A quantum entanglement source of a dual-channel periodically poled waveguide

Abstract

The present application is applicable to the field of quantum technology and relates to a quantum entanglement source of a dual-path periodically polarized waveguide. The quantum entanglement source includes a waveguide device consisting of a first PPLN waveguide and a second PPLN waveguide, and an entanglement device consisting of a first collimator, a second collimator, a half-wave plate, a fully transparent plate, and a beam shifter. The PPLN waveguides are both connected to a light source. The parameters of the two PPLN waveguides are the same, so that the two PPLN waveguides output photon pairs with the same parameters. The optical paths of the two collimators are arranged in parallel and have the same parameters. They are respectively connected to the output end of one PPLN waveguide. The other ends of the two collimators cover the half-wave plate and the fully transparent plate, respectively. Photons that pass through the half-wave plate and the fully transparent plate enter the beam shifter at the same time. The beam shifter is used to aggregate the two photon pairs to form interference and output them, so that the two photons can form entanglement. The use of PPLN waveguides improves the conversion efficiency of photons and improves the brightness and contrast of the entanglement source.

Description

Technical Field

[0001] The present application is applicable to the fields of quantum optics and quantum information technology, and in particular relates to a quantum entanglement source of a dual-path periodically polarized waveguide. Background Art

[0002] Communications security underpins the security of the national information economy and the well-being of the people. Quantum communication, based on quantum mechanics, is a communication method that is, in principle, provably unconditionally secure based on information theory. It offers absolute security that traditional communication methods lack, and holds enormous potential for application in areas such as national information security, military security, and financial security. In quantum communication, channel transmission attenuation is the square of the single link, limiting improvements in the ultimate bit rate. Increasing the brightness of the entangled source is one of the most direct means of increasing this rate.

[0003] Traditional entanglement sources use the spontaneous parametric down-conversion (SPDC) of periodically poled second-order nonlinear crystals to generate quantum entanglement, but they have disadvantages such as low yield, large size, large temperature control circuit size, complex design resulting in high temperature control difficulty, and low integration. Compared with bulk quantum optics, quantum integrated optics has the advantages of small size, high stability, strong controllability, and reconfigurability. Lithium niobate, as one of the most important nonlinear optical materials, is widely used in the preparation of quantum light sources and high-speed control of quantum states. With the introduction of micromachining technology, lithium niobate has become an important platform for quantum integrated optics, capable of realizing monolithic integrated entangled light sources with multiple degrees of freedom encoding. Periodically poled lithium niobate (PPLN) waveguides are the best choice for solving the problem of entanglement sources in the field of quantum information.

[0004] Currently, the most commonly used on-chip PPLN waveguide entanglement source is a type-II SPDC coupled with a polarization beam combiner. While highly integrated, this approach suffers from low conversion efficiency and significant brightness loss. Therefore, improving the entanglement formation efficiency while maintaining the conversion efficiency of the quantum entanglement source, thereby enhancing the brightness and contrast of the quantum entanglement source, has become an urgent issue. Summary of the Invention

[0005] In view of this, an embodiment of the present application provides a quantum entangled source of a dual-path periodically polarized waveguide to solve the problem of how to improve the entanglement formation efficiency while ensuring the conversion efficiency of the quantum entangled source, so as to improve the brightness and contrast of the quantum entangled source.

[0006] The present application provides a quantum entanglement source of a dual-path periodically polarized waveguide, the quantum entanglement source comprising a waveguide device and an entanglement device;

[0007] The waveguide device includes a first PPLN waveguide and a second PPLN waveguide, wherein the input end of the first PPLN waveguide and the input end of the second PPLN waveguide are both used to connect to a pump light source, the first PPLN waveguide and the second PPLN waveguide are waveguides with the same parameters, and the first PPLN waveguide and the second PPLN waveguide both output photon pairs with the same parameters;

[0008] The entanglement device includes a first collimator, a second collimator, a half-wave plate, a fully transparent plate and a beam shifter, wherein the optical paths of the first collimator and the second collimator are arranged in parallel and are collimators with the same parameters;

[0009] The output end of the first PPLN waveguide is connected to one end of the first collimator, and the other end of the first collimator covers the half-wave plate. The output end of the second PPLN waveguide is connected to one end of the second collimator, and the other end of the second collimator covers the full-transmittance plate.

[0010] The photons passing through the half-wave plate and the fully transparent plate enter the beam shifter at the same time. The beam shifter is used to aggregate the two photon pairs to form interference and output them.

[0011] In one embodiment, the quantum entanglement source further includes an output device, which includes a third collimator and an output optical fiber. One end of the third collimator is connected to the output of the beam shifter, and the other end of the third collimator is connected to the output optical fiber.

[0012] In one embodiment, the quantum entanglement source further includes a substrate and a temperature control device. The waveguide device, the entanglement device and the temperature control device are all fixed on the substrate. The temperature control device is used to control the temperature of the waveguide device and the entanglement device on the substrate.

[0013] In one embodiment, the temperature control device is a TEC temperature control driver, the waveguide module and the compensation device are arranged on one side of the substrate, and the TEC temperature control driver is arranged on the other side of the substrate.

[0014] In one embodiment, the half-wave plate and the fully transparent plate have the same thickness, and the photon entrance side of the beam displacer covers the half-wave plate and the fully transparent plate.

[0015] In one embodiment, both the first collimator and the second collimator are GLENS mirrors.

[0016] In one embodiment, the beam displacer is a birefringent crystal formed of BDY12U type yttrium vanadate, and the birefringent crystal is a cuboid, wherein the surface formed by the length and height of the cuboid covers the half-wave plate and the fully transparent plate.

[0017] In one embodiment, the first PPLN waveguide and the second PPLN waveguide are both ridge waveguides, titanium diffused waveguides, or proton exchange waveguides made of lithium niobate.

[0018] In one embodiment, the quantum entanglement source further includes an on-chip beam splitter, the input end of the on-chip beam splitter is connected to the pump light source, and the two ends of the on-chip beam splitter are spaced apart and connected to a PPLN waveguide composed of a Z-cut lithium niobate film with a thickness of 300 nm to 10 μm, forming the first PPLN waveguide and the second PPLN waveguide.

[0019] In one embodiment, during the fabrication of the PPLN waveguide, spontaneous parametric down-conversion adopts a type-0 mode, including:

[0020] Calculate the quasi-phase matching period required for parametric down-conversion and construct a polarization electrode model using COMSOL numerical simulation software;

[0021] According to the quasi-phase matching period, a periodically polarized electrode is prepared on a Z-cut lithium niobate film using UV mask exposure;

[0022] Based on the curves of inversion domain nucleation and lateral broadening as a function of polarization electrode configuration, polarization voltage, and polarization time, the optimal polarization electrode configuration is determined and selected.

[0023] Based on the optimal polarization electrode configuration, spontaneous parametric down-conversion is performed on a Z-cut lithium niobate film with a periodically polarized electrode by an applied electric field method to obtain a PPLN waveguide with uniform polarization and inversion domain duty ratios, which meets the requirements of first-order quasi-phase matching.

[0024] The quantum entanglement source of the dual-path periodic polarization waveguide of the present application has the following beneficial effects compared with the prior art: the quantum entanglement source of the present application includes a waveguide device and an entanglement device, the waveguide device includes a first PPLN waveguide and a second PPLN waveguide, the input end of the first PPLN waveguide and the input end of the second PPLN waveguide are both used to connect to a pump light source, the first PPLN waveguide and the second PPLN waveguide are waveguides with the same parameters, the first PPLN waveguide and the second PPLN waveguide both output photon pairs with the same parameters, the entanglement device includes a first collimator, a second collimator, a half-wave plate, a fully transparent plate and a beam shifter, the first collimator and the second collimator are waveguides with the same parameters, and the first PPLN waveguide and the second PPLN waveguide both output photon pairs with the same parameters. The optical paths of the collimators are set in parallel and have the same parameters. The output end of the first PPLN waveguide is connected to one end of the first collimator, and the other end of the first collimator is covered with a half-wave plate. The output end of the second PPLN waveguide is connected to one end of the second collimator, and the other end of the second collimator is covered with a fully transparent plate. The photons passing through the half-wave plate and the fully transparent plate enter the beam shifter at the same time. The beam shifter is used to aggregate the two photon pairs to form interference and output them, constructing a free-space integrated entanglement structure and realizing the entanglement formation of the waveguide output photons. The use of dual waveguides can improve the conversion efficiency of photons. At the same time, the two photons can form entanglement, which improves the brightness and contrast of the quantum entanglement source. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0026] Figure 1 This is a schematic structural diagram of a quantum entanglement source of a dual-path periodically polarized waveguide provided in Example 1 of the present application;

[0027] Figure 2 This is a schematic structural diagram of a quantum entanglement source of a dual-path periodically polarized waveguide provided in Example 2 of the present application;

[0028] Figure 3 This is a flow chart of a PPLN waveguide preparation process in a dual-path periodically polarized waveguide quantum entanglement source provided in Example 2 of the present application;

[0029] Figure 4 This is a schematic structural diagram of a quantum entanglement source of a dual-path periodically polarized waveguide provided in Example 3 of the present application;

[0030] Figure 5 This is a schematic structural diagram of a quantum entanglement source of a dual-path periodically polarized waveguide provided in Example 4 of the present application;

[0031] In the figure, 1 is the pump light source, 2 is the waveguide device, 3 is the entanglement device, 4 is the substrate, 5 is the temperature control device, 6 is the output device, 7 is the on-chip beam splitter, 201 is the first PPLN waveguide, 202 is the second PPLN waveguide, 301 is the first collimator, 302 is the second collimator, 303 is the half-wave plate, 304 is the fully transparent plate, 305 is the beam shifter, 601 is the third collimator, and 602 is the output optical fiber. DETAILED DESCRIPTION

[0032] In the following description, specific details such as specific system structures and techniques are provided for purposes of illustration rather than limitation to facilitate a thorough understanding of the embodiments of the present application. However, it will be apparent to those skilled in the art that the present application may be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to avoid obscuring the description of the present application with unnecessary detail.

[0033] It should be understood that when used in the present specification and the appended claims, the term "comprising" indicates the presence of described features, integers, steps, operations, elements and / or components, but does not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or collections thereof.

[0034] It will also be understood that the term "and / or" used in this specification and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.

[0035] As used in this specification and the appended claims, the term "if" can be interpreted as "when" or "upon" or "in response to determining" or "in response to detecting," depending on the context. Similarly, the phrase "if it is determined" or "if [described condition or event] is detected" can be interpreted as meaning "upon determination" or "in response to determining" or "upon detection of [described condition or event]" or "in response to detecting [described condition or event]," depending on the context.

[0036] In addition, in the description of the present application specification and the appended claims, the terms "first", "second", "third", etc. are only used to distinguish the descriptions and cannot be understood as indicating or implying relative importance.

[0037] References to "one embodiment" or "some embodiments" in this specification mean that a particular feature, structure, or characteristic described in conjunction with that embodiment is included in one or more embodiments of the present application. Thus, phrases such as "in one embodiment," "in some embodiments," "in other embodiments," and "in other embodiments" appearing in various places in this specification do not necessarily refer to the same embodiment, but rather mean "one or more but not all embodiments," unless otherwise specifically emphasized. The terms "including," "comprising," "having," and variations thereof all mean "including but not limited to," unless otherwise specifically emphasized.

[0038] It should be understood that the size of the serial numbers of the steps in the following embodiments does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0039] In order to illustrate the technical solution of the present application, specific embodiments are provided below.

[0040] Example 1

[0041] See also Figure 1 , is a schematic structural diagram of a quantum entanglement source of a dual-path periodically polarized waveguide provided in Example 1 of the present application, wherein the quantum entanglement source comprises a pump light source 1, a waveguide device 2, and an entanglement device 3, wherein the waveguide device 2 comprises a first PPLN waveguide 201 and a second PPLN waveguide 202, wherein the input end of the first PPLN waveguide 201 and the input end of the second PPLN waveguide 202 are both used to connect to the pump light source 1, and the entanglement device 3 comprises a first collimator 301, a second collimator 302, a half-wave plate 303, and a fully transparent plate 304 and the beam shifter 305, the output end of the first PPLN waveguide 201 is connected to one end of the first collimator 301, the other end of the first collimator 301 is covered with the half-wave plate 303, the output end of the second PPLN waveguide 202 is connected to one end of the second collimator 302, the other end of the second collimator 302 is covered with the full-transmittance plate 304, the photons passing through the half-wave plate 303 and the full-transmittance plate 304 enter the beam shifter 305 at the same time, and the beam shifter 305 is used to aggregate the two photon pairs to form interference and output them.

[0042] The first PPLN waveguide 201 and the second PPLN waveguide 202 are waveguides with the same parameters. Both the first PPLN waveguide 201 and the second PPLN waveguide 202 output photon pairs with the same parameters. The optical paths of the first collimator 301 and the second collimator 302 are arranged in parallel and are collimators with the same parameters.

[0043] The two PPLN waveguides mentioned above respectively generate longer V s V i Photon, then two-way V s V i Photons enter the corresponding collimator, 1 / 2 glass plate or full transparent plate respectively and form entanglement after beam deflection. Among them, the first collimator 301 is attached with a half glass plate 303, the second collimator 302 is attached with a full transparent plate 304 of the same thickness, and the beam deflector 305 is attached behind the half-wave plate 303 and the full transparent plate 304. After the half-wave plate 303, V s V i Photons are converted into H s H i Photons, passing through the V of the full transparent film 304 s V i The photon maintains its original polarization, that is, it is still V s V i The photons pass through the beam shifter 305, which brings the two beams together. This pair of photons forms interference, which can construct different maximum entangled states. In the polarization dimension, one of the following two maximum entangled states is constructed. The entanglement formula is as follows:

[0044]

[0045] Example 2

[0046] See also Figure 2 , which is a structural schematic diagram of a quantum entanglement source of a dual-path periodically polarized waveguide provided in Example 2 of the present application, wherein the two PPLN waveguides in the waveguide device 2 are formed using a Z-cut lithium niobate film. Based on Example 1, the quantum entanglement source also includes an on-chip beam splitter 7, the input end of the on-chip beam splitter 7 is connected to the pump light source 1, and the two ends of the on-chip beam splitter 7 are spaced apart and connected to the PPLN waveguide composed of the Z-cut lithium niobate film with a thickness of 300nm to 10μm, forming a first PPLN waveguide and a second PPLN waveguide.

[0047] In the second embodiment, the PPLN waveguide is a Z-cut lithium niobate film. During the manufacturing process of the Z-cut lithium niobate film, the spontaneous parametric down conversion adopts the type-0 mode, such as Figure 3 FIG. 1 is a flow chart of a process for preparing a PPLN waveguide in a quantum entanglement source of a dual-path periodically polarized waveguide provided in the second embodiment of the present application, comprising the following steps:

[0048] Step S31 , calculating the quasi-phase matching period required for parametric down-conversion, and constructing a polarization electrode model using COMSOL numerical simulation software.

[0049] Among them, in the polarization of lithium niobate thin films, it is necessary to construct a polarization electrode model through COMSOL numerical simulation software, simulate and study the polarization electric field distribution corresponding to different electrode configurations, and experimentally explore the influence mechanism of polarization electrode configuration, polarization voltage, polarization time, and polarization period on the nucleation and lateral broadening of inversion domains. The optimal polarization electrode configuration is selected, and the corresponding relationship between the external polarization electric field and polarization time and the movement of the internal domain structure is established to accurately control the lateral broadening of the inversion domain and reduce the merging of adjacent inversion domains.

[0050] Step S32 : preparing a periodically polarized electrode on the Z-cut lithium niobate film using UV mask exposure according to the quasi-phase matching period.

[0051] The processing process may include steps such as glue spinning, exposure (UV exposure, EBL), development, fixing, electrode plating, and high-voltage polarization (high-voltage polarization uses a polarization cycle for polarization). The exposure process uses double UV exposure technology to increase the exposure limit, thereby precisely controlling PVD to produce precise metal electrodes. Of course, if processing accuracy is not considered, a single exposure technology can also be used, and this application does not limit this.

[0052] Step S33 , determining and selecting the optimal polarization electrode configuration based on the curves of the variation of the inversion domain nucleation and lateral broadening with the polarization electrode configuration, polarization voltage and polarization time.

[0053] Step S34, based on the optimal polarization electrode configuration, spontaneous parametric down-conversion is performed on the Z-cut lithium niobate film with periodically polarized electrodes by an applied electric field method to obtain a PPLN waveguide with uniform polarization and inversion domain duty ratios, and the PPLN waveguide meets the requirements of first-order quasi-phase matching.

[0054] Among them, after periodic poling, the inversion domain structure is tested and characterized, requiring uniform polarization and uniform inversion domain duty cycle to meet the requirements of first-order quasi-phase matching.

[0055] Traditional periodically polarized second-order nonlinear crystals use a type-II mode for spontaneous parametric down-conversion, with a small piezoelectric constant d33 (i.e., low conversion capability and low entanglement source performance) and a large period (i.e., on the order of 10μm). In this application, the PPLN waveguide spontaneous parametric down-conversion adopts a type-0 mode, with a large piezoelectric constant d33 (i.e., several times that of type-II, with several times increased entanglement source performance) and a small period (i.e., within the order of 10, especially at 405nm, the period is 2.55μm), so the preparation is more difficult.

[0056] This application simulates and studies the polarization electric field distribution corresponding to different electrode configurations, calculates the quasi-phase matching period required for parametric down-conversion, and constructs a polarization electrode model using COMSOL numerical simulation software; and prepares periodic polarization electrodes on the Z-cut PPLN waveguide using standard semiconductor precision processing technology (UV mask exposure); experimentally explores the influence mechanism of polarization electrode configuration, polarization voltage and polarization time on the nucleation and lateral broadening of inversion domains, selects the optimal polarization electrode configuration, establishes the correspondence between the external polarization electric field and polarization time and the movement of the internal domain structure, accurately controls the lateral broadening of the inversion domain, and reduces the merging of adjacent inversion domains; performs periodic polarization and SPDC process on the Z-cut PPLN waveguide using the external electric field method; and tests and characterizes the inversion domain structure, requiring uniform polarization and uniform inversion domain duty cycle to meet the requirements of first-order quasi-phase matching.

[0057] The main indicator of entanglement in a quantum entangled source is entanglement visibility. Table 1 shows the contrast ratio of the quantum entangled source of the type-0 SPDC on-chip dual-periodic polarization thin-film lithium niobate waveguide. The contrast ratio of the quantum entangled source is as high as 104:1, which is much higher than the contrast ratio of the spatial optical path of the traditional optical platform (about 20:1).

[0058] Table 1

[0059]

[0060] In this embodiment, the first PPLN waveguide and the second PPLN waveguide are both made of lithium niobate ridge waveguide, titanium diffused waveguide or proton exchange waveguide.

[0061] For example, taking the ridge waveguide as an example, the processing process of the PPLN waveguide is as follows: Based on the optical waveguide theory, COMSOL numerical simulation and Lumerical simulation are used to construct a PPLN ridge waveguide structure model, and the influence of waveguide structure parameters such as waveguide width, ridge height, sidewall tilt angle, etc. on the effective refractive index of the light spot mode with different wavelengths inside the waveguide is simulated and studied, thereby guiding the design of the ridge waveguide structure parameters.

[0062] This application designs and manufactures a free-space integrated entanglement device for a dual-periodic-polarized lithium niobate waveguide quantum entanglement source. The device's volume is significantly reduced compared to the spatial optical path of an optical platform, and the final step is optical fiber input for pump light and optical fiber output for the entanglement source, significantly increasing integration. By adopting a type-0 dual-periodic-polarized thin-film lithium niobate waveguide photon pair generation system, high conversion efficiency and high brightness are achieved. Furthermore, this application designs and manufactures a type-0 SPDC on-chip dual-periodic-polarized thin-film lithium niobate waveguide photon pair generation system, which can achieve overall temperature control, reducing the difficulty of temperature control and integrating a small temperature control circuit, significantly reducing the system volume. Furthermore, this application designs and manufactures a free-space integrated entanglement device for a dual-periodic-polarized lithium niobate waveguide quantum entanglement source. The device's volume is significantly reduced compared to the spatial optical path of an optical platform, and the final step is optical fiber input for pump light and optical fiber output for the entanglement source, significantly increasing integration.

[0063] Example 3

[0064] See also Figure 4 , which is a structural schematic diagram of a quantum entanglement source of a dual-path periodically polarized waveguide provided in the third embodiment of the application. Based on the first embodiment, the quantum entanglement source also includes a substrate 4 and a temperature control device 5. The waveguide device 2, the entanglement device 3 and the temperature control device 5 are all fixed on the substrate. The temperature control device 5 is used to control the temperature of the waveguide device 2 and the entanglement device 3 on the substrate 4.

[0065] The temperature control device 5 achieves overall temperature control, reducing the difficulty of temperature control and making the system more stable. It also integrates a small temperature control circuit, significantly reducing the system size. Specifically, the temperature control device 5 is a TEC temperature control driver. The waveguide module 2 and the entanglement device 3 are arranged on one side of the substrate 4, and the TEC temperature control driver is arranged on the other side of the substrate 4. Furthermore, the plane formed by the two optical paths of the first PPLN waveguide 201 and the second PPLN waveguide 202 in the waveguide device 2 can be arranged perpendicular to or parallel to the substrate 4.

[0066] In the third embodiment, the temperature control device 5 is used to control the temperature of the whole system to increase the stability of the system. The TEC temperature control driver, waveguide device and entanglement device are respectively terminated at the upper and lower sides of the substrate 4. The temperature control circuit is small, which can effectively reduce the volume of the system.

[0067] Example 4

[0068] See also Figure 5, which is a schematic diagram of the structure of a dual-path periodically polarized waveguide quantum entanglement source provided in Example 4 of the present application. Based on Example 1, this quantum entanglement source also includes an output device 6, which includes a third collimator 601 and an output optical fiber 602. One end of the third collimator 601 is connected to the output of the beam shifter 305, and the other end of the third collimator 601 is connected to the output optical fiber 602. Using optical fibers to output the quantum entanglement source greatly increases the integration of the quantum entanglement source.

[0069] In one embodiment, the half-wave plate 303 and the fully transparent plate 304 have the same thickness, and the photon-entry side of the beam displacer 305 covers both the half-wave plate 303 and the fully transparent plate 304. In other words, the beam displacer 305 is directly connected to the half-wave plate 303 and the fully transparent plate 304, and can be secured using UV adhesive. This improves the integration and effectiveness of the entanglement device 3 and prevents photon leakage.

[0070] In one embodiment, both the first collimator 301 and the second collimator 302 are GLENS lenses, each having an antireflection coating on both ends. The GLENS lenses have a diameter of 1 mm and a length of 3.5 mm. The half-wave plate and the fully transparent plate are both square plates with a side length of 1 mm. Furthermore, the GLENS lenses are G2P10 (NA = 0.5) GLENS lenses with antireflection coatings on both ends.

[0071] In one embodiment, the beam shifter 305 is a birefringent crystal formed of BDY12U type yttrium vanadate, and the birefringent crystal is a rectangular parallelepiped with a length, width and height of 12 mm, 6 mm and 4 mm, respectively. The length and height of the rectangular parallelepiped form a surface covering the half-wave plate 303 and the fully transparent plate 304.

[0072] In this fourth embodiment and its further implementation methods, the front end of the dual-path PPLN waveguide is coupled with an optical fiber and fixed, the rear end of the dual-path PPLN waveguide is respectively coupled with two GLENS and fixed, one collimating lens is attached to a 1 / 2 glass slide, and the other collimating lens is attached to a fully transparent chip of the same thickness, and a beam displacer is added and fixed at the rear. The size of the beam displacer covers the two collimating lenses, and the rear end of the beam displacer is coupled to the collimator and the output optical fiber according to the light output position.

[0073] As can be seen, this fourth embodiment integrates the free-space optical components of the entanglement device 3, such as the two GLENS, half-glass, and beam displacer, on a single substrate, using a TEC for temperature control. The overall system design process is as follows: First, the dimensions of the compensation device's free-space optical components, such as the two GLENS, half-glass, and beam displacer, as well as the beam propagation parameters and corresponding electric field distribution of the output spot pattern, are designed using FDTD numerical simulation software and CAD design software. The coupling efficiency of each unit is calculated, and the parameters with the highest coupling efficiency are explored. Second, based on the simulated optimal parameters, the PPLN waveguide is patch-protected, coupled to the entanglement device, and secured. This waveguide is then fixed to the substrate using thermal adhesive. The TEC temperature control module and circuit are then installed, and finally, packaging is performed.

[0074] Traditional periodically polarized crystals can be simply glued together and fixed to free-space optical devices (i.e., mm-scale), but this results in low light confinement and entanglement source performance (brightness is inversely proportional to the beam waist). The mode field diameter of the PPLN waveguides in this application is at the micron or even nanometer level, making coupling with free-space optical devices very difficult. Lumerical FDTD numerical simulation software was used to simulate the electric field distribution corresponding to the output spot pattern of the two PPLN waveguides, calculate the coupling efficiency of the PPLN waveguides with the free-space optical device, and explore the parameters with the highest coupling efficiency. The two PPLN waveguides were precisely controlled through a real-time monitoring system, thereby precisely controlling their output spot pattern to match the spot pattern of the compensation device. The waveguides were then fixed with UV glue, achieving high-efficiency coupling of the PPLN waveguides.

[0075] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included in the scope of protection of the present application.

Claims

1. A quantum entanglement source of a dual-path periodically polarized waveguide, characterized in that: The quantum entanglement source includes a waveguide device and an entanglement device; The waveguide device includes a first PPLN waveguide and a second PPLN waveguide, wherein the input end of the first PPLN waveguide and the input end of the second PPLN waveguide are both used to connect to a pump light source, the first PPLN waveguide and the second PPLN waveguide are waveguides with the same parameters, and the first PPLN waveguide and the second PPLN waveguide both output photon pairs with the same parameters; The entanglement device includes a first collimator, a second collimator, a half-wave plate, a fully transparent plate and a beam shifter, wherein the optical paths of the first collimator and the second collimator are arranged in parallel and are collimators with the same parameters; The output end of the first PPLN waveguide is connected to one end of the first collimator, and the other end of the first collimator covers the half-wave plate. The output end of the second PPLN waveguide is connected to one end of the second collimator, and the other end of the second collimator covers the full-transmittance plate. The photons passing through the half-wave plate and the fully transparent plate simultaneously enter the beam shifter, and the beam shifter is used to aggregate the two photon pairs to form interference and output them; The first PPLN waveguide and the second PPLN waveguide are formed by using a Z-cut lithium niobate film; During the fabrication of the PPLN waveguide, spontaneous parametric down-conversion adopts a type-0 mode, including: Calculate the quasi-phase matching period required for parametric down-conversion and construct a polarization electrode model using COMSOL numerical simulation software; According to the quasi-phase matching period, a periodically polarized electrode is prepared on a Z-cut lithium niobate film using UV mask exposure; Based on the curves of inversion domain nucleation and lateral broadening as a function of polarization electrode configuration, polarization voltage, and polarization time, the optimal polarization electrode configuration is determined and selected. Based on the optimal polarization electrode configuration, spontaneous parametric down-conversion is performed on a Z-cut lithium niobate film with a periodically polarized electrode by an applied electric field method to obtain a PPLN waveguide with uniform polarization and inversion domain duty ratios, which meets the requirements of first-order quasi-phase matching.

2. The quantum entanglement source according to claim 1, characterized in that The quantum entanglement source also includes a substrate and a temperature control device. The waveguide device, the entanglement device and the temperature control device are all fixed on the substrate. The temperature control device is used to control the temperature of the waveguide device and the entanglement device on the substrate.

3. The quantum entanglement source according to claim 2, characterized in that The temperature control device is a TEC temperature control driver. The waveguide module and the entanglement device are arranged on one side of the substrate, and the TEC temperature control driver is arranged on the other side of the substrate.

4. The quantum entanglement source according to claim 1, characterized in that The quantum entanglement source further includes an output device, which includes a third collimator and an output optical fiber. One end of the third collimator is connected to the output of the beam shifter, and the other end of the third collimator is connected to the output optical fiber.

5. The quantum entanglement source according to claim 1, characterized in that The half-wave plate and the fully transparent plate have the same thickness, and the photon entrance side of the beam displacer covers the half-wave plate and the fully transparent plate.

6. The quantum entanglement source according to claim 5, characterized in that The first collimator and the second collimator are both GLENS mirrors.

7. The quantum entanglement source according to claim 6, characterized in that The beam shifter is a birefringent crystal formed of BDY12U type yttrium vanadate, wherein the surface formed by the length and height of the birefringent crystal covers the half-wave plate and the fully transparent plate.

8. The quantum entanglement source according to claim 1, characterized in that The first PPLN waveguide and the second PPLN waveguide are both ridge waveguides, titanium diffused waveguides or proton exchange waveguides made of lithium niobate.

9. The quantum entanglement source according to claim 1, characterized in that The quantum entanglement source also includes an on-chip beam splitter, the input end of the on-chip beam splitter is connected to the pump light source, and the two ends of the on-chip beam splitter are spaced apart and connected to a PPLN waveguide composed of a Z-cut lithium niobate film with a thickness of 300nm to 10μm, forming the first PPLN waveguide and the second PPLN waveguide.

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