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Development of a portable laser-cooled ytterbium ion microwave atomic clock

Sean Mulholland

Published Jan 1, 2019• 1 citation
Publisher

Abstract

Portable microwave clocks that incorporate laser cooling promise to bring significantly improved performance compared to commercially available microwave clocks that are based on thermal atoms. This thesis reports on the development of a portable, laser-cooled ytterbium-171 trapped-ion microwave frequency standard from design to testing.
DOI
10.5287/ora-bpqggg20x

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A portable microwave clock incorporating laser-cooling of171Yb+promises to bring significantly improved performance compared to existing thermal commercial devices. We report on progress in the development of a compact microwave frequency standard designed to fit in a 19-inch rack enclosure that comprises laser, electronics and physics package subsystems. Recent results in which a local oscillator was disciplined by laser-cooled trapped171Yb+ions are presented. Preliminary measurements have demonstrated a fractional frequency instability of 1 × 10-13at 10,000 s. Potential applications include telecommunications and financial operations time reference systems as well as navigation system integrity.
We report on the development of a portable, trapped-ion, microwave frequency standard based on the 12.6 GHz hyperfine transition in laser-cooled ytterbium-171 ions. The entire system fits into a 6U 19-inch rack unit (51×49×2851\times49\times2851×49×28 cm) and comprises laser, electronics and physics package subsystems. The performance of this development system is evaluated; the fractional frequency instability was measured to be 3.6×10−12/√τ3.6\times10^{-12}/\surd\tau3.6×10−12/√τ for averaging times between 30 s and 1500 s.
High performance transportable microwave atomic clocks are enablers for resilient GNSS-free timescales. We present progress towards a transportable laser-cooled ytterbium ion microwave clock. This system incorporates our recently developed multi-ion trap chamber and redesigned electronics and optics systems, offering reductions in size, weight and power, with improved performance over our previous system. The benchtop prototype version of this clock uses the new physics package design and reaches a short-term instability of 1.2 × 10−12/√τ. We present the transportable clock design as well as subsystem test results, which so far indicate that a similar performance to the benchtop version is achievable in a 3U rack-mounted package.
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.
Abstract As a candidate for the next generation microwave clock, microwave atomic clocks based on trapped ions are currently studied worldwide. Since 2010, our team at Tsinghua University has been committed to developing a high-performance 113Cd+ microwave ion clock. In 2021, the laser-cooled cadmium-ion microwave clock constructed in our laboratory demonstrated remarkable performance, with a short-term instability of 4.2 × 10 − 13 / τ and a relative frequency uncertainty of 1.8 × 10−14, improving nearly three orders of magnitude compared to the result from JPL. The 0-0 ground-state hyperfine splitting frequency of 113Cd+ was determined to be 15199862855.02799(27) Hz, currently the most accurate measurement result for this transition frequency. Apart from that, we introduced the sympathetic cooling technology into the experiment of cadmium-ion microwave clock. Using 40Ca+ as the coolant ion, we obtained Ramsey patterns with a free evolution time of up to 100 seconds, ultimately improving the short-term instability of the system to 3.48 × 10 − 13 / τ . Moreover, a microwave frequency standard based on laser-cooled 171Yb+ ions has also been developed in our laboratory since 2021. The short-term frequency instability was measured to be 8.5 × 10 − 13 / τ . The ground-state hyperfine splitting frequency of 171Yb+ was determined to be 12642812118.4674(8) Hz with a fractional frequency uncertainty of 6.33 × 10−14.
The stability and accuracy of atomic microwave frequency standards (atomic clocks) have been improving at a rate exceeding one order of magnitude per decade for the past 30 years. This sustained improvement has been driven mainly by the many and diverse technological applications requiring highly stable and accurate time and frequency standards. There is no reason to suspect that this rate of progress is slowing. Over the last decade, research and development in atomic frequency standards has tended increasingly towards the use of atom and ion trapping technologies to approach the supposedly ideal unperturbed atomic frequency reference consisting of a single atom or ion either motionless or in free-fall in a perfect, field-free vacuum. This work has been facilitated by the relatively recent development of techniques whereby laser light is used to cool, confine and manipulate atoms or ions. This paper reviews the current status of atomic microwave frequency standards based on trapped atoms and trapped ions, and attempts to explain the motivation for their development and the principles of their operation, with particular emphasis on the physics of factors which limit their performance.