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European Frequency and Time Forum

CPT atomic clock based on an antirelaxation-coated cell and quadrature-signal method of the light shift cancellation

S M Ignatovich, V. I. Vishnyakov, A. O. Makarov, Mikhail N Skvortsov, N. L. Kvashnin, V. A. Vasiliev, Sergey Nikitich Atutov, Denis V. Brazhnikov, Valeriy I. Yudin, А. В. Тайченачев, Sergei N Bagayev

Published Apr 1, 2018• 3 citations
Publisher

Abstract

High-stability and compact clocks are in great demand for many applications, for example, such as global navigation satellite systems (GNSS), broadband communication networks and high-secret lines, smart electricity grids, etc. Many applications of the clocks require small size and weight (V3, m-12per a day). The most promising technology involves approach based on the coherent populations trapping (CPT) phenomenon, because it can combine all the features listed. Here we present the recent progress of development a version of CPT atomic clocks, which involves an antirelaxation-coated cell and the quadrature-signal method for the light shift cancellation.

DOI
10.1109/eftf.2018.8409003

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High-stability and compact clocks are in great demand for many applications, for example, such as global navigation satellite systems (GNSS), broadband communication networks and high-secret lines, smart electricity grids, etc. Many applications of the clocks require small size and weight (V3, m-12per a day). The most promising technology involves approach based on the coherent populations trapping (CPT) phenomenon, because it can combine all the features listed. Here we present the recent progress of development a version of CPT atomic clocks, which involves an antirelaxation-coated cell and the quadrature-signal method for the light shift cancellation.
Atomic clocks based on coherent population trapping and constructive polarization modulation are promising for high performance and compact devices. We have demonstrated a cw mode CPT clock with a short-term fractional frequency stability at the level of 3. \pmb2×10-13/√{τ} for a few 100 s integration time. This Double-Modulation CPT technology can be reduced to a miniature and robust setup due to its linear architecture. This clock could find future applications in industry, telecommunications, instrumentation, or global navigation satellite systems.
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.
We propose a novel way of radiation wavelength stabilisation in atomic clocks based on coherent population trapping (CPT). It uses quadrature-phase component of the CPT signal within the CPT resonance feedback loop and automatic wavelength locking of radiation from a VCSEL to the atomic absorption line. We demonstrate advantages and limitations of the new method applied to vapour cells with buffer gas or anti-relaxation coating. Also provided are measurements of short- and long-term stability of a CPT-based atomic clock using the proposed method with various optical cells. Discussed are the prospects of this method in chip-scale atomic clocks.
We report on the mid-term stability progress of a table-top coherent population trapping (CPT) microcell atomic clock, previously limited by light-shift effects and variations of the cell's inner atmosphere. The light-shift contribution is now mitigated through the use of a pulsed symmetric auto-balanced Ramsey (SABR) interrogation technique, combined with setup temperature, laser power, and microwave power stabilization. In addition, Ne buffer gas pressure variations in the cell are now greatly reduced through the use of a micro-fabricated cell built with low permeation alumino-silicate glass (ASG) windows. Combining these approaches, the clock Allan deviation is measured to be 1.4 × 10 −12 at 10 5 s. This stability level at one day is competitive with the best current microwave microcell-based atomic clocks.
Field-grade atomic clocks capable of primary standard performance in compact physics packages would be of significant value in a variety of applications ranging from network synchronization and secure communications to GPS hold-over and inertial navigation. A cold-atom coherent population trapping (CACPT) clock featuring laser-cooled 87RB atoms and pulsed Ramsey interrogation is a strong candidate for this technology if the principal frequency shifts can be controlled and the performance degradation associated with miniaturization can be overcome. In this thesis, research focused on the development of this type of compact atomic clock is presented. To address the low atom numbers obtained in small cold-atom sources, experiments were performed in which an atomic beam was decelerated with bichromatic stimulated laser forces and loaded into a mm-scale magneto-optical trap, increasing the atom number by a factor of 12.5x. A CACPT clock using the high-contrast lin||lin optical interrogation technique was developed and achieved a stability of 7 × 10-13 after one hour of integration. Doppler shifts in the clock are explained using a simple kinematic model and canceled by interrogating the atoms with a counter-propagating CPT configuration. Finally, a thorough characterization of the AC-stark effect in lin||lin CPT was performed. Observed shifts are explained in terms of contributions from coherent CPT-generating couplings and population transfer effects caused by optical pumping from incoherent light. Measurements are compared with existing and new theoretical treatments, and a laser configuration is identified that reduces clock drift from light shifts to less than 10-14 for the current system.
In recent years, the coherent population trapping (CPT) phenomenon using a very small cell has attracted considerable attention for its potential application to ultra-compact atomic clock. For the miniaturization of CPT clock, it is important to clarify influence by the collision with atoms and cell walls. We study the effect of cell geometry on the CPT spectrum using a thin Cs vapor cell without buffer gas. When the cell length is below 1 mm and the laser intensity is sufficiently weak, the transit-time broadening is largely suppressed because only slow atoms have enough time to settle down on the dark state and contribute to the CPT signals. Furthermore, we show that introduction of buffer gas may broaden the CPT resonance in extremely thin cells. We also show that sub-Doppler signal which can be used for stabilization of laser frequency is obtained by using the CPT signal.