Abstract Optical atomic clocks with compact size, reduced weight and low power consumption have broad out-of-the-lab applications such as satellite-based geo-positioning and communication engineering. Here, we propose an active optical microclock based on the lattice-trapped atoms evanescently interacting with a whispering-gallery-mode microcavity. Unlike the conventional passive clock scheme, the active operation directly produces the optical frequency standard without the need of extra laser stabilization, substantially simplifying the clock configuration. The numerical simulation illustrates that the microclock’s frequency stability reaches 1.5 × 10 − 14 at 1 s of averaging, over one order of magnitude better than the recently demonstrated chip-scale optical clock that is built upon rubidium vapor cell and also more stable than current cesium fountain clocks and hydrogen masers. Our work extends the chip-scale clocks to the active fashion, paving the way towards the on-chip quantum micro-metrology, for example, the optical frequency comparison and synchronization between multiple microclocks through frequency microcombs.
Abstract Optical atomic clocks with compact size, reduced weight and low power consumption have broad out-of-the-lab applications such as satellite-based geo-positioning and communication engineering. Here, we propose an active optical microclock based on the lattice-trapped atoms evanescently interacting with a whispering-gallery-mode microcavity. Unlike the conventional passive clock scheme, the active operation directly produces the optical frequency standard without the need of extra laser stabilization, substantially simplifying the clock configuration. The numerical simulation illustrates that the microclock’s frequency stability reaches 1.5 × 10 − 14 at 1 s of averaging, over one order of magnitude better than the recently demonstrated chip-scale optical clock that is built upon rubidium vapor cell and also more stable than current cesium fountain clocks and hydrogen masers. Our work extends the chip-scale clocks to the active fashion, paving the way towards the on-chip quantum micro-metrology, for example, the optical frequency comparison and synchronization between multiple microclocks through frequency microcombs.
A whispering-gallery-mode microlaser in the bad-cavity limit serves as an active optical clock. In this clock scheme, an ensemble of two-level atoms is trapped in a two-color ring-shaped optical lattice, i.e., Lamb-Dicke regime, and evanescently interacts with an optical microcavity. The microclock is operated in the pulsed mode, where the atoms are periodically pumped into the upper state of the electric-dipole-forbidden clock transition, providing the optical gain. The numerical simulation shows that the fractional frequency instability of the microclock reaches $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ at 103 s of averaging, mainly limited by the lattice-induced frequency shifts. This miniature optical clock owns the high integration and can be potentially used to supply the time standard in a photonic network, paving the way towards the on-chip metrology.
Optical atomic clocks produce highly stable frequency standards and frequency combs bridge clock frequencies with hundreds of terahertz difference. In this paper, we propose a hybrid clock scheme, where a light source pumps an active optical clock through a microresonator-based nonlinear third harmonic process, serves as a passive optical clock via indirectly locking its frequency to an atomic transition, and drives a chip-scale microcomb whose mode spacing is stabilized using the active optical clock. The operation of the whole hybrid system is investigated through simulation analysis. The numerical results show: (i) The short-term frequency stability of the passive optical clock follows an Allan deviation of σ y ( τ ) = 9.3 × 10 −14 τ −1/2 with the averaging time τ , limited by the population fluctuations of interrogated atoms. (ii) The frequency stability of the active optical clock reaches σ y ( τ ) = 6.2 × 10 −15 τ −1/2 , which is close to the quantum noise limit. (iii) The mode spacing of the stabilized microcomb has a shot-noise-limited Allan deviation of σ y ( τ ) = 1.9 × 10 −11 τ −1/2 . Our hybrid scheme may be realized using recently developed technologies in (micro)photonics and atomic physics, paving the way towards on-chip optical frequency comparison, synthesis, and synchronization.
Microlasers, relying on the strong coupling between active particles and optical microcavity, exhibit fundamental differences from conventional lasers, such as multi-threshold/thresholdless behavior and nonclassical photon emission. As light sources, microlasers possess extensive applications in precision measurement, quantum information processing, and biochemical sensing. Here we propose a whispering-gallery-mode microlaser scheme, where ultracold alkaline-earth metal atoms, i.e., gain medium, are tightly confined in a two-color evanescent lattice that is in the ring shape and formed around a microsphere. To suppress the influence of the lattice-induced ac Stark shift on the moderately-narrow-linewidth laser transition, the red-detuned trapping beams operate at a magic wavelength while the wavelength of the blue-detuned trapping beam is set close to the other magic wavelength. The tiny mode volume and high quality factor of the microsphere ensure the strong atom-microcavity coupling in the bad-cavity regime. As a result, both saturation photon and critical atom numbers, which characterize the laser performance, are substantially reduced below unity. We explore the lasing action of the coupled system by using the Monte Carlo approach. Our scheme may be potentially generalized to the microlasers based on the forbidden clock transitions, holding the prospect for microscale active optical clocks in precision measurement and frequency metrology.
In this paper, we propose a chip-scale cesium four-level active optical clock based on a microfabricated atomic vapor cell. This millimeter-scale vapor cell is coated on both ends of the wall, and the coated walls directly construct the optical resonator. The microfabricated atomic vapor cell is also used to stabilize the frequency of the pump laser, to further reduce the influence of light shift caused by the frequency-stabilized pump laser on the linewidth of active optical frequency standard signal. We make a theoretical analysis and design an experimental scheme. The chip-scale active optical clock with small size and high performance has wide application in precision measurement and detection, and other fields, and it will aid to promote the definition of time unit second based on the optical clock.
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.
This paper presents a novel architecture for a high performance atomic clock based on the use of miniature optical whispering gallery mode (WGM) resonators. Following the approach of stabilizing a laser local oscillator to an optical transition in an atom or ion, as used in advanced atomic clock, a semiconductor laser is used for stabilization to the D1 line of Rb atoms, held in a small vapor cell. The laser is self-injection locked to a WGM resonator to reduce its linewidth. To produce the RF output of the clock, a second WGM resonator excited with a second cw semiconductor laser produces an optical frequency comb that is demodulated on a fast photodiode. Locking the resonator that generates the frequency comb to the laser stabilized to the Rb transition transfers the stability of the atomic transition to the RF output of the clock. In this way, a miniature all-optical atomic clock is realized. Details of the operation of the clock and application of the architecture to other atomic systems, such as a ytterbium ion, will be described.