On-chip dual-period poled lithium niobate quantum entanglement source

Abstract

The application is suitable for the field of quantum technology, and relates to a quantum entanglement source of on-chip double-period polarization lithium niobate. The quantum entanglement source comprises a waveguide device composed of a PPLN waveguide and an on-chip beam splitter, and an entanglement device composed of a fiber fixer, a first mode micro-nano fiber, a second mode micro-nano fiber and a fiber polarization beam combiner. An input end of the on-chip beam splitter is used for connecting a pump light source. The on-chip beam splitter injects photons into the PPLN waveguide to form two SPDC light paths. The modes of the two micro-nano fiber ends are 90 degrees different, and both are fixed by the fiber fixer, so that the lengths of the two optical fibers are the same. The input ends of the two micro-nano fibers are respectively connected to the output ends of the two PPLN waveguides. The output ends of the two micro-nano fibers are respectively connected to the two input ends of the fiber polarization beam combiner. The fiber polarization beam combiner is used for forming interference after the two optical waveguides are aggregated and outputting. Compared with the traditional free-space entanglement form, the fiber is easy to realize entanglement, and the occupied space of the whole optical path can be reduced.

Description

TECHNICAL FIELD

[0001] The present application is suitable for the field of quantum optics and quantum information technology, and particularly relates to a quantum entanglement source of on-chip double-periodic poled lithium niobate. BACKGROUND

[0002] Communication security is the guarantee of national information economic security and people's social life. Quantum communication is based on quantum mechanics and is a communication mode that is unconditionally secure in principle according to information theory, and has absolute security that traditional communication modes do not have, and has a huge application prospect in the security fields of national information security, military security, financial security, etc. In quantum communication, the channel transmission attenuation is the square of the single link, and there are some constraints in improving the final code rate, and increasing the brightness of the entanglement source is one of the most direct means to improve the final code rate.

[0003] Traditional entanglement sources use spontaneous parametric down-conversion (SPDC) of a periodically poled second-order nonlinear crystal to generate quantum entanglement, but have the disadvantages of low yield, large volume, large volume of temperature control circuit, great difficulty in temperature control due to complex design, low integration, etc. Compared with bulk quantum optics, quantum integrated optics has the characteristics of small volume, high stability, strong controllability, reconfigurability, etc. Lithium niobate, as one of the most important nonlinear optical materials, is widely used in the preparation of quantum light sources and the high-speed regulation of quantum states. With the introduction of microprocessing technology, lithium niobate has become an important platform for quantum integrated optics, and can realize single-chip integrated entangled light sources with multiple degrees of freedom encoding. Periodically poled lithium niobate (PPLN) waveguide is the best choice to solve the problem of entanglement source in the field of quantum information.

[0004] Existing quantum entanglement sources need to use lenses and other optical paths to form a device with a large volume, which is not convenient to use, and the effect of quantum entanglement using lenses is poor, the requirements for the parameters of the lenses and the sealing of the device are high, and it is not easy to implement. Therefore, how to reduce the implementation difficulty of an entanglement source with a small volume and good quantum entanglement effect has become a problem to be solved. SUMMARY

[0005] In view of this, the embodiments of the present application provide a quantum entanglement source of on-chip double-periodic poled lithium niobate to solve the problem of how to reduce the implementation difficulty of an entanglement source with a small volume and good quantum entanglement effect.

[0006] The present application provides a quantum entanglement source of on-chip double-periodic poled lithium niobate, which comprises a pump light source, a waveguide device and an entanglement device.

[0007] The waveguide device comprises a PPLN waveguide and an on-chip beam splitter, an input end of the on-chip beam splitter is connected to the pump light source, the on-chip beam splitter divides the pump light source into two light paths into the PPLN waveguide, and two SPDC light paths are formed in the PPLN waveguide;

[0008] The entanglement device comprises a fiber fixer, a first mode micro-nano fiber, a second mode micro-nano fiber, and a fiber polarization beam combiner, the first mode micro-nano fiber and the second mode micro-nano fiber have a mode difference of 90° at the end, and the first mode micro-nano fiber and the second mode micro-nano fiber are both fixed by using the fiber fixer so that the lengths of the two fibers are the same.

[0009] An output end of the first mode micro-nano fiber is connected to one of the SPDC light paths output from the PPLN waveguide, an input end of the second mode micro-nano fiber is connected to the other of the SPDC light paths output from the PPLN waveguide, and the output ends of the first mode micro-nano fiber and the second mode micro-nano fiber are respectively connected to two input ends of the fiber polarization beam combiner, and the fiber polarization beam combiner is used to aggregate the two SPDC light paths to form interference and output.

[0010] In an embodiment, the fiber fixer is a fiber array, the fiber array comprises N parallel and same-length fiber fixing grooves, all the fiber fixing grooves are arranged at equal intervals, the interval of the two SPDC light paths in the PPLN waveguide is an integer multiple of the interval distance of the adjacent two fiber fixing grooves, the fiber fixing grooves are used to fix the first mode micro-nano fiber and the second mode micro-nano fiber, and N is an integer greater than 1.

[0011] In an embodiment, the first mode micro-nano fiber and the second mode micro-nano fiber are both polarization maintaining fibers.

[0012] In an embodiment, the fiber fixer is a fixing structure in the shape of a panda eye, the fixing structure arranges the first mode micro-nano fiber and the second mode micro-nano fiber adjacently and in parallel, and the interval between the cross-sectional centers of the first mode micro-nano fiber and the second mode micro-nano fiber is the same as the interval of the two SPDC light paths in the PPLN waveguide.

[0013] In an embodiment, the on-chip beam splitter is a Y-type beam splitter.

[0014] In an embodiment, the quantum entanglement source further comprises a substrate sheet and a temperature control device, the waveguide device, the entanglement device, and the temperature control device are all fixed on the substrate sheet, and the temperature control device is used to control the temperature of the waveguide device and the entanglement device on the substrate sheet.

[0015] In an 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 sheet, and the TEC temperature control driver is arranged on the other side of the substrate sheet.

[0016] In an embodiment, the quantum entanglement source further comprises an output optical fiber, and an output end of the optical fiber polarization beam combiner is connected to the output optical fiber.

[0017] In an embodiment, the PPLN waveguide is a ridge waveguide, a titanium diffusion waveguide or a proton exchange waveguide of lithium niobate material.

[0018] In an embodiment, the PPLN waveguide is a Z-cut lithium niobate thin film with a thickness of 5 to 600 microns, and the PPLN waveguide is obtained by periodically poling with a poling period of 2.55 to 18.9 microns and a duty cycle of 50%.

[0019] The quantum entanglement source of the on-chip double-period poling lithium niobate of the present application has the following beneficial effects compared with the prior art: the quantum entanglement source of the present application comprises a pump light source, a waveguide device and an entanglement device, the waveguide device comprises a PPLN waveguide and an on-chip beam splitter, the input end of the on-chip beam splitter is connected to the pump light source, the on-chip beam splitter divides the pump light source into two light paths entering the PPLN waveguide, and two SPDC light paths are formed in the PPLN waveguide, the entanglement device comprises an optical fiber fixer, a first mode micro-nano optical fiber, a second mode micro-nano optical fiber and an optical fiber polarization beam combiner, the modes at the ends of the first mode micro-nano optical fiber and the second mode micro-nano optical fiber are different by 90°, the first mode micro-nano optical fiber and the second mode micro-nano optical fiber are both fixed by the optical fiber fixer so that the lengths of the two optical fibers are the same, the output end of the first mode micro-nano optical fiber is connected to one of the two SPDC light paths output from the PPLN waveguide, the input end of the second mode micro-nano optical fiber is connected to the other of the two SPDC light paths output from the PPLN waveguide, the output ends of the first mode micro-nano optical fiber and the second mode micro-nano optical fiber are respectively connected to the two input ends of the optical fiber polarization beam combiner, and the optical fiber polarization beam combiner is used to aggregate the two light paths to form interference and output, which realizes the formation of two SPDC light paths on the PPLN waveguide, converts one of the SPDC light paths by optical fiber output, finally forms coupled output, and obtains the quantum entanglement source. Compared with the traditional free-space entanglement form, the use of optical fiber facilitates entanglement and reduces the occupied space of the entire optical path, thereby reducing the volume of the entanglement source. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description only constitute some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort.

[0021] Figure 1 is a structure schematic diagram of a quantum entanglement source of on-chip double-period polarization lithium niobate provided by the embodiment one of the present application;

[0022] Figure 2 is a structure schematic diagram of a quantum entanglement source after PPLN waveguide using Z-cut lithium niobate film provided by the embodiment one of the present application;

[0023] Figure 3 is a yield diagram of a quantum entanglement source at different temperatures provided by the embodiment one of the present application;

[0024] Figure 4 is a structure schematic diagram of a fiber fixer of a quantum entanglement source provided by the embodiment two of the present application;

[0025] Figure 5 is a structure schematic diagram of a fiber fixer of a quantum entanglement source provided by the embodiment three of the present application;

[0026] Figure 6 is a structure schematic diagram of a quantum entanglement source of on-chip double-period polarization lithium niobate provided by the embodiment four of the present application;

[0027] In the figure, 1 is a pump light source, 2 is a waveguide device, 3 is an entanglement device, 4 is a substrate sheet, 5 is a temperature control device, 201 is a PPLN waveguide, 202 is an on-chip beam splitter, 2011 is a first SPDC light path, 2012 is a second SPDC light path, 301 is a fiber fixer, 302 is a first mode micro-nano fiber, 303 is a second mode micro-nano fiber, 304 is a fiber polarization beam combiner, 3011 is a fiber array, and 3012 is a panda eye-shaped fixing structure. DETAILED DESCRIPTION

[0028] In the following description, specific details are set forth in order to provide a thorough understanding of the embodiments of the present application. However, persons skilled in the art will understand that the present application can be practiced without these specific details. In other instances, well-known systems, devices, circuits, and methods have not been described in detail so as not to obscure the description of the present application.

[0029] It should be understood that the word “comprise” or variations such as “comprises” or “comprising”, when used in this specification and in the accompanying claims, specify the presence of stated features, integers, steps, operations, elements, components and / or groups but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0030] It should also be understood that the term “and / or” when used in this specification and in the following claims is to be interpreted as “one or the other or both” and / or “any combination of the items in the list.

[0031] As used in this specification and in the claims, the terms “if’ and “when” can be interpreted to mean “upon” or “in response to a determination” or “in response to a detection” depending on the context. Similarly, the phrase “if it is determined” or “if [a described condition or event] is detected” can be interpreted to mean “upon a determination” or “in response to a determination” or “upon detecting [the described condition or event]” or “in response to detecting [the described condition or event]”, depending on the context.

[0032] In addition, the terms “first”, “second”, “third”, etc. as used in the description of the application and the following claims are only used to distinguish descriptions and cannot be understood as indicating or implying relative importance.

[0033] Reference in the specification to “one embodiment” or “some embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the application. The appearances of the phrase “in one embodiment” or “in some embodiments” in various places in the specification are not necessarily all referring to the same embodiment, although it can. The terms “comprise”, “comprises”, “comprising”, “include”, “includes”, “including” and “contain”, “contains”, “containing” and variants thereof are meant to be construed as “including but not limited to” unless otherwise noted.

[0034] It should be understood that the size of the serial number of each step in the following embodiments does not mean the order of execution, and the execution order of each process should be determined according to its function and inherent logic, and should not constitute any limitation on the implementation process of the embodiments of the application.

[0035] In order to illustrate the technical solutions of the application, the following specific embodiments are described.

[0036] Embodiment one

[0037] Reference is made to Figure 1A structure schematic diagram of a quantum entanglement source of on-chip dual-period polarization lithium niobate provided by the embodiment one of the present application, the quantum entanglement source comprises a pump light source 1, a waveguide device 2 and an entanglement device 3, the waveguide device comprises a PPLN waveguide 201 and an on-chip beam splitter 202, the input end of the on-chip beam splitter 202 is connected to the pump light source 1, the entanglement device 3 comprises a fiber fixer 301, a first mode micro-nano fiber 302, a second mode micro-nano fiber 303 and a fiber polarization beam combiner 304.

[0038] The two output ends of the on-chip beam splitter 202 respectively inject photons into the PPLN waveguide 2, and two SPDC light paths are formed in the PPLN waveguide 2, that is, Figure 1 a first SPDC light path 2011 and a second SPDC light path 2012. The modes at the ends of the first mode micro-nano fiber 302 and the second mode micro-nano fiber 303 are different by 90°, and the first mode micro-nano fiber 302 and the second mode micro-nano fiber 303 are both fixed by the fiber fixer 301, so that the lengths of the first mode micro-nano fiber 302 and the second mode micro-nano fiber 303 are the same. For example, the on-chip beam splitter is a Y-type beam splitter.

[0039] The output end of the first mode micro-nano fiber 302 is connected to the output of one SPDC light path, that is, the first SPDC light path 2012 from the PPLN waveguide 201, the input end of the second mode micro-nano fiber 303 is connected to the output of the other SPDC light path, that is, the second SPDC light path 2011 from the PPLN waveguide 201, and the output end of the first mode micro-nano fiber 302 and the output end of the second mode micro-nano fiber 303 are respectively connected to the two input ends of the fiber polarization beam combiner 304, and the fiber polarization beam combiner 304 is used to aggregate the two SPDC light paths to form interference and output.

[0040] The two SPDC light paths described above respectively generate longer |V> s |V> i photons from the light sent by the pump light source 1 through the SPDC process, then the two |V> s |V> i photons respectively enter the first mode micro-nano fiber 302 and the second mode micro-nano fiber 303, and after mode conversion of the |V> s |V> i photons in the first mode micro-nano fiber 302 or the second mode micro-nano fiber 303, |H> s |H> i photons are obtained, |V> s |V> i photons and |H> s |H> iPhotons enter the polarization beam combiner 304, making two beams of light converge together, and the pair of photons forms interference, which can construct different maximum entangled states, and construct one of the following two maximum entangled states in the polarization dimension:

[0041]

[0042] In the present application, the PPLN waveguide 201 can be micron or even nanometer, so the periodic polarization of the PPLN waveguide 201 and the fiber coupling entanglement are exponentially increased compared with the traditional scheme. In order to realize the above-mentioned PPLN waveguide 201, Z-cut lithium niobate film can be used, as shown in the figure, which is a quantum entanglement source after the PPLN waveguide 201 of Z-cut lithium niobate film. In the process of manufacturing the Z-cut lithium niobate film, the spontaneous parametric down-conversion adopts type-0 mode, which is: Figure 2

[0043] 1. Calculate the quasi-phase matching period required for parametric down-conversion, and build a polarization electrode model through COMSOL numerical simulation software.

[0044] Among them, in the polarization of lithium niobate film, the polarization electrode model needs to be built through COMSOL numerical simulation software, the polarization electric field distribution corresponding to different electrode configurations is simulated, the influence mechanism of polarization electrode configuration, polarization voltage, polarization time and polarization period on the nucleation and lateral broadening of the reversed domain is explored through experiments, the optimal polarization electrode configuration is selected, the correspondence between the external polarization electric field and the polarization time and the internal domain structure movement is established, the lateral broadening of the reversed domain is accurately controlled, and the merging of adjacent reversed domains is reduced.

[0045] 2. According to the quasi-phase matching period, use UV mask to expose on the Z-cut lithium niobate film to prepare periodic polarization electrodes.

[0046] Among them, the process can include glue spinning, exposure (UV exposure, EBL), development, fixing, electrode plating, high voltage polarization (high voltage polarization adopts polarization period for polarization) and other steps, double ultraviolet exposure technology is used in the exposure process to increase the exposure limit, and then the PVD is accurately controlled to accurately manufacture the metal electrode. Of course, without considering the processing precision, one-time exposure technology can also be used, which is not limited in the present application.

[0047] 3. Based on the change curve of the nucleation and lateral broadening of the reversed domain with the polarization electrode configuration, polarization voltage and polarization time, the optimal polarization electrode configuration is determined.

[0048] ​4. Based on the optimal polarization electrode configuration, the Z-cut lithium niobate thin film of the prepared periodic polarization electrode is subjected to spontaneous parametric down-conversion by an external electric field method to obtain a PPLN waveguide with uniform polarization and inversion domain duty cycle, and the PPLN waveguide meets the requirement of first-order quasi-phase matching.

[0049] In the periodic polarization, the inversion domain structure is tested and characterized, and the polarization is required to be uniform, and the inversion domain duty cycle is required to be uniform to meet the requirement of first-order quasi-phase matching.

[0050] The traditional periodic polarization second-order nonlinear crystal spontaneous parametric down-conversion adopts a type-II mode, the piezoelectric constant d33 is small (i.e. the conversion ability is low, and the entanglement source performance is low), and the period is large (i.e. the order is 10 μm). In the present application, the PPLN waveguide spontaneous parametric down-conversion adopts a type-0 mode, the piezoelectric constant d33 is large (i.e. several times of the type-II mode, and the entanglement source performance is increased several times), and the period is small (i.e. the order is within 10, especially at 405 nm, the period is 2.55 μm), so the preparation difficulty is higher.

[0051] In the present application, the polarization electric field distribution corresponding to different electrode configurations is simulated, the quasi-phase matching period required for SPDC is calculated, the polarization electrode model is constructed by COMSOL numerical simulation software; and the periodic polarization electrode is prepared on the Z-cut PPLN waveguide through standard semiconductor precision processing technology (UV mask exposure); the influence mechanism of the polarization electrode configuration, the polarization voltage and the polarization time on the inversion domain nucleation and lateral broadening is explored through experiments, the optimal polarization electrode configuration is selected, the correspondence between the external polarization electric field and the polarization time and the internal domain structure movement is established, the lateral broadening of the inversion domain is accurately controlled, and the merging of adjacent inversion domains is reduced; the Z-cut PPLN waveguide is subjected to periodic polarization by an external electric field method, and the SPDC process is performed; the inversion domain structure is tested and characterized, and the polarization is required to be uniform, and the inversion domain duty cycle is required to be uniform to meet the requirement of first-order quasi-phase matching.

[0052] In an embodiment, the PPLN waveguide 201 adopts a Z-cut lithium niobate thin film, the thin film thickness is 5 μm to 600 μm, and the PPLN waveguide is obtained by periodic polarization under the condition that the polarization period is 2.55 μm to 18.9 μm and the duty cycle is 50%.

[0053] In the present application, the Z-cut lithium niobate thin film is used, and the contrast of the quantum entanglement source system is as high as 103:1, which is more than twice of the traditional PPKDP crystal (50:1).

[0054] As shown in FIG. 1, the PPLN waveguide 201 is prepared by periodically polarizing the Z-cut lithium niobate thin film 202. Figure 3As shown, the yield of the quantum entanglement source at different temperatures is shown, wherein the abscissa is the temperature, the unit is ℃, and the ordinate is the yield, the unit is G / mw, the yield of the quantum entanglement source in the application can reach 6G pairs / mW (at 44℃), which is more than 100 times higher than the traditional PPKDP crystal (the yield is 30M pairs / mW / s), and the quantum entanglement source in the application can efficiently provide quantum entangled photon pairs at different temperatures.

[0055] Embodiment two

[0056] Reference Figure 4 A structure diagram of a fiber fixer of a quantum entanglement source provided in embodiment two of the application is shown, the fiber fixer 301 is a fiber array 3011, the fiber array 3011 includes N parallel and same-length fiber fixing grooves, all the fiber fixing grooves are arranged at equal intervals, the interval of the two-way SPDC light path in the PPLN waveguide is an integer multiple of the interval distance of the adjacent two fiber fixing grooves, the fiber fixing groove is used for fixing the first mode micro-nano fiber 302 and the second mode micro-nano fiber 303, N is an integer greater than 1.

[0057] Specifically, the first mode micro-nano fiber 302 and the second mode micro-nano fiber 303 are both polarization maintaining fibers.

[0058] At this time, according to the quasi-phase matching equation, the planned period is determined, and the lithium niobate thin film is periodically polarized, for example, the input light is 405nm continuous laser, the parametric light is 810nm, and the calculated polarization period is 2.55μm, and the duty cycle is 50%.

[0059] Among them, the periodic polarization is carried out by glue spinning, exposure, development, fixing, electrode plating and voltage polarization, the polarization period is 2.55μm, and the duty cycle is 50%.

[0060] The periodically polarized lithium niobate thin film is made into a periodically polarized thin film lithium niobate waveguide (i.e. PPLN waveguide) through a micro-nano processing process.

[0061] The PPLN waveguide 201 is electron beam evaporated with SiO2 as a protective layer, and then cut and polished.

[0062] The front-end coupling fiber of the PPLN waveguide 201 is used to connect the on-chip beam splitter 202, and the rear-end coupling fiber is output to the fiber polarization beam combiner 304 entanglement device through the fiber array 3011 for interference, combined with other devices that can be set, and then packaged, so as to obtain a quantum entanglement light source.

[0063] The traditional period polarization crystal is directly fixed together with the free space optical device (i.e. mm level), but the ability of binding light is low, and the entanglement source performance is low (the brightness is inversely proportional to the beam waist); in the application, the distance between the two SPDC light paths is 100 um, and the two SPDC light paths need to be coupled with the entanglement device at the same time, which is very difficult. The Lumerical FDTD numerical simulation software is used to simulate the electric field distribution corresponding to the output spot mode of the two SPDC light paths, calculate the coupling efficiency of the two SPDC light paths, and explore the highest coupling efficiency parameters; by accurately controlling the two SPDC light paths through the real-time monitoring system, the output spot mode is accurately controlled to match the spot mode of the entanglement device, and the ultraviolet glue is fixed, so as to realize the high-efficiency coupling of the waveguide.

[0064] Embodiment three

[0065] Reference Figure 5 A structure diagram of a quantum entanglement source optical fiber fixer provided for embodiment three of the application is shown in the figure, the optical fiber fixer 301 is a panda eye-shaped fixing structure 3012, the first mode micro-nano optical fiber 302 and the second mode micro-nano optical fiber 303 are arranged adjacent to each other in the panda eye-shaped fixing structure 3012, and the interval between the cross-sectional center of the first mode micro-nano optical fiber 301 and the cross-sectional center of the second mode micro-nano optical fiber 303 is the same as the interval of the two SPDC light paths in the PPLN waveguide 201. Figure 5 For example, the interval between the cross-sectional centers of the two optical fibers is 0.125 um, and the diameter of the corresponding fixing structure 3012 can be 1.8 um, the two optical fibers present a panda eye shape in the cladding layer, and the modes of the ends of the two optical fibers differ by 90 degrees.

[0066] Specifically, the first mode micro-nano optical fiber 302 and the second mode micro-nano optical fiber 303 are both polarization maintaining optical fibers.

[0067] At this time, according to the quasi-phase matching equation, the planned period is determined, the lithium niobate thin film is periodically polarized, for example, the input light is 780 nm continuous laser, the parametric light is 1560 nm, and the calculated polarization period is 18.9 um, and the duty cycle is 50%.

[0068] The polarization period is 2.55 um, and the duty cycle is 50%.

[0069] The periodically polarized lithium niobate thin film is made into a periodically polarized thin film lithium niobate waveguide (i.e. PPLN waveguide) through a micro-nano processing process.

[0070] The PPLN waveguide 201 is electron beam evaporated with SiO2 as a protective layer, and then cut and polished.

[0071] The front end of the PPLN waveguide 201 is coupled with the optical fiber for connecting the on-chip beam splitter 202, and the rear end is assembled with the optical fiber output to the fiber polarization beam combiner 304 entanglement device for interference, combined with other devices that can be set, and then packaged to obtain the quantum entanglement light source.

[0072] In an embodiment, the quantum entanglement source further comprises an output optical fiber, and the output end of the fiber polarization beam combiner is connected with the output optical fiber.

[0073] Embodiment Four

[0074] Referring to Figure 6 A structure schematic diagram of a quantum entanglement source of a double-period on-chip PPLN is provided for the fourth embodiment, and on the basis of the first embodiment, the quantum entanglement source further comprises a substrate sheet 4 and a temperature control device 5, the waveguide device 2, the entanglement device 3 and the temperature control device 5 are fixed on the substrate sheet, and the temperature control device 5 is used for temperature control of the waveguide device 2 and the entanglement device 3 on the substrate sheet 4.

[0075] The temperature control device 5 can realize overall temperature control, reduce the difficulty of temperature control, integrate a small temperature control circuit, and greatly reduce the system volume. Specifically, 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 sheet, and the TEC temperature control driver is arranged on the other side of the substrate sheet. In addition, the plane formed by the two SPDC light paths of the PPLN waveguide 201 in the waveguide device 2 can be arranged vertically or horizontally by the substrate sheet 4.

[0076] In an embodiment, the PPLN waveguide 201 is a ridge waveguide, a titanium diffusion waveguide or a proton exchange waveguide of lithium niobate material. This embodiment takes the ridge waveguide as an example to introduce the processing process of the basic PPLN waveguide.

[0077] Based on the theory of optical waveguide, COMSOL numerical simulation and Lumerical simulation are used to construct a PPLN ridge waveguide structure model, to simulate the influence of waveguide structure parameters such as waveguide width, ridge height and side wall inclination angle on the effective refractive index of different wavelength light spot modes in the waveguide, and to guide the design of ridge waveguide structure parameters.

[0078] The application designs and manufactures a double-period polarization lithium niobate waveguide quantum entanglement source, the volume of which is greatly reduced compared with the space optical path of an optical platform, and finally the pump light is input by an optical fiber and the entanglement source is output by an optical fiber, which greatly increases the integration. By using a type-0 double-period polarization thin film lithium niobate waveguide photon pair generation system, the conversion efficiency is high and the brightness is large; and the application designs and manufactures a type-0 SPDC on-chip double-period polarization thin film lithium niobate waveguide photon pair generation system, which can realize overall temperature control, reduce the temperature control difficulty, integrate a small temperature control circuit, greatly reduce the system volume, and in addition, the application designs and manufactures a free space integrated entanglement device of the double-period polarization lithium niobate waveguide quantum entanglement source, the volume of the device is greatly reduced compared with the space optical path of an optical platform, and finally the pump light is input by an optical fiber and the entanglement source is output by an optical fiber, which greatly increases the integration.

[0079] The above examples are only used to illustrate the technical solutions of the application, but not to limit them; although the application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the application, and should be included in the protection scope of the application.

Claims

1. A quantum entanglement source of on-chip dual-period poled lithium niobate, characterized in that, The quantum entanglement source comprises a pump light source, a waveguide device and an entanglement device; The waveguide device comprises a PPLN waveguide and an on-chip beam splitter, an input end of the on-chip beam splitter is connected to the pump light source, the on-chip beam splitter divides the pump light source into two light paths into the PPLN waveguide, and two SPDC light paths are formed in the PPLN waveguide; The entanglement device comprises a fiber fixer, a first mode micro-nano fiber, a second mode micro-nano fiber and a fiber polarization beam combiner, the first mode micro-nano fiber and the second mode micro-nano fiber have a mode difference of 90° at the end, and the first mode micro-nano fiber and the second mode micro-nano fiber are both fixed by using the fiber fixer so that the lengths of the two fibers are the same; An output end of the first mode micro-nano fiber is connected to one SPDC light path output from the PPLN waveguide, an input end of the second mode micro-nano fiber is connected to another SPDC light path output from the PPLN waveguide, and the output end of the first mode micro-nano fiber and the output end of the second mode micro-nano fiber are respectively connected to two input ends of the fiber polarization beam combiner, and the fiber polarization beam combiner is used to aggregate the two SPDC light paths to form interference and output. The PPLN waveguide is formed by using a Z-cut lithium niobate thin film; In the process of manufacturing the PPLN waveguide, type-0 mode is used for spontaneous parametric down-conversion, which comprises the following steps: A quasi-phase matching period required for parametric down-conversion is calculated, and a polarization electrode model is constructed by using COMSOL numerical simulation software; According to the quasi-phase matching period, a periodic polarization electrode is prepared on the Z-cut lithium niobate thin film by using UV mask exposure; Based on the change curves of the inversion domain nucleation and lateral broadening with the polarization electrode configuration, polarization voltage and polarization time, the optimal polarization electrode configuration is determined; Based on the optimal polarization electrode configuration, the Z-cut lithium niobate thin film with the prepared periodic polarization electrode is subjected to spontaneous parametric down-conversion by using an external electric field method, so as to obtain a PPLN waveguide with uniform polarization and inversion domain duty cycle, and the PPLN waveguide meets the requirement of first-order quasi-phase matching.

2. The quantum entanglement source of claim 1, wherein, The fiber fixer is a fiber array, the fiber array comprises N parallel and same-length fiber fixing grooves, all the fiber fixing grooves are arranged at equal intervals, the interval of the two SPDC light paths in the PPLN waveguide is an integer multiple of the interval distance of the adjacent two fiber fixing grooves, the fiber fixing grooves are used for fixing the first mode micro-nano fiber and the second mode micro-nano fiber, and N is an integer greater than 1.

3. The quantum entanglement source of claim 2, wherein, The first mode micro-nano fiber and the second mode micro-nano fiber are both polarization maintaining fibers.

4. The quantum entanglement source of claim 1, wherein, The fiber fixer is a panda eye-shaped fixing structure, the fixing structure arranges the first mode micro-nano fiber and the second mode micro-nano fiber adjacently and in parallel, and the interval between the cross-sectional center of the first mode micro-nano fiber and the cross-sectional center of the second mode micro-nano fiber is the same as the interval of the two SPDC light paths in the PPLN waveguide.

5. The quantum entanglement source of claim 1, wherein, The on-chip beam splitter is a Y-type beam splitter polarization maintaining fiber.

6. The quantum entanglement source of claim 5, wherein, The quantum entanglement source further comprises a substrate sheet and a temperature control device, the waveguide device, the entanglement device and the temperature control device are fixed on the substrate sheet, and the temperature control device is used for temperature control on the waveguide device and the entanglement device on the substrate sheet.

7. The quantum entanglement source of claim 6, wherein, The temperature control device is a TEC temperature control driver, the waveguide device and the entanglement device are arranged on one side of the substrate sheet, and the TEC temperature control driver is arranged on the other side of the substrate sheet.

8. The quantum entanglement source of claim 1, wherein, The quantum entanglement source further comprises an output optical fiber, and an output end of the optical fiber polarization beam combiner is connected to the output optical fiber.

9. The quantum entanglement source of claim 1, wherein, The PPLN waveguide is a ridge waveguide, a titanium diffusion waveguide or a proton exchange waveguide of lithium niobate material.

10. The quantum entanglement source of any of claims 1 to 9, wherein, The PPLN waveguide adopts a Z-cut lithium niobate thin film, the thickness of the thin film is 5-600 microns, and the PPLN waveguide is obtained by using a periodic polarization with a polarization period of 2.55-18.9 microns and a duty cycle of 50%. The quantum entanglement source further comprises an output optical fiber, and an output end of the optical fiber polarization beam combiner is connected to the output optical fiber. The PPLN waveguide is a ridge waveguide, a titanium diffusion waveguide or a proton exchange waveguide of lithium niobate material. The PPLN waveguide adopts a Z-cut lithium niobate thin film, the thickness of the thin film is 5-600 microns, and the PPLN waveguide is obtained by using a periodic polarization with a polarization period of 2.55-18.9 microns and a duty cycle of 50%.

Images