阅读说明：本技术 量子纠缠纯化装置及方法 (Quantum entanglement purification device and method ) 是由 刘超 周宗权 李传锋 于 2020-06-16 设计创作，主要内容包括：一种量子纠缠纯化装置及方法,装置包括：量子纠缠建立模块,用于建立量子存储器之间的量子纠缠态；局域操作模块,用于将电磁脉冲分别作用于量子存储器上,使电磁脉冲与量子存储器的量子能级共振；激光模块,用于读取量子存储器中的激发使之放出反斯托克斯光子；局域探测模块,用于探测反斯托克斯光子,其中,量子存储器与局域探测模块一一对应；中央控制模块,用于根据局域探测模块的探测结果判断量子纠缠纯化是否成功,在未成功情况下控制量子纠缠建立模块、局域操作模块、激光模块及局域探测模块重复进行量子纠缠纯化。该装置及方法不限制初始量子纠缠态的保真度,可以通过控制量子纠缠纯化的实施次数实现高保真度的量子纠缠态。(A quantum entanglement purifying device and method, the device comprises: the quantum entanglement establishing module is used for establishing quantum entanglement states among the quantum memories; the local operation module is used for respectively acting the electromagnetic pulse on the quantum memory to enable the electromagnetic pulse to resonate with the quantum energy level of the quantum memory; the laser module is used for reading excitation in the quantum memory to enable the quantum memory to emit anti-Stokes photons; the local detection module is used for detecting anti-Stokes photons, wherein the quantum memories correspond to the local detection modules one to one; and the central control module is used for judging whether the quantum entanglement purification is successful or not according to the detection result of the local detection module and controlling the quantum entanglement establishing module, the local operation module, the laser module and the local detection module to repeatedly carry out the quantum entanglement purification under the unsuccessful condition. The device and the method do not limit the fidelity of the initial quantum entanglement state, and can realize the high-fidelity quantum entanglement state by controlling the implementation times of quantum entanglement purification.)

1. A quantum entanglement purification device, comprising:

the quantum entanglement establishing module is used for establishing quantum entanglement states among the quantum memories;

the local operation module is used for respectively acting electromagnetic pulses on the quantum memories to enable the electromagnetic pulses to resonate with quantum energy levels of the quantum memories;

a laser module for reading the excitation in the quantum memory to cause it to emit anti-stokes photons;

the local detection module is used for detecting the anti-Stokes photons, wherein the quantum memories correspond to the local detection modules one by one;

and the central control module is used for judging whether the quantum entanglement purification is successful or not according to the detection result of the local detection module, and controlling the quantum entanglement establishing module, the local operation module, the laser module and the local detection module to repeatedly carry out the quantum entanglement purification under the condition of unsuccessful quantum entanglement purification.

2. The quantum entanglement purification device according to claim 1, wherein the central control module is further configured to determine whether the fidelity of the quantum entanglement state after the quantum entanglement purification is successful is higher than a preset value;

and under the condition that the fidelity is lower than a preset value, controlling the quantum entanglement establishing module, the local operation module, the laser module and the local detection module to repeatedly carry out quantum entanglement purification.

3. The quantum entanglement purification device of claim 1, wherein the local operation module comprises a generation module and an application module;

the generating module is used for generating the electromagnetic pulse; the applying module is used for applying the electromagnetic pulse to the quantum memory, wherein the applying module comprises a silver-plated copper coil or a three-dimensional resonant cavity.

4. The quantum entanglement purification device of claim 3, wherein the silver-plated copper coil is wound on the quantum memory to form a solenoid structure.

5. The quantum entanglement purification device of claim 1, wherein the electromagnetic pulse comprises a radio frequency or microwave pulse.

6. The quantum entanglement purification device of claim 1, wherein the quantum memory comprises at least four quantum energy levels.

7. The quantum entanglement purification device of claim 1, wherein the electromagnetic pulse resonates with two quantum energy levels of the quantum memory.

8. A quantum entanglement purification method based on the quantum entanglement purification device of any one of claims 1 to 7, comprising:

s1, establishing quantum entanglement states among the quantum memories;

s2, applying electromagnetic pulses to the quantum memories respectively to make the electromagnetic pulses resonate with quantum energy levels of the quantum memories;

s3, reading the excitation in the quantum memory to make it emit anti-Stokes photons;

s4, detecting the anti-Stokes photons;

s5, judging whether the quantum entanglement purification is successful or not according to the detection result, if so, executing operation S6, and if not, repeatedly executing operations S1-S4;

and S6, judging whether the fidelity of the quantum entanglement state after the quantum entanglement purification is successful is higher than a preset value, if not, repeatedly executing the operation S2-S5, and if so, ending the execution.

9. The quantum entanglement purification method according to claim 8, wherein the determining whether the quantum entanglement purification is successful according to the detection result comprises:

for each of the quantum memories, if the anti-stokes photon is not detected, the quantum entanglement purification is successful.

10. The quantum entanglement purification method of claim 8, wherein in operation S1, the establishing the quantum entanglement state between the quantum memories comprises:

single-photon entangled states between quantum memories are established based on entanglement generation or entanglement swapping in the DLCZ scheme.

Technical Field

The disclosure relates to the field of quantum information communication, in particular to a quantum entanglement purification device and method.

Background

Quantum communication can realize unconditional safe communication based on the physics principle, however, quantum communication longer than 500 kilometers cannot be realized by the single photon direct transmission mode because the loss of photons in the optical fiber increases exponentially along with the increase of the transmission distance. Quantum relay based communication schemes can effectively address this problem. Chunloming et al proposed a quantum relay scheme (DLCZ scheme) based on single-photon entanglement between atomic ensembles in 2001, which has the advantage that it is insensitive to photon loss, thereby facilitating quantum entanglement over long distances. The scheme has wide application in a plurality of physical systems, and has extremely profound influence on the practical application of quantum communication.

Assume A, B that a quantum entanglement state is established between two quantum memories, denoted as:

|0>_A(B)indicating that all electrons in the quantum memory a (or B) are on the ground state 4, which may be referred to as no excitation in the quantum memory a (or B), which is not readable. L 1>_A(B)Indicating that one electron in the quantum memory a (or B) is in another metastable ground state 2, which may be referred to as an excited state in the quantum memory a (or B), which can be read. The quantum memory a (or B) can release anti-stokes photons by pumping a laser that resonates with the metastable and optically excited states.

Due to the fact that the practical conditions are not ideal, such as the loss of the channel, the single photon detector does not have 100% detection efficiency and full photon number resolution, some noise is inevitably introduced, which leads to A, B the situation where both memories have one excitation (this state is written as |11 >_AB) And occurs. The quantum entanglement states described above can thus be represented by the following density matrix (ignoring vacuum terms that do not affect fidelity):

wherein p is₁Is |11 >_ABRelative toProbability of occurrence of a state. The fidelity F of the quantum entanglement state is 1/(1+ p)₁). Obviously, |11>_ABThe higher the proportion of states, the lower the fidelity of the quantum entangled state. However, in practical quantum technology applications (such as quantum computing and quantum invisible transport), there are high requirements on the fidelity of quantum entangled states. Therefore, it is very important to purify the quantum entangled state to improve the fidelity of the quantum entangled state.

Disclosure of Invention

Technical problem to be solved

In order to solve the technical problems, the present disclosure provides a quantum entanglement purification apparatus and method, which are used for solving the problem of low fidelity of quantum entanglement state caused by non-ideal actual conditions in the prior art.

(II) technical scheme

According to a first aspect of the present disclosure, there is provided a quantum entanglement purification device comprising: the quantum entanglement establishing module is used for establishing quantum entanglement states among the quantum memories; the local operation module is used for respectively acting the electromagnetic pulse on the quantum memory to enable the electromagnetic pulse to resonate with the quantum energy level of the quantum memory; the laser module is used for reading excitation in the quantum memory to enable the quantum memory to emit anti-Stokes photons; the local detection module is used for detecting anti-Stokes photons, wherein the quantum memories correspond to the local detection modules one to one; and the central control module is used for judging whether the quantum entanglement purification is successful or not according to the detection result of the local detection module and controlling the quantum entanglement establishing module, the local operation module, the laser module and the local detection module to repeatedly carry out the quantum entanglement purification under the unsuccessful condition.

Optionally, the central control module is further configured to determine whether the fidelity of the quantum entanglement state after the quantum entanglement purification is successful is higher than a preset value; and under the condition that the fidelity is lower than the preset value, controlling the quantum entanglement establishing module, the local operation module, the laser module and the local detection module to repeatedly carry out quantum entanglement purification.

Optionally, the local operation module comprises a generation module and an application module; the generating module is used for generating electromagnetic pulses; the applying module is used for applying electromagnetic pulses to the quantum memory, wherein the applying module comprises a silver-plated copper coil or a three-dimensional resonant cavity.

Optionally, a silver-plated copper coil is wound on the quantum memory to form a solenoid structure.

Optionally, the electromagnetic pulse comprises a radio frequency or microwave pulse.

Optionally, the quantum memory comprises at least four quantum energy levels.

Optionally, the electromagnetic pulse resonates with two quantum energy levels of the quantum memory.

According to another aspect of the present disclosure, there is provided a quantum entanglement purification method based on a quantum entanglement purification device, including: s1, establishing quantum entanglement states among the quantum memories; s2, respectively applying the electromagnetic pulse on the quantum memory to make the electromagnetic pulse and the quantum energy level of the quantum memory resonate; s3, reading excitation in the quantum memory to make it emit anti-Stokes photon; s4, detecting anti-Stokes photons; s5, judging whether the quantum entanglement purification is successful or not according to the detection result, if so, executing operation S6, and if not, repeatedly executing operations S1-S4; and S6, judging whether the fidelity of the quantum entanglement state after the quantum entanglement purification is successful is higher than a preset value, if not, repeatedly executing the operations S2-S5, and if so, ending the execution.

Optionally, the determining whether the quantum entanglement purification is successful according to the detection result comprises: for each quantum memory, if no anti-stokes photon is detected, quantum entanglement purification is successful.

Optionally, in operation S1, establishing the quantum entanglement states between the quantum memories includes: single-photon entangled states between quantum memories are established based on entanglement generation or entanglement swapping in the DLCZ scheme.

(III) advantageous effects

The utility model provides a quantum entanglement purification device and method, which has the following beneficial effects: the device and the method do not limit the fidelity of the initial quantum entanglement state in the quantum entanglement purification process, and can enable the fidelity of the final quantum entanglement state to meet the application requirements by controlling the implementation times of the quantum entanglement purification.

Drawings

For a more complete understanding of the present disclosure and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present disclosure, and together with the description serve to explain the principles of the disclosure. Wherein:

fig. 1 schematically shows a block diagram of a quantum entanglement purification device according to an embodiment of the present disclosure;

fig. 2 schematically illustrates a flow diagram of a quantum entanglement purification method of an embodiment of the present disclosure;

FIG. 3 schematically shows a detailed diagram of an embodiment of local operation and local detection in a quantum entanglement purification method;

figure 4 schematically shows a graph of quantum entanglement fidelity as a function of the number of purification successes.

Detailed Description

Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. It should be understood that the description is illustrative only and is not intended to limit the scope of the present disclosure. In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the disclosure. It may be evident, however, that one or more embodiments may be practiced without these specific details. Moreover, in the following description, descriptions of well-known structures and techniques are omitted so as to not unnecessarily obscure the concepts of the present disclosure.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The terms "comprises," "comprising," and the like, as used herein, specify the presence of stated features, steps, operations, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, or components.

The device comprises a quantum entanglement establishing module, a local operation module, a laser module, a local detection module and a central control module, and can realize a high-fidelity quantum entanglement state through the interaction of all functional modules. The following description will take the purification of quantum entanglement states between two quantum memories as a specific example.

Fig. 1 schematically shows a structural block diagram of a quantum entanglement purification apparatus according to an embodiment of the present disclosure. As shown in fig. 1, the apparatus may include, for example:

and the quantum entanglement establishing module is used for establishing quantum entanglement states between the quantum memories. In some embodiments of the present disclosure, the quantum memory comprises at least four quantum energy levels, and the atomic ensemble may be selected¹⁴³Nd ion doped yttrium silicate crystals. In a possible arrangement of this embodiment, single-photon entangled states between quantum memories can be established using entanglement generation or entanglement swapping in the DLCZ scheme.

And the local operation module is used for respectively acting the electromagnetic pulse on the quantum memory to enable the electromagnetic pulse to resonate with the quantum energy level of the quantum memory. Specifically, the local operation module may include a generation module and an application module. The generation module is used to generate an electromagnetic pulse, wherein the electromagnetic pulse may include, but is not limited to, a radio frequency or microwave pulse. The applying module is used for applying electromagnetic pulses to the quantum memory, and the applying module comprises but is not limited to a silver-plated copper coil or a three-dimensional resonant cavity. In one possible implementation of this embodiment, the application module selects a silver-plated copper coil, which is wound around the quantum memory to form a solenoid structure, and can effectively apply various electromagnetic pulses. The applied electromagnetic pulse may resonate with two quantum energy levels of the quantum memory.

And the laser module is used for exciting the quantum memory to emit Stokes photons and reading the excitation in the quantum memory to emit anti-Stokes photons. The stokes photon is excited by a weakly detuned laser pulse and the detection of a stokes photon means that an electron in the quantum memory is excited and falls onto another metastable ground state 2. Striking a strong laser pulse that resonates with the metastable ground state and the optically excited state causes the electron to be excited to the optically excited state and then fall back to the initial excited state, during which the quantum memory emits an anti-stokes photon. Anti-stokes photons and stokes photons are strictly correlated.

A local detection module for detecting anti-stokes photons. The local area detection module can comprise a single photon detector and a classical communication module. The local detection module is used for detecting anti-Stokes photons emitted by the quantum memory, and the classical communication module is used for sending a detection result to the central control module. The quantum memories correspond to the single-photon detectors one by one, namely one single-photon detector detects that one quantum memory emits anti-Stokes photons.

And the central control module is used for judging whether the quantum entanglement purification is successful or not according to the detection result of the local detection module and controlling the quantum entanglement establishing module, the local operation module, the laser module and the local detection module to repeatedly carry out the quantum entanglement purification under the unsuccessful condition. Meanwhile, under the condition that the quantum entanglement purification is successful, judging whether the fidelity of the quantum entanglement purified quantum entanglement is higher than a preset value or not; and under the condition that the fidelity is lower than the preset value, controlling the quantum entanglement establishing module, the local operation module, the laser module and the local detection module to repeatedly carry out quantum entanglement purification until the fidelity of the quantum entanglement state is higher than the preset value. The preset value corresponding to the fidelity can be set according to the actual application requirements, and the invention is not limited.

The basis of successful quantum entanglement purification is as follows: if the single photon detectors corresponding to the quantum memories do not detect the anti-Stokes photons, the quantum entanglement purification is successful, otherwise, the quantum entanglement purification fails.

The quantum entanglement purification device does not limit the fidelity of the initially established quantum entanglement states in the process of purifying the quantum entanglement states to improve the fidelity of the quantum entanglement states.

Based on the quantum entanglement purification device, the embodiment of the present disclosure further provides a quantum entanglement purification method, as shown in fig. 2, the method may include:

and S1, establishing quantum entanglement states among the quantum memories.

In the above operation S1, a single-photon entangled state between the quantum memories is established using entanglement generation or entanglement swapping in the DLCZ scheme.

S2, the electromagnetic pulses are applied to the quantum memories, respectively, and the electromagnetic pulses resonate with quantum levels of the quantum memories.

S3, reading the excitation in the quantum memory causes it to emit anti-stokes photons.

S4, detecting the anti-stokes photon.

And S5, judging whether the quantum entanglement purification is successful or not according to the detection result, if so, executing operation S6, otherwise, repeatedly executing operations S1-S4, reestablishing the initial quantum entanglement state, and performing subsequent steps until the purification process is successful.

And S6, judging whether the fidelity of the quantum entanglement state after the quantum entanglement purification is successful is higher than a preset value, if not, repeatedly executing the operations S2-S5, and if so, ending the execution.

For details of the method embodiment, please refer to the device embodiment, which is not described herein.

The quantum entanglement purification apparatus and method described above are further illustrated by a more specific example below. FIG. 3 is a detailed diagram of the local operation and local detection in the quantum entanglement purification method, and as shown in FIG. 2, two quantum memories A, B have 4 energy levels of 1, 2, 3, and 4, where 1 is an optically excited state⁴F_3/2And 4 is the ground state⁴I_9/2And 2 and 3 are metastable states corresponding to the nuclear spin fine energy level of the ground state. By adopting an optical pumping method, the population numbers on the 2 and 3 energy levels can be transferred to the 4 energy level, so that the initialization of the population numbers is realized. Assuming that multiple entanglement exchanges between A, B eventually successfully establish an initial single-photon entangled state, wherein only 10% of the opportunities are to obtain the target state, the initial single-photon entangled state between A, B is represented asFidelity F⁰10%. To improve the fidelity of the quantum entangled state, it is necessary to reduce the non-linear luminance11>_ABRelative toProportion of states, which can be achieved by quantum entanglement purification.

Specifically, first, A, B two quantum memories were each subjected to a pi/2 pulse through a silver plated copper coil, which resonated with two metastable states (2 and 3) of the quantum memory. Then, a strong laser pulse is applied, which resonates with metastable state 2 and optically excited state 1, the excitation of the two quantum memories in metastable state 2 is separately read A, B, and anti-stokes photons emitted by the two quantum memories are separately detected A, B with two single photon detectors. Only when the two quantum memories can not detect the anti-Stokes photons, the purification process is successful, and the purified quantum state is reserved and the subsequent steps are carried out; otherwise, the quantum state after purification is abandoned, the initial quantum entanglement state is reestablished, and the subsequent steps are carried out until the purification process is successful. After the first time of purification, the product is purified,the probability of being retained after purification is 1/2, then |11>_ABThe probability of being retained is 1/4, thus, |11>_ABRelative toThe proportion of states is reduced to 1/2. If the purification process is successful, then the quantum entanglement state retained isIts fidelity is increased to F¹1/6. After the second purification process is successful, the retained quantum entanglement state isIts fidelity is increased to F²2/7. By summarizing the above results, it can be obtained that the quantum entanglement state retained after the nth purification process is successful isIts fidelity becomes Fⁿ＝1/(1+(10/2ⁿ))。

Fig. 4 schematically shows a graph of the fidelity of the quantum entangled state as a function of the number of successful purifications, and as shown in fig. 4, the fidelity of the quantum entangled state can exceed 90% after 7 purifications, that is, the fidelity as high as possible can be achieved by increasing the number of purifications within a range allowed by conditions. If the fidelity of the final quantum entanglement state is required to be not less than 90%, the experiment can be stopped after the 7 th purification process is successful through the central control module. The specific number of purifications can be selected according to the requirements of actual fidelity.

To sum up, the embodiments of the present disclosure provide a quantum entanglement purification apparatus and method, which do not limit the fidelity of the initial quantum entanglement state, and can realize high fidelity of the quantum entanglement state by controlling the number of times of quantum entanglement purification, thereby satisfying application requirements.

The above-mentioned embodiments are intended to illustrate the objects, aspects and advantages of the present disclosure in further detail, and it should be understood that the above-mentioned embodiments are only illustrative of the present disclosure and are not intended to limit the present disclosure, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present disclosure should be included in the scope of the present disclosure.