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Applied Physics Letters

High contrast dark resonances in a cold-atom clock probed with counterpropagating circularly polarized beams

Xiaochi Liu, Valeriy I. Yudin, А. В. Тайченачев, John E. Kitching, Elizabeth A. Donley

Published Nov 27, 2017• 44 citations
Publisher

Abstract

A cold-atom coherent population trapping clock based on σ+−σ− interrogation realized by counter-propagating optical fields of opposite circular polarization is presented. The simultaneous use of σ+ and σ− polarizations prevents atoms from being trapped in the end magnetic sublevels, significantly enhancing the contrast over interrogation with a single circular polarization. Because the system is based on cold atoms and there is very little relaxation, nearly complete dark states are created, and coherent population trapping resonances with maximum contrast are observed. A frequency stability of 1.3×10−11/√τ is achieved, which averages down to 2×10−13 after a 40 000 s integration period.

DOI
10.1063/1.5001179

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A cold-atom coherent population trapping clock based on σ+−σ− interrogation realized by counter-propagating optical fields of opposite circular polarization is presented. The simultaneous use of σ+ and σ− polarizations prevents atoms from being trapped in the end magnetic sublevels, significantly enhancing the contrast over interrogation with a single circular polarization. Because the system is based on cold atoms and there is very little relaxation, nearly complete dark states are created, and coherent population trapping resonances with maximum contrast are observed. A frequency stability of 1.3×10−11/√τ is achieved, which averages down to 2×10−13 after a 40 000 s integration period.
We present results from an atomic clock that employs a beam of cold 87Rb atoms and spatially separated (Ramsey) coherent population trapping interrogation of the hyperfine clock transition at 6.834 GHz. The cold atomic beam is generated through the use of a 2D+-magneto-optical trap. The interrogation is performed on the D2 line of 87Rb, and the optical fields use a counter-propagating σ+–σ− probing scheme. The use of cold atoms allows for relatively narrow Ramsey fringes even for a small spatial separation between the two interrogation zones (4.6 cm). The resulting clock has a short-term stability of 3×10−11/τ.
We present a coherent population trapping clock system based on laser-cooled 87Rb atoms. The clock consists of a frequency-stabilized CPT interrogation laser and a cooling laser as well as a compact magneto-optical trap, a high-performance microwave synthesizer, and a signal detection system. The resonance signal in the continuous wave regime exhibits an absorption contrast of ∼ 50%. In the Ramsey interrogation method, the linewidth of the central fringe is 31.25 Hz. The system achieves fractional frequency stability of 2.4 × 10 − 11 / τ , which goes down to 1.8 × 10−13 at 20000 s. The results validate that cold atom interrogation can improve the long-term frequency stability of coherent population trapping clocks and holds the potential for developing compact/miniature cold atoms clocks.
Miniature atomic clocks based on coherent population trapping (CPT) states in thermal atoms are an important component in many field applications, particularly where satellite frequency standards are not accessible. Cold-atom CPT clocks promise improved accuracy and stability over existing commercial technologies. Here we demonstrate a cold-atom CPT clock based on 87{}^{87}87Rb using a high-contrast double-$\ensuremath{\Lambda}$ configuration. Doppler frequency shifts are explained using a simple model and canceled by interrogating the atoms with counterpropagating light beams. We realize a compact cold-atom CPT clock with a fractional frequency stability of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$, thus demonstrating the potential of these devices. We also show that the long-term stability is currently limited by the second-order Zeeman shift to $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at 1000 s.
A review of a cold-atom clock based on coherent population trapping that highlights recent progress will be presented. Improvements in the coherence of the interrogation spectrum have resulted in the generation of dark states in the cold Rb atoms with essentially 100 % transmission – evidence that decoherence in this system is negligible. This improvement in coherence has resulted in improved short-term stability at the level of 1.5E-11 fractional frequency stability for a one second integration period. In combination with improved interrogation schemes, the improved spectrum has also resulted in dramatically smaller light shifts and improved long-term frequency stability – with the clock averaging down to the level of 1E-13 fractional frequency stability on time scales of over 10,000 seconds.
Traditional coherent population trapping (CPT) atomic clocks use single circularly polarized coherent multichromatic light to interact with atoms for preparing CPT dark states. While this configuration is simple, it suffers from the trap states and involves only the ±1st-order sidebands in the CPT resonance, excluding the carrier and other sidebands. This results in a lower CPT signal amplitude and higher laser noise, limiting the frequency stability of CPT atomic clocks. To address this issue, we developed and experimentally validated a scheme utilizing left- and right-circularly polarized light based on multiple coherent superpositions. This scheme significantly enhances the amplitude of the CPT resonance. Furthermore, by incorporating a Michelson interferometer, the polarization direction of the ±1st-order sidebands in the coherent multichromatic light is adjusted from parallel to perpendicular, enabling the filtering of one sideband to be detected while eliminating other unwanted sidebands and effectively suppressing laser noise. The results show that, under identical conditions, the short-term frequency stability of this method is theoretically improved by a factor of seven compared to the traditional circularly polarized light scheme, providing an effective pathway for the development of high-performance CPT atomic clocks.
We propose a constructive polarization modulation scheme for atomic clocks based on coherent population trapping (CPT). In this scheme, the polarization of a bichromatic laser beam is modulated between two opposite circular polarizations to avoid trapping the atomic populations in the extreme Zeeman sublevels. We show that if an appropriate phase modulation between the two optical components of the bichromatic laser is applied synchronously, the two CPT dark states which are produced successively by the alternate polarizations add constructively. Measured CPT resonance contrasts up to 20% in one-pulse CPT and 12% in two-pulse Ramsey-CPT experiments are reported, demonstrating the potential of this scheme for applications to high performance atomic clocks.