of freedom, the above-mentioned controlled-NOT gate device includes: an unbalanced Mach-Zehnder interferometer for performing a controlled-NOT operation on the above-mentioned single photon, and regulating the photon state characteristics of the above-mentioned single photon in the time stamp degree of freedom according to the photon state characteristics of the above-mentioned single photon in the polarization degree of freedom, so as to obtain a purified single photon with consistent photon state characteristics in the time stamp degree of freedom and unchanged photon state characteristics in the polarization degree of freedom.

[0012] According to an embodiment of the present disclosure, in the case where the first degree of freedom is the timestamp degree of freedom and the second degree of freedom is the polarization degree of freedom, the controlled-NOT gate device includes: an electro-optic polarization controller for performing a controlled-NOT operation on the single photon, and regulating 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, so as to obtain a purified single photon with consistent photon state in the polarization degree of freedom and unchanged photon state in the timestamp degree of freedom; and a polarizer for post-selecting the polarization degree of freedom of the purified single photon, and outputting a purified single photon with horizontal polarization in the polarization degree of freedom or a purified single photon with vertical polarization in the polarization degree of freedom.

[0013] According to an embodiment of the present disclosure, the relay node further includes: a synchronization signal generating device for synchronously sending the pulse signal to the first hyper-entangled source and the second hyper-entangled source.

[0014] According to another aspect of the present disclosure, there is provided a quantum relay method for single-copy entanglement purification, which is applied to the above-mentioned quantum relay system. The quantum relay method includes: using a relay node to synchronously send a pulse signal to a first hyper-entangled source and a second hyper-entangled source; using the first hyper-entangled source to generate a first hyper-entangled quantum pair in response to the pulse signal; using the second hyper-entangled source to generate a second hyper-entangled quantum pair in response to the pulse signal; using a first node and the relay node to perform entanglement purification on a first idler photon and a first signal photon in the first hyper-entangled quantum pair respectively; using a second node and the relay node to perform entanglement purification on a second idler photon and a second signal photon in the second hyper-entangled quantum pair respectively, so that the relay node realizes quantum relay between the first node and the second node based on the purified first signal photon and the purified second signal photon.

[0015] According to an embodiment of the present disclosure, based on the principle of quantum entanglement purification using hyperentanglement, the first node and the relay node improve the entanglement degrees of the first signal photon and the first idler photon in the first hyperentangled quantum pair through entanglement purification, and the second node and the relay node improve the entanglement degrees of the second signal photon and the second idler photon in the second hyperentangled quantum pair through entanglement purification. This can better protect the quantum state, effectively enhance the resistance to channel depolarization noise during the quantum relay process, and enhance the security and reliability of quantum communication. Compared with the technical solution of using two physical bits for entanglement purification, in the quantum relay system with single-copy entanglement purification according to the embodiment of the present disclosure, entanglement purification is respectively performed on the first idler photon and the first signal photon, and entanglement purification is respectively performed on the second idler photon and the second signal photon. Through single-copy entanglement purification, the efficiency of entanglement purification can be effectively improved, quantum communication tasks can be effectively and reliably supported, the communication efficiency meets the actual requirements, and it helps to realize the practical application of long-distance quantum communication. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Through the following description of the embodiments of the present disclosure with reference to the accompanying drawings, the above content and other objects, features, and advantages of the present disclosure will become clearer. In the drawings:

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

[0018] Figure 2 Schematically shows a schematic diagram of the principle of a quantum relay system with single-copy entanglement purification according to an embodiment of the present disclosure;

[0019] Figure 3 Schematically shows a schematic diagram of the principle of a quantum relay system with single-copy entanglement purification according to another embodiment of the present disclosure;

[0020] Figure 4 Schematically shows a schematic diagram of the principle of a controlled-NOT gate operation of an unbalanced Mach-Zehnder interferometer according to an embodiment of the present disclosure;

[0021] Figure 5 Schematically shows a schematic diagram of the principle of a quantum relay system with single-copy entanglement purification according to still another embodiment of the present disclosure; and

[0022] Figure 6 Schematically shows a schematic diagram of the principle of a multi-user node of a quantum relay system with single-copy entanglement purification according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present disclosure. In the following detailed description, for the sake of explanation, numerous specific details are set forth in order to provide a comprehensive understanding of the embodiments of the present disclosure. However, it is obvious that one or more embodiments can also be implemented without these specific details. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessarily obscuring the concepts of the present disclosure.

[0024] The terms used herein are merely for describing specific embodiments and are not intended to limit the present disclosure. The terms "comprising", "including" and the like used herein indicate the presence of the 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.

[0025] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those of ordinary skill in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification, and should not be interpreted in an idealized or overly rigid manner.

[0026] In the case of using expressions such as "at least one of A, B, and C, etc.", generally, it should be interpreted according to the meaning commonly understood by those of ordinary skill in the art (for example, "a system having at least one of A, B, and C" should include, but is not limited to, a system having only A, only B, only C, having A and B, having A and C, having B and C, and / or having A, B, and C, etc.).

[0027] Figure 1 A schematic diagram showing the principle of a quantum relay system for single-copy entanglement purification according to an embodiment of the present disclosure is schematically illustrated.

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

[0029] According to an embodiment of the present disclosure, the relay node is configured to synchronously send pulse signals to the first hyperentanglement source and the second hyperentanglement source. The first hyperentanglement source is configured to generate a first hyperentangled quantum pair in response to the pulse signal. The second hyperentanglement source is configured to generate a second hyperentangled quantum pair in response to the pulse signal. The first node is configured to perform entanglement purification on the first idler photon and the first signal photon in the first hyperentangled quantum pair with the relay node. The second node is configured to perform entanglement purification on the second idler photon and the second signal photon in the second hyperentangled quantum pair with the relay node, so that the relay node realizes quantum relay between the first node and the second node based on the purified first signal photon and the purified second signal photon.

[0030] A hyper-entangled state represents a quantum state in which entanglement simultaneously exists in multiple degrees of freedom (e.g., polarization, spatial mode, orbital angular momentum, frequency, timestamp, etc.) in a quantum system. In one example, the two quantum states of the first hyper-entangled quantum pair can be quantum states in which entanglement simultaneously exists in the polarization degree of freedom and the orbital angular momentum degree of freedom.

[0031] According to an embodiment of the present disclosure, based on the principle of quantum entanglement purification of hyper-entanglement, the first node and the 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, and the second node and the 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, which can better protect the quantum state, effectively improve the resistance to channel depolarization noise in the quantum relay process, and enhance the security and reliability of quantum communication. Compared with the technical solution of using two physical bits for entanglement purification, the quantum relay system with single-copy entanglement purification in the embodiment of the present disclosure respectively performs entanglement purification on the first idler photon and the first signal photon, and respectively performs entanglement purification on the second idler photon and the second signal photon. Through single-copy entanglement purification, the efficiency of entanglement purification can be effectively improved, the quantum communication task can be effectively and reliably supported, the communication efficiency meets the actual requirements, and it helps to realize the practical application of long-distance quantum communication.

[0032] Figure 2 The schematic diagram of the principle of the quantum relay system with single-copy entanglement purification according to an embodiment of the present disclosure is schematically shown.

[0033] As Figure 2 shown, the quantum relay system with single-copy entanglement purification includes a plurality of controlled-NOT gate devices.

[0034] According to an embodiment of the present disclosure, the controlled-NOT gate device is used to perform a controlled-NOT gate operation on a single photon, so that the photon state characteristic of the single photon in the first degree of freedom is retained, and the photon state characteristic in the second degree of freedom is eliminated.

[0035] As Figure 2 shown, the relay 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. The first controlled-NOT gate device is connected to the first hyper-entanglement source to receive the first signal photon; the second controlled-NOT gate device is connected to the second hyper-entanglement source to receive the second signal photon; the third controlled-NOT gate device is connected to the first hyper-entanglement source to receive the first idler photon; the fourth controlled-NOT gate device is connected to the second hyper-entanglement source to receive the second idler photon.

[0036] According to an embodiment of the present disclosure, the first controlled-NOT gate device and the third controlled-NOT gate device respectively perform entanglement purification on the first signal photon and the first idler photon, so that the photon state characteristics of the first signal photon and the first idler photon in the first degree of freedom are retained, and the photon state characteristics in the second degree of freedom are eliminated, improving the quantum state entanglement purity of the first signal photon and the first idler photon in the first degree of freedom. The second controlled-NOT gate device and the fourth controlled-NOT gate device respectively perform entanglement purification on the second signal photon and the second idler photon, so that the photon state characteristics of the second signal photon and the second idler photon in the first degree of freedom are retained, and the photon state characteristics in the second degree of freedom are eliminated, improving the quantum state entanglement purity of the second signal photon and the second idler photon in the first degree of freedom.

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

[0038] According to an embodiment of the present disclosure, the Bell state measurement device is configured to perform a Bell state measurement on the purified first signal photon and the purified second signal photon in the first degree of freedom to obtain a Bell state measurement result.

[0039] According to an embodiment of the present disclosure, a Bell state projection measurement is performed in the first degree of freedom according to the quantum communication protocol in the first degree of freedom. In a quantum entanglement distribution network, Bell state measurement is used to establish entanglement between non-adjacent nodes. Through quantum entanglement swapping, two independent qubits can be entangled.

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

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

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

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

[0044] According to an embodiment of the present disclosure, the first node and the second node are configured to perform entanglement swapping according to the Bell state measurement result, and complete quantum communication according to the first quantum state measurement result and the second quantum state measurement result.

[0045] According to an embodiment of the present 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, and the entanglement fidelity of the photon state decreases. The first node and the relay node respectively perform single-copy entanglement purification on the first idler photon and the first signal photon through a controlled-NOT gate device. The purified first signal photon and the purified first idler photon obtained are entangled only in the first degree of freedom, and the fidelity is improved. The second node and the relay node respectively perform single-copy entanglement purification on the second idler photon and the second signal photon through a controlled-NOT gate device. The purified second signal photon and the purified second idler photon obtained are entangled only in the first degree of freedom, and the fidelity is improved.

[0046] According to an embodiment of the present disclosure, the relay node performs a Bell state measurement on the purified first signal photon and the second signal photon, and informs the first node and the second node of the Bell state measurement result in a classical communication manner. The Bell state is the maximum entanglement state of two qubits, and there are four orthogonal Bell states in total. Through the Bell state measurement, the joint state of two qubits can be projected onto one of the four Bell states, thereby determining their entanglement relationship. The first node and the second node perform a local unitary transformation operation on their respective idler photons according to the Bell state measurement result to obtain one of the four Bell states agreed upon before communication, establish a definite entanglement relationship, and complete the entanglement swapping process. The local unitary transformation refers to an operation performed on a local part (e.g., a single qubit) of a quantum system, which does not affect the overall entanglement property of the system. The first node and the second node perform a quantum state measurement on their respective entangled pairs, converting the entangled qubits into correlated classical information, and completing the quantum communication process.

[0047] In one example, the classical communication manner includes optical fiber communication, wireless communication, and satellite communication.

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

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

[0050] AsFigure 3 As shown, according to an embodiment of the present 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 controlled-NOT gate device can be an unbalanced Mach-Zehnder interferometer.

[0051] According to an embodiment of the present disclosure, the unbalanced Mach-Zehnder interferometer is used to perform a controlled-NOT operation on a single photon, and regulates 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 states in the timestamp degree of freedom and unchanged photon states in the polarization degree of freedom.

[0052] Figure 4 Schematically shows a schematic diagram of the principle of the controlled-NOT operation of the unbalanced Mach-Zehnder interferometer according to an embodiment of the present disclosure.

[0053] As Figure 4 shown, in one example, the unbalanced 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 low-purity quantum entangled state. According to the principle of entanglement purification, a controlled-NOT gate is a two-qubit quantum logic gate, where one qubit serves as the control qubit and the other serves as the target qubit. When the control qubit is in a specific state (usually ), the target qubit will perform a NOT operation, that is, flip its state; if the control qubit is in the state, the target qubit remains unchanged.

[0055] As Figure 4 shown, in one example, the polarization degree of freedom is determined as the control qubit, and the timestamp degree of freedom is determined as the target qubit. The quantum state of a single photon (e.g., the first signal photon) includes , , , . Define as the horizontal polarization state (H), as the vertical polarization state (V), as the late photon state (L) that arrives late in the timestamp, is the early photon state (E) arriving early in the timestamp. Before performing single-copy entanglement purification, it is necessary to calibrate the polarization reference frame of the incident polarized photon state with an unbalanced Mach-Zehnder interferometer. Set the horizontally polarized state to travel through the short arm and the vertically polarized state to travel through the long arm; in addition, the arm length difference of the unbalanced Mach-Zehnder interferometer needs to be set to be the same as the time domain interval of the pulse signal. When the incident polarized photon state is the horizontally polarized state, based on the timestamp bit of traveling through the short arm, that is, define After the state passes through the unbalanced Mach-Zehnder interferometer, it is still Then The photon state of the state incident on the unbalanced Mach-Zehnder interferometer also does not change in the timestamp degree of freedom, that is, it is still . When the incident polarized photon state is the vertically polarized state, The state will experience an additional delay of the arm length difference, and the photon state in the timestamp degree of freedom has flipped, changing to state; while The state will experience an additional delay and change to state. Among them, indicates that the late-arriving first signal photon is even later in the timestamp after passing through the long arm. In the embodiment of the present application, since the late photon originally polarized vertically did not successfully implement the CNOT operation, so what is implemented is the partial-CNOT (p-CNOT) operation, that is, the partial controlled-NOT gate, indicating that finally only the CNOT operation on part of the degrees of freedom is successful. Finally, a post-selection operation needs to be performed to screen out the successful events and discard the failed events. It should be emphasized that although the success rate of the p-CNOT operation is less than 100%, the final entanglement purification effect can be made unaffected through the post-selection operation.

[0056] Table 1 shows the transformation process of the controlled-NOT gate operation of the unbalanced Mach-Zehnder interferometer according to the embodiment of the present disclosure.

[0057] Table 1

[0058]

[0059] According to the embodiment of the present disclosure, by measuring the late photon state traveling through the short arm or the early photon state traveling through the long arm, a controlled-NOT gate operation with the polarization degree of freedom as the control bit and the timestamp degree of freedom as the target bit is realized.

[0060] Such as Figure 3As shown, in one example, the unbalanced Mach-Zehnder interferometer of the first controlled-NOT gate device includes a polarization beam splitter 1 and a polarization beam splitter 2. Similarly, the second controlled-NOT gate device includes a polarization beam splitter 3 and a polarization beam splitter 4; the third controlled-NOT gate device includes a polarization beam splitter 5 and a polarization beam splitter 6; the fourth controlled-NOT gate device includes a polarization beam splitter 7 and a polarization beam splitter 8.

[0061] As Figure 3 shown, according to an embodiment of the present 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 the controlled-NOT gate operation, it is necessary to perform a Bell state measurement on the purified first signal photon and the purified second signal photon in the polarization degree of freedom, and perform a quantum state measurement on the purified first idler photon and the purified second idler photon in the polarization degree of freedom, respectively.

[0062] Figure 5 Schematically shows a schematic diagram of the principle of a single-copy entanglement purification quantum relay system according to another embodiment of the present disclosure.

[0063] As Figure 5 shown, the first node can be represented as the Alice side, the second node can be represented as the Bob side, and the relay node can be represented as the Charlie side. The Charlie side synchronously sends pulse signals through the first servo signal and the second servo signal to excite the first hyperentanglement source and the second hyperentanglement source, respectively. The first hyperentanglement source sends a first signal photon to the Charlie side through the first signal channel and a first idler photon to the Alice side through the first idler channel. The second hyperentanglement source sends a second signal photon to the Charlie side through the second signal channel and a second idler photon to the Bob side through the second idler channel.

[0064] As Figure 5 shown, according to an embodiment of the present 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 controlled-NOT gate device may include an electro-optic polarization controller and a polarizer.

[0065] According to an embodiment of the present disclosure, the electro-optic polarization controller is used to perform a controlled-NOT gate operation on a single photon, regulate 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, and obtain a purified single photon with consistent photon state characteristics in the polarization degree of freedom and unchanged photon state characteristics in the timestamp degree of freedom. The polarizer is used to perform post-selection on the polarization degree of freedom of the purified single photon, and output a purified single photon with a horizontal polarization degree of freedom or a purified single photon with a vertical polarization degree of freedom.

[0066] According to an embodiment of the present disclosure, the timestamp degree of freedom is determined as the control bit, and the polarization degree of freedom is determined as the target bit. According to the arrival time of the timestamp photon state of the incident single photon (e.g., the first signal photon), a control voltage is applied to cause the polarization photon state of the incident timestamp photon state to flip or not. In one example, an electro-optic polarization controller is set to not flip the polarization photon state of the single photon that arrives early (e.g., the first signal photon), and to flip the polarization photon state of the single photon that arrives late (e.g., the first signal photon).

[0067] Table 2 shows the transformation process of the controlled-NOT gate operation of the electro-optic polarization controller according to another embodiment of the present disclosure.

[0068] Table 2

[0069]

[0070] According to an embodiment of the present disclosure, after the controlled-NOT gate operation, a polarizer is used to perform post-selection on the single photon (e.g., the first signal photon) in the polarization degree of freedom, and only the single photon (e.g., the first signal photon) with horizontal polarization or vertical polarization is allowed to pass through.

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

[0072] As Figure 5 shown, according to an embodiment of the present disclosure, the Bell state measurement device is a timestamp Bell state measurement device. The first quantum state measurement device and the second quantum state measurement device are timestamp quantum state measurement devices. After the controlled-NOT gate operation, it is necessary to perform Bell state measurement on the purified first signal photon and the purified second signal photon in the timestamp degree of freedom, and perform quantum state measurement on the purified first idler photon and the purified second idler photon in the timestamp degree of freedom respectively.

[0073] According to an embodiment of the present disclosure, the relay node further includes a synchronization signal generating device. The synchronization signal generating device is used to synchronously send pulse signals to the first super-entangled source and the second super-entangled source.

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

[0075] According to an embodiment of the present disclosure, the synchronization signal generating device at the Charlie side can be a mode-locked pulsed seed laser. The mode-locked pulsed seed laser is used to generate a pulsed signal with coherent characteristics between front and rear pulses. The pulsed signal is respectively sent to the first servo channel and the second servo channel on both sides through a first beam splitter.

[0076] According to an embodiment of the present disclosure, the synchronization signal generating device can also be a pulsed seed laser and an interferometer. The pulsed seed laser is used to generate pulsed seeds, and the interferometer is used to transform the pulsed seeds into a pulsed signal with coherent characteristics between front and rear pulses.

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

[0078] According to an embodiment of the present disclosure, the first hyper-entangled source includes a first polarization entanglement unit. The second hyper-entangled source includes a second polarization entanglement unit.

[0079] According to an embodiment of the present disclosure, the first polarization entanglement unit is used to respond to the pulsed signal, generate a first pair of photons that are entangled in the polarization degree of freedom, and based on the coherent characteristics of the pulsed signal, make the first pair of photons entangled in the timestamp degree of freedom, thereby generating a first hyper-entangled quantum pair that is entangled in the polarization-timestamp degree of freedom. The second polarization entanglement unit is used to respond to the pulsed signal, generate a second pair of photons that are entangled in the polarization degree of freedom, and based on the coherent characteristics of the pulsed signal, make the second pair of photons entangled in the timestamp degree of freedom, thereby generating a second hyper-entangled quantum pair that is entangled in the polarization-timestamp degree of freedom.

[0080] It should be noted that the specific structures of the first polarization entanglement unit and the second polarization entanglement unit are not limited. A Sagnac ring structure based on PPKTP (periodically poled potassium titanyl phosphate) can be used, or any other method capable of realizing polarization entanglement can be used.

[0081] The single-copy entanglement purification device according to the embodiment of the present disclosure can not only achieve high-efficiency entanglement purification, but also be compatible with optical fiber channels and is suitable for the actual deployment of quantum repeaters.

[0082] The present disclosure also provides a quantum repeater method for single-copy entanglement purification, which is applied to a relay node of a quantum repeater system for single-copy entanglement purification.

[0083] The quantum relay method for single-copy entanglement purification includes: using a relay node to synchronously send pulse signals to a first hyper-entangled source and a second hyper-entangled source; using the first hyper-entangled source to generate a first hyper-entangled quantum pair in response to the pulse signal; using the second hyper-entangled source to generate a second hyper-entangled quantum pair in response to the pulse signal; using a first node and the relay node to perform entanglement purification on a first idler photon and a first signal photon in the first hyper-entangled quantum pair respectively; using a second node and the relay node to perform entanglement purification on a second idler photon and a second signal photon in the second hyper-entangled quantum pair respectively, so that the relay node realizes quantum relay between the first node and the second node based on the purified first signal photon and the purified second signal photon.

[0084] Figure 6 Schematically shows a schematic diagram of the principle of a multi-user node of a quantum relay system for single-copy entanglement purification according to an embodiment of the present disclosure.

[0085] As Figure 6 shown, in one example, the relay node Charlie side includes a Bell state measurement device and a plurality of controlled-NOT gate devices. The relay node synchronously sends a pulse signal with a wavelength of about 1550 nm to the hyper-entangled source 1 and the hyper-entangled source 2 through a seed light distribution link. The hyper-entangled source 1 and the hyper-entangled source 2 perform preprocessing on the pulse signal, including dispersion compensation, optical amplification, frequency doubling, etc., to obtain pump light with a wavelength of about 775 nm. Under the action of the pump light, the hyper-entangled source 1 and the hyper-entangled source 2 simultaneously generate hyper-entangled quantum pairs that are entangled in two degrees of freedom respectively. The wavelength of the hyper-entangled quantum pair is about 1550 nm. The hyper-entangled source 1 and the hyper-entangled source 2 respectively send one photon in the hyper-entangled quantum pair 1 and the hyper-entangled quantum pair 2 back to the controlled-NOT gate device 10 and the controlled-NOT gate device 20 of the relay node through the backbone link, and send the other photon to two user Alice sides and Bob sides through the access link. For example, the Alice side includes a quantum state measurement device 11 and a controlled-NOT gate device 11, and the Bob side includes a quantum state measurement device 21 and a controlled-NOT gate device 21. For example, the Alice side includes a quantum state measurement device 12 and a controlled-NOT gate device 12, and the Bob side includes a quantum state measurement device 21 and a controlled-NOT gate device 21. For example, the Alice side includes a quantum state measurement device 13 and a controlled-NOT gate device 13, and the Bob side includes a quantum state measurement device 22 and a controlled-NOT gate device 22.

[0086] As Figure 6As shown, in one example, the relay node Charlie synchronously sends a pulse signal to the hyper-entanglement source 2 and the hyper-entanglement source 3 through the seed optical distribution link. The hyper-entanglement source 2 and the hyper-entanglement source 3 generate hyper-entangled quantum pairs in response to the pulse signal respectively. The hyper-entanglement source 2 and the hyper-entanglement source 3 respectively send one photon of their respective hyper-entangled quantum pairs back to the controlled-NOT gate device 20 and the controlled-NOT gate device 30 of the relay node through the backbone link, and send the other photon to the two users, the Alice side and the Bob side, through the access link. For example, the Alice side includes a quantum state measurement device 21 and a controlled-NOT gate device 21, and the Bob side includes a quantum state measurement device 31 and a controlled-NOT gate device 31. For example, the Alice side includes a quantum state measurement device 22 and a controlled-NOT gate device 22, and the Bob side includes a quantum state measurement device 31 and a controlled-NOT gate device 31.

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

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

Claims

1. A quantum relay system for single-copy entanglement purification, characterized in that: The quantum relay system comprises 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 a pulse signal to the first super-entangled source and the second super-entangled source; 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 purify the entanglement of 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 purify the entanglement of the second idler photon and the second signal photon in the second super-entangled quantum pair with the relay node respectively, 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 relay system according to claim 1, characterized in that: The quantum relay system comprises: A plurality of controlled NOT gate devices, used for performing a controlled NOT gate operation on a single photon, so that the photon state characteristics of the single photon in a first degree of freedom are retained, and the photon state characteristics of the single photon in a second degree of freedom are eliminated; Wherein, the relay node comprises a first controlled NOT gate device and a second controlled NOT gate device, the first node comprises a third controlled NOT gate device, and the second node comprises a fourth controlled NOT gate device; Among them, 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; the fourth control NOT gate device is connected to the second super-entangled source to receive the second idler photon.

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

4. The quantum relay system according to claim 3, characterized in that: The first node comprises: 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 to obtain a first quantum state measurement result.

5. The quantum relay system according to claim 4, characterized in that: The second node comprises: 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 a second quantum state measurement result.

6. The quantum relay system according to claim 5, characterized in that: The first node and the second node are used to implement entanglement exchange according to the Bell state measurement result, and to complete quantum communication according to the first quantum state measurement result and the second quantum state measurement result.

7. The quantum relay system according to claim 2, characterized in that: In the case where the first degree of freedom is a polarization degree of freedom and the second degree of freedom is a timestamp degree of freedom, the control NOT gate device includes: The unequal-arm Mach-Zehnder interferometer is used to perform a controlled NOT gate operation on the single photon, and regulate the photon state characteristics of the single photon at the timestamp degree of freedom according to the photon state characteristics of the single photon at the polarization degree of freedom, so as to obtain a purified single photon with a consistent photon state at the timestamp degree of freedom and an unchanged photon state at the polarization degree of freedom.

8. The quantum relay system according to claim 2, characterized in that: In the case where 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 control a NOT gate operation on the single photon, and to regulate 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, so as to obtain a purified single photon with a consistent photon state in the polarization degree of freedom and an unchanged photon state in the timestamp degree of freedom; and The polarizer is used to post-select the polarization degree of freedom of the purified single photon, and output the purified single photon whose polarization degree of freedom is horizontal polarization or the purified single photon whose polarization degree of freedom is vertical polarization.

9. The quantum relay 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 quantum relay method for single copy entanglement purification, applied to the quantum relay system as claimed in any one of claims 1 to 9, characterized in that: The quantum relay method comprises: Using a relay node to synchronously send a pulse signal to the first super-entangled source and the second super-entangled source; generating a first super-entangled quantum pair in response to the pulse signal using the first super-entangled source; generating a second super-entangled quantum pair using the second super-entangled source in response to the pulse signal; Utilizing the first node and the relay node to entangle and purify the first idler photon and the first signal photon in the first super-entangled quantum pair respectively; The second node and the relay node are used to purify the entanglement of the second idler photon and the second signal photon in the second super-entangled quantum pair respectively, so that the relay node realizes quantum relay between the first node and the second node based on the purified first signal photon and the purified second signal photon.

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