Fully collinear polarization entangled light source system

Abstract

The invention relates to a fully collinear polarization entangled light source system, which realizes collinear series layout of all optical elements along a single optical axis by splitting and combining pump light by adopting a light beam shifter, and does not need precise alignment of non-collinear angles in structure, thereby simplifying the assembly and debugging process of the system and improving the production efficiency. The sensitivity to mechanical vibration is reduced, and the long-term operation stability of the system is enhanced. Meanwhile, the collinear layout effectively compresses the length of an optical path, the overall physical size of the light source system is effectively reduced, and a feasible technical approach is provided for achieving compact, modularized and automatic batch production of the entangled light source.

Description

Technical Field

[0001] This application relates to the field of quantum optics technology, and in particular to a fully collinear polarization entangled light source system. Background Technology

[0002] In existing technologies, entangled light sources are a core resource for quantum technology, playing an indispensable role in many cutting-edge branches of quantum information science. In the field of quantum computing, high-quality entangled light sources are the foundation for building distributed quantum computing networks and realizing the distribution of entangled qubits over long distances. In the field of quantum precision measurement, entangled photon pairs can break through the standard quantum limit set by shot noise, and their ultra-high sensitivity has been successfully applied to cutting-edge technologies such as quantum radar, quantum imaging, and gravitational wave detection, effectively improving the ability to detect weak signals. In the field of quantum communication, the nonlocality of entangled photon pairs is the physical foundation for realizing quantum key distribution (QKD) and ensuring unconditional security of communication.

[0003] Currently, two-photon polarization entangled light sources are mainly based on the Sagnac interferometer structure. These sources are highly adaptable, compatible with photon pairs generated by type 0, type I, and type II spontaneous parametric down-conversion crystals. However, despite the widespread success of Sagnac interferometer-based entangled light sources in academic research, their inherent structure suffers from several insurmountable drawbacks: First, the traditional Sagnac architecture requires the optical path to form a closed loop in space, which involves the non-collinear arrangement of multiple optical components such as mirrors, dichroic mirrors, and polarizing beam splitters. In particular, several key components need to be precisely installed and aligned at a 45° angle, resulting in a complex optical path structure, a long optical path, and a large amount of space consumption.

[0004] Secondly, the open, long optical path composed of multiple discrete components is extremely sensitive to mechanical vibrations and changes in ambient temperature. Any minute mechanical jitter or thermal drift can cause a change in the optical path difference between the two optical paths within the loop, leading to relative phase instability in the entangled states and severely reducing the fidelity and quality of entangled photon pairs. This sensitivity to environmental disturbances poses a serious challenge to the reliability and long-term stability of the light source in practical applications outside the laboratory.

[0005] Finally, due to its complex, non-collinear layout, the construction of the Sagnac light source requires experienced technicians to manually align multiple optical components with precision in multiple degrees of freedom. The entire process is time-consuming and labor-intensive, and consistency is difficult to guarantee. This highly manual assembly method greatly hinders the large-scale, low-cost production of entangled light sources, making it difficult to meet the future demands of quantum technology industrialization for core components. Summary of the Invention

[0006] To address the aforementioned issues, this application provides a compact and highly stable fully collinear polarization entangled light source system.

[0007] To achieve the above objectives, the fully collinear polarization entangled light source system designed in this application includes: A pump light source, used to provide a pump beam; A first beam shifter is configured to receive the pump beam and split the pump beam into a first optical path and a second optical path that are parallel to each other and propagate in the same direction according to its polarization state. A nonlinear crystal, disposed in the first and second optical paths, is used to generate photon pairs through a spontaneous parametric downconversion process. A polarization control element is disposed in the first optical path and / or the second optical path to control the polarization state of a photon pair, such that the polarization state of the photon pair output from the first optical path is orthogonal to the polarization state of the photon pair output from the second optical path. The second beam shifter is configured to combine photons from the first and second optical paths into a single output beam.

[0008] Preferably, the first beam shifter, the nonlinear crystal, the polarization control element, and the second beam shifter are arranged collinearly along the propagation direction of the pump beam.

[0009] Preferably, the optical surfaces of the first beam shifter, the nonlinear crystal, the polarization control element, and the second beam shifter are all perpendicular to the propagation direction of the pump beam.

[0010] Preferably, the first beam shifter, the nonlinear crystal, the polarization control element, and the second beam shifter are bonded together with optical adhesive to form an integrated optical component.

[0011] Preferably, the polarization control element includes: a first half-wave plate disposed in the first optical path and located between the first beam shifter and the nonlinear crystal, for unifying the polarization state of the pump light in the first and second optical paths; and a second half-wave plate disposed in the first optical path and located between the nonlinear crystal and the second beam shifter, for making the polarization state of the photon pair generated by the first optical path orthogonal to the polarization state of the photon pair generated by the second optical path.

[0012] Preferably, the nonlinear crystal is a periodically polarized crystal.

[0013] Preferably, it further includes a filter disposed downstream of the second beam shifter for filtering out residual pump beam.

[0014] Preferably, the filter further includes a phase compensator disposed downstream of the filter, the phase compensator being used to adjust the relative phase between the first optical path photon pair and the second optical path photon pair in the beam filtered by the filter.

[0015] Preferably, it further includes a wavelength division multiplexer for spatially separating photon pairs in the output beam according to wavelength.

[0016] Preferably, it further includes an input polarization preparation unit, which is disposed between the pump light source and the first beam shifter, and is used to adjust the polarization direction of the pump beam to a preset angle.

[0017] The fully collinear polarization entangled light source system designed in this application achieves a collinear series layout of all optical elements along a single optical axis by using a beam shifter to split and combine the pump light. This structure eliminates the need for precise alignment at non-collinear angles, simplifying the system's assembly and debugging process, reducing sensitivity to mechanical vibration, and enhancing long-term operational stability. Simultaneously, the collinear layout effectively compresses the optical path length, significantly reducing the overall physical size of the light source system, providing a feasible technical approach for achieving compact, modular, and automated mass production of entangled light sources. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structure of the fully collinear polarization entangled light source system provided in the embodiments of this application.

[0019] Figure 2 This is a schematic diagram of the structure of a fully collinear polarization entangled light source system provided in another embodiment of this application.

[0020] Among them: pump beam 10, first beam shifter 20, first optical path 30, second optical path 40, nonlinear crystal 50, polarization control element 60, first half-wave plate 61, second half-wave plate 62, second beam shifter 70, filter 80, phase compensator 90, wavelength division multiplexer 100, input polarization preparation unit 110, first polarization-maintaining fiber collimator 120, and fiber coupling device 130. Detailed Implementation

[0021] The preferred embodiments of this application are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit this application.

[0022] This embodiment describes a fully collinear polarization entanglement light source system, designed to replace the traditional, complex Sagnac ring interferometer based on a fully collinear translational interferometer structure. Please refer to... Figure 1The system, along the beam propagation direction, includes, in sequence, a pump source, a first beam shifter 20, a nonlinear crystal 50, a polarization control element 60, and a second beam shifter 70.

[0023] Specifically, the pump source is used to provide a pump beam 10 with stable linear polarization. In one specific implementation, the pump source can be a narrow-linewidth laser with a center wavelength of 780 nm, which guides the beam through a single-mode polarization-maintaining fiber and converts the diverging beam in the fiber into a collimated free-space beam through a first polarization-maintaining fiber collimator 120, so as to ensure that the output pump beam 10 has an extremely high polarization extinction ratio and a stable polarization direction, thereby laying the foundation for subsequent precise polarization manipulation.

[0024] In a specific example, such as Figure 1 As shown, it also includes an input polarization preparation unit 110. In this embodiment, the input polarization preparation unit 110 is specifically a half-wave plate (HWP) disposed downstream of the polarization-maintaining fiber collimator 120. This unit is used to precisely rotate the polarization direction of the input linearly polarized pump beam 10 to a preset angle, such as 45°, so that the subsequent first beam shifter 20 can distribute the energy of the pump beam 10 to two orthogonal polarization components with a preset intensity ratio.

[0025] The first beam displacementr (BD) 20 is configured to receive the pump beam 10. Specifically, the first beam displacementr 20 splits the pump beam 10 into two optical paths according to its polarization state: one polarization component, for example, horizontally polarized H, passes directly through, forming the first optical path 30; while the other polarization component, orthogonal to it, for example, vertically polarized V, is laterally shifted by a fixed distance, forming the second optical path 40. The split first optical path 30 and the second optical path 40 are spatially parallel to each other and propagate in the same direction.

[0026] A nonlinear crystal 50 is disposed in the first optical path 30 and the second optical path 40, specifically on the path that the first optical path 30 and the second optical path 40 both pass through, and is used to generate photon pairs through a spontaneous parametric down-conversion process. In this embodiment, the nonlinear crystal 50 is preferably a type 0 phase-matched periodically polarized crystal (PPLN) to achieve a highly efficient spontaneous parametric down-conversion (SPDC) process, that is, two parallel pump beams pass through the nonlinear crystal 50 in the same direction, and each pump photon has a certain probability of splitting into a pair of photons with lower energy and longer wavelength (|VV> state).

[0027] A polarization control element 60 is disposed in the first optical path 30 and / or the second optical path 40 to control the polarization state of photon pairs, such that the polarization state of the photon pairs output from the first optical path 30 is orthogonal to the polarization state of the photon pairs output from the second optical path 40. In this embodiment, the polarization control element 60 is specifically implemented by two independent half-wave plates (HWPs): The first half-wave plate 61 is disposed in the first optical path 30 and located between the first beam shifter 20 and the nonlinear crystal 50. Its main function is to polarize the pump light in the first optical path 30, for example, by rotating it 90° from H to V polarization. After this operation, the pump light entering the two optical paths of the nonlinear crystal 50 is V polarized, thereby ensuring that the initial photon pairs they generate are all in the |VV> state.

[0028] The second half-wave plate 62 is disposed in the first optical path 30 and located between the nonlinear crystal 50 and the second beam shifter 70. Its main function is to rotate the |VV> state photon pairs generated by the downconversion of the first optical path 30 by 90°, so that their polarization state changes to the |HH> state. At this time, the photon pairs of the first optical path 30 are in the |HH> state, while the photon pairs of the second optical path 40 are still in the |VV> state, and their polarization states are mutually orthogonal.

[0029] The second beam displacementr (BD) 70 is configured to combine photon pairs from the first optical path 30 and the second optical path 40 into a single output beam. Specifically, the second beam displacementr 70 has the same or similar structure as the first beam displacementr 20, but functions in the opposite way. It receives photon pairs (|HH> and |VV>) with orthogonal polarization states from the first optical path 30 and the second optical path 40, and spatially recombines them into a collinear output beam.

[0030] In this embodiment, the first beam shifter 20, the nonlinear crystal 50, the polarization control element 60, and the second beam shifter 70 are arranged collinearly along the propagation direction of the pump beam 10. Furthermore, these elements are all cuboid or cylindrical structures with flat optical surfaces; that is, the optical surfaces of the first beam shifter 20, the nonlinear crystal 50, the polarization control element 60, and the second beam shifter 70 are all perpendicular to the propagation direction of the pump beam 10. This series-connected layout completely eliminates the 45° angle reflection and alignment required in traditional Sagnac structures, effectively simplifying the assembly process. Moreover, since the surfaces of all optical elements are perpendicular to the incident direction of the pump light, the construction and debugging of the entire entangled light source are more convenient, making the engineering and mass production of this entangled light source possible. Specifically, each element can be positioned using a multi-axis optical displacement stage or automatically assembled using an electromechanical optical coupling system.

[0031] In a preferred embodiment, such as Figure 2 As shown, the aforementioned core optical components, such as the first beam shifter 20, the nonlinear crystal 50, the first half-wave plate 61, the second half-wave plate 62, and the second beam shifter 70, can utilize their highly flat outer surfaces and be bonded together with refractive index-matched optical adhesive to form a single, integrated optical assembly. This modular design utilizes the mechanical precision of the components to achieve self-alignment, completing the complex optical path alignment work in the prefabrication stage. This results in a final system with extremely high mechanical stability and resistance to environmental interference, while maintaining a very compact size.

[0032] In some embodiments, such as Figure 1 As shown, the output of this light source system also includes a series of post-processing components: A filter 80, typically a long-pass filter, is positioned after the second beam shifter 70 to filter out the high-energy, unconverted residual pump beam 10, allowing only long-wavelength entangled photon pairs to pass through. In specific implementations, such as... Figure 2 As shown, filter 80 can be a filter sheet bonded together with the aforementioned optical components using refractive index matching adhesive, further simplifying the process of building the entire entangled light source.

[0033] A phase compensator 90 is disposed downstream of the filter 80. In this embodiment, the phase compensator 90 is preferably a compensating crystal (CC), whose main function is to precisely control the relative phase φ between the two paths |HH> and |VV>, compensating for phase deviations caused by factors such as component dispersion, in order to obtain a high-fidelity entangled state. In specific implementations, similarly, such as... Figure 2 As shown, the compensation crystal (CC) can be bonded to the aforementioned optical components using refractive index matching adhesive, further simplifying the process of building the entire entangled light source.

[0034] In addition, the light source system includes an optical fiber coupling device 130, for example, using a polarization-maintaining fiber collimator in reverse, and a wavelength division multiplexer 100. The entangled photon pair, compensated by the phase compensator 90, is coupled into a single-mode fiber by the optical fiber coupling device 130 and then fed into the wavelength division multiplexer 100. The wavelength division multiplexer 100 deterministically separates the two photons (signal photon and idler photon) in the photon pair to two different output fiber ports based on the wavelength difference (e.g., 1559 nm and 1561 nm) for subsequent quantum experiments.

[0035] In summary, the working principle of the fully collinear translational interferometer structure based on a beam displacementr (BD) provided in this application is summarized as follows: In summary, the working principle of the fully collinear polarization entangled light source system based on a beam shifter provided in this application embodiment is as follows: First, the linearly polarized pump beam 10 from the pump source is polarized to a preset angle by an input polarization preparation unit 110 (e.g., a half-wave plate); then, the first beam shifter 20 splits the pump beam 10 into a first optical path 30 (e.g., horizontally polarized H) and a second optical path 40 (e.g., vertically polarized V) according to the polarization state.

[0036] Next, under the action of the polarization control element 60, the first half-wave plate 61 located in the first optical path 30 adjusts the pump light in the optical path to the same polarization state (e.g., vertically polarized V) as the pump light in the second optical path 40. Thereafter, the pump beams from the first optical path 30 and the second optical path 40 are incident on the same nonlinear crystal 50 (e.g., a type O periodically polarized crystal), and photon pairs (e.g., |VV> state) are generated through a spontaneous parametric down-conversion (SPDC) process. Afterward, also under the action of the polarization control element 60, the second half-wave plate 62 located in the first optical path 30 adjusts the polarization state of the photon pairs generated in the optical path (e.g., from |VV> state to |HH> state), so that they are orthogonal to the polarization state of the photon pairs in the second optical path 40.

[0037] Finally, the photon pairs from the two optical paths are combined by the second beam shifter 70, and the residual pump beam 10 is filtered out by a filter 80 before being output to a single-mode fiber through an optical fiber coupling device 130.

[0038] In the spontaneous parametric downconversion process, the generated photon pairs need to follow the law of conservation of energy. The energy of a single photon is inversely proportional to its wavelength. Therefore, 1 / λp = 1 / λi + 1 / λs, where λp is the wavelength of the pump photon, and λs and λi are the wavelengths of the signal photon and the idler photon (the signal photon and the idler photon are used to refer to the two photons in the photon pair, and the names themselves have no practical meaning). Therefore, the two photons in the photon pair can be separated by a wavelength division multiplexer 100.

[0039] Taking a two-channel wavelength division multiplexer 100 as an example: when a 780nm pump beam 10 is incident on a nonlinear crystal 50, a polarization entangled photon pair with wavelengths of 1561nm and 1559nm can be generated. After this photon pair is coupled into the wavelength division multiplexer 100 via a single-mode fiber, if the center wavelengths of its output channels are set to 1561nm and 1559nm respectively, spatial separation of the photon pair can be achieved. Thus, the entangled states finally measured in the two output channels of the wavelength division multiplexer 100 are: The fully collinear polarization entangled light source system provided in this application embodiment achieves a collinear series layout of all optical elements along a single optical axis by using a beam shifter to split and combine the pump light. This structure eliminates the need for precise alignment at non-collinear angles, simplifying the system's assembly and debugging process, reducing sensitivity to mechanical vibration, and enhancing long-term operational stability. Simultaneously, the collinear layout effectively compresses the optical path length, significantly reducing the overall physical size of the light source system, providing a feasible technical approach for achieving compact, modular, and automated mass production of entangled light sources.

[0040] In the description of this application, it should be noted that the terms "vertical", "up", "down", "horizontal", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and 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 this application.

[0041] In the description of this application, it should also be noted that, unless otherwise expressly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0042] Finally, it should be noted that the above descriptions are merely preferred embodiments of this application and are not intended to limit this application. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A fully collinear polarization entangled light source system, characterized in that, include: A pump light source, used to provide a pump beam; A first beam shifter is configured to receive the pump beam and split the pump beam into a first optical path and a second optical path that are parallel to each other and propagate in the same direction according to its polarization state. A nonlinear crystal, disposed in the first and second optical paths, is used to generate photon pairs through a spontaneous parametric downconversion process. A polarization control element is disposed in the first optical path and / or the second optical path to control the polarization state of a photon pair, such that the polarization state of the photon pair output from the first optical path is orthogonal to the polarization state of the photon pair output from the second optical path. The second beam shifter is configured to combine photons from the first and second optical paths into a single output beam.

2. The fully collinear polarization entangled light source system according to claim 1, characterized in that, The first beam shifter, the nonlinear crystal, the polarization control element, and the second beam shifter are arranged collinearly along the propagation direction of the pump beam.

3. The fully collinear polarization entangled light source system according to claim 1, characterized in that, The optical surfaces of the first beam shifter, the nonlinear crystal, the polarization control element, and the second beam shifter are all perpendicular to the propagation direction of the pump beam.

4. The fully collinear polarization entangled light source system according to any one of claims 1 to 3, characterized in that, The first beam shifter, the nonlinear crystal, the polarization control element, and the second beam shifter are bonded together with optical adhesive to form an integrated optical component.

5. The fully collinear polarization entangled light source system according to claim 1, characterized in that, The polarization control element includes: a first half-wave plate disposed in the first optical path and located between the first beam shifter and the nonlinear crystal, for unifying the polarization state of the pump light in the first and second optical paths; and a second half-wave plate disposed in the first optical path and located between the nonlinear crystal and the second beam shifter, for making the polarization state of the photon pair generated by the first optical path orthogonal to the polarization state of the photon pair generated by the second optical path.

6. The fully collinear polarization entangled light source system according to claim 1, characterized in that, The nonlinear crystal is a periodically polarized crystal.

7. The fully collinear polarization entangled light source system according to claim 1, characterized in that, It also includes a filter, located downstream of the second beam shifter, for filtering out residual pump beams.

8. The fully collinear polarization entangled light source system according to claim 7, characterized in that, It also includes a phase compensator disposed downstream of the filter, the phase compensator being used to adjust the relative phase between the first optical path photon pair and the second optical path photon pair in the beam filtered by the filter.

9. The fully collinear polarization entangled light source system according to claim 1, characterized in that, It also includes a wavelength division multiplexer for spatially separating photon pairs in the output beam according to wavelength.

10. The fully collinear polarization entangled light source system according to claim 1, characterized in that, It also includes an input polarization preparation unit, which is disposed between the pump light source and the first beam shifter, and is used to adjust the polarization direction of the pump beam to a preset angle.

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