A quantum entanglement source based on a double-periodically polarized thin-film lithium niobate waveguide

Abstract

The present invention is applicable to the field of quantum physics technology, and in particular relates to a quantum entanglement source based on a dual-periodic polarized thin-film lithium niobate waveguide. The entanglement source comprises: a waveguide module, the waveguide module comprising a first PPLN waveguide and a second PPLN waveguide that are perpendicular and coupled to each other, the first PPLN waveguide being used to receive a pump light source, and the second PPLN waveguide outputting associated photon pairs; a compensation device, one end of the compensation device being fixedly connected to the output end of the second PPLN waveguide, being used to receive the photon pairs output by the second PPLN waveguide, perform spatial and / or temporal compensation on the photon pairs, and interfere with the compensated phase-matched photon pairs to form an entangled state, which is output through the other end of the compensation device; and a temperature control device being used to adjust the temperature of the waveguide module and the compensation device. The quantum entanglement source of the present invention is a single-path entanglement system that does not require waveguide consistency. It is suitable for short-wavelength PPLN waveguide entanglement sources and has higher entanglement brightness.

Description

Technical Field

[0001] The present invention is applicable to the field of quantum physics technology, and in particular relates to a quantum entanglement source based on a double-periodic polarized thin-film lithium niobate waveguide. Background Art

[0002] Quantum key distribution, through the transmission of quantum states, is the only known unconditionally secure communication method that is untapped and unbreakable. The transmission attenuation in a quantum key distribution channel is the square of the single link, limiting the ultimate bit rate. Increasing the brightness of the entangled source is one of the most direct ways to increase the ultimate bit rate.

[0003] Traditional entanglement sources use the spontaneous parametric down-conversion of periodically poled second-order nonlinear crystals to generate quantum entanglement, but they have disadvantages such as low photon pair yield, large system size, difficulty in system temperature control (large temperature control circuit), and low integration. Compared with bulk quantum optics (free space), quantum integrated optics has the characteristics 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. Therefore, periodically poled lithium niobate (PPLN) waveguides are the best choice to solve the problem of entanglement sources in the field of quantum information.

[0004] Currently, the commonly used PPLN waveguide entanglement source is a dual-path one. For example, the Chinese utility model patent document with authorization publication number CN 209182627 U discloses an entanglement source based on a dual PPLN waveguide, comprising a light source, a polarization splitting unit, a polarization control unit, two PPLN waveguides, and an entangled photon generation unit. The polarization splitting unit splits the pump light into two pump light components with mutually perpendicular polarization directions. The polarization control unit rotates the polarization direction of one of the pump light components by 90 degrees before entering the PPLN waveguide. The two PPLN waveguides are used to generate correlated first and second photon pairs based on the parametric down-conversion effect under the action of the pump light components. The entangled photon generation unit simultaneously receives the two photon pairs and outputs two polarization-entangled photon pairs. This entangled source achieves an all-fiber structure, reducing the integrated volume of the entanglement source and making it widely used.

[0005] However, the above-mentioned PPLN waveguide entanglement source has high requirements on waveguide consistency because it is based on a dual-path setting, and it is difficult to achieve a short-wavelength entanglement source. Summary of the Invention

[0006] In view of this, an embodiment of the present invention provides a quantum entangled source based on a dual-periodic polarization thin-film lithium niobate waveguide to solve the problem that existing quantum entangled sources are difficult to achieve short-wavelength entangled sources.

[0007] The present application provides a quantum entanglement source based on a double-periodic polarization thin-film lithium niobate waveguide, comprising:

[0008] The waveguide module includes a first PPLN waveguide and a second PPLN waveguide that are perpendicular to each other and coupled to each other. The first PPLN waveguide is used to receive a pump light source, and the second PPLN waveguide outputs associated photon pairs. The perpendicularity includes that the Y axis of the first PPLN waveguide is parallel to the Y axis of the second PPLN waveguide, the Y axis of the first PPLN waveguide and the Y axis of the second PPLN waveguide are the transmission direction of light, the X axis of the first PPLN waveguide is parallel to the Z axis of the second PPLN waveguide, and the Z axis of the first PPLN waveguide is parallel to the X axis of the second PPLN waveguide. The first PPLN waveguide and the second PPLN waveguide are coupled to each other at an optimal coupling position, which is the position corresponding to the highest coupling efficiency. The coupling efficiency is determined according to the spot power output by the first PPLN waveguide and the second PPLN waveguide.

[0009] a compensation device, one end of which is fixedly connected to the output end of the second PPLN waveguide, for receiving the photon pairs output by the second PPLN waveguide, performing spatial and / or temporal compensation on the photon pairs, and interfering the compensated phase-matched photon pairs to form an entangled state, which is output through the other end of the compensation device;

[0010] The temperature regulating device is used to regulate the temperature of the waveguide module and the compensation device.

[0011] Compared with the prior art, the quantum entanglement source based on the dual-periodic polarization thin film lithium niobate waveguide of the present invention has the following advantages: the waveguide module of the quantum entanglement source of the present invention adopts two mutually perpendicular and coupled PPLN waveguides, which generate |V> s |V> i and |H> s |H> i The photon pair is a single-path entangled system with no requirement for waveguide consistency, which reduces process difficulty and cost and increases yield. Moreover, based on a small-period PPLN waveguide, it is suitable for a short-wavelength PPLN waveguide entanglement source with a small mode field diameter and a small beam waist. It has a stronger optical confinement capability, and the entangled source has higher performance and increased entanglement brightness under the same light intensity.

[0012] Furthermore, the preparation process of the first PPLN waveguide and the second PPLN waveguide includes: determining the polarization period according to the wavelength of the pump light source and the quasi-phase matching equation; periodically polarizing the lithium niobate film according to the polarization period, and the exposure method during the periodic poling process is double ultraviolet UV exposure to increase the exposure limit; processing the periodically poled lithium niobate film into a PPLN waveguide, and the exposure method during the processing is EBL exposure.

[0013] Furthermore, it also includes a fiber coupling device, which is connected to the other end of the compensation device to achieve fiber output, and the first PPLN waveguide is connected to the pump light source through the optical fiber.

[0014] Further, the first PPLN waveguide and the second PPLN waveguide include a ridge waveguide, a titanium diffused waveguide, or a proton exchange waveguide.

[0015] Furthermore, the optimal coupling position is determined by numerical simulation software and actual position adjustment: the initial coupling position is determined by numerical simulation software, and the relative positions of the XZ plane of the first PPLN waveguide and the XZ plane of the second PPLN waveguide are adjusted around the initial coupling position by adjusting the platform to determine the optimal coupling position; the initial coupling position is the position with the highest calculated coupling efficiency, and the calculated coupling efficiency is obtained by calculating the electric field distribution corresponding to the light spots output by the first PPLN waveguide and the second PPLN waveguide.

[0016] Furthermore, the first PPLN waveguide and the second PPLN waveguide are coupled and connected by ultraviolet glue, and the second PPLN waveguide and the compensation device are fixedly connected by ultraviolet glue.

[0017] Furthermore, in order to reduce the volume of the entanglement source, the temperature regulating device includes a substrate and a TEC driving module arranged on the substrate, and the waveguide module, the compensation device and the optical fiber coupling device are all integrated on the substrate.

[0018] Furthermore, the TEC driving module, the waveguide module, the compensation device and the optical fiber coupling device are respectively arranged on the outer surfaces of two opposite surfaces of the substrate, and the contact area between the TEC driving module and the substrate is greater than or equal to the contact area between the waveguide module, the compensation device and the optical fiber coupling device and the substrate.

[0019] Furthermore, before integrating the waveguide module, the compensation device, and the optical fiber coupling device on the substrate, a simulation step is also included using numerical simulation software. During the simulation, the size of the compensation device and the relative position of the compensation device and the waveguide module are adjusted so that the waveguide module and the compensation device are coaxially coupled in the optical path and the coupled waveguide module and the compensation device are on the same horizontal plane.

[0020] Furthermore, the compensation device is a birefringent crystal. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0022] Figure 1 This is a block diagram of the quantum entanglement source based on a double-periodic polarized thin-film lithium niobate waveguide according to the present invention;

[0023] Figure 2 This is a schematic diagram of the quantum entanglement source based on a double-periodic polarized thin-film lithium niobate waveguide of the present invention;

[0024] Figure 3 This is a process flow chart for preparing a PPLN waveguide according to the present invention;

[0025] Figure 4a is a cross-sectional view of the light spot mode of the horizontally placed waveguide of the present invention;

[0026] Figure 4b is a cross-sectional view of the light spot mode of a vertically placed waveguide of the present invention;

[0027] Figure 5 This is a scatter plot of the yield of the quantum entanglement source based on the double-periodic polarization thin film lithium niobate waveguide of the present invention;

[0028] In the figure, 1 is the pump light source, 2 is the horizontally placed PPLN waveguide, 3 is the vertically placed PPLN waveguide, 4 is the compensation device, 5 is the coupling device, 6 is the housing, 7 is the substrate, and 8 is the TEC driver module. DETAILED DESCRIPTION

[0029] In the following description, specific details such as particular system structures and techniques are provided for purposes of illustration, not limitation, to facilitate a thorough understanding of the embodiments of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

[0030] 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 groups thereof.

[0031] It will also be understood that the term "and / or" used in the present description and appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.

[0032] As used in the present specification and the appended claims, the term "if" may 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" may 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.

[0033] In addition, in the description of the present 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.

[0034] References to "one embodiment" or "some embodiments" in the present 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 invention. Thus, phrases such as "in one embodiment," "in some embodiments," "in other embodiments," and "in yet 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 mean "including but not limited to," unless otherwise specifically emphasized.

[0035] It should be understood that the order of execution of the steps in the following embodiments does not necessarily mean the order in which they are executed. The order in which each process is executed should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0036] In order to illustrate the technical solution of the present invention, specific embodiments are provided below.

[0037] The present invention proposes a quantum entanglement source based on a dual-periodic polarization thin-film lithium niobate waveguide, which includes a waveguide module, a temperature adjustment device, a compensation device and a coupling device. One end of the waveguide module is connected to a pump light source, and the other end is connected to one end of the compensation device; the other end of the compensation device is connected to the coupling device.

[0038] The waveguide module includes a first PPLN waveguide and a second PPLN waveguide that are perpendicular to each other and coupled. Perpendicularity here means that the Y axis of the first PPLN waveguide is parallel to the Y axis of the second PPLN waveguide, the Y axis of the first PPLN waveguide and the Y axis of the second PPLN waveguide are the transmission direction of light, the X axis of the first PPLN waveguide is parallel to the Z axis of the second PPLN waveguide, and the Z axis of the first PPLN waveguide is parallel to the X axis of the second PPLN waveguide. The quantum entanglement source of the present invention is described in detail below using the example of the first PPLN waveguide being placed horizontally (i.e., PPLN waveguide 2 is placed horizontally with its Z axis pointing vertically upward) and the second PPLN waveguide being placed vertically (i.e., PPLN waveguide 3 is placed vertically with its X axis pointing vertically upward). As other embodiments, the first PPLN waveguide can be placed vertically and the second PPLN waveguide can be placed horizontally, or they can be placed in other directions rather than horizontal or vertical directions. As long as the two PPLN waveguides are perpendicular to each other, the present invention does not limit this.

[0039] The structure of the quantum entanglement source of the present invention is as follows Figure 1 As shown, it includes a horizontally placed PPLN waveguide 2 and a vertically placed PPLN waveguide 3. The horizontally placed PPLN waveguide 2 receives the polarized pump light emitted by the pump light source 1 through an optical fiber, and the polarization angle between the polarization light and the Z axis is 45°. The vertically placed PPLN waveguide 3 outputs the associated photon pairs. The horizontally placed PPLN waveguide 2 and the vertically placed PPLN waveguide 3 are coupled and connected at the optimal coupling position to achieve the highest coupling efficiency. The coupling connection is performed by ultraviolet glue. In practice, the coupling efficiency here exceeds 80%.

[0040] Compensation device 4 such as Figure 2 As shown, the compensation device 4 is a birefringent crystal (all existing compensation devices 4 can be used and the present invention does not limit it). One end of the compensation device 4 is fixedly connected to the output end of the vertically placed PPLN waveguide 3, and is used to receive the photon pairs output by the vertically placed PPLN waveguide 3, perform spatial and / or temporal compensation on the photon pairs, and interfere with the compensated phase-matched photon pairs to form an entangled state that is output through the other end of the compensation device 4; the vertically placed PPLN waveguide 3 and the compensation device 4 are fixedly connected by ultraviolet glue.

[0041] The coupling device 5 is a fiber coupling device, which is connected to the other end of the compensation device 4 to achieve fiber output.

[0042] The temperature regulating device is used to regulate the temperature of the horizontally placed PPLN waveguide 2, the vertically placed PPLN waveguide 3, the compensation device 4, and the coupling device 5. The temperature regulating device includes a substrate 7 and a TEC driver module 8 disposed on the substrate 7. The horizontally placed PPLN waveguide 2, the vertically placed PPLN waveguide 3, the compensation device 4, and the coupling device 5 are all integrated on the substrate 7. The TEC driver module 8 and the waveguide module, the compensation device 4, and the coupling device 5 are respectively disposed on the outer surfaces of two opposing sides of the substrate 7. The contact area between the TEC driver module 8 and the substrate 7 is greater than or equal to the contact area between the waveguide module, the compensation device 4, the coupling device 5, and the substrate. Figure 1 As can be seen, to reduce the size of the quantum entanglement source, the TEC driver module 8 is positioned below the substrate 7. The horizontally placed PPLN waveguide 2, the vertically placed PPLN waveguide 3, the compensation device 4, and the coupling device 5 are positioned above the substrate 7, with the TEC driver module 8 and substrate 7 having the same dimensions. Furthermore, to prevent interference, the substrate 7, TEC driver module 8, the horizontally placed PPLN waveguide 2, the vertically placed PPLN waveguide 3, the compensation device 4, and the coupling device 5 are positioned within a housing 6. Two holes are provided in the housing 6 for the optical fibers to pass through. The substrate 7 is made of a material with good thermal conductivity, such as metal, and each device is secured to the substrate 7 with thermally conductive adhesive.

[0043] The process of the above-mentioned quantum entanglement source generating entangled state is as follows Figure 2 As shown:

[0044] First, the pump light source 1 generates short-wavelength pump light, and the polarization direction of the pump light is at an angle of 45° with the Z-axis of the horizontally placed PPLN waveguide 2 (45° is the optimal angle for the pump light, but other angles can also be used). The vertically polarized component of the pump light passes through the horizontally placed PPLN waveguide 2 and generates longer-wavelength photon pairs through spontaneous parametric conversion type-0 mode (SPDC). s |V> i The horizontal polarization component of the pump light passes through the vertically placed PPLN waveguide 3 and generates a photon pair |H> through the spontaneous parametric conversion type-0 mode s |H> i , photon pair |V> s |V> i The vertical placement of the PPLN waveguide 3 generates dispersion, so the vertical placement of the PPLN waveguide 3 outputs the associated |V> s |V> i and |H> s |H> i Photon pairs.

[0045] Then, depending on the specific nonlinear optical process, each pair of photons can have different forms of correlation in time, frequency, momentum, polarization and other dimensions, so |V> s |V> i The photon pairs are compensated by the compensation device 4 to achieve coaxial transmission, and the compensated phase-matched photon pairs are interfered in the compensation device 4 to form an entangled state which is output through the other end of the compensation device 4.

[0046] The above entangled source can construct different maximum entangled states; the following two maximum entangled states |Φ are constructed in the polarization dimension ± >:

[0047]

[0048] The conventional periodically poled second-order nonlinear crystal spontaneous parametric down-conversion adopts type-II mode, and the piezoelectric constant d 33 Small (low conversion ability, low entanglement source performance), large period (tens of μm), mature periodic polarization technology; the PPLN waveguide spontaneous parametric down conversion of the present invention adopts type-0 mode, d 33 The entanglement source performance is increased several times, the period is small (micrometer-level or even nanometer-level PPLN waveguide is used, generally within 10μm, especially the polarization period is only 2.55μm at a wavelength of 405nm), the periodic polarization and the PPLN waveguide manufacturing process are complex, and the preparation process of the PPLN waveguide of the present invention is as follows: Figure 3 As shown, the steps include:

[0049] Step S101 determines the polarization period according to the wavelength and the quasi-phase matching equation.

[0050] In this step, the quasi-phase matching equation is:

[0051] k1+k2+K Q -k3=0;

[0052] Wherein, k1 is the photon momentum of the first parametric light; k2 is the photon momentum of the second parametric light; K Q is the phase mismatch factor; k3 is the photon momentum of the pump light; λ1 is the wavelength of the first parametric light; λ2 is the wavelength of the second parametric light; λ3 is the wavelength of the pump light; n1 is the relative refractive index of the first parametric light in the waveguide; n2 is the relative refractive index of the second parametric light in the waveguide; n3 is the relative refractive index of the pump light in the waveguide; Λ is the polarization period.

[0053] Step S102 performs periodic polarization on the lithium niobate film according to the polarization period.

[0054] In this step, the lithium niobate film (LNOI) is a Z-cut lithium niobate with a thickness of 300nm-10μm, which is periodically poled by an applied electric field method. Specifically, the periodic poling includes three parts: polarization electrode design, micro-nano processing and testing.

[0055] a. Polarization electrode design.

[0056] 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 should be selected, and the corresponding relationship between the external polarization electric field and polarization time and the movement of the internal domain structure should be established to accurately control the lateral broadening of the inversion domain and reduce the merging of adjacent inversion domains.

[0057] b. Micro-nano processing.

[0058] The micro-nano processing process includes: glue throwing, exposure (UV ultraviolet exposure, EBL), development, fixing, electrode plating, high-voltage polarization (high-voltage polarization uses polarization cycle for polarization) and other steps.

[0059] 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 a concern, a single exposure technology can also be used, and the present invention does not limit this.

[0060] c. Testing.

[0061] 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.

[0062] Step S103 processes the periodically poled lithium niobate film to form a basic PPLN waveguide.

[0063] In this step, the periodically poled thin-film lithium niobate waveguide (PPLN) adopts a ridge waveguide, a titanium diffused waveguide, or a proton exchange waveguide. This embodiment takes the ridge waveguide as an example to introduce the processing process of the basic PPLN waveguide.

[0064] a. Before micro-nano processing, the ridge waveguide structure parameters need to be designed.

[0065] Based on the optical waveguide theory, COMSOL numerical simulation and Lumerical simulation were used to construct a PPLN ridge waveguide structure model. 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 was simulated and studied, thereby guiding the design of ridge waveguide structure parameters.

[0066] b. The micro-nano processing process includes: stripping, EBL exposure, development, fixing, ICP etching or cutting to obtain ridge waveguide.

[0067] Experiments were conducted to investigate the key factors influencing waveguide surface roughness during EBL exposure and ICP etching, including the electron beam exposure dose, photoresist mask hardening temperature and time, etching gas type and flow rate, and etching power. Subsequently, ultrasonic cleaning with RCA solution was performed to fabricate a PPLN ridge waveguide structure. While EBL exposure was used in this process, other existing exposure methods, such as UV exposure, could also be employed, without limitation in this disclosure.

[0068] In step S104 , the base PPLN waveguide is plated to form a protective layer.

[0069] In this step, SiO2 is deposited as a protective layer for the basic PPLN waveguide using coating techniques such as electron beam evaporation or plasma.

[0070] In step S105 , the coated PPLN waveguide is cut and the end faces of both ends are polished to finally form a PPLN waveguide to be coupled.

[0071] The mode field diameter of the horizontally placed PPLN waveguide 2 and the vertically placed PPLN waveguide 3 of the present invention is in the micron level or even the nanometer level. The beam waist is very small, the light confinement ability is high, and the entanglement source performance is much higher under the same light intensity (the brightness is inversely proportional to the beam waist). At the same time, this entanglement source is a single-channel coaxial output. The coupling between waveguides with a small mode field is very difficult. In addition, the present invention is a horizontally placed PPLN waveguide 2 and a vertically placed PPLN waveguide, such as Figure 4a and Figure 4b As shown, the mode fields are in different directions, and it is difficult to achieve a high coupling efficiency between the two PPLN waveguides. Therefore, before the horizontally placed PPLN waveguide 2 and the vertically placed PPLN waveguide 3 are coupled and connected, the optimal coupling position needs to be found. The process of determining the optimal coupling position is as follows:

[0072] First, the initial coupling position is determined by Lumerical FDTD numerical simulation software.

[0073] The electric field distribution corresponding to the output spot mode of the horizontally placed PPLN waveguide 2 and the vertically placed PPLN waveguide 3 was simulated using Lumerical FDTD numerical simulation software. The computational coupling efficiency of the two PPLN waveguides was calculated (the integral intensity of the spot overlap was calculated). The relative position relationship between the horizontally placed PPLN waveguide 2 and the vertically placed PPLN waveguide 3 was explored when the computational coupling efficiency was the highest, and the initial coupling position was found.

[0074] Secondly, the horizontally placed PPLN waveguide 2 and the vertically placed PPLN waveguide 3 are protected by patch protection. On the coupling adjustment platform, the initial coupling position is used as the initial position, and the light spot output by the vertically placed PPLN waveguide 3 is monitored in real time. The relative positions of the XZ plane of the first PPLN waveguide and the XZ plane of the second PPLN waveguide are adjusted around the initial coupling position by adjusting the platform to determine the optimal coupling position, thereby accurately controlling the output light spot pattern to match the light spot pattern of the horizontally placed waveguide, thereby obtaining the position with the highest transmittance (maximum light power, which is proportional to the output light power), which is also the optimal coupling position.

[0075] After finding the optimal coupling position, ultraviolet glue is used to fix the horizontally placed PPLN waveguide 2 and the vertically placed PPLN waveguide 3, thereby achieving high-efficiency coupling of the two PPLN waveguides, with a coupling efficiency of more than 80%.

[0076] In the above embodiment, the coupled horizontally placed PPLN waveguide 2 and the vertically placed PPLN waveguide 3 (hereinafter referred to as the waveguide module), the compensation device 4, and the coupling device 5 are integrated above the substrate 7. In order to better achieve small-volume integration, the size of the compensation device 4 and the relative position of the compensation device 4 and the waveguide module need to be determined. The determination process is as follows:

[0077] The dimensions of the compensation device 4 were adjusted using CAD software, and the relative positions of the compensation device 4 and the waveguide module were then adjusted using Lumerical FDTD numerical simulation software. Combined with the design of the electric field distribution corresponding to the beam propagation parameters and the output spot pattern, the waveguide module and compensation device 4 were coaxially coupled in the optical path, with the coupled waveguide module and compensation device 4 being on the same horizontal plane.

[0078] While performing the Lumerical FDTD numerical simulation, the optical parameters of the compensation device 4 also need to be determined based on the optical parameters of the waveguide module, such as the NA mode field and transmittance of the NA optical fiber of the collimating lens.

[0079] After the size of the compensation device 4 and the relative positions of the compensation device 4 and the waveguide module are determined, these components are fixed on the substrate 7 by using thermal conductive glue.

[0080] Of course, without considering the integrated volume, the size of the compensation device 4 may not be adjusted or simulated, or the integrated structure may not be adopted, and the devices may be set separately. The present invention does not impose any restrictions on this.

[0081] In the above embodiment, in order to better integrate, the waveguide, the pump light source 1 and the coupling device 5 are all in the form of optical fibers. As other implementations, when integration is not considered, free space coupling can also be used, and the present invention does not impose any restrictions.

[0082] The quantum entanglement source of the present invention is described below with two specific examples.

[0083] The preparation process of the quantum entanglement source in Example 1 is as follows:

[0084] 1. Based on the quasi-phase matching equation, the input light is a 780nm continuous laser and the parametric light is 1560nm. The calculated polarization period is 18.9μm, the duty cycle is 50%, and the Z-cut 5μm thick lithium niobate film is periodically poled through spinning, double UV exposure, development, fixing, electrode plating, and high-voltage poling.

[0085] 2. The periodically poled lithium niobate film was fabricated into a PPLN waveguide by spinning, EBL exposure, development, fixing, and cutting. The PPLN waveguide was 20 mm long and 4.5 μm wide.

[0086] 3. Use electron beam evaporation technology to coat the PPLN waveguide with SiO2 as a protective layer, followed by cutting and polishing;

[0087] 4. Place the two PPLN waveguides horizontally and vertically, and then perform patch protection and coupling connection;

[0088] 5. Place the front end of the horizontally placed PPLN waveguide 2 coupled with an optical fiber to connect to the pump light source 1, and place the back end of the PPLN waveguide 3 vertically coupled with the compensation device 4 and the optical fiber coupling device;

[0089] 6. Fix the temperature regulating device to the overall device and then package it.

[0090] The preparation process of the quantum entanglement source of Example 2 is as follows:

[0091] 1. Based on the quasi-phase matching equation, the input light is a 405nm continuous laser and the parametric light is 810nm. The calculated polarization period is 2.55μm and the duty cycle is 50%. Z-cut 600nm thick lithium niobate films are periodically poled by spinning, double UV exposure, development, fixing, electrode coating, and voltage poling.

[0092] 2. The periodically poled lithium niobate film was fabricated into a PPLN waveguide through steps such as spinning, EBL exposure, development, fixing, and ICP etching. The PPLN waveguide had a length of 10 mm and a width of 1.5 μm.

[0093] 3. Use electron beam evaporation technology to coat the PPLN waveguide with SiO2 as a protective layer, followed by cutting and polishing;

[0094] 4. Place the two PPLN waveguides horizontally and vertically, and then perform patch protection and coupling connection;

[0095] 5. Place the front end of the horizontally placed PPLN waveguide 2 coupled with an optical fiber to connect to the pump light source 1, and place the back end of the PPLN waveguide 3 vertically coupled with the compensation device 4 and the optical fiber coupling device;

[0096] 6. Fix the temperature regulating device to the overall device and then package it.

[0097] The two main indicators of quantum entanglement sources are yield and entanglement visibility. Through the measurement of single photon detectors and TDC counters, Figure 5 This is a graph showing the system yield and stability of the quantum entangled source from Example 1. The quantum entangled source's yield is 40G pairs / mW / s, over 1,000 times higher than the conventional PPKDP crystal (30M pairs / mW / s). Furthermore, during a 12-hour stability test, the quantum entangled source consistently provided entangled photon pairs. Furthermore, as shown in Table 1, the system contrast ratio of the quantum entangled source is as high as 370:1, seven times that of the conventional PPKDP crystal (50:1).

[0098] Table 1. Contrast ratio of quantum entangled source system of double periodically polarized thin film lithium niobate waveguide

[0099]

[0100] In summary, the quantum entanglement source integrated system of the single-channel dual-period polarized thin-film lithium niobate waveguide of the present invention adopts two PPLN waveguides that are perpendicular to each other and coupled to each other, which improves the entanglement effect of the small-period entanglement source and makes the entanglement source brighter. It adopts optical fiber input pump light and optical fiber output entanglement source, and the integrated design greatly reduces the volume of the entanglement source.

[0101] The single-channel quantum entanglement source integrated system has no consistency requirements for PPLN, achieves overall temperature control, reduces the difficulty of temperature control, and integrates a small temperature control circuit, greatly reducing the system volume; and the yield and entanglement visibility are far greater than traditional crystal entanglement source solutions such as PPKTP.

[0102] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention 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 invention, and should all be included in the scope of protection of the present invention.

Claims

1. A quantum entanglement source based on a double-periodic polarization thin-film lithium niobate waveguide, characterized in that: include: The waveguide module includes a first PPLN waveguide and a second PPLN waveguide that are perpendicular to each other and coupled to each other. The first PPLN waveguide is used to receive a pump light source, and the second PPLN waveguide outputs associated photon pairs. The perpendicularity includes the Y-axis of the first PPLN waveguide and the Y-axis of the second PPLN waveguide being parallel, the Y-axis of the first PPLN waveguide and the Y-axis of the second PPLN waveguide being the transmission direction of light, the X-axis of the first PPLN waveguide and the Z-axis of the second PPLN waveguide being parallel, and the Z-axis of the first PPLN waveguide and the X-axis of the second PPLN waveguide being parallel. The first PPLN waveguide and the second PPLN waveguide are coupled to each other at an optimal coupling position, which is the position corresponding to the highest coupling efficiency. The coupling efficiency is determined according to the spot power output by the first PPLN waveguide and the second PPLN waveguide; a compensation device, one end of which is fixedly connected to the output end of the second PPLN waveguide, for receiving the photon pairs output by the second PPLN waveguide, performing spatial and / or temporal compensation on the photon pairs, and interfering the compensated phase-matched photon pairs to form an entangled state, which is output through the other end of the compensation device; A temperature regulating device, used for regulating the temperature of the waveguide module and the compensation device; Lumerical FDTD numerical simulation software was used to simulate the electric field distribution corresponding to the output spot pattern of the first and second PPLN waveguides, and the computational coupling efficiency of the two PPLN waveguides was calculated. The relative position relationship between the first and second PPLN waveguides when the computational coupling efficiency was the highest was explored to obtain the initial coupling position. The first and second PPLN waveguides are patch-protected. On the coupling adjustment platform, the initial coupling position is used as the initial position, and the light spot output through the second PPLN waveguide is monitored in real time. The relative positions of the XZ plane of the first PPLN waveguide and the XZ plane of the second PPLN waveguide are adjusted around the initial coupling position by the adjustment platform to determine the position with the highest transmittance as the optimal coupling position.

2. The quantum entanglement source based on a double-periodic polarization thin-film lithium niobate waveguide according to claim 1, characterized in that: The preparation process of the first PPLN waveguide and the second PPLN waveguide includes: determining the polarization period according to the wavelength of the pump light source and the quasi-phase matching equation; periodically polarizing the lithium niobate film according to the polarization period, and exposing the periodic poling film by double ultraviolet (UV) exposure to increase the exposure limit; processing the periodically poled lithium niobate film into a PPLN waveguide, and exposing the film by EBL exposure during the processing.

3. The quantum entanglement source based on a double-periodic polarization thin-film lithium niobate waveguide according to claim 1, characterized in that: It also includes an optical fiber coupling device, which is connected to the other end of the compensation device to achieve optical fiber output, and the first PPLN waveguide is connected to the pump light source through the optical fiber.

4. The quantum entanglement source based on a double-periodic polarization thin-film lithium niobate waveguide according to claim 1, characterized in that: The first PPLN waveguide and the second PPLN waveguide include a ridge waveguide, a titanium diffused waveguide, or a proton exchange waveguide.

5. The quantum entanglement source based on a double-periodic polarization thin-film lithium niobate waveguide according to claim 1, characterized in that: The first PPLN waveguide and the second PPLN waveguide are coupled and connected by ultraviolet glue, and the second PPLN waveguide and the compensation device are fixedly connected by ultraviolet glue.

6. The quantum entanglement source based on a double-periodic polarization thin-film lithium niobate waveguide according to claim 3, characterized in that: The temperature regulating device comprises a substrate and a TEC driving module arranged on the substrate, and the waveguide module, the compensation device and the optical fiber coupling device are all integrated on the substrate.

7. The quantum entanglement source based on a double-periodic polarization thin-film lithium niobate waveguide according to claim 6, characterized in that: The TEC driving module, the waveguide module, the compensation device and the optical fiber coupling device are respectively arranged on the outer surfaces of two opposite surfaces of the substrate, and the contact area between the TEC driving module and the substrate is greater than or equal to the contact area between the waveguide module, the compensation device and the optical fiber coupling device and the substrate.

8. The quantum entanglement source based on a double-periodic polarization thin-film lithium niobate waveguide according to claim 6, characterized in that: Before integrating the waveguide module, the compensation device, and the optical fiber coupling device on the substrate, a simulation step is also included. During the simulation, the size of the compensation device and the relative position of the compensation device and the waveguide module are adjusted so that the waveguide module and the compensation device are coaxially coupled in the optical path and the coupled waveguide module and the compensation device are on the same horizontal plane.

9. The quantum entanglement source based on a double-periodic polarization thin-film lithium niobate waveguide according to claim 1, characterized in that: The compensation device is a birefringent crystal.

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