Entanglement Source Simulation Method and Apparatus, Electronic Device, and Medium

Abstract

The present disclosure provides a method, apparatus, electronic device, computer-readable storage medium, and computer program product for simulating an entanglement source, relating to the field of quantum computers, and particularly to the field of quantum entanglement technology. The implementation solution is as follows: determining a first quantum state vector representation of a target entangled state to be prepared by a target entanglement source; determining a quantum state vector corresponding to the target entanglement source based on the characteristic parameters of the target entanglement source and the first quantum state vector representation; determining second coincidence probabilities of respective second coincidence terms caused by the target entanglement source during the spontaneous parametric down-conversion process after measurement based on the quantum state vector; and determining a first coincidence count of the target entanglement source under respective second coincidence terms based on the second coincidence probabilities, where the first coincidence count is used to determine the figure of merit of the quantum entangled state prepared by the target entanglement source.

Description

Technical Field

[0001] The present disclosure relates to the field of quantum computers, particularly to the field of quantum entanglement technology, and specifically to a method, device, electronic device, computer-readable storage medium, and computer program product for simulating an entanglement source. Background Art

[0002] Quantum entanglement is a core resource for quantum science and technologies such as quantum computing, quantum communication, and quantum sensing, and the preparation of quantum entanglement states can be achieved through an entanglement source.

[0003] Using the spontaneous parametric down-conversion process of a barium metaborate (BBO) crystal to prepare an entanglement source is relatively easy to obtain and operate. When actually building an entanglement source, due to factors such as complex real physical processes and environmental noise, before the actual construction is completed, it cannot be ensured that the overall quality of the entanglement source meets the expectations of actual applications or experiments. In terms of yield, this will affect the construction and R & D efficiency of optical quantum devices. Therefore, the simulation of the entanglement source is very important.

[0004] Currently, the simulation method of the entanglement source usually only considers the first-order term of the spontaneous parametric down-conversion of the BBO crystal. Summary of the Invention

[0005] The present disclosure provides a method, device, electronic device, computer-readable storage medium, and computer program product for simulating an entanglement source.

[0006] According to one aspect of the present disclosure, there is provided a method for simulating an entanglement source, including: determining a first quantum state vector representation of a target entangled state to be prepared by a target entanglement source, where the target entanglement source includes a barium metaborate (BBO) crystal for converting pump light into two parametric lights with a spatial distribution of either an intersecting type or a laser type in the spontaneous parametric down-conversion process, where the elements in the first quantum state vector representation are determined based on the first coincidence probabilities of respective first coincidence terms triggered in the spontaneous parametric down-conversion process by the target entanglement source, where the first coincidence terms are used to characterize the polarization states of photons in the two parametric lights, and the spontaneous parametric down-conversion process includes a first-order process and a second-order process; determining a quantum state vector corresponding to the target entanglement source based on the characteristic parameters of the target entanglement source and the first quantum state vector representation, where the characteristic parameters include the down-conversion probability of the BBO crystal in the spontaneous parametric down-conversion process; determining the second coincidence probabilities of respective second coincidence terms triggered in the spontaneous parametric down-conversion process by the target entanglement source obtained after measurement based on the quantum state vector, where the second coincidence terms are used to characterize the polarization states of photons in the two parametric lights, and the second coincidence terms are triggered by performing a measurement in a first measurement basis on the target entanglement source; and determining a first coincidence count of the target entanglement source under respective second coincidence terms based on the second coincidence probabilities, where the first coincidence count is used to determine the quality factor of the quantum entangled state prepared by the target entanglement source.

[0007] According to another aspect of the present disclosure, there is provided an entanglement source simulation device, including: a first determination unit configured to determine a first quantum state vector representation of a target entangled state to be prepared by a target entanglement source, wherein the target entanglement source includes a barium metaborate (BBO) crystal for converting pump light into two parametric lights with a spatial distribution of either an intersecting type or a laser type during the spontaneous parametric down-conversion process, wherein the elements in the first quantum state vector representation are determined based on first coincidence probabilities of respective first coincidence terms caused during the spontaneous parametric down-conversion process of the target entanglement source, wherein the first coincidence terms are used to characterize the polarization states of photons in the two parametric lights, and wherein the spontaneous parametric down-conversion process includes a first-order process and a second-order process; a second determination unit configured to determine a quantum state vector corresponding to the target entanglement source based on characteristic parameters of the target entanglement source and the first quantum state vector representation, wherein the characteristic parameters include the down-conversion probability of the BBO crystal during the spontaneous parametric down-conversion process; a third determination unit configured to determine second coincidence probabilities of respective second coincidence terms caused during the spontaneous parametric down-conversion process of the target entanglement source obtained after measurement based on the quantum state vector, wherein the second coincidence terms are used to characterize the polarization states of photons in the two parametric lights, and the second coincidence terms are caused by performing a measurement in a first measurement basis on the target entanglement source; and a simulation unit configured to determine first coincidence counts of the target entanglement source under respective second coincidence terms based on the second coincidence probabilities, and the first coincidence counts are used to determine a quality factor of the quantum entangled state prepared by the target entanglement source.

[0008] According to another aspect of the present disclosure, there is provided an electronic device, including: at least one processor; and a memory communicatively connected to the at least one processor; the memory stores instructions executable by the at least one processor, and when the instructions are executed by the at least one processor, the at least one processor is enabled to execute the method described in the present disclosure.

[0009] According to another aspect of the present disclosure, there is provided a non-transitory computer-readable storage medium storing computer instructions for causing a computer to execute the method described in the present disclosure.

[0010] According to another aspect of the present disclosure, there is provided a computer program product including a computer program which, when executed by a processor, implements the method described in the present disclosure.

[0011] According to one or more embodiments of the present disclosure, higher-order terms (i.e., second-order processes) of the spontaneous parametric down-conversion process are considered, making the simulation results of the entangled light source more accurate; and, the quantum state initialization and measurement processes are separated, making it easier to extend to the simulation of multi-photon interferometers composed of multiple entangled sources, and the coincidence counts for preparing the target quantum entangled state can be accurately and efficiently obtained in the simulation results, and further a series of quality factors such as its fidelity can be obtained.

[0012] It should be understood that the content described in this part is not intended to identify the key or important features of the embodiments of the present disclosure, nor is it used to limit the scope of the present disclosure. Other features of the present disclosure will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The drawings exemplarily show embodiments and form a part of the specification, and are used together with the written description of the specification to explain the exemplary embodiments. The shown embodiments are for illustrative purposes only and do not limit the scope of the claims. In all the drawings, the same reference numerals refer to similar but not necessarily identical elements.

[0014] Figure 1 Schematic diagrams showing three typical parametric light spatial distributions according to embodiments of the present disclosure are shown;

[0015] Figure 2a and 2b Schematic diagrams of parametric light of the intersecting type and Beam-Like type BBO entangled sources according to embodiments of the present disclosure are respectively shown;

[0016] Figure 3 A flowchart of an entangled source simulation method according to an embodiment of the present disclosure is shown;

[0017] Figure 4 A flowchart of an entangled source simulation method according to an exemplary embodiment of the present disclosure is shown;

[0018] Figure 5a and 5b Schematic diagrams of simulation data of the intersecting type and Beam-Like type BBO entangled sources according to embodiments of the present disclosure are respectively shown;

[0019] Figure 6 A structural block diagram of an entangled source simulation device according to an embodiment of the present disclosure is shown; and

[0020] Figure 7 A structural block diagram of an exemplary electronic device capable of implementing the embodiments of the present disclosure is shown. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] The following describes exemplary embodiments of the present disclosure with reference to the accompanying drawings. Various details of the embodiments of the present disclosure are included to facilitate understanding, and they should be considered merely exemplary. Therefore, those of ordinary skill in the art should recognize that various changes and modifications can be made to the embodiments described herein without departing from the scope of the present disclosure. Similarly, descriptions of well-known functions and structures are omitted in the following description for clarity and conciseness.

[0022] In the present disclosure, unless otherwise specified, the terms "first", "second", etc. are used to describe various elements and are not intended to limit the positional relationship, temporal relationship, or importance relationship of these elements. Such terms are only used to distinguish one element from another. In some examples, the first element and the second element may refer to the same instance of the element, and in certain cases, based on the context description, they may also refer to different instances.

[0023] The terms used in the description of various examples in the present disclosure are only for the purpose of describing specific examples and are not intended to be restrictive. Unless the context clearly indicates otherwise, if the number of elements is not specifically limited, the element may be one or more. In addition, the term "and / or" used in the present disclosure covers any one of the listed items and all possible combinations.

[0024] Embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings.

[0025] The method of preparing entangled states using the spontaneous parametric down-conversion process has been widely used in fields such as optical quantum computing, optical quantum communication, optical quantum sensing, and non-locality verification. Especially in recent years, with the development of entanglement source technology and the maturity of optical chip technology, people have achieved verification of the computational superiority of optical quantum computing in directions such as boson sampling and quantum randomness. Due to the advantages of the relatively simple construction method and high stability of the barium metaborate (BBO) entanglement source, the BBO entanglement source is widely used as the preferred choice for quantum state preparation in optical quantum computers and optical quantum communication.

[0026] Based on the different spatial distributions of the parametric light generated by the down-conversion process, the BBO crystal-based entanglement source can be divided into an intersecting type, a collinear type, and a Beam-Like type. In addition, according to the different paths of the parametric light, it can be further divided into a single-path entanglement source and a double-path entanglement source. The single-path entanglement source is such as the collinear BBO entanglement light source, and the double-path entanglement sources are such as the intersecting type and the Beam-Like type BBO entanglement sources. In particular, the intersecting type and the Beam-Like type BBO entanglement sources are usually used as the light source of an optical quantum computer or for preparing quantum states such as GHZ.

[0027] However, the quantum states generated by the spontaneous parametric down-conversion process are not perfect because, in addition to the first-order down-conversion process, there are also higher-order down-conversion processes. Theory and experiments have shown that the existence of higher-order terms leads to a trade-off relationship between the coincidence counting rate and the contrast of the entanglement source. Therefore, during the preliminary design, actual setup, experiment, or actual operation, it is necessary to repeatedly adjust the laser power to change the down-conversion probability in order to find the optimal comprehensive performance. In addition, for a multi-photon interferometer composed of multiple entanglement sources, the propagation of photons in multiple paths is very complex. Regarding the existing simulation algorithms for two-path BBO entanglement sources, when considering the second-order terms of spontaneous parametric down-conversion, it is impossible to obtain simulation results with an effective calculation method.

[0028] The core process of the entanglement source based on the BBO crystal is as follows: When the pump light wave vector forms certain special angles with the BBO optical axis, the spontaneous parametric down-conversion process (Spontaneous Parametric Down-Conversion, SPDC) will occur. Through this process, a pump light beam can split into two beams of light with a certain probability, which are respectively called the ordinary light and the extraordinary light, and these two beams of light are usually called the parametric lights.

[0029] As the angle between the pump light wave vector and the BBO optical axis changes, the spatial distribution of the parametric lights will also change accordingly. Figure 1 Three typical spatial distributions of the parametric lights are shown, namely: the intersecting type, the collinear type, and the Beam-Like type. These three typical spatial distributions are all common methods for building the entanglement source device. From the perspective of the propagation path of the parametric lights, they can be divided into two-path entanglement light sources and single-path entanglement light sources. The two-path entanglement light sources include the intersecting type and the Beam-Like type, and the single-path entanglement light sources include the collinear type. The collinear BBO light source is usually used to prepare W states, Cluster states, etc.; the two-path BBO entanglement source is usually used for the preparation of the initial state of an optical quantum computer or for the preparation of quantum states such as GHZ.

[0030] The parametric lights of the intersecting type BBO entanglement source are as Figure 2aAs shown in the figure. Among them, the parametric light of the intersecting-type BBO entanglement source presents the shape of two light cones, and the cross-section is the shape of two intersecting rings, which is also the origin of the so-called "intersecting-type" name. Theory and experiments have proved that the polarization directions of the photons from the two light cones are completely orthogonal. That is, if the photons in one light cone are horizontally polarized |H>, then the photons in the other light cone are vertically polarized |V>. In the actual hardware device, the photons at the two intersection points of the ring propagate along their respective optical links and are finally collected by two beam couplers to collect the photons in the optical links. It should be noted that only when photons from the two light cones are collected simultaneously will a two-photon coincidence count be generated. If the two detected photons come from the same light cone, no coincidence count will be generated.

[0031] The parametric light of the Beam-Like type entanglement source is as Figure 2b shown in the figure. Among them, the Beam-Like type entanglement source does not have only one BBO crystal. It is composed of two BBO crystals bonded together with ultraviolet glue, and the second crystal is flipped 180° relative to the first crystal. Only when the photons in two paths come from the down-conversion process of the same BBO crystal will a two-photon coincidence count be generated.

[0032] It can be seen that whether it is the intersecting-type BBO entanglement source or the Beam-Like type BBO entanglement source, their parametric lights are clearly two optical links (or optical paths, which can be expressed as qumode). In addition, no matter which type of entanglement source it is, the spontaneous parametric down-conversion process is a high-order process. Specifically, the first-order process corresponds to: within one pump pulse, there is exactly one photon in each of the two optical paths; the second-order process corresponds to: there are exactly two photons in each of the two optical paths, and so on. In each pump pulse, there is a probability that a certain-order spontaneous parametric down-conversion process occurs, and the occurrence probabilities corresponding to different orders are different.

[0033] Taking only the first-order term process as an example, as Figure 2a shown in the figure, if a photon from the upper light cone is detected on the path propagating from the left intersection point, then a photon from the lower light cone must be detected on the path propagating from the right intersection point to generate a coincidence count. At this time, the polarization state is |HV> (assuming the upper light cone is horizontally polarized and the lower light cone is vertically polarized); if a photon from the lower light cone is detected on the path propagating from the left intersection point, then a photon from the upper light cone must be detected on the path propagating from the right intersection point to generate a coincidence count. At this time, the polarization state is |VH>. Considering only the first-order term, the quantum state form at this time is the Bell state,

[0034]

[0035] In formula (1), |H> represents the horizontal polarization of a photon, and |V> represents the vertical polarization of a photon. In the upper light cone, photons in the horizontal polarization state can be detected by one detector (such as D1), and photons in the vertical polarization state can be detected by another detector (such as D2). In the lower light cone, photons in the horizontal polarization state can be detected by one detector (such as D3), and photons in the vertical polarization state can be detected by a detector (such as D4). |HV> represents the case where, when split into two parametric light beams (i.e., two light cones, ordinary light and extraordinary light respectively), the upper light cone is horizontally polarized and the lower light cone is vertically polarized. |VH> represents the case where, when split into two parametric light beams (i.e., two light cones, ordinary light and extraordinary light respectively), the upper light cone is vertically polarized and the lower light cone is horizontally polarized.

[0036] Generally, when simulating an entanglement source, only the probability of the first-order term is used to obtain coincidence counts or simulate the sampling process. Assume that the repetition frequency of the laser for outputting pump light is M, and the cumulative measurement time is t (unit: second), then it means that within the time t, there are a total of tM pump light pulses. Assume that for an intersecting BBO entanglement source, the probability of first-order spontaneous parametric down-conversion is p (how much probability there is for a first-order process in one pulse), then there is the following relationship:

[0037] B HV = B VH = ptM Formula (2)

[0038] Among them, B HV and B VH respectively represent that the eigenstates |HV> and |VH> are the coincidence counts obtained within the time t.

[0039] Similarly, currently, the simulation method of the Beam-Like type BBO entanglement source usually only considers the first-order term of spontaneous parametric down-conversion. Still assume that the repetition frequency of the pump light is M, and the cumulative measurement time is t (unit: second); Figure 2b the down-conversion probability of BBO1 shown in is p1, the down-conversion probability of BBO2 is p2, and the coincidence counts of the two coincidence terms |HV> and |VH> within 1 second are B HV and B VH , then the simulation can be carried out according to the following relationship. Generally, the simulation result refers to the coincidence count of the entanglement source. Finally, two non-zero coincidence counts B HV and B VH can be obtained, which are expressed by formulas (3) and (4):

[0040] B HV = p1Mη 2 Formula (3)

[0041] B VH = p2Mη2 Formula (4)

[0042] As described above, the results of existing simulation methods are obtained only under the first-order term of spontaneous parametric down-conversion, while the actual spontaneous parametric down-conversion process is a process that includes higher-order terms and follows a thermal distribution. Specifically, the first-order process corresponds to: within a pump pulse, each of the ordinary light and extraordinary light links contains one photon; the second-order process corresponds to: each of the ordinary light and extraordinary light links down-converts two photons within a pulse, and so on. Strictly speaking, the higher-order term processes including the second-order term will all affect the final result. Therefore, the existing simulation schemes will cause deviations in simulation accuracy.

[0043] In addition, the simulation method for a Beam-Like type BBO entanglement source may include: (1) calculating the eigenstate probabilities of each term in the first-order and second-order terms according to the input down-conversion probability; (2) using the detector efficiency to convert the second-order term quantum state into a first-order term. For example, assuming the detector efficiency is η, the occurrence probability of |2H; 2V> in the second-order term is Then there is:[[]]

[0044]

[0045] Correspondingly, the coincidence count due to the second-order term process can be calculated as:[[]]

[0046]

[0047] From the perspective of the simulation logic of the optical quantum structure, generally, it can be divided into quantum state initialization, quantum gate operation, and measurement function. In the above-mentioned simulation algorithm considering the second-order term, the calculation process is very complicated and not scalable. Specifically, the calculation of each part of the entanglement source in the algorithm is not clearly segmented, and the simulation of quantum state initialization and measurement processes is combined together, and then the simulation of quantum gate operation is carried out; if the second-order term is converted into a quantum state represented by the first-order term in this step of entanglement source simulation, the result of two-photon interference that will occur later cannot be analyzed.

[0048] Therefore, according to an embodiment of the present disclosure, an entanglement source simulation method is provided. Figure 3 The flowchart of the entanglement source simulation method according to an embodiment of the present disclosure is shown. As Figure 3 shown, method 300 includes:[[]]

[0049] Determine the first quantum state vector representation of the target entangled state to be prepared by the target entanglement source, where the target entanglement source includes a barium metaborate (BBO) crystal, which is used to convert pump light into two parametric lights with a spatial distribution of either an intersecting type or a laser type in the spontaneous parametric down-conversion process (step 310); based on the characteristic parameters of the target entanglement source and the first quantum state vector representation, determine the quantum state vector corresponding to the target entanglement source, where the characteristic parameters include the down-conversion probability of the BBO crystal in the spontaneous parametric down-conversion process (step 320); based on the quantum state vector, determine the second coincidence probability of each second coincidence term obtained after measurement in the spontaneous parametric down-conversion process of the target entanglement source, where the second coincidence term is used to characterize the polarization state of photons in the two parametric lights, and the second coincidence term is caused by performing a measurement in the first measurement basis on the target entanglement source (step 330); and based on the second coincidence probability, determine the first coincidence count of the target entanglement source under each second coincidence term, where the first coincidence count is used to determine the quality factor of the quantum entangled state prepared by the target entanglement source (step 340).

[0050] In the present disclosure, the elements in the first quantum state vector representation are determined based on the first coincidence probability of each first coincidence term caused by the target entanglement source in the spontaneous parametric down-conversion process, where the first coincidence term is used to characterize the polarization state of photons in the two parametric lights, and the spontaneous parametric down-conversion process includes a first-order process and a second-order process.

[0051] According to an embodiment of the present disclosure, considering the higher-order terms (i.e., the second-order process) of the spontaneous parametric down-conversion process makes the simulation results of the entanglement light source more accurate; and separating the quantum state initialization and measurement processes makes it easier to extend to the simulation of a multi-photon interferometer composed of multiple entanglement sources, and the coincidence count of preparing the target quantum entangled state can be accurately and efficiently obtained in the simulation results, and further a series of quality factors such as its fidelity can be obtained.

[0052] According to some embodiments, the target entanglement source includes two optical paths respectively used to measure the two parametric lights. In the spontaneous parametric down-conversion process, when a pump light forms a certain angle with the optical axis of the BBO crystal, the phase matching condition can be satisfied. At this time, the high-frequency pump light will be converted (''split'') into two parametric lights with a certain probability, which are respectively called ordinary light and extraordinary light. Among them, the target entanglement source may include two optical links (i.e., optical paths), one optical path for transmitting ordinary light and the other optical path for transmitting extraordinary light.

[0053] Determining the first quantum state vector representation corresponding to the target entanglement source includes: determining the second quantum state vector representation and the third quantum state vector representation corresponding to each of the two optical paths, where the elements in the second quantum state vector representation are determined based on the third coincidence probabilities of the respective third coincidence terms corresponding to the current optical path for the first-order process, and the elements in the third quantum state vector representation are expressions of the fourth coincidence probabilities of the respective fourth coincidence terms corresponding to the current optical path for the second-order process; based on the corresponding second quantum state vector representation and third quantum state vector representation, determining the fourth quantum state vector representation corresponding to each of the two optical paths, where the elements in the fourth quantum state vector representation are determined based on the fifth coincidence probabilities of the respective fifth coincidence terms including the first-order process and the second process in the current optical path; and based on the fourth quantum state vector representations corresponding to each of the two optical paths, determining the first quantum state vector representation corresponding to the target entanglement source.

[0054] That is to say, the quantum state vector representation of each optical path in the first-order process can be determined first, and then the quantum state vector representation of this optical path in the second-order process can be determined. Furthermore, it is extended from one optical path to two optical paths to obtain the quantum state vector representation corresponding to the target entanglement source. Thus, it is easier to extend to the case of multiple optical paths, making the simulation method of a single entanglement source easier to be extended to the simulation of a multi-photon interferometer composed of multiple entanglement sources.

[0055] According to some embodiments, determining the fourth quantum state vector representation corresponding to each of the two optical paths includes: determining the fourth quantum state vector representation based on the direct sum representation between the corresponding second quantum state vector representation and third quantum state vector representation; and determining the first quantum state vector representation corresponding to the target entanglement source based on the fourth quantum state vector representations corresponding to each of the two optical paths includes: determining the first quantum state vector representation corresponding to the target entanglement source based on the direct product representation between the fourth quantum state vector representations corresponding to each of the two optical paths.

[0056] Exemplarily, in a quantum circuit based on qubits (quantum bits), a single qubit can be represented by a 2-dimensional vector, and the quantum state composed of multiple qubits can be represented by taking the direct product of the vectors corresponding to the single qubits. In a quantum circuit based on Fock states, the number of photons in a single optical path qumode (i.e., an optical link, which can be simply referred to as an optical path) can be represented by only one element (i.e., 1-dimensional), and the quantum state composed of multiple optical paths can be represented by taking the direct sum of the vectors corresponding to the single optical paths.

[0057] Different from the conventional representation methods described above, even when only considering a single entangled light source, the situation is very complex, including different order terms of a single optical path and calculation methods between different optical paths. Therefore, it is far from enough to only use the direct sum or direct product. Therefore, in the above embodiments, the direct sum is used to represent between different down-conversion orders of the same optical path (also called spatial mode, or mode), and the direct product is used to represent between multiple optical paths.

[0058] By combining the direct product and the direct sum, the quantum state vector representation formula of the two-path target entangled source is regularly represented, further making the simulation method of a single entangled source easier to be extended to the simulation of a multi-photon interferometer composed of multiple entangled sources.

[0059] In some embodiments, specifically, since only the second-order term of spontaneous parametric down-conversion is considered at most, therefore, for a single optical path, its quantum state is represented as:

[0060]

[0061] where P represents probability, H represents the horizontal polarization of a photon, and V represents the vertical polarization of a photon, that is, P 1H represents the probability of one photon with horizontal polarization, P 1H1V represents the probability of one photon with horizontal polarization and one photon with vertical polarization, P 2H represents the probability of two photons with horizontal polarization, P 2V The superscript represents the probability of two photons with vertical polarization, and T represents matrix transpose.

[0062] As Figure 2a and 2b shown, whether it is an intersecting BBO entangled source or a Beam-Like BBO entangled source, its idler light is obviously two optical paths. Therefore, the quantum state vector representation formula can be written in the form of a direct product. Considering the truncation number is 2 (that is, considering the high-order term process with the highest order being the second-order term), the quantum state vector corresponding to the entangled source is a 25-dimensional column vector, that is:

[0063]

[0064]

[0065]

[0066]

[0067]

[0068] where, by way of example, P 1H；2VRepresents the probability that there is 1 horizontally polarized photon in one path and 2 vertically polarized photons in another path.

[0069] After obtaining the first quantum state vector representation, the quantum state vector corresponding to the target entanglement source can be further determined based on the characteristic parameters of the target entanglement source. The characteristic parameters include the down-conversion probability of the BBO crystal in the spontaneous parametric down-conversion process.

[0070] According to some embodiments, determining the quantum state vector corresponding to the target entanglement source based on the characteristic parameters of the target entanglement source and the first quantum state vector representation includes: determining the sixth coincidence probability of each sixth coincidence term triggered by the target entanglement source in the first-order process based on the characteristic parameters of the target entanglement source; determining the seventh coincidence probability of each seventh coincidence term triggered in the second-order process based on the characteristic parameters of the target entanglement source; and determining the quantum state vector corresponding to the target entanglement source based on the first quantum state vector representation, the sixth coincidence probability, and the seventh coincidence probability.

[0071] In some examples, when considering the second-order term of the intersecting BBO entanglement source, the quantum state form corresponding to its idler light can be directly obtained, that is:

[0072]

[0073] where r represents the interaction coefficient between the pump light and the BBO crystal in the nonlinear process; Represents no down-conversion, Represents the quantum state of the first-order term, Represents the quantum state of the second-order term, which has the following forms respectively:

[0074]

[0075]

[0076]

[0077] where, The phase of the second term of is π, and the coefficient is -1.

[0078] The above formula means that: (1) the ratio of the probability of first-order down-conversion to the probability of no down-conversion is tanhr; (2) the ratio of the probability of second-order down-conversion to the probability of first-order down-conversion is still tanhr. It should be noted that the "interaction coefficient" r was used previously. In recent years, when actually constructing the entanglement source hardware, the academic and industrial circles often use p to represent the first-order or second-order down-conversion probability, and there is the following conversion relationship,

[0079] p = tanh2 r formula (11)

[0080] Therefore, by using the "down-conversion probability p" to replace the "interaction coefficient" r, we have:

[0081]

[0082] Compare |ψ> intersect and |ψ> source expressions, the quantum state form of the intersecting BBO entanglement source can be obtained by inputting the down-conversion probability p. Specifically,

[0083]

[0084] |ψ> source (15, 1) = |ψ> source (23, 1) = p

[0085] |ψ> source (19, 1) = -p

[0086] Among them, |ψ> source (x, 1) represents the x-th row element of the column vector represented by |ψ> source

[0087] In some embodiments, for the Beam-Like type BBO entanglement source, the down-conversion probability of the first BBO crystal is denoted as p1, and the down-conversion probability of the second BBO crystal is denoted as p2. The first-order term corresponds to each of the two optical paths having exactly one photon. At this time, there are two cases:

[0088] 1) The first BBO crystal undergoes a first-order down-conversion process with a probability of p1; the second crystal does not undergo a down-conversion process with a probability of The overall probability is written as:

[0089]

[0090] 2) The second BBO crystal undergoes a first-order down-conversion process with a probability of p2; the first crystal does not undergo a down-conversion process with a probability of The overall probability is written as:

[0091]

[0092] The second-order term corresponds to each of the two qumodes having exactly two photons. At this time, there are three cases:

[0093] 1) The first BBO crystal undergoes a second-order down-conversion process with a probability of The second crystal does not undergo a down-conversion process with a probability of The overall probability is written as: ​

[0094]

[0095] 2) The first BBO crystal undergoes a 1st-order down-conversion process with a probability of p1; the second BBO crystal also undergoes a 1st-order down-conversion process with a probability of p2. The overall probability is written as:

[0096]

[0097] 3) The second BBO crystal undergoes a 2nd-order down-conversion process with a probability of The first crystal does not undergo down-conversion, with probability The overall probability is written as:

[0098]

[0099] It should be noted that there are other combinations of down-conversion situations for two BBO crystals: for example, the first BBO crystal undergoes a 1st-order down-conversion process, and the second BBO crystal undergoes a 2nd-order down-conversion process. However, in this disclosure, no matter for the link of the entanglement source or the subsequent multi-photon interference link, the cutoff number is set to 2, that is, only the case where a single path contains at most 2 photons is considered. Therefore, the case where the number of photons in a single optical path is 3 is not considered. In summary, we have:

[0100]

[0101]

[0102]

[0103] |ψ> source (19,1)=p1p2

[0104]

[0105] Among them, |ψ> source (x,1) represents |ψ> source The element corresponding to the x-th row of the column vector.

[0106] As mentioned above, the corresponding quantum state vector can be obtained, but the simulation process is not complete. Finally, in order to simulate the coincidence counting sampling process of the entangled source, it is also necessary to obtain the probability of each eigenstate. As we all know, the probability can be obtained by the square of the coefficient before the eigenstate, but in actual hardware, the detector efficiency of each measurement end is different and not 100%. The final probability will also change accordingly, which requires adding a measurement function that includes the detector efficiency at the end.

[0107] According to some embodiments, the target entanglement source includes two optical paths for measuring the two beams of parametric light, each optical path including two detectors, and each detector is used to detect photons in the parametric light measured by the optical path that match the polarization state of the detector.

[0108] For the two detectors on each of the above optical paths, one is used to detect photons polarized in the horizontal direction, and the other is used to detect photons polarized in the vertical direction.

[0109] According to some embodiments, determining the second coincidence probability of each second coincidence term triggered by the target entanglement source during the spontaneous parametric down-conversion process after measurement includes: determining a first transformation matrix based on the detection efficiency of the detector, where the first transformation matrix represents the measurement process of performing the measurement in the first measurement basis on the target entangled state; and determining the second coincidence probability of each second coincidence term triggered by the target entanglement source during the spontaneous parametric down-conversion process after measurement based on the first transformation matrix and the quantum state vector.

[0110] By determining the first transformation matrix, a measurement function including the detector efficiency is introduced, thus conveniently simulating the coincidence counting sampling process of the actual entanglement source. And by changing the corresponding elements in the first transformation matrix, the effect of simulating different detector detection efficiencies can be conveniently achieved.

[0111] According to some embodiments, determining the first transformation matrix of the quantum state determined after performing the measurement in the first measurement basis with respect to the quantum state prepared by the target entanglement source includes: for each of the two optical paths, determining a second transformation matrix corresponding to the optical path, where the second transformation matrix represents the measurement process of performing the measurement in the first measurement basis on the single-bit quantum state prepared by the optical path; and determining the first transformation matrix based on the second transformation matrices corresponding to the two optical paths respectively.

[0112] In this embodiment, since a joint measurement is not performed on the two optical links, the measurement of a single optical path can be analyzed first and then extended to the two optical paths. For a single optical path with a truncation number of 2, as described above, its quantum state vector can be represented as a 5-dimensional column vector; for a single bit encoded by polarization, its quantum state vector can be represented as a 2-dimensional column vector. Therefore, in principle, for the conversion from a single path to a single bit, it can be achieved through a 2×5 matrix.

[0113] Specifically, assume that the detection efficiencies of the two detectors placed on a certain optical path are E H and E V , respectively, then the matrix is:

[0114]

[0115] Among them, E H represents the detection efficiency of the detector for detecting photons polarized in the horizontal direction. E V represents the detection efficiency of the detector for detecting photons polarized in the vertical direction. Then, there is a relationship:

[0116] P single qubit = EP single mode Formula (14)

[0117] Among them, P single qubit and P single mode respectively represent the probability vectors obtained after squaring the elements in the quantum state vectors of a single qubit and a single path. For an entanglement source with 2 optical paths, the transformation matrix at its measurement end can be written as:

[0118]

[0119] Among them, E H，1 represents the detection efficiency of the detector for detecting photons polarized in the horizontal direction on one optical path (i.e., path "1"). E H，2 represents the detection efficiency of the detector for detecting photons polarized in the horizontal direction on the other optical path (i.e., path "2").

[0120] Therefore, the second coincidence probability of each finally obtained second coincidence term can be calculated as:

[0121] P final = EP source Formula (16)

[0122] Among them, P source represents the probability vector formed after squaring the elements of |ψ> source . It should be noted that P source is a 25-dimensional vector, while P final is a 4-dimensional vector, which is expressed as:

[0123] P final = [P H；H P H；V P V；H P V；V T

[0124] Among them, it represents the probabilities of actually obtaining the eigenstates |H; H>, |H; V>, |V; H>, |V; V>.

[0125] According to P final ​, the sampling process of the target entangled source in each coincidence item (i.e., the second coincidence item) can be simulated. Specifically, in some embodiments, the repetition frequency of the laser for outputting pump light can be set to R; in addition, within the cumulative time t (in seconds), the cumulative coincidence count obtained by sampling is the vector B, and its form is:

[0126] B = [B H；H B H；V B V；H B V；V T

[0127] In some examples, "numpy.random.choice" in Python can be used to simulate the random sampling process. Optionally, this process can also be implemented in other languages. It should be noted that regardless of the language used to implement this process, the following need to be input: (1) the probability distribution of the eigenstates, i.e., P final ; (2) the number of sampling times, such as the number of sampling times can be calculated as Rt. The coincidence counts of each final coincidence item are stored in the vector B.

[0128] It should be noted that the following relationship will exist in the final result:

[0129]

[0130] But there is not necessarily a relationship:

[0131] B i；j = RtP i；j

[0132] In an exemplary embodiment according to the present disclosure, for ease of understanding, the steps are described through a flowchart as shown in Figure 4 shown.

[0133] The first step: Calculate the quantum state vector corresponding to the two-path entangled source. Specifically, 1. Input the down-conversion probability of the spontaneous parametric down-conversion process (i.e., step 401). If it is an intersecting BBO entangled source, input the down-conversion probability p; if it is a Beam-Like entangled source, input the down-conversion probabilities p1 and p2 of two BBO crystals. 2. Calculate the eigenstate probabilities of the first-order spontaneous parametric down-conversion process. 3. Calculate the eigenstate probabilities of the second-order spontaneous parametric down-conversion process. 4. According to the results of 2 and 3, obtain the 25-dimensional quantum state vector of the target entangled state prepared by the target entangled source (i.e., step 401).

[0134] ​Step 2: Determine the actual occurrence probability vector after the measurement process. 1. Input the detector efficiency corresponding to each of the 4 detectors (i.e., step 403). 2. Obtain the transformation matrix E at the measurement end (i.e., step 404). 3. Calculate the probability vector P after measurement based on the result of the first step, i.e., the quantum state vector final (i.e., step 406).

[0135] Step 3: Simulate the sampling process of the target entangled source. 1. Input the laser repetition frequency R and the cumulative measurement time t (i.e., step 405). 2. Calculate the number of samplings based on the laser repetition frequency and the measurement time, i.e., Rt. 3. Based on the result P final from the second step and the number of samplings Rt, implement the random sampling process to obtain the eigenstate counts for each item (i.e., step 407).

[0136] To verify the effect according to the embodiments of the present disclosure, the sampling results of two types of two-path entangled sources are simulated, and the contrast and coincidence counts are shown. Among them, for the intersecting BBO entangled source, the simulation parameters are set as follows: the value of the down-conversion probability ranges from 0.001 to 0.025, with a step size of 0.002; the detector efficiency is set to 0.6 for all; the cumulative acquisition time for coincidence counts is 0.5 seconds; the pump laser repetition frequency is 8×10 7 Hz. The simulation results are shown in Table 1.

[0137]

[0138] Table 1: Sampling coincidence counts for each item of the intersecting BBO entangled source at different down-conversion probabilities

[0139] For the Beam-Like BBO entangled source, the simulation parameters are set as follows: the value of the down-conversion probability for both of the two BBO crystals ranges from 0.001 to 0.025, with a step size of 0.002; the detector efficiency is set to 0.6 for all; the cumulative acquisition time for coincidence counts is 0.5 seconds; the pump laser repetition frequency is 8×10 7 Hz. The simulation results are shown in Table 2.

[0140]

[0141]

[0142] Table 2: Sampling coincidence counts for each item of the Beam-Like BBO entangled source at different down-conversion probabilities

[0143] For the sampling counts at each down-conversion probability, calculate at Contrast and coincidence counts in the measurement basis. For a two-path entanglement source, as the down-conversion probability increases, the coincidence counts gradually increase, but the contrast gradually decreases. If only first-order spontaneous parametric down-conversion is considered, the contrast does not change with down-conversion and is always the ideal case 1. Figure 5a and 5b respectively show schematic diagrams of simulation data of an intersecting type and a Beam-Like type BBO entanglement source according to an embodiment of the present disclosure, in which the results of simulated sampling that better conform to physical principles are clearly shown. From the simulation effect, the solution according to the embodiment of the present disclosure helps to compress the characteristic parameters of the hardware into a narrower range before actually building the device, thereby reducing the time cost and economic cost of later hardware parameter adjustment and optimization.

[0144] Compared with the methods in the industry, the solution according to the embodiment of the present disclosure considers the higher-order terms in the spontaneous parametric down-conversion process, and the simulation results of the entangled light source are more accurate; in addition, by combining the direct product and the direct sum, the quantum state form of the two-path entanglement source is regularly represented, and this method makes it easier to extend the simulation method of a single entangled source to the simulation of a multi-photon interferometer composed of multiple entangled sources, facilitating the simulation of the multi-photon interferometer directly using matrix multiplication.

[0145] According to the solution of the embodiment of the present disclosure, the calculation principle is closer to the real physical process. By considering the second-order term in the spontaneous parametric down-conversion process, simulated sampling is performed on the two-path entanglement source, improving the reliability of the simulation sampling results; representing the quantum state corresponding to the entanglement source in a more regular vector form can provide an extensibility basis for the implementation of simulating a large-scale multi-photon interferometer; before actually building, designing, and optimizing the entanglement source, the two-path BBO entanglement source can be simulated, the sampling process within a certain time can be simulated, and the cumulative coincidence counts can be obtained. The simulation process only needs to be carried out on a computer, without actually building and testing the hardware, with low simulation overhead, efficiently accelerating the hardware development efficiency of the entangled light source, and reducing the time cost and economic cost of hardware parameter adjustment and optimization.

[0146] According to an embodiment of the present disclosure, as Figure 6As shown, an entanglement source simulation device 600 is also provided, including: a first determination unit 610 configured to determine a first quantum state vector representation of a target entangled state to be prepared by a target entanglement source, where the target entanglement source includes a barium metaborate (BBO) crystal for converting pump light into two parametric lights with a spatial distribution of either an intersecting type or a laser type during the spontaneous parametric down-conversion process, where the elements in the first quantum state vector representation are determined based on the first coincidence probabilities of respective first coincidence terms triggered during the spontaneous parametric down-conversion process of the target entanglement source, where the first coincidence terms are used to characterize the polarization states of photons in the two parametric lights, and where the spontaneous parametric down-conversion process includes a first-order process and a second-order process; a second determination unit 620 configured to determine a quantum state vector corresponding to the target entanglement source based on the characteristic parameters of the target entanglement source and the first quantum state vector representation, where the characteristic parameters include the down-conversion probability of the BBO crystal during the spontaneous parametric down-conversion process; a third determination unit 630 configured to determine the second coincidence probabilities of respective second coincidence terms triggered during the spontaneous parametric down-conversion process of the target entanglement source obtained after measurement based on the quantum state vector, where the second coincidence terms are used to characterize the polarization states of photons in the two parametric lights, and where the second coincidence terms are triggered by performing a measurement in a first measurement basis on the target entanglement source; and a simulation unit 640 configured to determine a first coincidence count of the target entanglement source under respective second coincidence terms based on the second coincidence probabilities, where the first coincidence count is used to determine the quality factor of the quantum entangled state prepared by the target entanglement source.

[0147] Here, the operations of the above-mentioned respective units 610 to 640 of the entanglement source simulation device 600 are respectively similar to the operations of steps 310 to 340 described above, and will not be elaborated here.

[0148] According to an embodiment of the present disclosure, an electronic device, a readable storage medium, and a computer program product are also provided.

[0149] Referring to Figure 7 , a block diagram of an electronic device 700 that can be used as a server or a client of the present disclosure will now be described. It is an example of a hardware device that can be applied to various aspects of the present disclosure. The electronic device is intended to represent various forms of digital electronic computer devices, such as, a laptop computer, a desktop computer, a workbench, a personal digital assistant, a server, a blade server, a mainframe computer, and other suitable computers. The electronic device can also represent various forms of mobile devices, such as, a personal digital processor, a cellular phone, a smart phone, a wearable device, and other similar computing devices. The components shown herein, their connections and relationships, and their functions are merely examples and are not intended to limit the implementation of the present disclosure described herein and / or required.

[0150] As Figure 7 shown, the electronic device 700 includes a computing unit 701, which can perform various appropriate actions and processes according to a computer program stored in a read-only memory (ROM) 702 or a computer program loaded from a storage unit 708 into a random access memory (RAM) 703. In the RAM 703, various programs and data required for the operation of the electronic device 700 can also be stored. The computing unit 701, the ROM 702, and the RAM 703 are connected to each other via a bus 704. An input / output (I / O) interface 705 is also connected to the bus 704.

[0151] Multiple components in the electronic device 700 are connected to the I / O interface 705, including: an input unit 706, an output unit 707, a storage unit 708, and a communication unit 709. The input unit 706 can be any type of device capable of inputting information into the electronic device 700. The input unit 706 can receive input digital or character information and generate key signal inputs related to user settings and / or function controls of the electronic device, and can include, but is not limited to, a mouse, a keyboard, a touch screen, a trackpad, a trackball, a joystick, a microphone, and / or a remote control. The output unit 707 can be any type of device capable of presenting information and can include, but is not limited to, a display, a speaker, a video / audio output terminal, a vibrator, and / or a printer. The storage unit 708 can include, but is not limited to, a magnetic disk, an optical disk. The communication unit 709 allows the electronic device 700 to exchange information / data with other devices via a computer network such as the Internet and / or various telecommunication networks, and can include, but is not limited to, a modem, a network card, an infrared communication device, a wireless communication transceiver, and / or a chipset, such as a Bluetooth device, an 802.11 device, a WiFi device, a WiMax device, a cellular communication device, and / or the like.

[0152] The computing unit 701 can be various general-purpose and / or special-purpose processing components with processing and computing capabilities. Some examples of the computing unit 701 include, but are not limited to, a central processing unit (CPU), a graphics processing unit (GPU), various dedicated artificial intelligence (AI) computing chips, various computing units running machine learning model algorithms, a digital signal processor (DSP), and any suitable processor, controller, microcontroller, etc. The computing unit 701 executes the various methods and processes described above, such as method 300. For example, in some embodiments, method 300 can be implemented as a computer software program tangibly embodied in a machine-readable medium, such as storage unit 708. In some embodiments, part or all of the computer program can be loaded and / or installed onto the electronic device 700 via the ROM 702 and / or the communication unit 709. When the computer program is loaded into the RAM 703 and executed by the computing unit 701, one or more steps of the method 300 described above can be executed. Alternatively, in other embodiments, the computing unit 701 can be configured to execute method 300 in any other suitable manner (e.g., by means of firmware).

[0153] Various embodiments of the systems and techniques described above in this document can be implemented in digital electronic circuitry, integrated circuit systems, field-programmable gate arrays (FPGA), application-specific integrated circuits (ASIC), application-specific standard products (ASSP), systems-on-a-chip (SOC), complex programmable logic devices (CPLD), computer hardware, firmware, software, and / or combinations thereof. These various embodiments can include: implemented in one or more computer programs that can be executed and / or interpreted on a programmable system including at least one programmable processor, which can be a special or general-purpose programmable processor that receives data and instructions from a storage system, at least one input device, and at least one output device, and transmits the data and instructions to the storage system, the at least one input device, and the at least one output device.

[0154] The program code for implementing the methods of the present disclosure can be written in any combination of one or more programming languages. These program codes can be provided to a processor or controller of a general-purpose computer, a special-purpose computer, or other programmable data processing device, such that when the program codes are executed by the processor or controller, the functions / operations specified in the flowcharts and / or block diagrams are implemented. The program code can be executed entirely on the machine, partially on the machine, as a stand-alone software package partially on the machine and partially on a remote machine, or entirely on a remote machine or server.

[0155] In the context of this disclosure, a machine-readable medium can be a tangible medium that can contain or store a program for use by or in connection with an instruction execution system, apparatus, or device. The machine-readable medium can be a machine-readable signal medium or a machine-readable storage medium. The machine-readable medium can include, but is not limited to, electronic, magnetic, optical, electromagnetic, infrared, or semiconductor systems, apparatus, or devices, or any suitable combination of the foregoing. More specific examples of the machine-readable storage medium would include an electrical connection based on one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

[0156] In order to provide interaction with a user, the systems and techniques described herein can be implemented on a computer having: a display device (e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor) for displaying information to the user; and a keyboard and a pointing device (e.g., a mouse or a trackball) by which the user can provide input to the computer. Other kinds of devices can also be used to provide interaction with the user; for example, the feedback provided to the user can be any form of sensory feedback (e.g., visual feedback, auditory feedback, or tactile feedback); and input from the user can be received in any form (including acoustic input, speech input, or tactile input).

[0157] The systems and techniques described herein can be implemented in a computing system that includes backend components (such as, for example, a data server), or a computing system that includes middleware components (such as, for example, an application server), or a computing system that includes frontend components (such as, for example, a user computer having a graphical user interface or a web browser through which the user can interact with an implementation of the systems and techniques described herein), or a computing system that includes any combination of such backend components, middleware components, or frontend components. The components of the system can be interconnected by any form or medium of digital data communication (such as, for example, a communication network). Examples of communication networks include: a local area network (LAN), a wide area network (WAN), the Internet, and a blockchain network.

[0158] A computer system may include a client and a server. The client and the server are generally far from each other and usually interact via a communication network. The relationship between the client and the server is generated by computer programs running on respective computers and having a client-server relationship with each other. The server may be a cloud server, a server of a distributed system, or a server incorporating a blockchain.

[0159] It should be understood that various forms of the processes shown above can be used, with steps reordered, added, or deleted. For example, the steps recited in this disclosure can be executed in parallel, sequentially, or in a different order, as long as the desired results of the technical solutions disclosed in this disclosure can be achieved, and this is not limited herein.

[0160] Although embodiments or examples of the present disclosure have been described with reference to the accompanying drawings, it should be understood that the above methods, systems, and devices are merely exemplary embodiments or examples, and the scope of the present invention is not limited by these embodiments or examples, but is only defined by the authorized claims and their equivalent scope. Various elements in the embodiments or examples can be omitted or replaced by their equivalent elements. In addition, the steps can be executed in an order different from that described in this disclosure. Further, various elements in the embodiments or examples can be combined in various ways. Importantly, with the evolution of technology, many of the elements described herein can be replaced by equivalent elements that emerge after this disclosure.

Claims

1. An entanglement source simulation method, comprising: Determining a first quantum state vector representation of a target entangled state to be prepared by a target entanglement source, wherein the target entanglement source includes a barium metaborate (BBO) crystal, which is used to convert pump light into two parametric lights with a spatial distribution of either an intersecting type or a laser type during the spontaneous parametric down-conversion process, wherein the elements in the first quantum state vector representation are determined based on the first coincidence probabilities of respective first coincidence terms triggered during the spontaneous parametric down-conversion process by the target entanglement source, wherein the first coincidence terms are used to characterize the polarization states of photons in the two parametric lights, and wherein the spontaneous parametric down-conversion process includes a first-order process and a second-order process; Determining a quantum state vector corresponding to the target entanglement source based on the characteristic parameters of the target entanglement source and the first quantum state vector representation, wherein the characteristic parameters include the down-conversion probability of the BBO crystal during the spontaneous parametric down-conversion process; Determining the second coincidence probabilities of respective second coincidence terms triggered during the spontaneous parametric down-conversion process by the target entanglement source after measurement based on the quantum state vector, wherein the second coincidence terms are used to characterize the polarization states of photons in the two parametric lights, and the second coincidence terms are triggered by performing a measurement under a first measurement basis on the target entanglement source; and Determining a first coincidence count of the target entanglement source under respective second coincidence terms based on the second coincidence probabilities, wherein the first coincidence count is used to determine the quality factor of the quantum entangled state prepared by the target entanglement source.

2. The method according to claim 1, wherein, The target entanglement source includes two optical paths respectively used to measure the two parametric lights, and wherein determining a first quantum state vector representation of a target entangled state to be prepared by a target entanglement source includes: Determining a second quantum state vector representation and a third quantum state vector representation corresponding to each of the two optical paths, wherein the elements in the second quantum state vector representation are determined based on the third coincidence probabilities of respective third coincidence terms triggered by the first-order process in the current optical path, and the elements in the third quantum state vector representation are expressions of the fourth coincidence probabilities of respective fourth coincidence terms triggered by the second-order process in the current optical path; Determining a fourth quantum state vector representation corresponding to each of the two optical paths based on the corresponding second quantum state vector representation and third quantum state vector representation, wherein the elements in the fourth quantum state vector representation are determined based on the fifth coincidence probabilities of respective fifth coincidence terms triggered by the first-order process and the second-order process in the current optical path; and Determining a first quantum state vector representation corresponding to the target entanglement source based on the fourth quantum state vector representations corresponding to each of the two optical paths.

3. The method according to claim 2, wherein Determining the fourth quantum state vector representations corresponding to each of the two optical paths includes: determining the fourth quantum state vector representations based on the direct sum representation between the corresponding second quantum state vector representation and third quantum state vector representation; and Determining the first quantum state vector representation corresponding to the target entanglement source based on the fourth quantum state vector representations corresponding to the two optical paths includes: determining the first quantum state vector representation corresponding to the target entanglement source based on the direct product representation between the fourth quantum state vector representations corresponding to the two optical paths.

4. The method according to claim 1, wherein, The target entanglement source includes two optical paths for measuring the two beams of parametric light, each optical path including two detectors, and each detector is used to detect photons in the parametric light measured by the optical path that match the polarization state of the detector. Among them, Determining the second coincidence probability of each second coincidence term triggered by the target entanglement source during the spontaneous parametric down-conversion process after measurement includes: Based on the detection efficiency of the detector, determining a first transformation matrix, where the first transformation matrix is used to represent the measurement process of performing the measurement in the first measurement basis on the target entangled state; and Based on the first transformation matrix and the quantum state vector, determining the second coincidence probability of each second coincidence term triggered by the target entanglement source during the spontaneous parametric down-conversion process after measurement.

5. The method according to claim 4, wherein, Determining the first transformation matrix based on the detection efficiency of the detector includes: For each of the two optical paths, determining a second transformation matrix corresponding to the optical path based on the detection efficiency of the detector, where the second transformation matrix is used to represent the measurement process of performing the measurement in the first measurement basis on the single-bit quantum state prepared by the optical path; and Based on the second transformation matrices corresponding to the two optical paths respectively, determining the first transformation matrix.

6. The method according to claim 1, wherein Determining the quantum state vector corresponding to the target entanglement source based on the characteristic parameters of the target entanglement source and the first quantum state vector representation includes: Based on the characteristic parameters of the target entanglement source, determining the sixth coincidence probability of each sixth coincidence term triggered by the target entanglement source during the first-order process; Based on the characteristic parameters of the target entanglement source, determining the seventh coincidence probability of each seventh coincidence term triggered during the second-order process; and Based on the first quantum state vector representation, the sixth coincidence probability, and the seventh coincidence probability, determining the quantum state vector corresponding to the target entanglement source.

7. An entanglement source simulation device, including: A first determination unit configured to determine the first quantum state vector representation of the target entangled state to be prepared by the target entanglement source, where the target entanglement source includes a barium metaborate (BBO) crystal, which is used to convert pump light into two beams of parametric light with any one of an intersecting type and a laser type in the spatial distribution during the spontaneous parametric down-conversion process. Among them, the elements in the first quantum state vector representation are determined based on the first coincidence probability of each first coincidence term triggered by the target entanglement source during the spontaneous parametric down-conversion process, where the first coincidence term is used to characterize the polarization state of photons in the two beams of parametric light, and the spontaneous parametric down-conversion process includes a first-order process and a second-order process; A second determination unit, configured to determine a quantum state vector corresponding to the target entanglement source based on the characteristic parameters of the target entanglement source and the first quantum state vector representation formula, where the characteristic parameters include the down-conversion probability of the BBO crystal in the spontaneous parametric down-conversion process; A third determination unit, configured to determine, based on the quantum state vector, a second coincidence probability of each second coincidence item caused by the target entanglement source in the spontaneous parametric down-conversion process after measurement, where the second coincidence item is used to characterize the polarization state of photons in the two beams of parametric light, and the second coincidence item is caused by performing a measurement in the first measurement basis on the target entanglement source; and A simulation unit, configured to determine a first coincidence count of the target entanglement source under each second coincidence item based on the second coincidence probability, where the first coincidence count is used to determine the quality factor of the quantum entanglement state prepared by the target entanglement source.

8. The apparatus according to claim 7, wherein The target entanglement source includes two optical paths respectively for measuring the two beams of parametric light, and among them, the first determination unit includes: A first determination subunit, configured to determine a second quantum state vector representation formula and a third quantum state vector representation formula corresponding to each of the two optical paths, where the elements in the second quantum state vector representation formula are determined based on the third coincidence probability of each third coincidence item caused by the first-order process in the current optical path, and the elements in the third quantum state vector representation formula are expressions of the fourth coincidence probability of each fourth coincidence item caused by the second-order process in the current optical path; A second determination subunit, configured to determine a fourth quantum state vector representation formula corresponding to each of the two optical paths based on the corresponding second quantum state vector representation formula and third quantum state vector representation formula, where the elements in the fourth quantum state vector representation formula are determined based on the fifth coincidence probability of each fifth coincidence item caused by the first-order process and the second-order process in the current optical path; and A third determination subunit, configured to determine a first quantum state vector representation formula corresponding to the target entanglement source based on the fourth quantum state vector representation formulas corresponding to the two optical paths.

9. The apparatus according to claim 8, wherein, The second determination subunit includes: a unit configured to determine the fourth quantum state vector representation formula based on the direct sum representation between the corresponding second quantum state vector representation formula and third quantum state vector representation formula; and The third determination subunit includes: a unit configured to determine the first quantum state vector representation formula corresponding to the target entanglement source based on the direct product representation between the fourth quantum state vector representation formulas corresponding to the two optical paths.

10. The device according to claim 7, wherein, The target entanglement source includes two optical paths for measuring the two beams of parametric light, each optical path includes two detectors, and each detector is used to detect photons in the parametric light measured by the optical path that match the polarization state of the detector. Among them, the third determination unit includes: A fourth determination subunit, configured to determine a first transformation matrix based on the detection efficiency of the detector, where the first transformation matrix represents the measurement process of performing the first measurement basis on the target entanglement state; and A fifth determination subunit, configured to determine, based on the first transformation matrix and the quantum state vector, the second coincidence probability of each second coincidence item obtained after measurement and caused by the target entanglement source in the spontaneous parametric down-conversion process.

11. The device according to claim 10, wherein The fourth determination subunit includes: A unit configured to determine, for each of the two optical paths and based on the detection efficiency of the detector, the second transformation matrix corresponding to the optical path, where the second transformation matrix is used to represent the measurement process of performing the first measurement basis on the single-bit quantum state prepared for the optical path; and A unit configured to determine the first transformation matrix based on the second transformation matrices corresponding to the two optical paths respectively.

12. The device according to claim 7, wherein The second determination unit includes: A sixth determination subunit, configured to determine, based on the characteristic parameters of the target entanglement source, the sixth coincidence probability of each sixth coincidence item caused by the target entanglement source in the first-order process; A seventh determination subunit, configured to determine, based on the characteristic parameters of the target entanglement source, the seventh coincidence probability of each seventh coincidence item caused in the second-order process; and An eighth determination subunit, configured to determine the quantum state vector corresponding to the target entanglement source based on the first quantum state vector representation, the sixth coincidence probability, and the seventh coincidence probability.

13. An electronic device, comprising: At least one processor; And A memory communicatively connected to the at least one processor; Wherein The memory stores instructions executable by the at least one processor, and when the instructions are executed by the at least one processor, the at least one processor is enabled to execute the method according to any one of claims 1-6.

14. A non-transitory computer-readable storage medium storing computer instructions, wherein, The computer instructions are used to cause the computer to execute the method according to any one of claims 1-6.

15. A computer program product, comprising a computer program, wherein, The computer program, when executed by a processor, implements the method according to any one of claims 1-6.

Images