Cold atom two-photon transition device based on diffuse reflection cooling

Abstract

This invention discloses a cold atom two-photon transition physics device based on diffuse reflection cooling, comprising a glass chamber, a diffuse reflection coating, an optical unit, and a detection unit. The glass chamber is an ultra-high vacuum cavity sealed and connected to an external ultra-high vacuum system and an atomic source. Its outer wall is covered with a diffuse reflection coating, on which are formed apertures for cooling light, re-pumping light, excitation light, and fluorescence output. The optical unit includes cooling light, re-pumping light, and excitation light. The cooling light and re-pumping light form a global diffuse reflection cooling light field through the diffuse reflection coating, preparing three-dimensionally distributed cold atom clusters within the cavity. The excitation light passes through the cavity and is reflected back to form a bidirectional opposing light path, constituting a two-photon transition excitation light field. The detection unit collects, filters, and converges the fluorescence signal before outputting it through a photodetector. This invention features a simple structure, small size, and low power consumption, and can output clock transition spectra with high signal-to-noise ratio and narrow linewidth, providing a novel technical solution for small-scale, high-precision optical frequency standards.

Description

Technical Field

[0001] This invention belongs to the field of optical atomic frequency standard technology, specifically relating to a cold atom two-photon transition physics device based on diffuse reflection cooling. Background Technology

[0002] Optical frequency standards (such as optical lattice clocks and ion clocks) are characterized by their extremely high frequency stability (up to 10). -18 -10 -19 Lightning-scale optical atomic clocks have become a core supporting technology for the next generation of time standards. However, traditional optical atomic clocks are bulky, consume a lot of power, and are extremely sensitive to the environment, limiting their practical application and promotion. Miniaturized optical frequency standards combine the advantages of small size, low power consumption, and high stability, and have a wide range of applications. They can improve navigation, positioning, and timing accuracy, and support the development of high-precision time synchronization in fields such as verification of general relativity, detection of gravitational waves, and quantum communication networks.

[0003] Optical frequency standards based on rubidium atom two-photon transitions have received widespread attention over the past few decades due to their relatively mature atomic manipulation technology, narrow natural linewidth, and potential for miniaturization applications. In 2021, Newman et al. at the National Institute of Standards and Technology (NIST) used silicon-based photonic chip technology to realize a compact optical atomic clock system with highly integrated microcells and microcavity optical frequency combs, achieving a frequency stability better than 1.8 × 10⁻⁶ Hz over 100 seconds. -13 However, to date, research on two-photon transition optical frequency references for rubidium atoms has focused more on hot atom systems, which suffer from problems such as large atomic collision broadening and short coherence time, limiting further improvements in spectral signal-to-noise ratio and frequency stability.

[0004] In China, various device schemes have been proposed for optical frequency standards and cold atom preparation. For example, Chen Jingbiao et al. proposed an active optical clock scheme using cold atoms in their patent "A Long Strip-Shaped Cold Atom Active Optical Clock Based on Diffuse Reflection Cooling and Its Implementation Method" (CN115327880B). However, its atomic gas chamber is not a vacuum cavity, limiting the coherence time of the cold atoms. Wang Ji et al. proposed a two-photon transition optical frequency standard device using cold atoms in their patent "A Rubidium Cold Atom Gas Chamber Two-Photon Transition Optical Frequency Standard Device" (CN121356577A). However, its two-photon transition excitation light adopts a unidirectional propagation structure, resulting in a significant heating effect on the cold atoms. Wan Jinyin et al. proposed a device for preparing cold atoms using diffuse reflection in their patent "Quasi-One-Dimensional Cold Atom Source Preparation Device and Method" (CN114005571A). However, it is limited by its quasi-one-dimensional nature and cannot provide three-dimensionally distributed cold atoms. All of the aforementioned patents exhibit limiting factors that restrict the improvement of optical clock performance. Summary of the Invention

[0005] To address the limitations of existing cold atom two-photon transition devices, such as limited atomic coherence time, significant excitation light heating effect, and insufficient spatial dimensionality of cold atoms, this invention provides a cold atom two-photon transition physics device based on diffuse reflection cooling. This device utilizes diffuse reflection to cool atoms and detect two-photon transition spectra, aiming to obtain clock transition spectra with high signal-to-noise ratio and narrow linewidth, while simultaneously meeting the practical requirements of miniaturization, low power consumption, and simple operation. The device constructs an ultra-high vacuum cavity connected to an external system, and within the cavity, uses diffuse reflection cooling to prepare high-density, long-coherence-time, three-dimensionally distributed cold atom clusters. Combined with high signal-to-noise ratio two-photon transition detection technology, it outputs clock transition spectra with high signal-to-noise ratio and narrow linewidth.

[0006] The technical solution of the present invention is as follows: This invention provides a cold atom two-photon transition physics device based on diffuse reflection cooling, comprising a glass gas chamber, a diffuse reflection coating, an optical unit, and a detection unit.

[0007] The glass gas chamber is a cavity structure with a connecting through hole at the bottom. The connecting through hole is sealed to the external ultra-high vacuum system and atomic source through a transition connector to form an ultra-high vacuum working cavity. The outer wall of the glass gas chamber is uniformly covered with a diffuse reflection coating. Several cooling light and heavy pump light transmission holes, excitation light incident light transmission holes, excitation light exit light transmission holes, and fluorescence output light transmission holes are opened on the diffuse reflection coating. The optical unit includes a cooling light, a re-pump light, and an excitation light. The cooling light and the re-pump light are configured as multiple beams of equal intensity, which are incident forward into the glass chamber through corresponding light-passing holes of the cooling light and the re-pump light. Under the action of the diffuse reflection coating, a global diffuse reflection cooling light field is formed to form a three-dimensional spatially distributed cold atom cluster inside the glass chamber. The excitation light passes sequentially through the excitation light incident light-passing hole, the glass chamber, and the excitation light exit light-passing hole, and then is incident on an external retroreflector. After being reflected by the retroreflector, it is transmitted in the reverse direction along the original light path, forming a coaxially aligned bidirectional opposing light path with the incident excitation light, constituting a two-photon transition excitation light field. The two-photon transition excitation light field at least partially overlaps with the three-dimensional spatially distributed cold atom cluster. The detection unit includes a filter, a converging lens, and a photodetector arranged sequentially along the fluorescence output optical path; the fluorescence signal generated by the two-photon transition is diffusely reflected and transmitted in the glass chamber, and then output through the fluorescence output aperture; the fluorescence signal is filtered and converged sequentially and then coupled to the photodetector, which converts it into an electrical signal for output.

[0008] Furthermore, the glass chamber can be made of high-transmittance ultraviolet-visible-infrared optical glass. Furthermore, the diffuse reflection coating is prepared using high-purity barium sulfate material, with a wavelength range of 350nm-850nm. The diffuse reflectance within the range is not less than 97%, and the optical performance remains stable within a temperature range of -40℃ to 90℃.

[0009] Furthermore, the transition connector includes a transition glass and a stainless steel flange, which connects to the external ultra-high vacuum chamber. Atomic sources enable sealed connections.

[0010] Furthermore, the cooling light and the re-pumping light are combined into a collinear beam by a beam combiner before incident.

[0011] Furthermore, the diameter of the fluorescence output aperture is larger than the diameter of the cooling light and re-pump light apertures and the excitation light incident and outgoing apertures.

[0012] Furthermore, the cooling light and re-pumping light transmission aperture and the excitation light incident aperture are disposed in the glass gas chamber. The excitation light emission aperture is located on the left end face of the glass gas chamber, and the fluorescence output aperture is located on the upper end face of the glass gas chamber.

[0013] Furthermore, the excitation light is coupled out from a polarization-maintaining single-mode fiber, and its wavelength matches that of the target atom in a two-beam configuration. Sub-transition energy level structure.

[0014] Furthermore, the photodetector is a photomultiplier tube.

[0015] Furthermore, the detection unit also includes a collimating lens group, which is disposed in front of the filter.

[0016] Furthermore, the cold atom cluster completely envelops the two-photon transition excitation light field.

[0017] The cold atom two-photon transition physics device based on diffuse reflection cooling provided by this invention has the following beneficial effects: This invention employs a structure that combines diffuse reflection cooling with a sealed connection to an external ultra-high vacuum system. This overcomes the limitation of coherence time caused by non-vacuum cavities, effectively reduces background gas collisions, extends the coherence time of cold atoms, and provides a fundamental guarantee for narrow linewidth clock transition spectral lines.

[0018] This invention utilizes diffuse reflection cooling to form three-dimensional spatially distributed cold atom clusters within a glass chamber, overcoming the limitation that quasi-one-dimensional structures cannot achieve three-dimensional cold atom distribution. This significantly increases the spatial overlap volume between cold atoms and excitation light, thereby enhancing the intensity of two-photon transition fluorescence signals and the spectral signal-to-noise ratio.

[0019] This invention employs a bidirectional excitation optical path structure, which solves the problem of significant heating effect of unidirectional excitation light on cold atoms, reduces the heating of cold atoms by excitation light, prolongs the effective interaction time between atoms and the light field, and is beneficial for narrowing spectral linewidth and improving frequency stability.

[0020] This invention employs an all-optical diffuse reflection cooling mechanism, which eliminates the need for an external magnetic field compared to traditional magneto-optical traps. This avoids interference from the magnetic field on atomic energy levels and spatial distribution. Furthermore, the system is insensitive to incident laser polarization and power fluctuations, exhibiting higher robustness and environmental adaptability.

[0021] This invention features a compact overall structure and eliminates the need for a complex magnetic field system. Compared to traditional magneto-optical traps and thermal atom two-photon transition systems, it significantly reduces system complexity and size, achieving miniaturization, lightweight design, and low power consumption. By combining the synergistic effect of three-dimensional cold atom clusters and bidirectional excitation light fields, it effectively reduces atomic collision broadening and can output clock transition spectral lines with high signal-to-noise ratio and narrow linewidth, providing an ideal physical platform for portable high-precision optical frequency standards. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the cross-section of a cold atom two-photon transition physics device and a cold atom cluster based on diffuse reflection cooling, according to an embodiment of the present invention.

[0023] Figure 2 This is a schematic diagram of the light-transmitting hole layout of the diffuse reflection coating according to an embodiment of the present invention. Figure 2 (a) is an opening on the left end face. Figure 2 (b) is an opening on the right end face. Figure 2 (c) is an opening on the upper end face.

[0024] In the figure: (1) is a glass gas chamber, (2) is a diffuse reflection coating, (3) is a stainless steel flange, (4) is a cooling light and a heavy pump light, (5) is a cold atom cluster, (6) is an excitation light, (7) is a retroreflector, (8) is a filter, (9) is a converging lens, (10) is a photomultiplier tube (PMT), (11) is fluorescence, a, b, c, d, e, f are the first to sixth light-passing holes, and g is the seventh light-passing hole. Detailed Implementation

[0025] The preferred embodiments of the two-photon optical clock system based on diffuse reflection cooling according to the present invention will be described in detail below with reference to the accompanying drawings. Those skilled in the art should understand that the following embodiments are only for explaining the present invention and are not intended to limit the scope of protection of the present invention; all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0026] Example 1 As shown in Figure 1, the two-photon transition physics system based on diffuse reflection cooling provided in this embodiment mainly includes: a glass gas chamber 1, a diffuse reflection coating 2, a stainless steel flange 3, a cooling light and a re-pumping light 4, a cold atom cluster 5, an excitation light 6, a retroreflector 7, a filter 8, a converging lens 9, and a photomultiplier tube 10. These components work together to sequentially achieve cold atom preparation, two-photon transition excitation, and fluorescence signal detection.

[0027] The glass chamber 1 is a cubic structure with a side length of 50 mm, made of high-transmittance ultraviolet-visible-infrared optical glass, possessing excellent optical transmittance and structural stability. Serving as the core cavity for atomic cooling and two-photon transitions, it accommodates cold atomic clusters and provides a stable optical channel for beam propagation and fluorescence emission. The outer wall of the glass chamber 1 is uniformly coated with a diffuse reflectance coating 2, prepared from high-purity barium sulfate. After process optimization, the coating exhibits a diffuse reflectance of no less than 97% in the wavelength range of 350 nm to 850 nm, and shows no optical performance degradation, coating peeling, or chemical changes within a wide temperature range of -40℃ to 90℃. To address issues such as chemical stability, the glass chamber 1 possesses excellent optical and chemical stability, laying a solid foundation for the construction of a global diffuse reflection cooling light field. A 10mm diameter through-hole is opened on the lower surface of the glass chamber 1. The through-hole is sealed to the external ultra-high vacuum cavity and atomic source through a transition connector (not shown in the figure). The transition connector specifically includes a transition glass and a stainless steel flange 3, ensuring the formation and maintenance of a stable ultra-high vacuum cavity environment inside the glass chamber 1, reducing atomic loss and decoherence caused by background gas collisions, and meeting the vacuum conditions required for cold atom preparation and two-photon transition detection.

[0028] The diffuse reflection coating 2 has multiple pre-set light-transmitting holes, each with a clearly defined function and adapted to the optical path requirements. Six light-transmitting holes with a diameter of 2mm are marked as first light-transmitting hole a, second light-transmitting hole b, third light-transmitting hole c, fourth light-transmitting hole d, fifth light-transmitting hole e, and sixth light-transmitting hole f, respectively, which are specifically used for the incident cooling light and re-pumping light, as well as the directional incident and emitted excitation light. A seventh light-transmitting hole g with a diameter of 15mm is also set up as the main channel for collecting fluorescence signals. The aperture of this light-transmitting hole is adapted to the fluorescence collection optical path, ensuring fluorescence collection efficiency while maintaining the integrity of the internal diffuse reflection light field as much as possible, and realizing the coordinated adaptation of beam entry and exit, fluorescence collection and diffuse reflection cooling.

[0029] like Figure 2 As shown, the first light-transmitting hole a, the second light-transmitting hole b, the third light-transmitting hole c, the fourth light-transmitting hole d, and the fifth light-transmitting hole e are disposed on the left end face of the glass gas chamber 1 (see Figure 1). Figure 2 (a) , wherein the first to fourth light-transmitting holes a, b, c, and d are used for the incident cooling light and the re-pumping light, and the fifth light-transmitting hole e is used for the incident excitation light; the sixth light-transmitting hole f is located on the right end face (see Figure 2(b) is used to excite the emission of light; the seventh light-transmitting aperture g is located on the upper end face (see Figure 2 (c) serves as the main channel for collecting fluorescence signals. The layout and number of each light-transmitting aperture can be adaptively adjusted according to the actual optical path requirements.

[0030] The optical unit includes cooling light, re-pump light, and excitation light. Four beams of equal power cooling light and re-pump light 4 are incident into the glass gas chamber 1 through the first to fourth light-passing holes a, b, c, and d, respectively. The cooling light and re-pump light are combined into collinear beams by a beam combiner before incident, and their spatial paths overlap. After entering the glass gas chamber, they undergo multiple diffuse reflections by the diffuse reflection coating, jointly constructing a diffusely reflected cooling light field distributed throughout the entire space. The glass gas chamber 1 is filled with alkali metal atoms (such as rubidium atoms). These atoms are cooled and trapped in the diffusely reflected cooling light field, achieving efficient production of cold atoms and forming a high-density cold atom cluster 5 with a three-dimensional spatial distribution inside the glass gas chamber 1. This cold atom cluster spatially envelops or passes through the propagation area of ​​the excitation light 6, thereby increasing the effective interaction volume and fluorescence detection signal intensity during the two-photon transition process, thus improving the signal-to-noise ratio in the subsequent detection process.

[0031] The excitation light 6 is coupled out by a polarization-maintaining single-mode fiber. The wavelength of the excitation light is precisely matched to the two-photon transition energy level structure of the target atom. The excitation light 6 is transmitted along the optical path, passing through the fifth light-passing hole e, the inner cavity of the glass gas chamber 1, and the sixth light-passing hole f in sequence, and then incident on the retroreflection mirror 7 outside the glass gas chamber 1. After being reflected by the mirror surface of the retroreflection mirror 7, the excitation light is transmitted in the opposite direction along the original optical path, and passes through the sixth light-passing hole f, the inner cavity of the glass gas chamber 1, and the fifth light-passing hole e again, realizing the bidirectional back-and-forth propagation of the excitation light. By precisely adjusting the pitch and deflection parameters of the optical path, it is ensured that the forward excitation beam and the reverse retroreflection beam are coaxially and precisely aligned in the glass gas chamber 1 to form a standing wave field, which reduces the heating effect on the cold atom cluster 5, and at the same time precisely acts on the cold atom cluster 5, triggering the narrow linewidth clock transition of the atom, providing the core transition spectral line for the high-precision optical frequency standard.

[0032] The detection unit includes a filter 8, a converging lens 9, and a photomultiplier tube 10 (PMT). The fluorescence 11 generated by two-photon transitions undergoes multiple diffuse reflections within the glass gas cell 1 via the diffuse reflection coating 2, with the majority escaping through the seventh aperture g, which occupies the largest area of ​​the total opening. After the background light is filtered out by the filter 8, the fluorescence 11 is focused by the converging lens 9 and precisely coupled to the photosensitive incident window of the photomultiplier tube 10. The photomultiplier tube 10 converts the weak fluorescence signal into an electrical signal. It should be noted that, depending on the fluorescence signal intensity and optical path layout, a collimating lens group can be added before the filter 8 to further improve the fluorescence collection efficiency. This collimating lens group can be selected for inclusion or exclusion based on actual needs.

[0033] The optical path design and diffuse reflection cooling method described in this embodiment overcome the dependence of traditional magneto-optical traps on external magnetic fields, simplify the system structure, reduce sensitivity to parameters such as cooling laser power and polarization, effectively extend the coherence time of cold atoms, and achieve a narrower clock transition spectral linewidth in experiments. Simultaneously, the combination of the two-photon transition process and a high signal-to-noise ratio fluorescence detection unit effectively improves the detection sensitivity and frequency stability of the clock transition signal.

[0034] In summary, the cold atom two-photon transition physics device based on diffuse reflection cooling provided by this invention has a compact structure, high stability, excellent cooling efficiency and detection sensitivity, and is suitable for high-precision optical frequency standards and precision physical measurement and other technical fields, and has great practical value and application prospects.

Claims

1. A cold atom two-photon transition physics device based on diffuse reflection cooling, characterized in that, include: Glass air chamber, diffuse reflection coating, optical unit and detection unit; The glass gas chamber is a cavity structure with a connecting through hole at the bottom. The connecting through hole is sealed to the external ultra-high vacuum system and atomic source through a transition connector to form an ultra-high vacuum working cavity. The outer wall of the glass gas chamber is uniformly covered with a diffuse reflection coating. Several cooling light and heavy pump light transmission holes, excitation light incident light transmission holes, excitation light exit light transmission holes, and fluorescence output light transmission holes are opened on the diffuse reflection coating. The optical unit includes a cooling light, a re-pump light, and an excitation light. The cooling light and the re-pump light are configured as multiple beams of equal intensity, which are incident forward into the glass chamber through corresponding light-passing holes of the cooling light and the re-pump light. Under the action of the diffuse reflection coating, a global diffuse reflection cooling light field is formed to form a three-dimensional spatially distributed cold atom cluster inside the glass chamber. The excitation light passes sequentially through the excitation light incident light-passing hole, the glass chamber, and the excitation light exit light-passing hole, and then is incident on an external retroreflector. After being reflected by the retroreflector, it is transmitted in the reverse direction along the original light path, forming a coaxially aligned bidirectional opposing light path with the incident excitation light, constituting a two-photon transition excitation light field. The two-photon transition excitation light field at least partially overlaps with the three-dimensional spatially distributed cold atom cluster. The detection unit includes a filter, a converging lens, and a photodetector arranged sequentially along the fluorescence output optical path; the fluorescence signal generated by the two-photon transition is diffusely reflected and transmitted in the glass chamber, and then output through the fluorescence output aperture; the fluorescence signal is filtered and converged sequentially and then coupled to the photodetector, which converts it into an electrical signal for output.

2. The cold atom two-photon transition physics device based on diffuse reflection cooling according to claim 1, characterized in that, The glass chamber is made of high-transmittance ultraviolet-visible-infrared optical glass.

3. The cold atom two-photon transition physics device based on diffuse reflection cooling according to claim 1, characterized in that, The diffuse reflective coating is prepared using high-purity barium sulfate material, with a diffuse reflectance of not less than 97% in the wavelength range of 350nm-850nm, and maintains stable optical performance in the temperature range of -40℃ to 90℃.

4. The cold atom two-photon transition physics device based on diffuse reflection cooling according to claim 1, characterized in that, The transition connector includes a transition glass and a stainless steel flange.

5. The cold atom two-photon transition physics device based on diffuse reflection cooling according to claim 1, characterized in that, The cooling light and the re-pump light are combined into a collinear beam by a beam combiner before incident.

6. The cold atom two-photon transition physics device based on diffuse reflection cooling according to claim 1, characterized in that, The diameter of the fluorescence output aperture is larger than the diameter of the cooling light and re-pump light apertures and the excitation light incident and outgoing apertures.

7. The cold atom two-photon transition physics device based on diffuse reflection cooling according to claim 1, characterized in that, The excitation light is coupled out from a polarization-maintaining single-mode fiber, and its wavelength matches the two-photon transition energy level structure of the target atom.

8. The cold atom two-photon transition physics device based on diffuse reflection cooling according to claim 1, characterized in that, The photodetector is a photomultiplier tube.

9. The cold atom two-photon transition physics device based on diffuse reflection cooling according to claim 1, characterized in that, The detection unit also includes a collimating lens group, which is positioned in front of the filter.

10. The cold atom two-photon transition physics device based on diffuse reflection cooling according to claim 1, characterized in that, The cold atom cluster completely envelops the two-photon transition excitation light field.

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