We report on our progress towards a continuously-operating active optical clock on the3P0→1S0transition of neutral87Sr or88Sr atoms. Our approach is based on a ring-cavity in which an optical conveyor carries a continuous stream of atoms along the cavity mode. We estimate the performance of our system by simulating the superradiant output from two counter-propagating cavity modes interacting with the atoms. The impact of the position-dependent light shifts, collisional decoherence and loss of atoms trapped within the optical conveyor are analysed.
We report on our progress towards a continuously-operating active optical clock on the3P0→1S0transition of neutral87Sr or88Sr atoms. Our approach is based on a ring-cavity in which an optical conveyor carries a continuous stream of atoms along the cavity mode. We estimate the performance of our system by simulating the superradiant output from two counter-propagating cavity modes interacting with the atoms. The impact of the position-dependent light shifts, collisional decoherence and loss of atoms trapped within the optical conveyor are analysed.
We demonstrate the state-of-the-art technique of an active clock to provide a continuous superradiant lasing signal using an ensemble of trapped Cs atoms in an optical lattice. A magic wavelength of the proposed $|7{S}_{1/2};F=4,{M}_{F}=0\ensuremath{\rangle}\ensuremath{\rightarrow}|6{P}_{3/2};F=3,{M}_{F}=0\ensuremath{\rangle}$ clock transition in Cs atoms is identified at 1181 nm for constructing the optical lattice. Pertinent optical lines are also found for pumping and repumping atoms from their ground states. A fractional uncertainty about ${10}^{\ensuremath{-}15}$ in the clock frequency has been predicted by carrying out rigorous calculations of several atomic properties. The bad-cavity operational mode of the active clock is anticipated to improve remarkably its short-term stability by suppressing intrinsic thermal fluctuations. Thus, a composite clock system with better short-term and long-term stabilities can be built by combining the proposed active clock with another high-accuracy passive optical clock.
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.
SummaryWe present our progress on the construction of a171Yb-based active optical atomic clock. The ytterbium (Yb) atoms, initially in a collimated thermal beam generated in an oven, are decelerated using a Zeeman slower to the capture velocity (~ 10 ms-1) for a magneto-optical trap on the1S0→3P1transition at 556 nm. This allows us to generate an ensemble of N >106atoms at temperatures below 100 μK, which will be transported using an optical conveyor belt to an ultra-stable (σy~ 10-13) cavity of finesse on the order of 104. Here, the atoms will be prepared in order to generate a superradiant emission which will serve as the frequency reference on the1S0→3P0clock transition in171Yb.
The feasibility and potential performance of optical clocks based on all-optical atom chips using neutral strontium atoms have been investigated. The key benefits of all-optical atom chips are their ability to trap alkali-earth atoms, and how easily these atoms can be manipulated and examined. The planar configuration of the chip enables ring photonic resonators to be incorporated into its structure, making it a promising candidate for active optical atomic clocks. Using the 'magic wavelengths' of the trapping optical fields (813 nm and 390 nm for strontium atoms) minimises frequency shifts in the intercombination clock transition caused by variations in optical field intensity. However, this frequency shift can be compensated for using optical AC Stark shifts, which are generated by the trapping optical fields and have frequencies below the 'magic frequencies'. Furthermore, it has been demonstrated that atom chip clocks can operate using solely repulsive, blue-detuned evanescent light fields when the attraction of the trapped atoms towards the photonic waveguide is realised through van der Waals forces alone. According to estimates, this could result in the short-term fractional frequency stability of the strontium atom-chip optical clocks being limited to ~10^{-16} per second, with an accuracy of about 5 x 10^{-15} for fractional intensity stability in the trapping light fields of 0.001.
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.
Abstract The atomic clocks, whether operating at optical or microwave region, can be divided into two categories according to their working mode, namely the passive clocks and active clocks. The passive clocks, whose standard frequency is locked to an ultra-narrow atomic spectral line, such as laser cooled Cs beam or lattice trapped Sr atoms, depend on the spontaneous emission line. On the contrary, the active clocks, in which the atoms are used as the gain medium, are based on the stimulated emission radiation, their spectrum can be directly used as the frequency standard. Up to now, the active hydrogen maser has been the most stable microwave atomic clocks. Also, the Sr superradiant active atomic clock is prospects for a millihertz-linewidth laser. Moreover, the optical clocks are expected to surpass the performance of microwave clocks both in stability and uncertainty, since their higher working frequency. The active optical clock has the potential to improve the stability of the best clocks by 2 orders of magnitude. In this work, we introduce the development of active optical clocks, and their types is classified according to the energy-level structure of atoms for stimulated radiation.