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Journal of Physics Conference Series

Active optical clock: seventeen years of progress and next steps

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

Published Nov 1, 2024• 1 citation
Publisher

Abstract

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.

DOI
10.1088/1742-6596/2889/1/012035

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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.
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.
Active optical frequency standard, or active optical clock, is a new concept of optical frequency standard, where a weak feedback with phase coherence information in optical bad-cavity limitation is formed, and the continuous self-sustained coherent stimulated emission between two atomic transition levels with population inversion is realized. Through ten years of both theoretical and experimental exploration, the narrow linewidth and suppression of cavity pulling effect of active optical frequency standard have been initially proved. In this paper, after a simple review, we will mainly present the most recent experimental progresses of active optical frequency standards in Peking University, including 4-level cesium active optical frequency standards and active Faraday optical frequency standards. The future development of active optical frequency standards is also discussed.
Active optical clocks, a new class of optical atomic clocks, work through stimulated emission lasing, a unique mechanism that is distinct from laser absorption spectroscopy of conventional optical clocks. With coherent weak optical feedback from a Fabry-Perot resonator operating in the bad-cavity regime, active optical clocks sustain a lasing oscillation at an atomic transition frequency. They are used directly as quantum optical frequency standards with expected stability better by about two orders of magnitude than that of the best clocks. This review starts with a simplified account of historical developments and the limitations of critical techniques in optical atomic clocks. After an introduction of the basic mechanism underlying active optical clocks, we discuss the most recent experimental results for several configurations and experimental set-ups of active optical clocks. These include those based on a 2-level atomic beam and 3-level laser-cooled trapped-atom system, those based on a 4-level atom scheme, and the Faraday active optical clock. Moreover, following the successful lasing and preliminary results on a Faraday active optical frequency standard with Faraday atomic filter using thermal Cs cell, we describe an experimental scheme of a Faraday active optical clock with a Faraday atomic filter using magic-lattice-trapped Sr atoms at the 689-nm transition, and Faraday optical clocks operating in the good-cavity regime. We conclude with a comprehensive comparison of the different active optical clocks, prospects for active Faraday optical clocks, and potential applications of active optical clocks.
Atomic clocks can be broadly classified into passive and active types according to their mode of operation. In the microwave domain, active hydrogen masers provide excellent short-term stability and are widely used as flywheel oscillators in timekeeping systems along with the cesium fountain standards. In the optical domain, passive optical clocks have achieved unprecedented accuracy and stability, but at the cost of large and complex infrastructures associated with ultrastable cavities and the preparation of atoms in optical lattices or ion traps. By contrast, active optical clocks generate coherent clock light directly from an atomic transition in the bad-cavity regime, where cavity pulling is strongly suppressed, thereby reducing sensitivity to cavity fluctuations and potentially simplifying optical clock architectures. Over the past decade, active optical clocks have developed from a conceptual proposal into a rapidly advancing field that includes pulsed superradiant systems based on cold atoms and continuous-wave bad-cavity systems based on thermal atomic vapors. This review summarizes recent progress in active optical clocks and discusses future directions of the field.
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.
Abstract Active optical clocks take advantage of the substantial suppression of cavity pulling in the bad‐cavity lasing regime and feature the simple structure and excellent portability. In the extreme case, where the cavity mirror reflection approaches zero and cannot support any quasi‐bound modes for light waves, conventional bad‐cavity lasers become the cavityless lasing and the cavity pulling vanishes. Consequently, the laser frequency is determined entirely by the atomic transition. This work is dedicated to theoretically explore the potential application of the cavityless lasing in active optical clocks. Without loss of generality, Cs atoms with well‐established quantum control techniques are employed as the gain medium that is pumped by a 459‐nm laser. The clock laser wavelength is 1470 nm. The simulation shows that the frequency stability of the clock laser with feasible physical quantities reaches at 1 s of averaging time, one order of magnitude better than the recently demonstrated compact passive optical clocks that are built upon a two‐photon transition in a hot Rb vapor and upon the modulation transfer spectroscopy of thermal Cs atoms. Such compact cavityless lasing‐based active optical clocks have the broad application prospect in satellite navigation and communications and promise an architecture for the integrated photonic metrology.