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European Frequency and Time Forum

Active Optical Clock Signal Based on 87Rb Atoms Cooled by 780 NM Diffuse Laser

Jia Zhang, Xiaolei Guan, Xun Gao, Jianxiang Miao, Tiantian Shi, Jingbiao Chen

Published Jun 25, 2024• 0 citations
Publisher

Abstract

The cavity-pulling suppression advantage of active optical clocks (AOCs) helps to realize atomic clocks with superior performance. However, realization of continuous-wave cold-atom AOCs is a highly challenging task. In this work, we proposed an AOC scheme with diffuse-laser-cooled atoms. Combining the advantages of diffuse laser cooling to easily prepare cold-atom cloud with large volume on the scale of meters and the four-level structure to greatly reduce the light shift, it is expected to realize the continuous output of AOC superradiant signals. We experimentally realized a 100-cm-long cold87Rb atom cloud as the gain medium. The 1367 nm AOC superradiant signal with a power of 0.3$\mu \mathrm{W}$has been achieved without resonant cavity feedback. In the future, we will continue to advance the experiment and also conduct research on 420 nm blue light cooling.

DOI
10.1109/eftf61992.2024.10722193

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The cavity-pulling suppression advantage of active optical clocks (AOCs) helps to realize atomic clocks with superior performance. However, realization of continuous-wave cold-atom AOCs is a highly challenging task. In this work, we proposed an AOC scheme with diffuse-laser-cooled atoms. Combining the advantages of diffuse laser cooling to easily prepare cold-atom cloud with large volume on the scale of meters and the four-level structure to greatly reduce the light shift, it is expected to realize the continuous output of AOC superradiant signals. We experimentally realized a 100-cm-long cold87Rb atom cloud as the gain medium. The 1367 nm AOC superradiant signal with a power of 0.3μW\mu \mathrm{W}μWhas been achieved without resonant cavity feedback. In the future, we will continue to advance the experiment and also conduct research on 420 nm blue light cooling.
Abstract The concept of the active optical clock (AOC) was proposed seventeen years ago by prof. Jingbiao Chen. After early calculations and experiments that demonstrated the advantages of “cavity-pulling suppression” and “narrower linewidth”, the development of AOCs soon became an active competition involving many research groups that proposed schemes based on different atomic species, and different energy-level structures. In this paper, after a brief background introduction, we will mainly focus on the most recent experimental and theoretical progress of AOCs in Peking University. In addition, we give an outlook on AOCs based on cold atoms that can be operated continuously, and report a novel AOC scheme based on diffuse laser cooling that can be operated continuously for the first time, which is proposed and performed by Peking University. The AOC has great promise for applications in quantum metrology, as it can overcome the thermal noise of the reference cavity that limits conventional lasers, making AOC a promising candidate for future atomic optical clocks.
Two feasible schemes are proposed by laser cooling of alkali atoms to realize the narrow-linewidth, continual superradiant lasing. Utilizing the D1 gray molasses to realize a cloud of cold Cs atoms for the two-level Faraday active optical clock (AOC), the effect of Doppler broadening on the transmission bandwidth of Faraday atomic filter will be effectively suppressed to the level of natural linewidth. Thus, the influence of mechanical or thermal vibrations of cavity mirrors on the clock laser will be decreased by about 102 times, which is expected to further narrow the laser linewidth by two orders of magnitude compared with the thermal atomic Faraday AOC to Hz level. Moreover, the four-level AOC based on the narrow linewidth cooling of Cs atoms using the 6S-7P transition is proposed. The 455 nm laser at 6S1/2-7P3/2transition plays two roles simultaneously, one is the cooling laser for a magneto-optical trap (MOT), another is the pumping laser for population inversion of 1470 nm clock transition, which is helpful to improve the compactness of the system. In the four-level structure, the frequency of pumping laser is far detuning from that of clock laser, therefore, the light shift resulting from the 455 nm laser is negligible. This four-level AOC based on cold atoms will further reduce the collision shift in thermal atom system to achieve a high-precision optical frequency reference.
We proposed a scheme for realizing active Faraday optical frequency standards using diffuse laser cooling of alkali-metal atoms. A 1-m-long cold87Rb atomic cloud was prepared within a ϕ 2 cm ×$l$1 m vacuum glass tube, featuring a maximum optical density of 4. By utilizing this setup to construct the Faraday anomalous dispersion optical filter, we are able to achieve an ultra-narrowband laser gain close to the natural linewidth of the atomic transition and corresponding to the hyperfine transition frequency. This ultra-narrowband laser gain can directly determine the output frequency of the active Faraday optical frequency standard operating in the “bad cavity” regime, significantly suppressing the cavity-pulling effect. Furthermore, due to the suppression of atomic collision and Doppler frequency shifts by the cold atoms, it is expected to further narrow the laser linewidth of the cold-atom-based active Faraday optical frequency standard to Hz level.
To date, the laser cooling of rubidium atoms has inevitably relied on 780 nm cooling light corresponding to the first excited state 5 P 3 / 2 . Here, we demonstrate diffuse laser cooling of Rb 87 atoms using the 420 nm 5 S 1 / 2 → 6 P 3 / 2 transition without a preceding 780 nm cooling stage. A high-power 420 nm laser is used as the cooling light to produce cold atoms in a 1-m-long diffuse-cooling cell. Absorption spectroscopy yields a cold-atom density of approximately 1.4 × 10 11 m − 3 . We compare the cooling performance of the 420 nm and conventional 780 nm diffuse-cooling schemes and verify the feasibility of using the high-excited-state transition as an independent cooling channel in this large-volume diffuse-cooling system. This approach provides an alternative cooling/pumping pathway for continuous cold-atom active optical clocks, where compatibility with the clock-lasing process and reduced cooling-light-induced perturbations are important. It is also expected to open up research directions and application prospects in frontier fields such as Rydberg atoms, quantum information, and so on.
The active optical clock (AOC) takes advantage of the narrowed linewidth and cavity pulling suppression of the bad cavity laser, making it a candidate for the next generation of optical clock. We propose a new experimental method to measure the cavity-pulling effect of a four-level AOC based on thermal cesium atoms. After modulating the frequency of the 459 nm pumping laser, atoms in different velocity groups are pumped to the 7P1/2energy level. Therefore, population inversion is achieved in separate velocity groups, and sidebands are created in the gain profile. By measuring the central frequency of the beat spectrum between the zeroth order AOC output and the sidebands, the cavity-pulling coefficient can be calculated. This method has the advantage of not requiring any additional optical frequency reference, and can be adopted in AOC systems other than those using thermal atoms. Furthermore, our proposed method for cavity-pulling measurement can serve as a basis for the characterization of AOCs.
We demonstrate an optical frequency standard by locking the output frequency of an external cavity diode laser to the 780 nm D2 transition of diffuse laser cooled 87Rb atoms using frequency modulation spectroscopy (FMS). A 50-cm-long cold atomic cloud is obtained within a ϕ 2 cm ×l 50 cm vacuum glass tube, featuring greater simplicity and reliability. By analyzing the impact of power and detuning of cooling and repumping lasers, and power of frequency-modulated laser on FMS based on cold atoms, we choose the optimal parameter and lock the laser frequency to the 87Rb 52S1/2 (F = 2) → 52P3/2 (F′ = 3) cycling transition. The cold-atom-based optical frequency standard achieves an in-loop frequency stability of 3.3 × 10−15 at 1 s, with peak-to-peak frequency fluctuations being less than 50 Hz within 6000 s. This represents the integration of cold atoms into an optical frequency standard utilizing FMS for frequency stabilization. In contrast to the common cold-atom optical frequency standards, our approach requires fewer lasers and eliminates the need for a complex and expensive Pound–Drever–Hall (PDH) system. Its low complexity makes it a promising candidate for development into a space cold-atom clock, promoting advancements in space science and navigational positioning. Additionally, it possesses great potential to serve as an absolute wavelength standard.