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European Frequency and Time Forum

Progress Towards a Transportable Laser-Cooled Yb+Microwave Clock

M. A. Trigatzis, Martin Knapp, Sean Mulholland, Gary I. Hockley, G. P. Barwood, Guilong Huang, Hugh A. Klein, Patrick Robert Gill

Published Jun 25, 2024• 2 citations
Publisher

Abstract

We present progress towards a transportable laser-cooled ytterbium ion microwave clock. We have designed and built a new trap chamber which we describe in this paper. Initial results of Rabi spectroscopy and clock stability are presented, and the next steps are outlined.
DOI
10.1109/eftf61992.2024.10722583

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Stable and accurate timekeeping is essential in today’s commercial and military applications. For decades, the 5071A Cesium beam clock has been the mainstay for long-term holdover in strategic applications for its unmatched performance. However, is too large, power hungry and cumbersome for most mobile platforms. To enable improved performance in lower size, weight and power applications, a new physics solution is required over traditional clock technology. In this paper we will present our Ytterbium microwave ion clock progress as a small form factor, high performance clock for use in military and commercial applications where portability is critical. Our device targets maximum performance improvement over traditional methods while minimizing technical risk, leveraging existing manufacturing, and targeting the lowest possible size, power and cost. The Physics Package includes a linear quadrupole trap, an electron gun (e-gun) ionizer, a passive getter pump and a Yb oven. We use electron impact ionization to load Ytterbium-171 ions which are cooled down to room temperature using buffer gas. Interaction-time limited Rabi spectroscopy using microwave radiation at 12.6 GHz is performed. The resulting microwave Rabi linewidth is used for closed-loop clock operation. Concurrent work is underway to develop a 369 nm Distributed Bragg Reflector laser which is essential for robust operation of the clock. We will discuss the specifications, the detailed design of our Ytterbium Microwave Ion Clock, the first assembled Physics Package and related preliminary test results.
We present a transportable laser-cooled ion microwave clock based on the ground-state hyperfine splitting of 171 Yb + . The system achieves a systematic uncertainty of 7.8×10 −15 and a short-term frequency stability of 9.5×10 −13 / τ . Enhancements to the magnetic-field system suppress the decoherence process in the multi-body ion ensemble and reduce the second-order Zeeman frequency shift by a factor of two. The absolute microwave frequency is measured to be 12 642 812 118.468 6(3) Hz, consistent with the previous measurements and demonstrating twice the precision of the current state-of-the-art.
Abstract Compact atomic clocks provide the stable, traceable timing required for 5G/6G telecom networks, for resilient operation of critical infrastructure including power grids, data centres and financial systems, and for maintaining accurate synchronisation during GNSS interruptions or outages. Microwave clocks based on caesium beam tubes have been the industry-standard rack-mounted clock for many years but are now outperformed in short-to-midterm instability by emerging technologies. Vapour cell clocks suffer from long-term drift which must be calibrated out periodically and on startup. This limits their use for long-term holdover and in applications where the device needs to operate after a cold start. Additionally, optical clocks require a narrow-linewidth clock laser and optical frequency comb. The ytterbium ion microwave clock developed at the National Physical Laboratory (NPL) does not require these subsystems and combines this relative simplicity with the benefits of laser cooling (long-term stability, accuracy). In this paper, we describe an ion trap physics package design where its functionality has been evaluated in a benchtop clock and forms the basis of a new design for a compact, transportable clock. We describe the operation and performance of the benchtop clock, demonstrating a short-term fractional frequency instability of 1.4 × 10 − 12 /√τ. The design for a new transportable clock based on this new physics package is detailed, as are the validation tests of other key subsystems.