A Quantum Entanglement Source Based on a Monolithic Single-Cycle Polarized Thin-Film Lithium Niobate Waveguide

Abstract

The present application relates to a quantum entanglement source of an on-chip single-cycle polarization thin-film lithium niobate waveguide. The quantum entanglement source includes two light sources, an on-chip PPLN waveguide, and an on-chip beam combiner. One end of the on-chip PPLN waveguide is connected to one light source and is connected to the first input end of the on-chip beam combiner through a mode flipper. The other end thereof is connected to the other light source and the second input end of the on-chip beam combiner. Photon pairs after SPDC conversion by one light source through the on-chip PPLN waveguide enter the second input end. Photon pairs after SPDC conversion by the other light source through the on-chip PPLN waveguide enter the mode flipper for mode flipping, and after flipping, enter the first input end. By controlling the two light sources, phase compensation is performed on the incoming photon pairs, and the photon pairs after phase compensation are aggregated to form interference and output. Through an on-chip PPLN waveguide, two-way photon pairs are formed for entangled output. There is no need to consider the problem of waveguide consistency, the design is simpler, and the volume of the entanglement source can be reduced.

Description

Technical Field

[0001] This application is applicable to the fields of quantum optics and quantum information technology, and particularly relates to a quantum entanglement source of a single-cycle polarization thin-film lithium niobate waveguide on a chip. Background Art

[0002] Quantum key distribution is a currently known unconditionally secure communication method that cannot be wiretapped or decrypted by humans through the transmission of quantum states. In this scheme, the channel transmission attenuation is the square of a single link, and there are some restrictions in improving the final code rate. Improving the brightness of the entanglement source is one of the most direct means to increase the final code 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 volume, large volume of the temperature control circuit, high temperature control difficulty caused by complex design, and low integration. Compared with bulk quantum optics, quantum integrated optics has the characteristics of small volume, high stability, strong controllability, and reconfigurability. As one of the most important nonlinear optical materials, lithium niobate is widely used in the preparation of quantum light sources and the high-speed control of quantum states. With the introduction of microfabrication technology, lithium niobate has become an important platform for quantum integrated optics, capable of realizing a monolithic integrated entanglement light source with multi-degree-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] The existing quantum entanglement sources formed by PPLN waveguides require the use of dual-waveguides, which have high requirements for the consistency of the dual-waveguides, and each waveguide needs to be temperature-controlled separately, with a relatively complex design, resulting in high costs. Therefore, how to reduce the high requirements of quantum entanglement sources for waveguide consistency has become an urgent problem to be solved. Summary of the Invention

[0005] In view of this, the embodiments of this application provide a quantum entanglement source of a single-cycle polarization thin-film lithium niobate waveguide on a chip to solve the problem of how to reduce the high requirements of quantum entanglement sources for waveguide consistency.

[0006] This application provides a quantum entanglement source of a single-cycle polarization thin-film lithium niobate waveguide on a chip, and the quantum entanglement source includes a first pump light source, a second pump light source, an on-chip PPLN waveguide, and an on-chip beam combiner;

[0007] One end of the on-chip PPLN waveguide is connected to the first pump light source and is connected to the first input end of the on-chip beam combiner through a mode converter, and the other end of the on-chip PPLN waveguide is connected to the second pump light source and the second input end of the on-chip beam combiner;

[0008] The photon pairs after SPDC conversion by the first pump light source through the on-chip PPLN waveguide enter the second input end, and the photon pairs after SPDC conversion by the second pump light source through the on-chip PPLN waveguide enter the mode converter for mode conversion and then enter the first input end after conversion. By controlling the first pump light source and the second pump light source, phase compensation is performed on the photon pairs entering the first input end and the second input end, and the on-chip beam combiner is used to aggregate the photon pairs after phase compensation to form interference and output.

[0009] In one embodiment, the mode converter is an optical fiber with a 90° mode conversion.

[0010] In one embodiment, one end of the on-chip PPLN waveguide is a left-end Y waveguide, the first pump light source is connected to one branch of the left-end Y waveguide through an optical fiber, and the mode converter is connected to the other branch of the left-end Y waveguide.

[0011] In one embodiment, the other end of the on-chip PPLN waveguide is a right-end Y waveguide, the second pump light source is connected to one branch of the right-end Y waveguide through an optical fiber, and the other branch of the right-end Y waveguide is connected to the second input end of the on-chip beam combiner.

[0012] In one embodiment, the quantum entanglement source further includes a substrate chip and a temperature control device. The on-chip PPLN waveguide, the on-chip beam combiner, and the temperature control device are all fixedly arranged on the substrate chip, and the temperature control device is used to control the temperature of the on-chip PPLN waveguide and the on-chip beam combiner on the substrate chip.

[0013] In one embodiment, the temperature control device is a TEC temperature control driver. The on-chip PPLN waveguide and the on-chip beam combiner are arranged on one side of the substrate chip, and the TEC temperature control driver is arranged on the other side of the substrate chip.

[0014] In one embodiment, the quantum entanglement source further includes an output optical fiber, and the output end of the on-chip beam combiner is connected to the output optical fiber.

[0015] In one embodiment, the output optical fibers are all polarization-maintaining optical fibers.

[0016] In one embodiment, the on-chip PPLN waveguide is any one of a ridge waveguide, a titanium-diffused waveguide, and a proton-exchanged waveguide made of lithium niobate material.

[0017] In one embodiment, the on-chip PPLN waveguide is made of Z-cut lithium niobate thin film with a thickness of 300 nm to 5 μm, and the on-chip PPLN waveguide is obtained by periodic poling under the conditions of a poling period of 2.4 μm - 10 μm and a duty cycle of 40% - 60%.

[0018] The beneficial effects of the quantum entanglement source of the on-chip single-period poled thin-film lithium niobate waveguide of the present application compared with the prior art are as follows: The quantum entanglement source of the present application includes a first pump light source, a second pump light source, an on-chip PPLN waveguide, and an on-chip beam combiner. One end of the on-chip PPLN waveguide is connected to the first pump light source and is connected to the first input end of the on-chip beam combiner through a mode inverter. The other end of the on-chip PPLN waveguide is connected to the second pump light source and the second input end of the on-chip beam combiner. The photon pairs after SPDC conversion by the first pump light source through the on-chip PPLN waveguide enter the second input end. The photon pairs after SPDC conversion by the second pump light source through the on-chip PPLN waveguide enter the mode inverter for mode inversion and then enter the first input end after inversion. By controlling the first pump light source and the second pump light source, phase compensation is performed on the photon pairs entering the first input end and the second input end. The on-chip beam combiner is used to aggregate the photon pairs after phase compensation to form interference and output. The above-mentioned SPDC optical path in one path of the on-chip PPLN waveguide forms two paths of photon pairs for entangled output, without considering the problem of waveguide consistency. Compared with the traditional two-path waveguide, the design is simpler, the cost is lower, and the volume of the entanglement source can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0020] Figure 1 It is a schematic structural diagram of a quantum entanglement source of an on-chip single-period poled thin-film lithium niobate waveguide provided in Embodiment 1 of the present application;

[0021] Figure 2 It is a schematic structural diagram of a quantum entanglement source after using a PPLN waveguide made of Z-cut lithium niobate thin film provided in Embodiment 1 of the present application;

[0022] Figure 3 It is a yield diagram of a quantum entanglement source at different temperatures provided in Embodiment 1 of the present application;

[0023] Figure 4It is a schematic structural diagram of a quantum entanglement source of an on-chip single-cycle polarization thin-film lithium niobate waveguide provided by the second embodiment of the present application;

[0024] In the figure, 1 is the first pump light source, 2 is the second pump light source, 3 is the on-chip PPLN waveguide, 4 is the on-chip beam combiner, 5 is the mode inverter, 6 is the substrate wafer, and 7 is the temperature control device. Specific embodiments

[0025] In the following description, specific details such as specific system structures and technologies are presented for the purpose of illustration rather than limitation, so as to thoroughly understand the embodiments of the present application. However, those skilled in the art should clearly understand that the present application can also 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 unnecessary details from interfering with the description of the present application.

[0026] It should be understood that when used in the specification and appended claims of the present application, the term "comprising" indicates the presence of the described features, wholes, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components, and / or their combinations.

[0027] It should also be understood that the term "and / or" as used in the specification and appended claims of the present application refers to any combination and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0028] As used in the specification and appended claims of the present application, the term "if" can be interpreted as "when", "once", "in response to determining", or "in response to detecting" depending on the context. Similarly, the phrase "if determined" or "if [the described condition or event] is detected" can be interpreted as meaning "once determined", "in response to determining", "once [the described condition or event] is detected", or "in response to detecting [the described condition or event]" depending on the context.

[0029] In addition, in the description of the specification and appended claims of the present application, the terms "first", "second", "third", etc. are only used for distinguishing descriptions and cannot be understood as indicating or implying relative importance.

[0030] References to "one embodiment" or "some embodiments" or the like described in the specification of the present application mean that a particular feature, structure, or characteristic described in connection with that embodiment is included in one or more embodiments of the present application. Thus, statements such as "in one embodiment", "in some embodiments", "in other some embodiments", "in still other embodiments", etc. that appear in different places in this specification do not necessarily all refer to the same embodiment, but mean "one or more but not all embodiments", unless otherwise specifically emphasized. The terms "comprising", "including", "having" and their variants mean "including but not limited to", unless otherwise specifically emphasized.

[0031] It should be understood that the magnitude of the sequence numbers of the steps in the following embodiments does not mean the order of execution, and the order of execution of each process should be determined according to its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

[0032] In order to illustrate the technical solution of the present application, specific embodiments will be used for illustration below.

[0033] See Figure 1 , which is a schematic structural diagram of a quantum entanglement source of a single-chip single-cycle polarization thin-film lithium niobate waveguide provided in the first embodiment of the present application. The quantum entanglement source includes a first pump light source 1, a second pump light source 2, an on-chip PPLN waveguide 3, an on-chip beam combiner 4, and a mode inverter 5.

[0034] One end of the on-chip PPLN waveguide 3 is connected to the first pump light source 1 and is connected to the first input end of the on-chip beam combiner 4 through the mode inverter 5, and the other end of the on-chip PPLN waveguide 3 is connected to the second pump light source 2 and the second input end of the on-chip beam combiner 4.

[0035] The photon pairs generated by SPDC conversion of the first pump light source 1 through the on-chip PPLN waveguide 3 enter the second input end, and the photon pairs generated by SPDC conversion of the second pump light source 2 through the on-chip PPLN waveguide 3 enter the mode inverter 5 for mode inversion and then enter the first input end after inversion. By controlling the first pump light source 1 and the second pump light source 2, phase compensation is performed on the photon pairs entering the first input end and the second input end. The on-chip beam combiner 5 is used to aggregate the phase-compensated photon pairs to form interference and output.

[0036] Through the SPDC process, photons with longer wavelengths are generated from the light emitted by the first pump light source 1 and the second pump light source 2 respectively. After that, the light of the first pump light source 1 is converted into photons and enter the second input end of the on-chip beam combiner 5, and the light of the second pump light source 2 is converted into Photons enter the mode converter 5, where mode conversion occurs and then enter the first input end of the on-chip beam combiner 5. After the mode converter 5 performs mode conversion on the photon pairs, photon pairs, photon pairs and photon pairs enter the on-chip beam combiner 4, causing the two beams of light to converge. The phase is compensated by adjusting the two light sources. The output of the on-chip beam combiner 4 is , and these two photons form an interference, so different maximum entangled states can be constructed, and one of the following 2 maximum entangled states can be constructed in the polarization dimension. The entanglement formula is as follows:

[0037] .

[0038] Since both pairs of photons are based on the SPDC optical path of the on-chip PPLN waveguide 3, they have good waveguide consistency, thus reducing the problem of high design complexity caused by the high requirement for waveguide consistency when using two waveguides.

[0039] In this application, the on-chip PPLN waveguide 3 can be in the micron level or even the nanometer level. Therefore, the periodic polarization and fiber coupling entanglement corresponding to the on-chip PPLN waveguide 3 have an exponential increase in difficulty compared with traditional solutions. To implement the above-mentioned on-chip PPLN waveguide 3, a Z-cut lithium niobate thin film can be used, as Figure 2 shown, which is a quantum entanglement source after using a Z-cut lithium niobate thin film for the on-chip PPLN waveguide 3. Among them, the first pump light source 1 is pump P1, and the second pump light source 2 is pump P2. During the production process of this Z-cut lithium niobate thin film, the type-0 mode is adopted for spontaneous parametric down-conversion, and the conversion efficiency is high. Specifically:

[0040] 1. Construct a polarization electrode model through the COMSOL numerical simulation software to calculate the quasi-phase matching period required for parametric down-conversion.

[0041] Among them, in the polarization of the lithium niobate thin film, it is necessary to construct a polarization electrode model through the COMSOL numerical simulation software, simulate and study the polarization electric field distribution corresponding to different electrode configurations, explore the influence mechanism of the polarization electrode configuration, polarization voltage, polarization time, and polarization period on the nucleation and lateral broadening of the inverted domain through experiments, select the optimal polarization electrode configuration, establish the corresponding relationship between the externally applied polarization electric field and polarization time and the internal domain structure movement, precisely control the lateral broadening of the inverted domain, and reduce the merger of adjacent inverted domains.

[0042] 2. According to the quasi-phase matching period, use UV mask exposure to prepare a periodic polarization electrode on the Z-cut lithium niobate thin film.

[0043] Among them, the processing process may include spin coating, exposure (UV ultraviolet exposure, EBL), development, fixing, electrode plating, high-voltage polarization (high-voltage polarization is carried out using a polarization cycle), etc. The exposure process uses a dual ultraviolet exposure technique to increase the exposure limit, and then precisely controls PVD to fabricate precise metal electrodes. Of course, without considering the processing accuracy, a single exposure technique can also be used, and this application does not limit it.

[0044] 3. Based on the curves of the inversion domain nucleation and lateral broadening varying with the polarization electrode configuration, polarization voltage, and polarization time, determine and select the optimal polarization electrode configuration.

[0045] 4. Based on the optimal polarization electrode configuration, perform spontaneous parametric down-conversion on the Z-cut lithium niobate thin film with a prefabricated periodically poled electrode by the external electric field method to obtain an on-chip PPLN waveguide with uniform polarization and inversion domain duty cycle, and the on-chip PPLN waveguide meets the requirements of first-order quasi-phase matching.

[0046] Among them, after periodic polarization, 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.

[0047] The traditional periodic polarization second-order nonlinear crystal spontaneous parametric down-conversion uses the type-II mode, with a small piezoelectric constant d33 (i.e., low conversion ability and low entanglement source performance), and a large period (i.e., on the order of 10 μm). In this application, the on-chip PPLN waveguide spontaneous parametric down-conversion uses the type-0 mode, with a large piezoelectric constant d33 (i.e., several times that of the type-II mode and several times the increase in entanglement source performance), and a small period (i.e., within the order of 10 μm, especially the period is 2.55 μm at 405 nm), so the preparation difficulty is relatively high.

[0048] This application constructs a polarization electrode model through COMSOL numerical simulation software, simulates and studies the polarization electric field distributions corresponding to different electrode configurations, and calculates the quasi-phase matching period required for SPDC; and prepares a periodically poled electrode on the Z-cut on-chip PPLN waveguide through standard semiconductor precision processing technology (UV mask exposure); explores the influence mechanism of the polarization electrode configuration, polarization voltage, and polarization time on the inversion domain nucleation and lateral broadening through experiments, selects the optimal polarization electrode configuration, establishes the correspondence between the applied polarization electric field and polarization time and the internal domain structure movement, precisely controls the lateral broadening of the inversion domain, and reduces the merger of adjacent inversion domains; performs periodic polarization on the Z-cut on-chip PPLN waveguide by the external electric field method, the SPDC process; 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.

[0049] In one embodiment, the on-chip PPLN waveguide uses a Z-cut lithium niobate thin film with a thickness of 300 nm to 5 μm, and the on-chip PPLN waveguide is obtained by periodic poling under the conditions of a poling period of 2.55 μm and a duty cycle of 40%-60%.

[0050] For example, according to the quasi-phase matching equation, the planned period is determined, and the Z-cut lithium niobate thin film with a thickness of 0.6 μm or 5 μm is periodically poled. The calculated poling period is 2.55 μm and the duty cycle is 50%.

[0051] Among them, periodic poling is carried out by spin coating, exposure, development, fixing, electrode plating and voltage application for polarization. The poling period is 2.55 μm and the duty cycle is 50%.

[0052] The periodically poled lithium niobate thin film is fabricated into a periodically poled thin film lithium niobate waveguide (i.e., on-chip PPLN waveguide) through micro-nano processing technology.

[0053] Electron beam evaporation of SiO2 is used as a protective layer for the on-chip PPLN waveguide 3, and then cutting and polishing are carried out.

[0054] The on-chip PPLN waveguide 3 is coupled with an optical fiber and the mode flipper 305 is coupled to the corresponding port, and is coupled through the on-chip beam combiner 4. Combined with other devices that can be set, and then packaged, a quantum entanglement light source can be obtained.

[0055] Among them, the main index of entanglement generation of the quantum entanglement source is the entanglement visibility. Using the above-mentioned Z-cut lithium niobate thin film, the contrast ratio of the quantum entanglement source system is as high as 104:1, which is more than twice that of the traditional PPKDP crystal (50:1).

[0056] Such as Figure 3 shown, it is the yield diagram of the quantum entanglement source at different temperatures. Among them, the abscissa is the temperature, the unit is °C, and the ordinate is the yield, the unit is G / mw). In this application, the yield of the quantum entanglement source can reach 6 G pairs / mW (at 44 °C), which is more than 100 times higher than that of the traditional PPKDP crystal (the yield is 30 M pairs / mW / s). The quantum entanglement source in this application can efficiently provide quantum entanglement photon pairs at different temperatures.

[0057] In one embodiment, as Figure 2 shown, the mode flipper 5 is an optical fiber with a 90° mode flip.

[0058] In one embodiment, as Figure 2 shown, one end of the on-chip PPLN waveguide 3 is a left-end Y waveguide. The first pump light source 1 is connected to a branch of the left-end Y waveguide through an optical fiber, and the mode flipper 5 is connected to another branch of the left-end Y waveguide.

[0059] In one embodiment, as Figure 2 shown, the other end of the on-chip PPLN waveguide 3 is the right-end Y waveguide. The second pump light source 2 is connected to a branch of the right-end Y waveguide through an optical fiber, and the other branch of the right-end Y waveguide is connected to the second input end of the on-chip beam combiner.

[0060] In one embodiment, the on-chip PPLN waveguide 3 is any one of a ridge waveguide, a titanium-diffused waveguide, and a proton-exchanged waveguide made of lithium niobate material. In this embodiment, the ridge waveguide is taken as an example to introduce the processing process of the basic on-chip PPLN waveguide.

[0061] Based on the optical waveguide theory, a PPLN ridge waveguide structure model is constructed by using COMSOL numerical simulation and Lumerical simulation. The influence of waveguide structure parameters such as waveguide width, ridge height, and sidewall inclination angle on the effective refractive index of different wavelength spot modes inside the waveguide is simulated and studied, so as to guide the design of the ridge waveguide structure parameters.

[0062] This application designs and manufactures an on-chip single-period poled lithium niobate waveguide quantum entanglement source, the volume of which is greatly reduced compared with the spatial optical path of the optical platform, and finally the pump light is input through an optical fiber and the entanglement source is output through an optical fiber, greatly increasing the integration level. By adopting a type-0 on-chip single-period poled thin-film lithium niobate waveguide photon pair generation system, the conversion efficiency is high and the brightness is large.

[0063] See Figure 4 , which is a schematic structural diagram of a quantum entanglement source of an on-chip single-period poled thin-film lithium niobate waveguide provided for the second embodiment of the application. Further, on the basis of the first embodiment, the quantum entanglement source further includes a substrate wafer 6 and a temperature control device 7. The on-chip components such as the on-chip PPLN waveguide 3, the on-chip beam combiner 4, the mode inverter 5, and other optical fibers are all fixedly arranged on the substrate wafer. The temperature control device 7 is used to control the temperature of the on-chip components such as the on-chip PPLN waveguide 3, the on-chip beam combiner 4, the mode inverter 5, and other optical fibers on the substrate wafer 6.

[0064] Among them, the temperature control device 7 can realize overall temperature control, reduce the difficulty of temperature control, and integrates a small temperature control circuit, greatly reducing the system volume. Specifically, the temperature control device 7 is a TEC temperature control driver. The on-chip components such as the on-chip PPLN waveguide 3 and the on-chip beam combiner 3 are arranged on one side of the substrate wafer, and the TEC temperature control driver is arranged on the other side of the substrate wafer.

[0065] The present application designs and fabricates a photon pair generation system based on a single-period polarization thin-film lithium niobate waveguide on a chip, which can achieve overall temperature control, reduce the difficulty of temperature control, integrate a small temperature control circuit, and greatly reduce the system volume. In addition, the present application designs and fabricates a single-period polarization lithium niobate waveguide quantum entanglement source, the device volume is greatly reduced relative to the spatial optical path of the optical platform, and finally, the pump light is input through an optical fiber and the entanglement source is output through an optical fiber, greatly increasing the integration level.

[0066] The above embodiments are only used to illustrate the technical solutions of the present application, rather than limiting them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A quantum entanglement source of a single-chip single-cycle polarization thin-film lithium niobate waveguide, characterized in that, The quantum entanglement source includes a first pump light source, a second pump light source, an on-chip PPLN waveguide, and an on-chip beam combiner; One end of the on-chip PPLN waveguide is connected to the first pump light source and is connected to the first input end of the on-chip beam combiner through a mode inverter, and the other end of the on-chip PPLN waveguide is connected to the second pump light source and the second input end of the on-chip beam combiner; The photon pairs after SPDC conversion by the first pump light source through the on-chip PPLN waveguide enter the second input end, and the photon pairs after SPDC conversion by the second pump light source through the on-chip PPLN waveguide enter the mode inverter for mode inversion and then enter the first input end after inversion. By controlling the first pump light source and the second pump light source, phase compensation is performed on the photon pairs entering the first input end and the second input end. The on-chip beam combiner is used to aggregate the photon pairs after phase compensation to form interference and output.

2. The quantum entanglement source according to claim 1, wherein The mode inverter is an optical fiber with a 90° mode inversion.

3. The quantum entanglement source according to claim 1, wherein One end of the on-chip PPLN waveguide is a left-end Y waveguide. The first pump light source is connected to one branch of the left-end Y waveguide through an optical fiber, and the mode inverter is connected to the other branch of the left-end Y waveguide.

4. The quantum entanglement source according to claim 3, wherein The other end of the on-chip PPLN waveguide is a right-end Y waveguide. The second pump light source is connected to one branch of the right-end Y waveguide through an optical fiber, and the other branch of the right-end Y waveguide is connected to the second input end of the on-chip beam combiner.

5. The quantum entanglement source according to claim 4, wherein The quantum entanglement source further includes a substrate chip and a temperature control device. The on-chip PPLN waveguide, the on-chip beam combiner, and the temperature control device are all fixed on the substrate chip. The temperature control device is used to control the temperature of the on-chip PPLN waveguide and the on-chip beam combiner on the substrate chip.

6. The quantum entanglement source according to claim 5, wherein The temperature control device is a TEC temperature control driver. The on-chip PPLN waveguide and the on-chip beam combiner are arranged on one side of the substrate chip, and the TEC temperature control driver is arranged on the other side of the substrate chip.

7. The quantum entanglement source according to claim 6, wherein The quantum entanglement source further includes an output optical fiber. The output end of the on-chip beam combiner is connected to the output optical fiber.

8. The quantum entanglement source according to claim 7, wherein, The output optical fibers are all polarization-maintaining optical fibers.

9. The quantum entanglement source according to claim 1, wherein, The on-chip PPLN waveguide is any one of a ridge waveguide, a titanium-diffused waveguide, and a proton-exchanged waveguide made of lithium niobate material.

10. The quantum entanglement source according to any one of claims 1 to 9, characterized in that, The on-chip PPLN waveguide uses a Z-cut lithium niobate thin film with a film thickness of 300 nm to 5 μm. The on-chip PPLN waveguide is obtained by periodic polarization under the conditions of a polarization period of 2.4 μm - 10 μm and a duty cycle of 40% - 60%.

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