Polarization entangled photon pair generation method and device based on hybrid architecture

Abstract

The invention relates to a polarization entangled photon pair, in particular to a polarization entangled photon pair generation method and device based on a mixed framework, and the method comprises the steps: bidirectional excitation: a dual-wavelength polarization beam splitter (PBS) divides laser into two beams of light which are horizontally and vertically polarized, and the two beams of light are transmitted in the clockwise CW direction and the anticlockwise CCW direction of a Sagnac interference ring respectively; photon pair generation: the two beams of light are coupled into a waveguide, and in the waveguide, horizontal polarized light propagating along the clockwise CW direction and vertical polarized light propagating along the anticlockwise CCW direction respectively generate a pair of photon pairs; quantum interference and entanglement generation: after the two paths of photon pairs are output from the waveguide, the two paths of photon pairs converge at a dual-wavelength polarization beam splitter (PBS) again, the amplitudes of the two paths of photon pairs are coherently superposed by regulating and controlling the phase in a Sagnac interference ring, and an ideal polarization entanglement singlet state is generated at two output ports of the dual-wavelength polarization beam splitter (PBS); according to the invention, the defect that high brightness, high efficiency, high entanglement fidelity, high spectral purity and high photon indistinguishability cannot be synchronously realized can be effectively overcome.

Description

Technical Field

[0001] This invention relates to polarization entangled photon pairs, and more specifically to a method and apparatus for generating polarization entangled photon pairs based on a hybrid architecture. Background Technology

[0002] Polarization-entangled photon pairs are a core resource for quantum information science experiments and applications. Their performance mainly depends on key parameters such as brightness, entanglement fidelity, system efficiency (e.g., Klyshko efficiency), spectral purity, and photon indistinguishability. Currently, high-performance entangled sources are mainly based on the following two technical approaches, but both have inherent drawbacks that are difficult to overcome:

[0003] 1) Free-space optical entanglement sources: These are based on bulk nonlinear crystals (such as PPKTP, BBO, etc.) combined with interference structures (such as Sagnac interferometers, Mach-Zehnder interferometers, etc.). Their advantages are high entanglement fidelity (>99%) and high single-photon collection efficiency (>70%). However, their fundamental drawback lies in the inherent physical trade-off between brightness and single-mode fiber coupling efficiency: high brightness requires tight focusing of the pump light to enhance nonlinear interaction, but this results in downconverted photons having complex spatial modes, making it difficult to efficiently couple into single-mode fibers; conversely, adopting a weak focusing scheme to obtain high coupling efficiency will severely sacrifice brightness. This contradiction has led to the performance limit of such entanglement sources being fully explored, especially in the communication band where high brightness is difficult to achieve.

[0004] 2) Integrated optical entangled sources: Based on waveguides (such as PPLN, AlGaAs waveguides, etc.) or microstructured optical fibers, their core advantage lies in the strong optical field confinement and single-mode characteristics of the waveguide. Theoretically, they can resolve the contradiction between brightness and coupling efficiency and achieve extremely high brightness (several orders of magnitude higher than bulk optical sources). However, such entangled sources have serious shortcomings:

[0005] The system efficiency is extremely low: due to waveguide mode and fiber mode mismatch, insertion loss of integrated devices, etc., its end-to-end Klyshko efficiency is usually less than 5%;

[0006] Entanglement quality is limited: Waveguide end-face reflection and fabrication inhomogeneity introduce distinguishing information about photon paths, significantly degrading entanglement fidelity and photon indistinguishability;

[0007] The spectrum engineering is complex: It is extremely difficult to achieve both high spectral purity and high-dimensional entanglement (such as polarization entanglement) on an integrated platform.

[0008] In summary, there is a lack of a method and apparatus in the field that can simultaneously achieve high brightness, high efficiency, high entanglement fidelity, high spectral purity and high photon indistinguishability in generating polarized entangled photon pairs. This has become a bottleneck in the development of practical multiphoton quantum technology and quantum networks. Summary of the Invention

[0009] (a) Technical problems to be solved

[0010] In view of the above-mentioned shortcomings of the prior art, the present invention provides a method and apparatus for generating polarization entangled photon pairs based on a hybrid architecture, which can effectively overcome the defects of the prior art that cannot simultaneously achieve high brightness, high efficiency, high entanglement fidelity, high spectral purity and high photon indistinguishability.

[0011] (II) Technical Solution

[0012] To achieve the above objectives, the present invention provides the following technical solution:

[0013] A hybrid architecture-based method for generating polarization entangled photon pairs includes the following steps:

[0014] S1, Pump Injection: After spatial mode cleaning by pulsed pump laser, the polarization angle of the laser is set and it is incident on the dual-wavelength polarization beam splitter PBS;

[0015] S2, Bidirectional Excitation: The dual-wavelength polarization beam splitter PBS splits the laser into two beams with horizontal and vertical polarization, which propagate along the clockwise CW and counterclockwise CCW directions of the Sagnac interference ring, respectively.

[0016] S3. Photon pair generation: Two beams of light are coupled into a waveguide. Inside the waveguide, the horizontally polarized light propagating in the clockwise CW direction and the vertically polarized light propagating in the counterclockwise CCW direction generate a pair of photons respectively.

[0017] S4. Quantum Interference and Entanglement Generation: After the two photon pairs are output from the waveguide, they rejoin at the dual-wavelength polarization beam splitter PBS. By adjusting the phase within the Sagnac interference ring, the amplitudes of the two photon pairs are coherently superimposed, and an ideal polarization entangled singlet state is generated at the two output ports of the dual-wavelength polarization beam splitter PBS.

[0018] S5. Filtering and Detection: The polarization-entangled singlet states are filtered and then subjected to polarization analysis. Finally, a coincidence measurement is performed by a single-photon detector.

[0019] Preferably, in S3, photon pair generation occurs as follows: two beams of light are coupled into a waveguide. Within the waveguide, the horizontally polarized light propagating in the clockwise (CW) direction and the vertically polarized light propagating in the counterclockwise (CCW) direction each generate a photon pair, including:

[0020] Two beams of light are coupled into a waveguide through a coupling lens. Within the waveguide, a type II phase-matched parametric down-conversion process generates horizontally polarized light propagating in the clockwise CW direction. Photon pairs are generated from vertically polarized light propagating in the counterclockwise CCW direction. Photon pairs;

[0021] in, This represents a polarization-entangled photon pair where the signal photon s is horizontally polarized and the idle photon i is vertically polarized. This represents a polarization-entangled photon pair where the signal photon s is vertically polarized and the idle photon i is horizontally polarized.

[0022] Preferably, the waveguide is a periodically polarized potassium titanate phosphate (PPKTP) crystal waveguide. Through group velocity matching (GVM) design, the group velocity of the pump light is positioned precisely between that of the signal light and the idle light. This ensures that the pump's spectral width naturally aligns with the down-conversion phase-matching bandwidth, directly generating spectrally uncorrelated photon pairs without the need for subsequent narrowband filtering. This guarantees high spectral purity from the source and avoids efficiency losses associated with filtering, ensuring:

[0023] Spatial mode determinism: All downconversion photons are forced to be emitted into the same spatial mode;

[0024] Decoupling brightness and efficiency: Completely breaks the trade-off between brightness and coupling efficiency in bulk optics;

[0025] Eliminating spatial-spectral coupling allows spectral purity to be optimized independently of the spatial mode.

[0026] Preferably, residual reflection at the waveguide end face is the main factor leading to entanglement degradation. The reflected photons enter the wrong path and polarization state, directly reducing the entanglement fidelity. By setting a proprietary ion-assisted deposition broadband antireflection film at both end faces of the waveguide, the end face reflectivity at the working wavelength is suppressed to an extremely low level.

[0027] Preferably, in S4, quantum interference and entanglement generation: after the two photon pairs are output from the waveguide, they rejoin at the dual-wavelength polarization beam splitter PBS. By adjusting the phase within the Sagnac interference ring, the amplitudes of the two photon pairs are coherently superimposed, and an ideal polarization-entangled singlet state is generated at the two output ports of the dual-wavelength polarization beam splitter PBS, including:

[0028] After the two photon pairs are output from the waveguide, they rejoin at the dual-wavelength polarization beamsplitter (PBS). By adjusting the phase compensator (SBC) within the Sagnac interferometer ring, the amplitudes of the two photon pairs are coherently superimposed, generating ideal polarization-entangled singlet states at the two output ports of the PBS. :

[0029] ;

[0030] Among them, polarization entangled singlet states This represents a pair of polarized entangled photons.

[0031] Preferably, the bidirectional paths of the Sagnac interferometer ring will have minute small frequency differences due to the wavelength / polarization dependence of the optical elements, which destroys the indistinguishability of photons. By using polarization-resolved joint spectral intensity (JSI) measurement, the spectral interference fringes caused by the relative time delay and chirp between paths can be directly observed by measuring the joint spectrum under the diagonal basis, thereby accurately quantifying the path distinguishability.

[0032] Preferably, in S5, filtering and detection: the polarization-entangled singlet states are filtered and then subjected to polarization analysis, and finally, a coincidence measurement is performed by a single-photon detector, including:

[0033] After the polarization-entangled singlet states are filtered to remove the pump and fluorescence background, they enter the polarization analysis unit and are finally measured by the superconducting nanowire single-photon detector (SNSPD).

[0034] A hybrid architecture-based device for generating polarization entangled photon pairs includes a pump laser source, a mode cleaning and polarization control unit, a free-space Sagnac interference ring, a coupling lens, a waveguide, a filter array, a polarization analysis unit, and a single-photon detector.

[0035] A pump laser source is used to generate pulsed pump laser light.

[0036] The mode cleaning and polarization control unit includes a single-mode fiber and a half-wave plate (HWP). The single-mode fiber performs spatial mode cleaning on the pulsed pump laser, and the half-wave plate (HWP) sets the polarization angle of the laser.

[0037] The free-space Sagnac interferometer ring includes a dual-wavelength polarization beam splitter (PBS) and a phase compensator (SBC). The PBS splits the laser into two beams with horizontal and vertical polarization, which propagate along the clockwise (CW) and counterclockwise (CCW) directions of the Sagnac interferometer ring, respectively, and enter the waveguide through a coupling lens. The SBC modulates the phase within the Sagnac interferometer ring, causing the amplitudes of the two photon pairs output from the waveguide to coherently superimpose, and generating an ideal polarization entangled singlet state at the two output ports of the PBS.

[0038] A waveguide is placed on a Sagnac interference ring so that horizontally polarized light propagating in the clockwise CW direction and vertically polarized light propagating in the counterclockwise CCW direction each generate a pair of photons.

[0039] A filter array is used to filter polarization-entangled singlet states;

[0040] The polarization analysis unit, including a quarter-wave plate, a half-wave plate (HWP), and a polarization beam splitter (PBS), performs polarization analysis on the filtered polarization entangled singlet state.

[0041] A single-photon detector is used to perform coincidence measurements on filtered polarization-entangled singlet states.

[0042] Preferably, the waveguide is a periodically polarized potassium titanate phosphate (PPKTP) crystal waveguide. Through group velocity matching (GVM) design, the group velocity of the pump light is positioned precisely between that of the signal light and the idle light. This ensures that the pump's spectral width naturally aligns with the down-conversion phase-matching bandwidth, directly generating spectrally uncorrelated photon pairs without the need for subsequent narrowband filtering. This guarantees high spectral purity from the source and avoids efficiency losses associated with filtering, ensuring:

[0043] Spatial mode determinism: All downconversion photons are forced to be emitted into the same spatial mode;

[0044] Decoupling brightness and efficiency: Completely breaks the trade-off between brightness and coupling efficiency in bulk optics;

[0045] Eliminating space-spectrum coupling: This allows spectral purity to be optimized independently of the spatial mode;

[0046] The residual reflection at the waveguide end face is the main factor leading to entanglement degradation. The reflected photons enter the wrong path and polarization state, directly reducing the entanglement fidelity. By setting a proprietary ion-assisted deposition broadband antireflection film on the two end faces of the waveguide, the end face reflectivity at the working wavelength is suppressed to an extremely low level.

[0047] Preferably, the bidirectional paths of the Sagnac interferometer ring will have minute small frequency differences due to the wavelength / polarization dependence of the optical elements, which destroys the indistinguishability of photons. By using polarization-resolved joint spectral intensity (JSI) measurement, the spectral interference fringes caused by the relative time delay and chirp between paths can be directly observed by measuring the joint spectrum under the diagonal basis, thereby accurately quantifying the path distinguishability.

[0048] (III) Beneficial Effects

[0049] Compared with the prior art, the method and apparatus for generating polarization entangled photon pairs based on a hybrid architecture provided by the present invention have the following advantages:

[0050] 1) Comprehensive performance breakthrough: For the first time, high brightness (>3.5×10⁻⁶) is achieved simultaneously in the same device. 6 With high Klyshko efficiency (>43%), high entanglement fidelity (>95%), high spectral purity (>98%), and high HOM visibility (>82%), it successfully breaks the long-standing performance trade-off triangle.

[0051] 2) Excellent energy efficiency: It can operate with low pump power in the microwatt range and produce high brightness output, which greatly reduces system power consumption and the requirements for pump lasers, and improves practicality;

[0052] 3) Intrinsic high-quality photon generation: Through waveguide group velocity matching design, spectrally pure photons are generated directly from the source, avoiding the efficiency loss of more than 50% caused by subsequent filtering, and achieving both purity and efficiency.

[0053] 4) Extremely high system robustness and ease of use: The single-mode nature of the waveguide greatly reduces the sensitivity of the Sagnac interferometer ring to mechanical vibration and temperature drift, resulting in greater system alignment tolerance, better long-term stability, and easier operation and maintenance for non-professionals.

[0054] 5) Strong technical scalability: This hybrid architecture points to a clear path for future performance improvements. For example, by further optimizing the end-face coating (which can achieve entanglement fidelity of 98.8%) and improving the waveguide-fiber coupling design (such as using mode field adapters), it is expected to achieve entanglement fidelity greater than 99% and Klyshko efficiency greater than 80% in the future. Attached Figure Description

[0055] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0056] Figure 1 This is a schematic diagram of the process of the present invention;

[0057] Figure 2 This is a schematic diagram illustrating the relationship between the waveguide end-face reflectivity and the maximum achievable entanglement fidelity in this invention.

[0058] Figure 3 This is a schematic diagram of the process for diagnosing the indistinguishability of the polarization-resolved joint spectral intensity JSI measurement in this invention.

[0059] Figure 4 This is a schematic diagram of the joint spectral intensity (JSI) measured by a time-of-flight spectrometer in this invention;

[0060] Figure 5 This is a schematic diagram of the HOM interference curve in this invention;

[0061] Figure 6 This is a schematic diagram illustrating the relationship between single-channel counting rate, coincidence counting rate, and pump power in this invention.

[0062] Figure 7 This is a schematic diagram showing the relationship between the Klyshko efficiency of the signal light and idle light and the pump power in this invention.

[0063] Figure 8 This is a schematic diagram of the coincidence count rate interference curves under the two measurement basis vectors, the linear basis and the diagonal basis, in this invention;

[0064] Figure 9 This is a schematic diagram of the real part of the two-qubit density matrix reconstructed by the overcomplete quantum state tomography technique in this invention;

[0065] Figure 10 This is a schematic diagram of the imaginary part of the two-qubit density matrix reconstructed using supercomplete quantum state tomography in the invention. Detailed Implementation

[0066] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0067] The core of this invention lies in creatively combining the inherent high brightness, deterministic single-mode output, and intrinsic spectrum generation capabilities of "integrated waveguides" with the flexible and efficient interference and low-loss coupling advantages of "free-space optics." Through system-level collaborative design, it fundamentally breaks the performance trade-offs in traditional solutions and achieves synchronous optimization of all key indicators.

[0068] The following describes the specific process of the hybrid architecture-based polarization entangled photon pair generation method provided by this invention, using concrete examples (e.g.) Figure 1 (as shown) and technical effects.

[0069] S1, Pump Injection: After spatial mode cleaning with a pulsed pump laser (770nm), the polarization angle of the laser is set (45°) and incident on the dual-wavelength polarization beam splitter PBS.

[0070] S2. Bidirectional excitation: The dual-wavelength polarization beam splitter PBS splits the laser into two beams with horizontal and vertical polarization, which propagate along the clockwise CW and counterclockwise CCW directions of the Sagnac interference ring (the core component of this device, which not only provides inherent phase stability but also serves as a platform for quantum interference and polarization entanglement generation).

[0071] S3. Photon Pair Generation: Two beams of light are coupled into a waveguide. Within the waveguide, the horizontally polarized light propagating in the clockwise (CW) direction and the vertically polarized light propagating in the counterclockwise (CCW) direction each generate a photon pair, including:

[0072] Two beams of light are coupled into the waveguide through a coupling lens (an achromatic lens with a focal length of 6mm). Within the waveguide, a type II phase-matched parametric down-conversion process generates horizontally polarized light propagating in the clockwise CW direction. Photon pairs are generated from vertically polarized light propagating in the counterclockwise CCW direction. Photon pairs;

[0073] in, This represents a polarization-entangled photon pair where the signal photon s is horizontally polarized and the idle photon i is vertically polarized. This represents a polarization-entangled photon pair where the signal photon s is vertically polarized and the idle photon i is horizontally polarized.

[0074] In this application's technical solution, the waveguide employs a periodically polarized potassium titanate phosphate (PPKTP) crystal waveguide. Through group velocity matching (GVM) design, the group velocity of the pump light is precisely positioned between that of the signal light and the idle light. This ensures that the pump's spectral width naturally aligns with the downconversion's phase-matching bandwidth, thereby directly generating spectrally uncorrelated (i.e., pure-state) photon pairs. No subsequent narrowband filtering is required, guaranteeing high spectral purity from the source and avoiding efficiency losses caused by filtering. This ensures:

[0075] Spatial mode determinism: All downconversion photons are forced to be emitted into the same spatial mode;

[0076] Decoupling brightness and efficiency: Completely breaks the trade-off between brightness and coupling efficiency in bulk optics;

[0077] Eliminating spatial-spectral coupling allows spectral purity to be optimized independently of the spatial mode.

[0078] Residual reflection at the waveguide end face is the main factor leading to entanglement degradation. Reflected photons enter incorrect paths and polarization states, directly reducing entanglement fidelity (e.g., ...). Figure 2 As shown, by setting proprietary ion-assisted deposition broadband antireflection films on the two end faces of the waveguide, the end face reflectivity at the working wavelength (such as around 1550nm) is suppressed to an extremely low level.

[0079] For the optimized waveguide, its end-face reflectivity can be less than 0.06% (close to the measurement limit), theoretically supporting entanglement fidelity of over 99.9%.

[0080] Figure 2The figure shows a schematic diagram of the relationship between the waveguide end-face reflectivity and the maximum achievable entanglement fidelity in this invention. As shown in the figure, when the waveguide end-face reflectivity is less than 0.1%, the entanglement fidelity can exceed 99%, highlighting the importance of the waveguide end-face coating technology used in this invention.

[0081] S4. Quantum Interference and Entanglement Generation: After the two photon pairs are output from the waveguide, they rejoin at the dual-wavelength polarization beamsplitter (PBS). By adjusting the phase within the Sagnac interference ring, the amplitudes of the two photon pairs are coherently superimposed, generating ideal polarization-entangled singlet states at the two output ports of the PBS, including:

[0082] After the two photon pairs are output from the waveguide, they rejoin at the dual-wavelength polarization beamsplitter (PBS). By adjusting the phase compensator SBC (using a Soleil-Babinet compensator) within the Sagnac interferometer ring, the amplitudes of the two photon pairs are coherently superimposed, generating ideal polarization-entangled singlet states at the two output ports of the PBS. :

[0083] ;

[0084] Among them, polarization entangled singlet states This represents a pair of polarized entangled photons.

[0085] In this application's technical solution, the bidirectional path of the Sagnac interferometer ring exhibits minute microfrequency differences due to the wavelength / polarization dependence of the optical elements, disrupting photon indistinguishability. Therefore, a polarization-resolved joint spectral intensity (JSI) measurement is employed (e.g., Figure 3 As shown in the figure, by measuring the joint spectrum under the diagonal basis vectors, the spectral interference fringes caused by the relative time delay and chirp between paths can be observed intuitively, thereby accurately quantifying the path discrimination.

[0086] Based on the above diagnosis, waveguides with higher uniformity were selected and fabricated, and the dispersion matching of optical components was optimized to ensure that the bidirectional paths of the Sagnac interferometer ring are completely indistinguishable in the time and frequency domains.

[0087] S5. Filtering and Detection: After filtering, the polarization-entangled singlet states undergo polarization analysis, and finally, a coincidence measurement is performed by a single-photon detector, including:

[0088] After the polarization-entangled singlet states are filtered to remove the pump and fluorescence background, they enter the polarization analysis unit and are finally measured by the superconducting nanowire single-photon detector (SNSPD).

[0089] The following experimental data will be used to verify the superior overall performance achieved by this invention.

[0090] Figure 4This is a schematic diagram of the joint spectral intensity (JSI) measured by a time-of-flight spectrometer in this invention. Its shape is approximately a single peak and highly symmetrical, indicating that the signal light and idle light are uncorrelated in the spectrum. The calculated upper limit of its spectral purity is as high as 98%.

[0091] Figure 5 The diagram shows the HOM interference curve in this invention. By measuring the change in the quadruple coincidence count through the half-wave plate (HWP) of the polarization analysis unit, the HOM visibility without background subtraction is (82±2)%, which directly proves that the photons generated from this source have a high degree of indistinguishability, which is the key to the success of the multiphoton interference experiment.

[0092] Figure 4 and Figure 5 Proof: This invention successfully combines the two characteristics of high spectral purity and high photon indistinguishability, which are usually difficult to achieve simultaneously. This makes the invention not only an excellent entanglement source, but also a high-quality single-photon source, capable of meeting the stringent requirements of today's most advanced multiphoton quantum information experiments.

[0093] Figure 6 This diagram illustrates the relationship between single-channel count rate, coincidence count rate, and pump power in this invention. The luminance, calculated from the slope, reaches as high as (3.5 ± 0.1) × 10⁻⁶. 6 The power output per pair (s·mW) is 1 to 3 orders of magnitude higher than most bulk optical sources, and requires only microwatt-level pump power.

[0094] Figure 7 This diagram illustrates the relationship between the Klyshko efficiency of the signal light and idle light and the pump power in this invention. In the intermediate power range, the average Klyshko efficiencies are (38.0±0.5)% and (46.8±1.3)%, respectively, averaging about 43%. This efficiency value is much higher than that of existing integrated optical entangled sources (<5%), and even better than many high-brightness bulk optical sources, proving the high efficiency of this invention in photon collection and transmission.

[0095] Figure 8 This is a schematic diagram of the coincidence count rate interference curves under the two measurement basis vectors (H / V and D / A) in this invention. The visibility reaches (96.0±0.1)% and (94.3±0.1)% respectively, which is direct evidence of high-quality entanglement.

[0096] Figure 8 middle:

[0097] Alice0°, Alice45°, Alice90°, Alice135°: These indicate the fixed angles of the polarization analyzer for the optical path (conventionally called Alice) that transmits the "signal light".

[0098] Figure 9 , Figure 10 These figures show the real and imaginary parts of the two-qubit density matrix reconstructed using ultracomplete quantum state tomography in this invention, respectively. The entanglement fidelity between the matrix and the ideal singlet state reaches (95.78±0.04)%. These data collectively demonstrate that the polarization-entangled photon pairs generated by this invention have extremely high purity, fully meeting the requirements of the most stringent quantum information processing applications.

[0099] Figure 9 , Figure 10 middle:

[0100] HH, HV, VH, VV: These two axes together define the basis vectors of the density matrix, representing the four possible polarization combinations of the signal light (Alice) and the idle light (Bob):

[0101] HH: Both are horizontally polarized;

[0102] HV: Signal light is horizontally polarized, and idle light is vertically polarized;

[0103] VH: Signal light is vertically polarized, and idle light is horizontally polarized;

[0104] VV: Both are vertically polarized.

[0105] Combination Figures 8-10 This is the "final verdict" proving that the present invention has successfully generated high-quality polarization entangled photon pairs:

[0106] 1) High entanglement fidelity: The shape of the density matrix is ​​almost identical to that of the ideal singlet state, and the calculated entanglement fidelity is as high as 95.78%, far exceeding the limit that any classical system can simulate;

[0107] 2) High purity: Only the HV and VH components dominate the density matrix and have correct coherence, indicating that the generated state is the desired pure entangled state, rather than a mixed state;

[0108] 3) Practicality: Combining Figure 8 The high coincidence count rate demonstrates that the present invention can not only generate "high-quality" entanglement, but also "abundant" entanglement, fully meeting all needs from basic physics testing to quantum information applications.

[0109] Based on the above-disclosed method for generating polarization entangled photon pairs based on a hybrid architecture, this application also discloses a device for generating polarization entangled photon pairs based on a hybrid architecture, including a pump laser source, a mode cleaning and polarization control unit, a free-space Sagnac interference ring, a coupling lens, a waveguide, a filter array, a polarization analysis unit, and a single-photon detector.

[0110] A pump laser source is used to generate a pulsed pump laser (770nm).

[0111] The mode cleaning and polarization control unit includes a single-mode fiber and a half-wave plate (HWP). The single-mode fiber performs spatial mode cleaning on the pulsed pump laser, and the half-wave plate (HWP) sets the polarization angle of the laser (45°).

[0112] The free-space Sagnac interferometer ring (the core component of this device, which not only provides inherent phase stability but also serves as a platform for quantum interference and polarization entanglement generation) includes a dual-wavelength polarization beam splitter (PBS) and a phase compensator (SBC) (using a Soleil-Babinet compensator). The PBS splits the laser into two beams, one horizontally polarized and the other vertically polarized, which propagate along the clockwise (CW) and counterclockwise (CCW) directions of the Sagnac interferometer ring, respectively, and enter the waveguide through a coupling lens (using an achromatic lens with a focal length of 6 mm). The SBC modulates the phase within the Sagnac interferometer ring, causing the amplitudes of the two photon pairs output from the waveguide to coherently superimpose, and generating ideal polarization entangled singlet states at the two output ports of the PBS.

[0113] A waveguide is placed on a Sagnac interference ring so that horizontally polarized light propagating in the clockwise CW direction and vertically polarized light propagating in the counterclockwise CCW direction each generate a pair of photons.

[0114] A filter array is used to filter polarization-entangled singlet states;

[0115] The polarization analysis unit, including a quarter-wave plate, a half-wave plate (HWP), and a polarization beam splitter (PBS), performs polarization analysis on the filtered polarization entangled singlet state.

[0116] A single-photon detector is used to perform coincidence measurements on filtered polarization-entangled singlet states.

[0117] Specifically, the waveguide employs a periodically polarized potassium titanyl phosphate (PPKTP) crystal waveguide. Through group velocity matching (GVM) design, the group velocity of the pump light is positioned precisely between that of the signal light and the idler light. This ensures that the pump's spectral width naturally aligns with the down-conversion phase-matching bandwidth, directly generating spectrally uncorrelated photon pairs without the need for subsequent narrowband filtering. This guarantees high spectral purity from the source and avoids efficiency losses associated with filtering, ensuring:

[0118] Spatial mode determinism: All downconversion photons are forced to be emitted into the same spatial mode;

[0119] Decoupling brightness and efficiency: Completely breaks the trade-off between brightness and coupling efficiency in bulk optics;

[0120] Eliminating spatial-spectral coupling allows spectral purity to be optimized independently of the spatial mode.

[0121] Specifically, residual reflection at the waveguide end face is the main factor leading to entanglement degradation. Reflected photons enter the wrong path and polarization state, directly reducing entanglement fidelity. By setting proprietary ion-assisted deposition broadband antireflection films at both end faces of the waveguide, the end face reflectivity at the working wavelength is suppressed to an extremely low level.

[0122] Specifically, the bidirectional paths of the Sagnac interferometer ring will have minute small frequency differences due to the wavelength / polarization dependence of the optical elements, which will destroy the indistinguishability of photons. By using polarization-resolved joint spectral intensity (JSI) measurement, the spectral interference fringes caused by the relative time delay and chirp between paths can be directly observed by measuring the joint spectrum under the diagonal basis, thereby accurately quantifying the path discrimination.

[0123] To better illustrate the technical solution of this application, the following detailed explanation is provided in conjunction with two specific experiments.

[0124] Experiment 1: Performance Experiment of Polarization Entangled Photon Pair Generation Device

[0125] • Hardware configuration:

[0126] o-waveguide: 9mm long, single-mode, PPKTP crystal, type II phase matching, end face reflectivity approximately 2%;

[0127] o Pump laser source: 770nm pulsed pump laser, bandwidth 1.8nm;

[0128] o Interference structure: Free-space Sagnac interferometer ring, equipped with dual-wavelength polarization beam splitter PBS and phase compensator SBC;

[0129] o Detector: Superconducting nanowire single-photon detector (SNSPD).

[0130] •Experimental Results:

[0131] Entanglement fidelity: (95.78±0.04)%

[0132] oHOM visibility: (82±2)%%

[0133] Brightness: (3.5±0.1)×10 6 pairs / (s·mW);

[0134] oKlyshko efficiency: signal light (38.0±0.5)%, idle light (46.8±1.3)%;

[0135] o Spectral purity: >98% (measured using polarization-resolved joint spectral intensity JSI).

[0136] Experiment 2: Optimization Effect Experiment of Waveguide Endface Coating Technology

[0137] • Hardware configuration: Same as Experiment 1, but from a different batch. The key improvement is that the antireflection coating on the waveguide end face has better performance, with a reflectivity of <0.06%, but the waveguide coupling loss is slightly higher.

[0138] •Experimental Results:

[0139] Entanglement fidelity: (98.82±0.05)%

[0140] oHOM visibility: (89±2)%%

[0141] Brightness: (5.6±0.2)×10 6 pairs / (s·mW);

[0142] oKlyshko efficiency: signal light (26.3±0.3)%, idle light (28.2±0.5)%.

[0143] • Experimental conclusions: This strongly demonstrates that reducing the reflectivity of the waveguide end face is one of the most critical technical means to improve entanglement quality (including entanglement fidelity and HOM visibility). At the same time, it also shows that coupling efficiency is a technical parameter that can be optimized independently of entanglement quality, providing a clear direction for subsequent continuous improvement.

[0144] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention 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 will not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for generating polarization-entangled photon pairs based on a hybrid architecture, characterized in that: The method comprises the following steps: S1, pumping injection: after the spatial mode cleaning of the pulsed pump laser, the polarization angle of the laser is set and incident into a dual-wavelength polarization beam splitter PBS; S2, bidirectional excitation: the dual-wavelength polarization beam splitter PBS divides the laser into two beams of horizontally and vertically polarized light, which propagate along the clockwise CW and counterclockwise CCW directions of the Sagnac interference ring respectively; S3, photon pair generation: the two beams of light are coupled into the waveguide, and in the waveguide, the horizontally polarized light propagating along the clockwise CW direction and the vertically polarized light propagating along the counterclockwise CCW direction generate a pair of photon pairs respectively; S4, quantum interference and entanglement generation: after the two photon pairs are output from the waveguide, they are converged again at the dual-wavelength polarization beam splitter PBS, the phase in the Sagnac interference ring is adjusted, the amplitudes of the two photon pairs are coherently superimposed, and an ideal polarization entangled single state is generated at the two output ports of the dual-wavelength polarization beam splitter PBS; S5, filtering and detection: after the polarization entangled single state is filtered, polarization analysis is performed, and finally coincidence measurement is performed by a single-photon detector. 2.The hybrid-architecture-based method for generating polarization-entangled photon pairs according to claim 1, wherein: In S3, photon pair generation: the two beams of light are coupled into the waveguide, and in the waveguide, the horizontally polarized light propagating along the clockwise CW direction and the vertically polarized light propagating along the counterclockwise CCW direction generate a pair of photon pairs, including: The two beams of light are coupled into the waveguide by a coupling lens, and within the waveguide, the horizontally polarized light propagating in the clockwise (CW) direction generates photon pairs by a type-II phase-matched parametric down-conversion process and the vertically polarized light propagating in the counter-clockwise (CCW) direction generates photon pairs ​ wherein, denotes a polarization-entangled photon pair with the signal photon s being horizontally polarized and the idler photon i being vertically polarized, denotes a polarization-entangled photon pair with the signal photon s being vertically polarized and the idler photon i being horizontally polarized. 3.The hybrid-architecture-based method for generating polarization-entangled photon pairs according to claim 2, wherein: The waveguide adopts a periodically poled potassium titanyl phosphate PPKTP crystal waveguide, and group velocity matching GVM design is adopted to make the group velocity of the pump light exactly between the signal light and the idle light, so that the spectral width of the pump and the phase matching bandwidth of the down-conversion are naturally aligned, thereby directly generating spectrally uncorrelated photon pairs without subsequent narrowband filtering, ensuring: Spatial mode certainty: all down-converted photons are forced to emit into the same spatial mode; Decoupling brightness and efficiency: the trade-off between brightness and coupling efficiency in bulk optics is completely broken; Eliminate spatial-spectral coupling: the spectral purity can be optimized independently of the spatial mode.

4. The method for generating polarization entangled photon pairs based on a hybrid architecture according to claim 3, characterized in that: The residual reflection of the waveguide end face is the main factor causing entanglement degradation, the reflected photons enter the wrong path and polarization state, directly reducing the entanglement fidelity, by setting a special ion-assisted deposition broadband antireflection film on the two end faces of the waveguide, the end face reflectivity of the working wavelength is suppressed to a very low level. 5.The hybrid-architecture-based method for generating polarization-entangled photon pairs according to claim 1, wherein: In S4, quantum interference and entanglement generation: after the two photon pairs are output from the waveguide, they are converged again at the dual-wavelength polarization beam splitter PBS, the phase in the Sagnac interference ring is adjusted, the amplitudes of the two photon pairs are coherently superimposed, and an ideal polarization entangled single state is generated at the two output ports of the dual-wavelength polarization beam splitter PBS, including: After the two-photon pairs are output from the waveguide, they are converged again at the dual-wavelength polarization beam splitter PBS, the phase compensator SBC in the Sagnac interference loop is adjusted to make the amplitude of the two-photon pairs coherent superposition, and an ideal polarization entangled single state is generated at the two output ports of the dual-wavelength polarization beam splitter PBS : ； where the polarization entangled singlet state denotes a pair of polarization entangled photons.

6. The hybrid-architecture-based method for generating polarization-entangled photon pairs according to claim 5, wherein: The bidirectional path of the Sagnac interference ring has a small frequency difference due to the wavelength / polarization-dependent characteristics of the optical elements, which destroys the indistinguishability of the photons, and a polarization-resolved joint spectral intensity JSI measurement is adopted, by measuring the joint spectrum under the diagonal basis vector, the spectral interference fringes caused by the relative time delay and chirp between the paths can be directly observed, so as to accurately quantify the path distinguishability.

7. The hybrid architecture based method for generation of polarization-entangled photon pairs as claimed in claim 1 wherein: Filtering and detection in S5: the polarization entangled single state is filtered and then analyzed by polarization, and finally measured by a single photon detector, including: After the polarization entangled single state is filtered by the filter set to remove the pump and fluorescence background, it enters the polarization analysis unit, and finally the coincidence measurement is performed by the superconducting nanowire single photon detector SNSPD.

8. A hybrid-architecture-based polarization-entangled photon pair generation apparatus for performing the hybrid-architecture-based polarization-entangled photon pair generation method of claim 1, characterized by: It includes a pump laser source, a mode cleaning and polarization control unit, a free space Sagnac interference ring, a coupling lens, a waveguide, a filter set, a polarization analysis unit and a single photon detector; The pump laser source generates pulsed pump laser; The mode cleaning and polarization control unit includes a single mode fiber and a half wave plate HWP, the single mode fiber performs spatial mode cleaning on the pulsed pump laser, and the half wave plate HWP sets the polarization angle of the laser; The free space Sagnac interference ring includes a dual-wavelength polarization beam splitter PBS and a phase compensator SBC, the dual-wavelength polarization beam splitter PBS divides the laser into two beams of horizontally and vertically polarized light, which propagate along the clockwise CW and counterclockwise CCW directions of the Sagnac interference ring respectively, and enter the waveguide through the coupling lens, and the phase compensator SBC adjusts the phase in the Sagnac interference ring to make the amplitudes of the two photon pairs output by the waveguide superimpose coherently, and generate ideal polarization entangled single states at the two output ports of the dual-wavelength polarization beam splitter PBS; The waveguide is arranged on the Sagnac interference ring, so that the horizontally polarized light propagating along the clockwise CW direction and the vertically polarized light propagating along the counterclockwise CCW direction generate a pair of photons respectively; The filter set filters the polarization entangled single state; The polarization analysis unit includes a quarter wave plate, a half wave plate HWP and a polarization beam splitter PBS, which analyzes the polarization of the filtered polarization entangled single state; The single photon detector measures the coincidence of the filtered polarization entangled single state.

9. The hybrid architecture based polarization-entangled photon pair generation apparatus according to claim 8, wherein: The waveguide uses a periodic polarization PPKTP crystal waveguide, which is designed by group velocity matching GVM to make the group velocity of the pump light exactly between the signal light and the idle light, so that the spectral width of the pump is naturally aligned with the phase matching bandwidth of the down-conversion, thereby directly generating spectrally uncorrelated photon pairs without subsequent narrowband filtering, ensuring: Spatial mode certainty: all down-converted photons are forced to emit into the same spatial mode; Decoupling brightness and efficiency: breaking the trade-off between brightness and coupling efficiency in bulk optics; Eliminate spatial-spectral coupling: make spectral purity independent of spatial mode optimization; The residual reflection of the waveguide end face is the main factor causing entanglement degradation, reflected photons enter the wrong path and polarization state, directly reducing the entanglement fidelity, by setting a special ion-assisted deposition broadband antireflection film on both ends of the waveguide, the end face reflectivity of the working wavelength is suppressed to a very low level.

10. The hybrid architecture based polarization-entangled photon pair generation apparatus according to claim 8, wherein: The bidirectional path of the Sagnac interference ring has a micro frequency difference due to the wavelength / polarization dependent characteristics of the optical element, which destroys the photon indistinguishability. By using a polarization-resolved joint spectrum intensity JSI measurement, by measuring the joint spectrum under the diagonal base vector, the spectrum interference fringes caused by the relative time delay and the chirp between the paths can be directly observed, so as to accurately quantify the path distinguishability.

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