On-chip entangled photon source based on film lithium niobate

Abstract

The invention relates to an on-chip entangled photon source based on thin film lithium niobate, which comprises a coupled semiconductor laser, an on-chip beam splitter, a thin film lithium niobate photon chip formed by a first PPLN waveguide, a second PPLN waveguide and an on-chip beam combiner, the on-chip beam splitter outputs split beams to align to the light incident ends of the first PPLN waveguide and the second PPLN waveguide, the on-chip beam combiner inputs to align to the first PPLN waveguide and the second PPLN waveguide, the on-chip beam combiner performs polarization conversion on photon pairs entering through the second PPLN waveguide, then entangles the photon pairs entering through the first PPLN waveguide, outputs entangled photon pairs, and outputs the entangled photon pairs to the light incident ends of the first PPLN waveguide and the second PPLN waveguide. By means of the on-chip structure, the device size can be effectively reduced, beam splitting, spontaneous parametric down-conversion and polarization beam combination are formed in the chip, entangled photon pairs can be effectively and directly generated, and direct polarization entanglement is achieved on the integrated photon chip while the size of the entangled light source device is reduced.

Description

Technical Field

[0001] This application belongs to the field of integrated quantum photonics and nonlinear optics, and particularly relates to an on-chip entangled photon source based on thin-film lithium niobate. Background Technology

[0002] Currently, on-chip entangled photon pairs based on integrated photonics platforms (such as silicon-based, silicon nitride, and lithium niobate) are mainly achieved through nonlinear processes such as spontaneous parametric downconversion or spontaneous four-wave mixing, and are realized through optical pumping. Although this approach reduces the device size to some extent, it relies on discrete external pump lasers, making it difficult to achieve true miniaturization, low power consumption, and scalability of the entire device.

[0003] Existing hybrid integration of silicon nitride microring resonators and III-V lasers has for the first time realized an electrically pumped time-energy entangled source. However, due to the low efficiency of this nonlinear process and the narrow-bandwidth characteristics of the resonator, the generated entangled photon pairs suffer from orders of magnitude deficiencies in key performance indicators such as brightness (typically only about 10⁴ pairs / s / mW) and bandwidth (about 10 nm), making it difficult to support the application requirements of high-speed, multi-channel quantum information protocols. Furthermore, regarding entangled modes, polarization entanglement, which has a more complex structure and is more directly applied in quantum communication, faces even greater technical challenges in generating, manipulating, and stably outputting it on integrated photonic chips.

[0004] Therefore, how to achieve direct polarization entanglement on an integrated photonic chip while reducing the size of the entangled light source device has become an urgent problem to be solved. Summary of the Invention

[0005] In view of this, embodiments of this application provide an on-chip entangled photonic source based on thin-film lithium niobate, to provide corresponding components that reduce the size of the entangled light source device while realizing direct polarization entanglement on an integrated photonic chip.

[0006] This application provides an on-chip entangled photonic source based on thin-film lithium niobate, including a coupled semiconductor laser and a thin-film lithium niobate photonic chip, wherein the thin-film lithium niobate photonic chip includes an on-chip beam splitter, a first PPLN waveguide, a second PPLN waveguide, and an on-chip beam combiner.

[0007] The optical input end of the on-chip beam splitter is coupled to the optical output end of the semiconductor laser, the first optical output end of the on-chip beam splitter is aligned with the optical input end of the first PPLN waveguide, and the second optical output end of the on-chip beam splitter is aligned with the optical input end of the second PPLN waveguide.

[0008] The first incident end of the on-chip beam combiner is aligned with the light emitting end of the first PPLN waveguide, and the second incident end of the on-chip beam combiner is aligned with the light emitting end of the second PPLN waveguide. The on-chip beam combiner performs polarization conversion on the photon pair entering through the second PPLN waveguide and then entangles it with the photon pair entering through the first PPLN waveguide, outputting an entangled photon pair.

[0009] Optionally, the thin-film lithium niobate photonic chip further includes a first mode converter, which is disposed between the semiconductor laser and the thin-film lithium niobate photonic chip.

[0010] Optionally, the thin-film lithium niobate photonic chip further includes a second mode converter, which is disposed at the output end of the thin-film lithium niobate photonic chip to output the entangled photon pair.

[0011] Optionally, the on-chip beam combiner is a polarization rotation beam combiner, which includes a first transition region and a PRC main region connected together;

[0012] The first transition region serves as the light emitting end aligned with the first PPLN waveguide. The width of the first transition region decreases linearly along the photon propagation direction, and the width of the PRC main region is the same as the width of the end of the first transition region.

[0013] Optionally, the on-chip beam combiner is a polarization rotation beam combiner, which further includes a connected second transition region and a PRC end;

[0014] The second transition region serves as the light emitting end aligned with the front end of the second PPLN waveguide. The width of the second transition region decreases linearly along the photon propagation direction, and the width of the PRC end is the same as that of the end of the second transition region.

[0015] Optionally, the edge spacing between the PRC main area and the PRC end is less than or equal to a preset spacing threshold.

[0016] Optionally, the semiconductor laser is a distributed feedback laser, and the end face of the light emitting end of the distributed feedback laser is coupled to the end face of the light incident end of the on-chip beam splitter.

[0017] Optionally, the distributed feedback laser emits 780 nm horizontally polarized pump light, wherein the side-mode suppression ratio of the pump light is greater than 40 dB and the polarization extinction ratio is greater than 20 dB.

[0018] Optionally, the on-chip beam splitter is a multimode interference beam splitter, which is used to split the incoming light equally into the first PPLN waveguide and the second PPLN waveguide.

[0019] Optionally, both the first PPLN waveguide and the second PPLN waveguide are first-order Type-0 quasi-phase-matched waveguides.

[0020] The advantages of this application compared to the prior art are as follows: The device of this application includes a coupled semiconductor laser and a thin-film lithium niobate photonic chip. The thin-film lithium niobate photonic chip includes an on-chip beamsplitter, a first PPLN waveguide, a second PPLN waveguide, and an on-chip beam combiner. The light incident end of the on-chip beamsplitter is coupled to the light emitting end of the semiconductor laser. The first light emitting end of the on-chip beamsplitter is aligned with the light incident end of the first PPLN waveguide, and the second light emitting end of the on-chip beamsplitter is aligned with the light incident end of the second PPLN waveguide. The first incident end of the on-chip beam combiner is aligned with the light emitting end of the first PPLN waveguide, and the second incident end of the on-chip beam combiner is aligned with the light emitting end of the second PPLN waveguide. The on-chip beam combiner performs polarization conversion on the photon pairs entering through the second PPLN waveguide and then entangles them with the photon pairs entering through the first PPLN waveguide, outputting entangled photon pairs. By forming an on-chip structure using semiconductor lasers and thin-film lithium niobate photonic chips, the device size can be effectively reduced. Furthermore, beam splitting, spontaneous parametric down-conversion, and polarization combining can be achieved within the chip, enabling the direct generation of entangled photon pairs. This reduces the size of the entangled light source device while realizing direct polarization entanglement on an integrated photonic chip. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of an on-chip entangled photon source based on thin-film lithium niobate provided in Embodiment 1 of this application;

[0023] Figure 2 This is a schematic diagram of an on-chip entangled photon source based on thin-film lithium niobate, provided in Embodiment 2 of this application. Detailed Implementation

[0024] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of this application. However, those skilled in the art will understand that this application may also be implemented in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods have been omitted so as not to obscure the description of this application with unnecessary detail.

[0025] It should be understood that, when used in this application specification and the appended claims, the term "comprising" indicates the presence of the described features, integrals, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or a collection thereof.

[0026] It should also be understood that the term “and / or” as used in this application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0027] As used in this application specification and the appended claims, the term "if" may be interpreted, depending on the context, as "when," "once," "in response to determination," or "in response to detection." Similarly, the phrase "if determined" or "if detected [the described condition or event]" may be interpreted, depending on the context, as meaning "once determined," "in response to determination," "once detected [the described condition or event]," or "in response to detection [the described condition or event]."

[0028] Furthermore, in the description of this application and the appended claims, the terms "first," "second," "third," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0029] References to "one embodiment" or "some embodiments" as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in still other embodiments," etc., appearing in different parts of 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 "comprising," "including," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0030] To illustrate the technical solution of this application, specific embodiments are described below.

[0031] like Figure 1The diagram shown is a schematic representation of an on-chip entangled photon source based on thin-film lithium niobate according to Embodiment 1 of this application. The on-chip entangled photon source includes a coupled semiconductor laser and a thin-film lithium niobate photonic chip. The thin-film lithium niobate photonic chip includes an on-chip beamsplitter, a first PPLN waveguide, a second PPLN waveguide, and an on-chip beam combiner. The light incident end of the on-chip beamsplitter is coupled to the light emitting end of the semiconductor laser. The first light emitting end of the on-chip beamsplitter is aligned with the light incident end of the first PPLN waveguide, and the second light emitting end of the on-chip beamsplitter is aligned with the light incident end of the second PPLN waveguide. The first incident end of the on-chip beam combiner is aligned with the light emitting end of the first PPLN waveguide, and the second incident end of the on-chip beam combiner is aligned with the light emitting end of the second PPLN waveguide. The on-chip beam combiner performs polarization conversion on photon pairs entering through the second PPLN waveguide and then entangles them with photon pairs entering through the first PPLN waveguide, outputting entangled photon pairs.

[0032] Among them, the semiconductor laser includes, but is not limited to, a distributed feedback laser. The semiconductor laser is directly coupled to a thin-film lithium niobate photonic chip and performs three functions in a compact area of ​​the thin-film lithium niobate photonic chip: a beam splitter distributes the pump light generated by the laser into two waveguides, and the waveguides generate two sets of SPDC photon pairs with the same characteristics. The two-channel photon pairs are re-merged and polarization state is converted to form a polarization entangled state.

[0033] By hybrid integration of semiconductor lasers and thin-film lithium niobate photonic chips, on-chip light sources can be directly driven via electrical injection, completely eliminating the reliance on bulky external pump lasers. Furthermore, by fully utilizing the superior second-order nonlinear optical properties, flexible domain engineering, and broadband low-loss waveguide advantages of thin-film lithium niobate photonic chips, beam splitting, dual-channel periodically polarized lithium niobate waveguides, and beam combiner structures are designed to directly generate polarization-entangled photons on the chip without post-selection.

[0034] The above requires the SPDC process of the two waveguides to be completely symmetrical, and to achieve polarization reversal and coherent superposition at the output. In this way, the output state naturally forms the following polarization entangled photon pair:

[0035]

[0036] Where α is the amplitude through the first PPLN waveguide channel, and β is the amplitude through the second PPLN waveguide channel. The phase difference is caused by the optical path difference between the two waveguides. A pair of photons that are horizontally polarized. For vertically polarized photon pairs. Of course, α can also be the amplitude through the second PPLN waveguide channel, in which case β is the amplitude through the first PPLN waveguide channel.

[0037] like Figure 2 The diagram shown is a three-dimensional schematic of an on-chip entangled photon source based on thin-film lithium niobate according to Embodiment 2 of this application. The thin-film lithium niobate photonic chip further includes a first mode converter, which is disposed between the semiconductor laser and the thin-film lithium niobate photonic chip. This first mode converter effectively improves the coupling efficiency of the pump light, thereby ensuring the yield of subsequent entangled photon pairs.

[0038] In one implementation, such as Figure 2 As shown, the thin-film lithium niobate photonic chip also includes a second mode converter, which is disposed at the output end of the thin-film lithium niobate photonic chip to output the entangled photon pairs. The inclusion of this first mode converter can effectively reduce the loss of the entangled photon pairs, thereby ensuring the utilization rate of the entangled photon pairs.

[0039] In one embodiment, the on-chip beam combiner is a polarization rotation beam combiner, which includes a first transition region and a PRC main region connected together.

[0040] The first transition region serves as the light emitting end aligned with the first PPLN waveguide. The width of the first transition region decreases linearly along the photon propagation direction, and the width of the PRC main region is the same as the width of the end of the first transition region.

[0041] For example, the first transition region serves as the light emitting end aligned with the first PPLN waveguide. The width of the first transition region is linearly reduced from 1.41 µm to 1.40 µm, and the length of the first transition region is 250 µm. The length of the PRC main region is 6 mm, and the width of the PRC main region is 1.40 µm.

[0042] In one embodiment, the on-chip beam combiner is a polarization rotation beam combiner, which further includes a connected second transition region and a PRC end;

[0043] The second transition region serves as the light emitting end aligned with the front end of the second PPLN waveguide. The width of the second transition region decreases linearly along the photon propagation direction, and the width of the PRC end is the same as that of the end of the second transition region.

[0044] For example, the second transition region serves as the light output end aligned with the second PPLN waveguide. The width of the second transition region gradually decreases from 1.41 µm to 1 µm, and the length of the second transition region is 250 µm. The width of the PRC end decreases from 1 µm to 0.8 µm to achieve polarization rotation. The PRC end is connected to the beam combiner through an S-shaped bend section with a length of 50 µm and a width of 0.8 µm.

[0045] Optionally, the edge spacing between the PRC main region and the PRC end is less than or equal to a preset spacing threshold. This preset spacing threshold can be 0.5 µm, where the PRC end and the PRC main region are connected by a tight coupling region with a 0.5 µm spacing to minimize mode loss.

[0046] In one embodiment, the semiconductor laser is a distributed feedback laser (PPLN), with the output end face of the PPLN coupled to the input end face of the on-chip beam splitter. The PPLN is used as the electrically pumped source, and its output is horizontally polarized single-mode light (approximately 780 nm). The PPLN is connected to the thin-film lithium niobate photonic chip via butt-coupling to ensure efficient mode matching. Furthermore, the laser wavelength can be fine-tuned by temperature to facilitate matching the quasi-phase-matching conditions of the PPLN. Preferably, the PPLN emits 780 nm horizontally polarized pump light, wherein the pump light has a side-mode suppression ratio greater than 40 dB and a polarization extinction ratio greater than 20 dB.

[0047] In one embodiment, the on-chip beamsplitter is a multimode interference beamsplitter, which is used to split the incoming light equally into the first PPLN waveguide and the second PPLN waveguide, thereby obtaining ideal Bell-state entangled photon pairs.

[0048] In one embodiment, the first PPLN waveguide and the second PPLN waveguide are two waveguides with identical parameters. The dual-channel structure must be completely symmetrical in terms of geometry and nonlinear response to avoid phase drift or amplitude imbalance.

[0049] In one embodiment, both the first PPLN waveguide and the second PPLN waveguide are first-order Type-0 quasi-phase-matched waveguides. For example, both the first and second PPLN waveguides are first-order Type-0 quasi-phase-matched waveguides with a width of 1.41 µm, a length of 6 mm, and a period of 4.13 µm, ensuring optical path matching to maintain high entanglement.

[0050] In one embodiment, the center-to-center distance between the first PPLN waveguide and the second PPLN waveguide is 5 µm to maintain mode independence while achieving coherent coupling.

[0051] The aforementioned thin-film lithium niobate photonic chip is an x-cut thin-film lithium niobate with a thickness of 600 nm and a buried layer of 2 µm SiO2.

[0052] After the pump light enters the chip, it is first distributed to two channels with equal power by a 1×2 multimode interference beam splitter. Each channel is a 6 mm long, 4.13 µm period type-0 PPLN waveguide. The center-to-center distance between the two waveguides is about 5 µm to maintain mode independence and achieve coherent coupling. A pair of fundamental mode photons (TE0 mode) are generated in each waveguide through the SPDC process.

[0053] The two photon pairs are recombine in a polarization rotation combiner (PRC) and maintain horizontal polarization after passing through the first PPLN waveguide. The photon pairs in the second PPLN waveguide are converted to vertical polarization via a waveguide width-gradient structure (i.e., the second transition region and the PRC end in the polarization rotation beam combiner). The two ultimately superimpose in the same output waveguide, forming a polarization superposition state. (Horizontal polarization) corresponds to the TE0 mode. (Vertical polarization) corresponds to the TM0 mode.

[0054] When the multimode interferometric beam combiner achieves a 50:50 ratio, the nonlinear processes of the two PPLN waveguides are consistent, and the PRC channel loss is balanced, the maximum polarization entanglement state can be obtained, and the output is as follows:

[0055] This is the ideal Bell state entangled photon pair.

[0056] This structure allows the pump light to undergo Type-0 spontaneous parametric down-conversion (SPDC) in both waveguides, i.e. Pump photons Converted into signal photons With idle photons All three belong to the same polarization mode (such as TE0).

[0057] By uniformly splitting the light after multimode interference, the two waveguides are simultaneously pumped by the same laser source, thus ensuring strict synchronization of the time and frequency characteristics of the photon pairs.

[0058] In addition, the structural composition and dimensional parameters are shown in Table 1 below:

[0059] Table 1

[0060] area Structural Description parameter First transition zone The width was linearly reduced from 1.41 µm to 1.40 µm, and the length was 250 µm. <![CDATA[w1=1.41 µm, w2=1.40 µm, L1=250 µm]]> Second transition zone The width gradually decreases from 1.41 µm to 1.00 µm, with a length of 250 µm, followed by an S-shaped bend section with a length of 50 µm and a width of 1.00 µm. <![CDATA[w3=1.00 µm, L2=50 µm]]> PRC Main Area It has a width of 1.40 µm and a length of 6 mm. <![CDATA[L3=6000 µm]]> PRC end The width was reduced from 1.00 µm to 0.8 µm. <![CDATA[w4=1.0 µm → w5=0.8 µm]]>

[0061] Through this gradient waveguide design, the TE0 mode gradually evolves into the TM0 mode after passing through the second transition region and the PRC end, achieving polarization rotation. The TE0 mode output is maintained through the first transition region and the main region of the PRC, and finally coupled to the same output waveguide with the two waveguides.

[0062] In this application, the beam combiner adopts an adiabatic structure to achieve a smooth TE-TM mode transition, thereby obtaining a working bandwidth of up to 160 nm. All dimensions are optimized based on the electron beam lithography process of the thin-film lithium niobate platform to ensure reproducible manufacturing.

[0063] Design reference table 2 is as follows:

[0064] Table 2

[0065] Module Function Design Concept DFB laser Electric pump light source A single-mode 780 nm laser was used; the wavelength was tuned by temperature control to match the QPM conditions of the PPLN. Multimode interferometric beam splitter Pump light equal division Ensure uniform pumping across both channels to achieve a coherent SPDC process. Dual-channel PPLN waveguide Photon pair generation Type-0 SPDC; dual-channel symmetric design to ensure quantum state consistency. PRC polarization rotating beam combiner Polarization conversion + beam combining <![CDATA[Lower channel TE0→TM0 conversion, upper channel remains TE0; final output formed]]>

[0066] Photon pairs are generated by spatially symmetrical dual PPLN waveguides, and polarization orthogonal superposition is achieved within the same chip using a polarization rotation structure, thereby constructing an electrically pumped, on-chip, integrated polarization entangled light source.

[0067] The above structure allows for the following light source specifications to be obtained at room temperature without the need for cooling: luminance of 4.5 × 10¹ 0 The entangled state has a bandwidth of 73 nm and a fidelity greater than 96% (verified using multiple frequency bands). Quantum state tomography verification confirms that the generated entangled state approximates an ideal Bell state, making it suitable for scenarios such as multi-channel quantum key distribution and satellite quantum communication.

[0068] Compared to optical or electrical pumping using Si3N4 microrings, this application offers a brightness increase of six orders of magnitude and a bandwidth increase from approximately 10 nm to 73 nm. It also supports multi-channel multiplexing and multi-band entanglement, making it suitable for WDM quantum networks. Furthermore, it features a compact structure (approximately 15 × 20 mm²).

[0069] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions 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 this application, and should all be included within the protection scope of this application.

Claims

1. An on-chip entangled photon source based on thin-film lithium niobate, characterized in that, The device includes a coupled semiconductor laser and a thin-film lithium niobate photonic chip, wherein the thin-film lithium niobate photonic chip includes an on-chip beam splitter, a first PPLN waveguide, a second PPLN waveguide, and an on-chip beam combiner. The optical input end of the on-chip beam splitter is coupled to the optical output end of the semiconductor laser, the first optical output end of the on-chip beam splitter is aligned with the optical input end of the first PPLN waveguide, and the second optical output end of the on-chip beam splitter is aligned with the optical input end of the second PPLN waveguide. The first incident end of the on-chip beam combiner is aligned with the light emitting end of the first PPLN waveguide, and the second incident end of the on-chip beam combiner is aligned with the light emitting end of the second PPLN waveguide. The on-chip beam combiner performs polarization conversion on the photon pair entering through the second PPLN waveguide and then entangles it with the photon pair entering through the first PPLN waveguide, outputting an entangled photon pair.

2. The on-chip entangled photon source based on thin-film lithium niobate according to claim 1, characterized in that, The thin-film lithium niobate photonic chip also includes a first mode converter, which is disposed between the semiconductor laser and the thin-film lithium niobate photonic chip.

3. The on-chip entangled photon source based on thin-film lithium niobate according to claim 1, characterized in that, The thin-film lithium niobate photonic chip also includes a second mode converter, which is disposed at the output end of the thin-film lithium niobate photonic chip to output the entangled photon pair.

4. The on-chip entangled photon source based on thin-film lithium niobate according to claim 1, characterized in that, The on-chip beam combiner is a polarization rotating beam combiner, which includes a connected first transition region and a PRC main region. The first transition region serves as the light emitting end aligned with the first PPLN waveguide. The width of the first transition region decreases linearly along the photon propagation direction, and the width of the PRC main region is the same as the width of the end of the first transition region.

5. The on-chip entangled photon source based on thin-film lithium niobate according to claim 4, characterized in that, The on-chip beam combiner is a polarization rotating beam combiner, which also includes a connected second transition region and a PRC end; The second transition region serves as the light emitting end aligned with the front end of the second PPLN waveguide. The width of the second transition region decreases linearly along the photon propagation direction, and the width of the PRC end is the same as that of the end of the second transition region.

6. The on-chip entangled photon source based on thin-film lithium niobate according to claim 1, characterized in that, The edge spacing between the main PRC region and the end of the PRC is less than or equal to a preset spacing threshold.

7. The on-chip entangled photon source based on thin-film lithium niobate according to claim 1, characterized in that, The semiconductor laser is a distributed feedback laser, and the end face of the light emitting end of the distributed feedback laser is coupled to the end face of the light incident end of the on-chip beam splitter.

8. The on-chip entangled photon source based on thin-film lithium niobate according to claim 7, characterized in that, The distributed feedback laser emits 780 nm horizontally polarized pump light, wherein the side-mode suppression ratio of the pump light is greater than 40 dB and the polarization extinction ratio is greater than 20 dB.

9. The on-chip entangled photon source based on thin-film lithium niobate according to claim 1, characterized in that, The on-chip beamsplitter is a multimode interference beamsplitter, which is used to split the incoming light equally into the first PPLN waveguide and the second PPLN waveguide.

10. The on-chip entangled photon source based on thin-film lithium niobate according to any one of claims 1 to 9, characterized in that, Both the first PPLN waveguide and the second PPLN waveguide are first-order Type-0 quasi-phase-matched waveguides.

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