An integrated high-order orbital angular momentum entangled photon source system and its generation method

Abstract

This invention belongs to the field of quantum optics and quantum information processing technology, and discloses an integrated high-order orbital angular momentum entangled photon source system and its generation method. The system includes a pump laser, a polarization preprocessing module, an entanglement generation module, a wavefront and mode control module, and a detection and post-processing module. This design overturns the "post-stage cascaded" mode commonly used in existing technologies (i.e., adding OAM modulation devices to independent, separate modules); by placing OAM modulation inside the interferometer, the optical path is greatly shortened, making the entire device structure more compact and highly integrated. This avoids the complex alignment and high sensitivity to vibration / temperature drift caused by multiple independent modules, fundamentally enhancing the system's robustness and long-term stability. Simultaneously, it allows for arbitrary adjustment of the topological charge, reducing system complexity and potential losses.

Description

Technical Field

[0001] This invention relates to the fields of quantum optics and quantum information processing, and in particular to an integrated high-order orbital angular momentum entangled photon source system and its generation method. Background Technology

[0002] Quantum systems are typically encoded in two-dimensional space (such as the polarization degree of freedom of a photon), meaning that a single photon can only carry one bit of information, resulting in low communication efficiency. One effective way to increase the information carrying capacity is to encode photons in a higher-dimensional space, such as the orbital angular momentum (OAM) space of photons. Therefore, the orbital angular momentum properties of photons have gradually become a research hotspot.

[0003] Orbital angular momentum provides a crucial degree of freedom for photons. Beams carrying orbital angular momentum possess a helical phase wavefront, making them extremely sensitive to phase changes. In 2005, Torner et al. proposed digital helical imaging, which demonstrates that the intensity and phase information of an object are recorded in the orbital angular momentum spectrum of the transmitted or reflected beam; this spectrum can be analyzed to reconstruct object information. In 2013, Padgett's team reported the rotational Doppler effect of photon orbital angular momentum in *Science*: when a beam carrying orbital angular momentum is scattered by the rough surface of a rotating object, its frequency shifts. Detecting this frequency shift allows the calculation of the object's rotational angular velocity. Notably, the magnitude of the frequency shift is proportional to the topological charge of the photon's orbital angular momentum, meaning that increasing the topological charge can improve the accuracy of angular velocity measurements.

[0004] By combining orbital angular momentum entangled photon pairs with the rotational Doppler effect, it is even possible to remotely sense angular rotation under non-local conditions. These orbital angular momentum entangled light sources operate at the single-photon level, belonging to the category of weak light sources, and are particularly suitable for precision measurement scenarios that do not damage samples, such as biological detection, thus possessing significant application value.

[0005] Theoretically, there is no upper limit to the orbital angular momentum of a single photon, but during spontaneous parametric conversion, the weight of higher-order topological charges decreases rapidly with increasing betweenness. Therefore, Zelinger et al. invented a method to convert two-photon polarization-entangled states into higher-order orbital angular momentum-entangled states. This method achieves higher-order orbital angular momentum-entangled states with topological charge numbers as high as 300. However, this method first requires generating polarization entanglement, and then using a Sagnac interferometer to convert it into orbital angular momentum entanglement (e.g., ...). Figure 1The image shown is excerpted from Zelinger's article M. Krenn, J. Handsteiner, M. Fink, R. Fickler, R. Ursin, M. Malik, and A. Zeilinger, Proc. Natl. Acad. Sci. USA 113, 13648 (2016)).

[0006] Although this two-part method for generating high-order orbital angular momentum entangled states can effectively generate high-order (high topological charge) orbital angular momentum entangled states, it suffers from poor mechanical stability and high assembly difficulty due to its two-part structure. Furthermore, many optical components are placed at a relative 45-degree angle or irregular angles, making assembly even more challenging.

[0007] Therefore, it is crucial to realize a compact, stable, and arbitrarily adjustable high-order OAM entangled photon source. Summary of the Invention

[0008] This invention provides an integrated high-order orbital angular momentum entangled photon source system and method to address the problem in the prior art of how to achieve a compact, stable, and arbitrarily adjustable high-order OAM entangled photon source. To provide a basic understanding of some aspects of the disclosed embodiments, a brief summary is given below. This summary is not intended as a general commentary, nor is it intended to identify key / important components or describe the scope of protection of these embodiments. Its sole purpose is to present some concepts in a simple form as a prelude to the detailed description that follows.

[0009] According to a first aspect of the present invention, an integrated high-order orbital angular momentum entangled photon source system is provided.

[0010] In one embodiment, the system includes:

[0011] Pump lasers used to provide pump light for exciting nonlinear processes;

[0012] A polarization preprocessing module used to adjust the polarization state of the pump light and provide specific polarization input for subsequent nonlinear processes;

[0013] An entanglement generation module for generating entangled photon pairs and controlling entanglement properties through spontaneous parametric downconversion or quasi-phase matching of nonlinear crystals;

[0014] A wavefront and mode control module used to finely control the spatial wavefront and polarization mode of photons to achieve quantum state encoding or mode screening;

[0015] And a detection and post-processing module for separating the signal light and pump light residue to realize quantum state detection and analysis.

[0016] Based on the above scheme, the wavefront and mode control module includes:

[0017] Integrated OAM modulator for spatial light modulation;

[0018] A second beam shifter used to combine photon pairs generated by two pump beams in space;

[0019] And a 45° analyzer for screening specific polarization states by projection onto a 45° polarization base.

[0020] Based on the above scheme, the integrated OAM modulation device includes a spatial light modulator for spatial orbital angular momentum encoding of photons and a third half-wave plate for changing the polarization of the upper half-path photon pair so that the upper and lower beams can be spatially overlapped by a second beam shifter.

[0021] The first spatial light modulator is either an SLM or an SPP.

[0022] Based on the above scheme, the integrated OAM modulation device includes:

[0023] A spatial light modulator used for spatial orbital angular momentum encoding of photons;

[0024] A second quarter-wave plate used to change the polarization of photon pairs in the upper and lower halves;

[0025] The third and fourth quarter-wave plates are used to change the polarization of the photon pairs in the upper and lower halves, respectively, so that the circular polarization is changed back to linear polarization in which the polarization directions of the upper and lower halves are perpendicular to each other, so that the upper and lower beams can be spatially overlapped and output by the second beam shifter.

[0026] The second spatial light modulator uses a Q board.

[0027] Based on the above scheme, the polarization preprocessing module includes a first quarter-wave plate for converting the input pump light from non-linear polarization to linear polarization and a first half-wave plate for rotating the polarization direction.

[0028] Based on the above scheme, the entanglement generation module includes a nonlinear crystal for generating photon pairs with the same polarization and a second half-wave plate for further adjusting the photon polarization in conjunction with the nonlinear crystal.

[0029] Based on the above scheme, the nonlinear crystal includes a type I nonlinear crystal or a type O nonlinear crystal.

[0030] Based on the above scheme, the detection and post-processing module includes a dichroic mirror for wavelength-selective spectral splitting and filtering out pump light, and a quantum detection terminal for measuring photon polarization states to verify entanglement or to realize communication or quantum precision measurement.

[0031] According to a second aspect of the present invention, a method for generating an integrated high-order orbital angular momentum entangled photon source based on SLM / SPPSLM / SPP is provided, based on any one of the integrated high-order orbital angular momentum entangled photon source systems described in the present invention, wherein the first spatial light modulator is selected from SLM, SPPSLM, or SPP, and the method includes the following steps:

[0032] S1: Pump light splitting and polarization adjustment: The pump light is first split into two spatially separated beams by the first beam shifter, and the beam intensity ratio is adjusted by the combination of the first 1 / 4 wave plate and the first half wave plate.

[0033] S2: Photon pair generation in a nonlinear crystal: Two identically polarized pump beams are incident on a nonlinear crystal, generating photon pairs that are polarized with or perpendicular to the pump beams.

[0034] S3: OAM modulation: After the downconversion crystal, the two photon pairs are passed through the integrated OAM modulation device. Different holographic patterns are loaded on the upper and lower halves of the spatial light modulator, and different OAM topological charges are applied respectively.

[0035] S4: Path Coincidence and Polarization Erasure: The upper path photon pair is rotated back to the horizontal direction by the third half-wave plate, the two spatially separated photon paths are re-coincided by the second beam shifter, and the polarization information is erased by the 45° analyzer to establish entanglement between higher-order OAM topological charges.

[0036] S5: Photon Pair Separation Output: Using a dichroic mirror, the signal photon and idler photon are separated according to the principle of energy conservation, and the final output is an OAM entangled state with a high topological charge and an adjustable topological charge.

[0037] According to a second aspect of the present invention, a method for generating an integrated high-order orbital angular momentum entangled photon source based on a Q-plate is provided. The method, based on the integrated high-order orbital angular momentum entangled photon source system, wherein the second spatial light modulator is a Q-plate, includes the following steps:

[0038] S1: Pump light splitting and polarization adjustment: The pump light first passes through the first 1 / 4 wave plate and the first half wave plate to adjust the polarization state, and then passes through the first beam shifter to split into two spatially separated beams. The relative light intensity of the two separated beams is determined by the polarization state before passing through the first beam shifter.

[0039] S2: Photon pair generation in a nonlinear crystal: Two identically polarized pump beams are incident on a nonlinear crystal, generating photon pairs that are polarized with or perpendicular to the pump beams.

[0040] S3: OAM modulation: The polarization of the upper and lower photon pairs is converted into circularly polarized light through the second 1 / 4 glass plate. After the downconversion crystal, the two photon pairs are passed through the integrated OAM modulation device. Different patterns are loaded on the upper and lower halves of the second spatial light modulator, and different OAM topological charges are applied respectively.

[0041] S4: Path Coincidence and Polarization Erasure: The polarization of the upper and lower photon pairs is changed by the third and fourth quarter-wave plates, so that the circular polarization is changed back to linear polarization with the polarization directions of the upper and lower paths being perpendicular to each other, so that the upper and lower beams can be spatially coincided and output by the second beam shifter; then the polarization information is erased by the 45° analyzer to establish entanglement between higher-order OAM topological charges;

[0042] S5: Photon Pair Separation Output: Using a dichroic mirror, the signal photon and idler photon are separated according to the principle of energy conservation, and the final output is an OAM entangled state with a high topological charge and an adjustable topological charge.

[0043] The technical solutions provided by the embodiments of the present invention may include the following beneficial effects:

[0044] This invention directly integrates orbital angular momentum (OAM) modulation devices (such as spatial light modulators (SLMs) or Q-plates) into the core device for generating polarization entanglement—the displacement interferometer—specifically located in the optical path after the spontaneous parametric down-conversion (SPDC) crystal. This key design overturns the "post-stage cascade" mode commonly used in existing technologies (i.e., adding OAM modulation devices to independent, separate modules). By placing OAM modulation inside the interferometer, the optical path is significantly shortened, making the entire device structure more compact and highly integrated. This avoids the complex alignment and high sensitivity to vibration / temperature drift caused by multiple independent modules, fundamentally enhancing the system's robustness and long-term stability. Simultaneously, it allows for arbitrary adjustment of the topological charge, reducing system complexity and potential losses.

[0045] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit the invention. Attached Figure Description

[0046] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0047] Figure 1 This is a schematic diagram of a method in the prior art for converting a two-photon polarization entangled state into a higher-order orbital angular momentum entangled state, wherein (a) is the conversion from polarization entanglement to higher-order orbital angular momentum entanglement, and (b) is the specific optical path for converting polarization entanglement into orbital angular momentum entanglement using a non-collinear Sagnac interferometer.

[0048] Figure 2 This is a schematic diagram of an integrated high-order orbital angular momentum entangled photon source system according to an exemplary embodiment (showing that the integrated OAM modulation device is selected from SLM, SPP or SPPSLM).

[0049] Figure 3 This is a schematic diagram of the structure of an integrated high-order orbital angular momentum entangled photon source system according to an exemplary embodiment (showing the Q-board as the integrated OAM modulation device). Detailed Implementation

[0050] The following description and accompanying drawings fully illustrate specific embodiments described herein to enable those skilled in the art to practice them. Some embodiments may include or substitute parts and features of other embodiments. The scope of the embodiments herein encompasses the entire scope of the claims and all available equivalents thereof. Throughout this document, the terms “first,” “second,” etc., are used only to distinguish one element from another without requiring or implying any actual relationship or order between the elements. Indeed, a first element can also be referred to as a second element, and vice versa. Furthermore, the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a structure, apparatus, or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a structure, apparatus, or device. Without further limitation, an element defined by the phrase “comprising one…” does not exclude the presence of other identical elements in the structure, apparatus, or device that includes said element. The various embodiments described herein are presented in a progressive manner, with each embodiment focusing on its differences from other embodiments; similar or identical parts between embodiments can be referred to interchangeably.

[0051] The terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer" used in this document to indicate orientations or positional relationships are based on the orientations or positional relationships shown in the accompanying drawings. They are used solely for the convenience of describing the document and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. In the description herein, unless otherwise specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to mechanical or electrical connections, or internal connections between two elements; they can be direct connections or indirect connections through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0052] In this document, unless otherwise stated, the term "multiple" means two or more.

[0053] In this article, the character " / " indicates that the objects before and after it are in an "or" relationship. For example, A / B means: A or B.

[0054] In this article, the term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0055] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0056] This invention proposes a compact, stable, and arbitrarily adjustable high-order orbital angular momentum (OAM) entangled photon source generation method. Its core architecture is based on a displaced sagnac interferometer platform, directly integrating the OAM modulation device into the core device for polarization entangled photon generation.

[0057] Figure 2 A schematic diagram of an integrated high-order orbital angular momentum entangled photon source system is shown.

[0058] The integrated high-order orbital angular momentum entangled photon source system includes:

[0059] Pump lasers used to provide pump light for exciting nonlinear processes;

[0060] A polarization preprocessing module used to adjust the polarization state of the pump light and provide specific polarization input for subsequent nonlinear processes;

[0061] An entanglement generation module for generating entangled photon pairs and modulating entanglement properties through spontaneous parametric downconversion (SPDC) of nonlinear crystals (including or quasi-phase-matched (QPM) type nonlinear crystals).

[0062] A wavefront and mode control module used to finely control the spatial wavefront and polarization mode of photons to achieve quantum state encoding or mode screening;

[0063] And a detection and post-processing module for separating the signal light and pump light residue to realize quantum state detection and analysis.

[0064] As a specific implementation, the polarization preprocessing module includes a first quarter-wave plate 201 and a first half-wave plate 202 for polarization preprocessing of the pump light.

[0065] Specifically, the first quarter-wave plate 201 converts the input non-linear polarization to linear polarization, and the first half-wave plate 202 is used to rotate the polarization direction. Then, the incident pump light is separated spatially by the first beam shifter 203 (BD) according to the polarization direction. Essentially, this projects the incident polarization onto horizontal and vertical polarization beams respectively. Generally, to obtain the maximum entanglement state, the power of the two pump beams should be the same. Therefore, rotating the first half-wave plate 202 and the first quarter-wave plate 201 to 45 degrees of polarization is sufficient. However, in practice, a single quarter-wave plate or a single half-wave plate can also rotate the polarization to a state where the final projection onto the H-polarization and V-polarization beams each has 50% of the initial light intensity.

[0066] As a specific implementation, the entanglement generation module includes a nonlinear crystal for generating photon pairs with the same polarization, and a second half-wave plate 302 for further adjusting the photon polarization in conjunction with the nonlinear crystal.

[0067] Among them, the nonlinear crystals include type I nonlinear crystal 301 or type O nonlinear crystal 303.

[0068] Specifically, the upper and lower pump beams are vertically polarized. The upper polarized light is rotated by the second half-wave plate 302 to be the same as the lower polarized light. Then, the two pump beams with the same polarization are incident on the type 0 nonlinear crystal 303 (such as ppKTP) or the type I nonlinear crystal 301, respectively, to generate photon pairs with the same polarization as the pump beam (type 0 crystal) or perpendicular polarization (type I crystal).

[0069] The wavefront and mode control module includes an integrated OAM modulator 401 for spatial light modulation, a second beam shifter 402 for combining photon pairs generated by two pump lights in space, and a 45° analyzer 403 for screening specific polarization states by projection onto a 45° polarization base.

[0070] like Figure 2As shown, in a specific embodiment of the integrated OAM modulator 401, for an SLM, SPPSLM, or SPP device, the integrated OAM modulator 401 (SLM, SPPSLM, or SPP) includes a first spatial light modulator 401-1 for spatial orbital angular momentum encoding of photons and a third half-wave plate 401-2 for changing the polarization of the upper half-path photon pair so that the upper and lower beams can be spatially overlapped and output (beam combining) by a second beam shifter 402. Specifically, after the down-conversion crystal (i.e., after the photon pair is generated), the two photon pairs are passed through the integrated OAM modulator 401 respectively. For the first spatial light modulator 401-1 (SLM) and the first quarter-wave plate 201: different holographic patterns are loaded on their upper and lower halves, respectively, and different OAM topological charges (+1 and -1, or +2 and -2 or higher-order topological charges) are applied to the photons of the upper and lower paths; wherein, the photon pairs of the upper path are then rotated back to the horizontal direction (perpendicular to the polarization of the photons of the lower path) by the third half-wave plate 401-2. The spatially separated upper and lower photon paths are re-overlapped by the beam deflector 402, and the overlapped light passes through the 45° analyzer 403 to erase the polarization information and establish entanglement between higher-order OAM topological charges.

[0071] For the Q-plate device, since a typical Q-plate can only incident circularly polarized light, the optical path is slightly different: as another specific implementation of the integrated OAM modulation device, the integrated OAM modulation device 401 includes a second 1 / 4 glass plate 401-3 for changing the polarization of the upper and lower photon pairs to circularly polarized light, so that the upper and lower beams can be orbitally angular momentum modulated by a second spatial light modulator 401-4 that only works on circularly polarized orbital angular momentum encoding.

[0072] After passing through the Q-plate, the upper and lower paths are loaded with different orbital angular momentum, but their polarization is the same: circular polarization. By using a third quarter-wave plate 401-5 and a fourth quarter-wave plate 401-6 placed at two different angles in the upper and lower paths, the polarization of the lower path is rotated to vertical polarization, and the polarization of the upper path is rotated to horizontal polarization. The spatially separated upper and lower photon paths are re-overlapped by the second beam shifter 402. The overlapped light passes through a 45° analyzer 403 to erase the polarization information and establish entanglement between higher-order OAM topological charges.

[0073] As a specific implementation, the detection and post-processing module includes a dichroic mirror 501 for wavelength-selective spectral splitting and filtering out the pump light, and a quantum detection terminal 502 for measuring the polarization state of photons to verify entanglement or to achieve communication or quantum precision measurement. Specifically, using the dichroic mirror 501 (with a center wavelength approximately twice the pump light wavelength) or other dichroic devices, the signal photon and idler photon in the entangled photon pair are separated and output according to the principle of energy conservation.

[0074] The aforementioned system employs a displacement interferometer based on a nonlinear crystal (such as ppKTP or ppLN) as a platform for generating polarization-entangled photon pairs (such as Bell states). More importantly, OAM modulation devices (such as SLMs or Q-plates) are integrated directly within the displacement interferometer, in the optical path following the downconversion crystal (SPDC crystal). (α-BBO is a common beam displacer; a combination of two beam displacers can be called a displacement interferometer. The crystal used to generate downconversion photon pairs is ppKTP as shown in the figure; ppLN is also commonly placed in the middle of the displacement interferometer.) Subsequent optical components include beam displacers for path reconciliation, analyzers for erasing polarization information (such as those placed at 45°), and dichroic devices (such as dichroic mirrors) that separate signal photons and idler photons based on energy conservation.

[0075] Based on the above system, this invention also proposes an integrated high-order orbital angular momentum entangled photon source generation method based on SLM / SPPSLM / SPP.

[0076] S1: Pump Light Splitting and Polarization Adjustment: The pump light (e.g., 405nm) is first split into two spatially separated beams by a beam shifter (BD). The intensity ratio of the split beams is adjusted by a combination of a first half-wave plate (HWP) and a first quarter-wave plate (QWP). The output polarization of the upper layer is inherently V. For the first beam shifter 203, the polarization directly output and the polarization output after a certain offset distance are perpendicular to each other. The polarization of the lower layer pump light is rotated from horizontal (H) to vertical (V).

[0077] S2: Photon pair generation in nonlinear crystals: Two identically polarized pump beams are incident on a type 0 or type I nonlinear crystal (such as ppKTP) respectively, generating photon pairs that are polarized in the same way (type 0) or perpendicular to the pump beams (type I).

[0078] S3: OAM modulation: After the downconversion crystal, the two photon pairs are passed through an integrated OAM modulation device (SLM, SPPSLM, or spp). Different holographic patterns are loaded on the upper and lower halves of the SLM board, and different OAM topological charges (+l and -l) are applied respectively. This integration position is the core technical means to solve the problems of poor stability and redundant optical paths.

[0079] Specifically, a spatial light modulator (SLM) is a device that modulates the spatial distribution of light waves. Under the control of an electrically driven signal, it changes the amplitude or intensity, phase, and polarization state of the light distribution in space. SLMs are key devices in modern optics fields such as real-time optical information processing, adaptive optics, and optical computing. The principle is primarily based on the birefringence of liquid crystal molecules to modulate the phase of the incident light beam. By changing the voltage applied to the liquid crystal pixel molecules (a material with many pixels, like a display screen), different angles are created between the liquid crystal molecules and the electric field. This angle between the director of the liquid crystal molecules and the polarization direction of the incident light alters the effective refractive index of the liquid crystal, thus changing the optical path length and achieving phase modulation.

[0080] Spiral phase plate (SPP) creates different orbital angular momentum by etching different patterns onto the surface of a transparent material using an embossing process. Different patterns can also be used to achieve different orbital angular momentum for the upper and lower parts.

[0081] S4: Path Coincidence and Polarization Erasure: The upper path photon pair is rotated back to the horizontal direction by a half-wave plate (when using the q version, different 1 / 4 wave plates are used to rotate the upper and lower paths to horizontal and vertical polarization, respectively). The two spatially separated photon paths are re-coincided by the second beam shifter 402, and the polarization information is erased by the 45° analyzer 403 to establish entanglement between higher-order OAM topological charges.

[0082] S5: Photon Pair Separation Output: Using a dichroic mirror, the signal photon and idler photon are separated according to the principle of energy conservation, and the final output is an OAM entangled state with a high topological charge and an adjustable topological charge.

[0083] Based on the above system, this invention also proposes an integrated high-order orbital angular momentum entangled photon source generation method based on Q-plate.

[0084] S1: Pump light splitting and polarization adjustment: The pump light first passes through the first 1 / 4 wave plate 201 and the first half wave plate 202 to adjust the polarization state, and then passes through the first beam shifter 203 to split into two spatially separated beams. The relative light intensity of the two separated beams is determined by the polarization state before passing through the first beam shifter 203.

[0085] S2: Photon pair generation in a nonlinear crystal: Two identically polarized pump beams are incident on a nonlinear crystal, generating photon pairs that are polarized with or perpendicular to the pump beams.

[0086] S3: OAM modulation: The polarization of the upper and lower photon pairs is converted into circularly polarized light through the second 1 / 4 glass plate 401-3. After the downconversion crystal, the two photon pairs are passed through the integrated OAM modulation device 401. Different patterns are loaded on the upper and lower halves of the second spatial light modulator 401-4, and different OAM topological charges are applied respectively.

[0087] Q-plates (vortex retarders) are made of liquid crystal polymers (LCP). In the LCP layer, the fast axis orientation of the liquid crystal molecules is consistent radially along the substrate and gradually changes angularly. They have the same λ / 2 retardation across the entire device plane, making them single-wavelength devices. Vortex retarders possess polarization-dependent optical properties. Depending on the polarization state of the incident beam, they can be used to generate vector-polarized beams or vortex beams with a helical phase wavefront. However, the incident polarization of vortex retarders is generally circular, so a quarter-wave plate is added between the crystal and the Q-plate to convert the two incident linearly polarized photons into circularly polarized photons. After passing through the Q-plate, mutually perpendicular quarter-wave plates are added to the two paths, converting the upper half of the light from circular polarization to V-polarization and the lower half to H-polarization.

[0088] S4: Path Coincidence and Polarization Erasure: The polarization of the upper and lower photon pairs is changed by the third quarter-wave plate 401-5 and the fourth quarter-wave plate 401-6, so that the circular polarization is changed back to linear polarization with the polarization directions of the upper and lower paths being perpendicular to each other, so that the upper and lower beams can be spatially coincided and output by the second beam shifter 402; then the polarization information is erased by the 45° analyzer 403, and entanglement between higher-order OAM topological charges is established;

[0089] S5: Photon pair separation output: Using a dichroic mirror 501, the signal photon and idler photon are separated according to the principle of energy conservation, and the final output is an OAM entangled state with a high topological charge and an arbitrarily adjustable topological charge.

[0090] This invention provides a specific implementation of an integrated high-order orbital angular momentum entangled photon source system and method.

[0091] Core architecture:

[0092] A displacement interferometer based on nonlinear crystals (such as ppKTP, ppLN) is used as a platform for generating polarization entangled photon pairs (such as Bell states).

[0093] Key innovation: The OAM modulation device (such as an SLM or Q board, sqq) is integrated directly inside the displacement interferometer, in the optical path after the downconversion crystal (SPDC crystal). This is a fundamental difference from existing technologies.

[0094] Subsequent optical components include beam shifters for path reconciliation, analyzers for erasing polarization information (e.g., placed at 45°), and dichroic devices (e.g., dichroic mirrors) that separate signal photons and idler photons based on energy conservation.

[0095] Workflow and OAM modulation implementation:

[0096] The pump light (the appropriate pump wavelength is selected according to the wavelength of the downconversion photons required by the application) is first split into two spatially separated beams by a beam shifter 203 (using birefringence effect). The beam splitting distance is on the order of hundreds of micrometers to millimeters, and the polarization of the two beams is perpendicular to each other. The beam splitting intensity ratio can be adjusted by a combination of waveplates (HWP - half-wave plate and quarter-wave plate).

[0097] The lower pump light, initially horizontally polarized (labeled H), is rotated to the vertical direction (labeled V) by a half-wave plate. It is assumed that the upper pump light is initially vertically polarized. Here, it is assumed that the nonlinear crystal used only reacts with pump light in the vertically polarized direction. Alternatively, a nonlinear crystal that only reacts with pump light in the horizontally polarized direction can be used. In this case, the polarization of both the upper and lower pump lights should be horizontally polarized.

[0098] Two pump beams with the same polarization, one upper and one lower, are incident on a type 0 or type I nonlinear crystal (such as ppKTP) to generate photon pairs with the same polarization as the pump beam (type 0 crystal) or perpendicular polarization (type I crystal).

[0099] The key step of this invention is as follows: After the down-conversion crystal (i.e., after the photon pair is generated), the two photon pairs are passed through an integrated OAM modulation device. Different patterns are loaded / set on the upper and lower halves of the OAM modulation device (such as an SLM / Q board / SPP board), applying different topological charges (e.g., +l and -l) to the photons in the upper / lower paths of the interferometer. By loading different patterns onto the SLM in real time, or by replacing the Q board or SPP board, the topological charge (l value) of the output entangled state can be easily adjusted.

[0100] For SLM, Q board, spp: different patterns are loaded / etched on the upper and lower halves, respectively, and different OAM topological charges (+l and -l, or +2 and -2 or higher order topological charges) are applied to the photons of the upper and lower paths.

[0101] The photon pairs from the upper path are then rotated back to the horizontal direction (perpendicular to the polarization of the photons from the upper path) by another half-wave plate.

[0102] When using a Q plate, the upper and lower layers are connected by a quarter-wave plate, so that the upper layer is horizontally polarized and the lower layer is vertically polarized.

[0103] The spatially separated upper and lower photon paths are re-aligned using another beam shifter.

[0104] The superimposed light passes through a 45° analyzer to erase polarization information and establish entanglement between higher-order OAM topological charges.

[0105] Finally, using a dichroic mirror 501 (with a center wavelength approximately twice that of the pump light) or other color separation devices, the signal photon and idler photon in the entangled photon pair are separated and output according to the principle of energy conservation.

[0106] Final output: Obtain an orbital angular momentum entangled state with a high topological charge (large l value) and an arbitrarily adjustable topological charge (by changing the SLM loading pattern or selecting different Q plates).

[0107] In summary, the core difference between this invention and existing technologies lies in directly integrating the orbital angular momentum (OAM) modulation device (such as a spatial light modulator (SLM) or Q-plate) into the core device for generating polarization entanglement—the displacement interferometer—specifically located in the optical path after the spontaneous parametric down-conversion (SPDC) crystal. This key design overturns the existing technology's commonly used "post-stage cascaded independent OAM module" model (i.e., adding OAM modulation devices to independent, separate modules). It has the following advantages:

[0108] 1. Significantly improved compactness and stability: By placing OAM modulation inside the interferometer, the optical path is greatly shortened, making the entire device more compact and highly integrated. At the same time, it avoids the complex alignment and high sensitivity to vibration / temperature drift caused by multi-level independent modules, fundamentally enhancing the system's robustness and long-term stability.

[0109] 2. Achieve Arbitrary Adjustment of Topological Charge: By replacing the Q-plate or sqq-plate, the required high-order OAM topological charge (l-value) can be conveniently and quickly set and changed in experiments. Since the optical path is perpendicular to the Q-plate, component replacement is simple, unlike existing solutions where optical elements that change orbital angular momentum when light is incident at a small angle make it difficult to precisely set this angle when replacing components. Furthermore, the optical loss of the Q-plate and / or sqq-plate is lower than that of SLM, and the price is cheaper.

[0110] 3. Reduced system complexity and potential losses: Single-platform integration simplifies the optical path structure, significantly reducing the difficulty and workload of installation, debugging, and alignment. Compared to existing technologies that require cascading multiple independent optical components, this solution reduces the number of components, potentially lowering insertion loss and thus improving the collection efficiency of entangled photon pairs.

[0111] This invention is not limited to the structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of this invention is limited only by the appended claims.

Claims

1. An integrated high-order orbital angular momentum entangled photon source system, characterized in that, Comprise: a pump laser for providing pump light to excite nonlinear process; a polarization pre-processing module for adjusting the polarization state of pump light to provide specific polarization input for subsequent nonlinear process; an entanglement generation module for generating entangled photon pairs by spontaneous parametric down-conversion through nonlinear crystal and regulating entanglement characteristics; a wavefront and mode regulation module for fine regulation of spatial wavefront and polarization mode of photons to achieve quantum state encoding or mode filtering; and a detection and post-processing module for separating signal light from residual pump light to achieve quantum state detection and analysis; The incident pump light is split into two spatially separated beams by a first beam displacer (203), and the two beams of pump light with the same polarization are incident into the nonlinear crystal to generate photon pairs with the same or perpendicular polarization as the pump light. Then, the two down-converted photon pairs pass through the integrated OAM modulation device (401), and the photon pairs from the upper path recombine the two spatially separated paths by a second beam displacer (402).

2. The integrated high-order orbital angular momentum entangled photon source system of claim 1, wherein, The wavefront and mode regulation module comprises: an integrated OAM modulation device (401) for spatial light modulation; a second beam displacer (402) for combining the photon pairs generated by the two pump lights in space; and a 45° polarizer (403) for projecting and filtering specific polarization states at 45° polarization basis.

3. The integrated high-order orbital angular momentum entangled photon source system of claim 2, wherein, The integrated OAM modulation device (401) comprises a first spatial light modulator (401-1) for spatial orbital angular momentum encoding of photons and a third half-wave plate (401-2) for changing the polarization of the upper half of the photon pairs so that the upper and lower beams can be spatially recombined by the second beam displacer (402). The first spatial light modulator (401-1) is selected from SLM or SPP.

4. The integrated high-order orbital angular momentum entangled photon source system of claim 1, wherein, The polarization pre-processing module comprises a first 1 / 4 wave plate (201) for converting the input pump light from non-linear polarization to linear polarization and a first half-wave plate (202) for rotating the polarization direction.

5. The integrated high-order orbital angular momentum entangled photon source system of claim 1, wherein, The entanglement generation module comprises a nonlinear crystal for generating photon pairs with the same polarization and a second half-wave plate (302) for further adjusting the polarization of photons in cooperation with the nonlinear crystal.

6. The integrated high-order orbital angular momentum entangled photon source system of claim 5, wherein, The nonlinear crystal comprises a type I nonlinear crystal (301) or a type 0 nonlinear crystal (303).

7. The integrated high-order orbital angular momentum entangled photon source system of claim 1, wherein, The detection and post-processing module comprises a dichroic mirror (501) for wavelength-selective light splitting and filtering out pump light and a quantum detection terminal (502) for measuring the polarization state of photons to verify entanglement or achieve communication or quantum precision measurement.

8. The integrated high-order orbital angular momentum entangled photon source system of claim 2, wherein, The integrated OAM modulation device (401) comprises: a second 1 / 4 wave plate (401-3) for changing the polarization of the upper and lower photon pairs to circularly polarized light, so that the upper and lower beams can be subjected to orbital angular momentum modulation by a second spatial light modulator (401-4) that only acts on circular polarization; a second spatial light modulator (401-4) for spatial orbital angular momentum encoding of photons; Third 1 / 4 wave plate (401-5) and fourth 1 / 4 wave plate (401-6) are respectively used to change the polarization of the upper and lower two paths of photon pairs, so that the polarization changes from circular polarization to linear polarization, and the upper and lower two paths of photon pairs can be output in space coincidence in the second beam displacer (402); Wherein, the second spatial light modulator (401-4) is selected as a Q plate. 9.A method for generating an integrated high-order orbital angular momentum entangled photon source based on SLM / SPP SLM / SPP, characterized in that, The integrated high-order orbital angular momentum entangled photon source system according to any one of claims 1-7, wherein the first spatial light modulator (401-1) is selected as an SLM or an SPPSLM or an SPP, comprising the following steps: S1: Pump light splitting and polarization adjustment: the pump light first passes through the first 1 / 4 wave plate (201) and the first half wave plate (202) to adjust the polarization state, and then passes through the first beam displacer (203) to split into two spatially separated beams, the relative light intensity of the two separated beams being determined by the polarization state before passing through the first beam displacer (203); S2: Photon pair generation in nonlinear crystal: two beams of pump light with the same polarization are incident into the nonlinear crystal to generate photon pairs with the same or perpendicular polarization as the pump light; S3: OAM modulation: after the down-conversion crystal, the two paths of photon pairs pass through the integrated OAM modulation device (401), the upper half and the lower half of the first spatial light modulator (401-1) are loaded with different holographic patterns, and different OAM topological charges are respectively applied; S4: Path coincidence and polarization erasure: the upper path of photon pairs passes through the third half wave plate (401-2) to rotate the polarization back to the horizontal direction, the second beam displacer (402) is used to recombine the two spatially separated paths of photon pairs, and the 45° polarizer (403) is used to erase the polarization information to establish the entanglement between high-order OAM topological charges; S5: Photon pair separation and output: the dichroic mirror (501) is used to separate the signal photons and the idler photons according to the principle of energy conservation, and finally the OAM entangled state with high topological charge and adjustable topological charge is output. 10.A method for generating a Q-plate based integrated high-order orbital angular momentum entangled photon source, the method comprising: The integrated high-order orbital angular momentum entangled photon source system according to any one of claims 8, wherein the second spatial light modulator (401-4) is selected as a Q plate, comprising the following steps: S1: Pump light splitting and polarization adjustment: the pump light first passes through the first 1 / 4 wave plate (201) and the first half wave plate (202) to adjust the polarization state, and then passes through the first beam displacer (203) to split into two spatially separated beams, the relative light intensity of the two separated beams being determined by the polarization state before passing through the first beam displacer (203); S2: Photon pair generation in nonlinear crystal: two beams of pump light with the same polarization are incident into the nonlinear crystal to generate photon pairs with the same or perpendicular polarization as the pump light; S3: OAM modulation: the polarization of the upper and lower two paths of photon pairs is converted into circularly polarized light by the second 1 / 4 wave plate (401-3), and after the down-conversion crystal, the two paths of photon pairs pass through the integrated OAM modulation device (401), the upper half and the lower half of the second spatial light modulator (401-4) are loaded with different patterns, and different OAM topological charges are respectively applied; S4: Path superposition and polarization erasure: The polarization of the upper and lower two photon pairs is changed by the third 1 / 4 wave plate (401-5) and the fourth 1 / 4 wave plate (401-6), so that the circular polarization changes back to linear polarization, and the polarization directions of the upper and lower two paths are perpendicular to each other, so that the upper and lower two beams can be superposed in space by the second beam displacer (402); then the polarization information is erased by the 45° polarizer (403), and the entanglement between the high-order OAM topological charges is established; S5: Photon pair separation output: Use dichroic mirror (501) to separate signal photons and idler photons according to the principle of energy conservation, finally output OAM entangled state with high topological charge number and adjustable topological charge number.

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