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Towards an Accuracy Budget of Cold-Atom-Based Commercial Microwave Clocks

Luc Archambault, Floriane Sparma, L. Lorini, Arnaud Landragin, Bruno Desruelle, P. Rosenbusch, Bruno Pelle

Published Jun 25, 2024• 0 citations

Abstract

We present results of commercial microwave clocks developed by Exail Quantum Systems (formerly Muquans). The MuClocks are cold-atom-based turnkey products dedicated to continuous long-term operation. A frequency stability of 1×10-15 over more than a month has been demonstrated on several set-ups, competing with the frequency drift (~1×10-16/day) of the best masers. Recent stability evaluations show for the best clock a long-term frequency stability of 7×10-16 over two weeks and for the longest run ~2×10-15 over two months. IIn this contribution, we will present a preliminary accuracy budget, the targeted product accuracy being in the 5×10-15 range. Control and evaluation of the known frequency biases is on-going. Several frequency shifts have already been quantified: the Quadratic Zeeman shift, the cavity pulling including its asymmetry from nearby cavity resonances, microwave phase transients and the Ramsey pulling.

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We present results of commercial microwave clocks developed by Exail Quantum Systems (formerly Muquans). The MuClocks are cold-atom-based turnkey products dedicated to continuous long-term operation. A frequency stability of 1×10-15 over more than a month has been demonstrated on several set-ups, competing with the frequency drift (~1×10-16/day) of the best masers. Recent stability evaluations show for the best clock a long-term frequency stability of 7×10-16 over two weeks and for the longest run ~2×10-15 over two months. IIn this contribution, we will present a preliminary accuracy budget, the targeted product accuracy being in the 5×10-15 range. Control and evaluation of the known frequency biases is on-going. Several frequency shifts have already been quantified: the Quadratic Zeeman shift, the cavity pulling including its asymmetry from nearby cavity resonances, microwave phase transients and the Ramsey pulling.

The medium long-term frequency stability of the integrating sphere cold atom clock was improved. During the clock operation, Rb87 atoms were cooled and manipulated using cooling light diffusely reflected by the inner surface of a microwave cavity in the clock. This light heated the cavity and caused a frequency drift from the resonant frequency of the cavity. Power fluctuations of the cooling light led to atomic density variations in the cavity’s central area, which increased the clock frequency instability through the cavity pulling effect. We overcame these limitations with appropriate solutions. A frequency stability of 3.5×10−15 was achieved when the integrating time τ increased to 2×104 s.

We present the long-term stability of the integrating sphere cold atom clock (ISCAC) and analyze its systematic limitations. The relative frequency instability of 2.6×10−15 is reached for an averaging time of 2×105 s. The second-order Zeeman effect and the cavity pulling effect in ISCAC, which would induce the frequency drift from the clock transition, are analyzed. The analytical and experimental results indicate that the cavity pulling effect is the main contribution to the long-term frequency instability of the ISCAC. Further technical improvements to the microwave cavity are also discussed.