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Physical Review Applied

Toward a High-Stability Coherent Population Trapping Cs Vapor-Cell Atomic Clock Using Autobalanced Ramsey Spectroscopy

Moustafa Abdel Hafiz, Grégoire Coget, Michael Petersen, C. Rocher, Stéphane Guérandel, T. Zanon-Willette, Emeric de Clercq, Rodolphe Boudot

Published Jun 1, 2018• 66 citations
Publisher

Abstract

The next generation of alkali-vapor-cell atomic clocks are under development in labs around the world, with prototypes already demonstrating short-term fractional frequency stabilities one or two orders of magnitude better than for commercial clocks. However, stability is often spoiled at medium time scales (>100 s) by light-power-induced frequency-shift effects. The authors tackle this problem by applying autobalanced Ramsey interrogation, recently demonstrated on a Yb${}^{+}$ clock, to coherent-population-trapping systems---an approach that could really boost the development of vapor-cell clocks with white-noise-limited frequency stability over greatly extended time scales.

DOI
10.1103/physrevapplied.9.064002

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The next generation of alkali-vapor-cell atomic clocks are under development in labs around the world, with prototypes already demonstrating short-term fractional frequency stabilities one or two orders of magnitude better than for commercial clocks. However, stability is often spoiled at medium time scales (>100 s) by light-power-induced frequency-shift effects. The authors tackle this problem by applying autobalanced Ramsey interrogation, recently demonstrated on a Yb+{}^{+}+ clock, to coherent-population-trapping systems---an approach that could really boost the development of vapor-cell clocks with white-noise-limited frequency stability over greatly extended time scales.
We report a high-performance pulsed coherent population trapping (CPT) Cs cell atomic clock using the implementation of a symmetric auto-balanced Ramsey (SABR) interrogation sequence. The latter method is found to reduce the light-power induced frequency shift by an order of magnitude compared to a previous experiment using a simple auto-balanced Ramsey interrogation. The contribution of this shift to the clock frequency stability is now rejected in the 10−16 range at 104 s averaging time. Additional tricks, including a compensation method to reduce the laser amplitude noise contribution and the generation of novel error signals for local oscillator frequency and phase correction, have been implemented using a FPGA-based digital electronics board in order to improve the clock short-term stability by a of factor 2. The Allan deviation of the SABR-CPT clock, extracted from a selected 3 × 104 s-long dataset, is 2 × 10−13 τ−1∕2 and averages down to the level of 2.5 × 10−15 at 104 s. These results are encouraging to stimulate the development of hot cell CPT-based clocks for industrial, scientific, and instrumentation applications.
We demonstrate a high-performance coherent-population-trapping (CPT) Cs vapor cell atomic clock using the push-pull optical pumping technique in the pulsed regime, allowing the detection of high-contrast and narrow Ramsey-CPT fringes. The impact of several experimental parameters onto the clock resonance and short-term fractional frequency stability, including the laser power, the cell temperature, and the Ramsey sequence parameters, has been investigated. We observe and explain the existence of a slight dependence on laser power of the central Ramsey-CPT fringe line-width in the pulsed regime. We report also that the central fringe line-width is commonly narrower than the expected Ramsey line-width given by 1/(2TR), with TR the free-evolution time, for short values of TR. The clock demonstrates a short-term fractional frequency stability at the level of 2.3×10−13 τ−1/2 up to 100 s averaging time, mainly limited by the laser amplitude modulation noise. Comparable performances are obtained in the conventional continuous wave regime, with the use of an additional laser power stabilization setup. The pulsed interaction allows to reduce significantly the clock frequency sensitivity to laser power variations, especially for high values of TR. This pulsed CPT clock, ranking among the best microwave vapor cell atomic frequency standards, could find applications in telecommunication, instrumentation, defense or satellite-based navigation systems.
Vapor cell atomic clocks exhibit reduced frequency stability for averaging time between about one hundred and a few thousand seconds. Here we report a study on the impact of the main parameters on the mid-to-long term instability of a buffer-gas vapor cell Cs clock, based on coherent population trapping (CPT). The CPT signal is observed on the Cs D1 line transmission, using a double Λ scheme and a Ramsey interrogation technique. The effects on the clock frequency of the magnetic field, the cell temperature, and the laser intensities are reported. We show in particular that the laser intensity shift is temperature dependent. Along with the laser intensity ratio and laser polarization properties, this is one of the most important parameters.
We propose a preliminary theoretical investigation and demonstrate experimentally the application of the autobalanced Ramsey (ABR) interrogation protocol onto a pulsed hot vapor Cs cell microwave atomic clock based on coherent population trapping (CPT). This approach, based on the alternation of two successive Ramsey-CPT sequences with unequal free-evolution times and the subsequent management oftwo interconnected phase and frequency servo loops, is found to allow a reduction of the clock frequency sensitivity to laser power variations. This approach, combined with the implementation of advanced electronics laser power stabilization systems, yields the demonstration of a CPT-based Cs cell clock with a fractional frequency stability at the level of 3.1 × 10−13 τ−1/2 and averaging down to the level of 7 ×10−15 at 2000 s integration time.
The mid-term fractional frequency stability of miniaturized atomic clocks can be limited by light-shift effects. In this Letter, we demonstrate the implementation of a symmetric auto-balanced Ramsey (SABR) interrogation sequence in a microcell-based atomic clock based on coherent population trapping. Using this advanced protocol, the sensitivity of the clock frequency to laser power, microwave power, and laser frequency variations is reduced, at least by one order of magnitude, in comparison with continuous-wave or Ramsey interrogation schemes. Light-shift mitigation obtained with the SABR sequence benefits greatly to the clock Allan deviation for integration times between 102 and 105 s. These results demonstrate that such interrogation techniques are of interest to enhance the timekeeping performance of chip-scale atomic clocks.
A gas cell atomic frequency standard based on coherent population trapping (CPT) with Ramsey interrogation pulses is developed. The clock resonance is detected in a Cs cell with buffer gas using two phase-locked diode lasers tuned to the D1 line. A preliminary short-term stability of3.5×10−12t−1/23.5 \times 10^{-12}t^{-1/2}3.5×10−12t−1/2is reported. The CPT specificity allows a new interrogation–detection sequence, i.e., the continuous pulse train, which, with phase modulation, could lead to better short-term stability compared to usual continuous CPT clocks. The parameters optimizing the stability include a high Rabi frequency, a very short measurement time, and an interrogation time equal to the lifetime of the hyperfine coherence.