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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Mesoscopic physics in vapor-cell atomic clocks

James C. Camparo, Andrew Hudson

Published Jul 1, 2017• 3 citations
Publisher

Abstract

In the early 1980s, it was common to hear that Rb atomic clocks were limited to Sy(τ) = 10-11/τ1/2at best, and that their ultimate frequency stability could never be better than low-parts of 10-13. Twenty-first century Rb clocks put the lie to those pronouncements. Today, it is not uncommon for vapor-cell clocks to exhibit short-term stabilities at low-parts of 10-13/τ1/2, and ultimate stability levels in parts of 10-15. Here, we discuss what it will take to improve those clocks further (e.g., getting the long-term stability to break into the 10-16range). Specifically, it is our contention that next steps will require a better understanding of mesoscopic physics in vapor-phase systems: atomic phenomena occurring on a scale between the microscopic level of quantum dynamics and the macroscopic level of vapor temperature and pressure. Though at present we have some understanding of mesoscopic physics and its effects, much more needs to be understood. Further, while past and present advances have been driven by “one-off” phenomenological discoveries, it is our opinion that rapid advancement will be best facilitated by a more overarching vision of mesoscopic vapor-phase physics, which at present is lacking.

DOI
10.1109/fcs.2017.8088797

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In the early 1980s, it was common to hear that Rb atomic clocks were limited to Sy(τ) = 10-11/τ1/2at best, and that their ultimate frequency stability could never be better than low-parts of 10-13. Twenty-first century Rb clocks put the lie to those pronouncements. Today, it is not uncommon for vapor-cell clocks to exhibit short-term stabilities at low-parts of 10-13/τ1/2, and ultimate stability levels in parts of 10-15. Here, we discuss what it will take to improve those clocks further (e.g., getting the long-term stability to break into the 10-16range). Specifically, it is our contention that next steps will require a better understanding of mesoscopic physics in vapor-phase systems: atomic phenomena occurring on a scale between the microscopic level of quantum dynamics and the macroscopic level of vapor temperature and pressure. Though at present we have some understanding of mesoscopic physics and its effects, much more needs to be understood. Further, while past and present advances have been driven by “one-off” phenomenological discoveries, it is our opinion that rapid advancement will be best facilitated by a more overarching vision of mesoscopic vapor-phase physics, which at present is lacking.
Vapor-phase atomic clocks and atomic magnetometers are achieving levels of stability and measurement precision today that would have been considered unrealistic in the not too distant past. To make further progress, researchers will need a more detailed understanding of vapor-phase atomic physics, in particular, the manner in which mesoscopic variations of atomic perturbations map onto the observed resonant phenomena: length scales larger than that of homogeneous quantum dynamics, where atoms within some localized region of the vapor all experience the same optical and microwave fields and collisional perturbations; but smaller than that of the vapor as a whole, where the atomic system is described in terms of (for example) vapor pressure, heat content, and optical depth. In this paper, we discuss the issue of mesoscopic physics as it manifests itself in vapor-phase collision-shift gradients affecting the 0-0 hyperfine transition of 87Rb^{87}\mathrm{Rb}87Rb. We show that these gradients not only produce increased line broadening but lead to lineshape asymmetry and an increased sensitivity of the observed resonance to power-broadening. We develop a statistical theory describing the experimental results, and through that theory we show that mesoscopic atomic physics encompasses a broader class of phenomena than that described by the circumscribed term ``inhomogeneous broadening.''
Two-photon vapor cell-based optical clocks are strong candidates for next-generation portable atomic standards, offering simplicity, compactness, and high performance. Their narrow clock transitions with counter-propagating beams enable first-order Doppler-free operation. However, systematic perturbations such as the AC Stark shift, temperature-induced shift, and drifts resulting from the laser system pose challenges cause instabilities to medium- to long-term performance. This paper provides a comprehensive overview of Rb two-photon vapor cell optical standards, focusing on the long-term performance-limiting effects and potential mitigation strategies, aiming for clock stabilities better than 1 × 10−15 over the averaging time of a day and beyond.
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.
Abstract We describe three independent research directions focussed on the development of next-generation chip-scale atomic clocks, which combine small size, low power consumption and manufacturability with high frequency stability. The use of optical transitions in microfabricated vapor cells improves both short- and long-term frequency stability to near 10−13 at the cost of the added complexity of a chip-scale optical frequency comb. Chip-scale atomic beam microwave clocks have been successfully demonstrated and offer the potential for microsecond-level timing accuracy over one week. And finally, Sr vapor cells have been developed with lifetimes exceeding 250 hours and residual gas pressures below 1 Torr.
As is well known, the phase fluctuations of a laser can decrease the signal-to-noise ratio of a vapor-cell clock due to laser PM-to-AM conversion. In previous work, we demonstrated an easy means of eliminating this problem: use high-pressure resonance cells so that collisional dephasing keeps the atoms from following the laser's fluctuating phase. Though our phase-1 clock employing a 100 torr N2 cell had excellent short-term performance, the long-term performance left much to be desired, quite likely because the increased pressure in the resonance cell gave the clock a fairly large temperature sensitivity. Here, we discuss our progress towards a phase-2 laser-pumped Rb clock, where the temperature sensitivity will be much reduced.
Atomic clocks based on laser-cooled atoms have made tremendous advances in both accuracy and stability. However, advanced clocks have not found their way into widespread use because there has been little need for such high performance in real-world/commercial applications. The drive in the commercial world favours smaller, lower-power, more robust compact atomic clocks that function well in real-world non-laboratory environments. Although the high-performance atomic frequency references are useful to test Einstein's special relativity more precisely, there are not compelling scientific arguments to expect a breakdown in special relativity. On the other hand, the dynamics of gravity, evidenced by the recent spectacular results in experimental detection of gravity waves by the LIGO Scientific Collaboration, shows dramatically that there is new physics to be seen and understood in space–time science. Those systems require strain measurements at less than or equal to 10 −20 . As we discuss here, cold atom optical frequency references are still many orders of magnitude away from the frequency stability that should be achievable with narrow-linewidth quantum transitions and large numbers of very cold atoms, and they may be able to achieve levels of phase stability, Δ Φ / Φ total ≤ 10 −20 , that could make an important impact in gravity wave science. This article is part of the themed issue ‘Quantum technology for the 21st century’.