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IEEE Spectrum

Quantum Tech: Squeezing an Optical Atomic Clock Into a Briefcase: A Mind-Set Shift Propelled the Leap from Basic Science to Invention

Dina Genkina

Published Nov 1, 2024• 1 citation

Abstract

WALKING INTO Jun Ye's lab at the University of Colorado Boulder is a bit like walking into an electronic jungle. There are wires strung across the ceiling that hang down to the floor. Right in the middle of the room are four hefty steel tables with metal panels above them extending all the way to the ceiling. Slide one of the panels to the side and you'll see a dense mesh of vacuum chambers, mirrors, magnetic coils, and laser light bouncing around in precisely orchestrated patterns. ¶ This is one of the world's most precise and accurate clocks, and it's so accurate that you'd have to wait 40 billion years-or three times the age of the universe-for it to be off by one second. ¶ What's interesting about Ye's atomic clock, part of a joint endeavor between the University of Colorado Boulder and the National Institute of Standards and Technology (NIST), is that it is optical, not microwave like most atomic clocks. The ticking heart of the clock is the strontium atom, and it beats at a frequency of 429 terahertz, or 429 trillion ticks per second. It's the same frequency as light in the lower part of the red region of the visible spectrum, and that relatively high frequency is a pillar of the clock's incredible precision. Commonly available atomic clocks beat at frequencies in the gigahertz range, or about 10 billion ticks per second. Going from the microwave to the optical makes it possible for Ye's clock to be tens of thousands of times as precise.

DOI: 10.1109/mspec.2024.10749725

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WALKING INTO Jun Ye's lab at the University of Colorado Boulder is a bit like walking into an electronic jungle. There are wires strung across the ceiling that hang down to the floor. Right in the middle of the room are four hefty steel tables with metal panels above them extending all the way to the ceiling. Slide one of the panels to the side and you'll see a dense mesh of vacuum chambers, mirrors, magnetic coils, and laser light bouncing around in precisely orchestrated patterns. ¶ This is one of the world's most precise and accurate clocks, and it's so accurate that you'd have to wait 40 billion years-or three times the age of the universe-for it to be off by one second. ¶ What's interesting about Ye's atomic clock, part of a joint endeavor between the University of Colorado Boulder and the National Institute of Standards and Technology (NIST), is that it is optical, not microwave like most atomic clocks. The ticking heart of the clock is the strontium atom, and it beats at a frequency of 429 terahertz, or 429 trillion ticks per second. It's the same frequency as light in the lower part of the red region of the visible spectrum, and that relatively high frequency is a pillar of the clock's incredible precision. Commonly available atomic clocks beat at frequencies in the gigahertz range, or about 10 billion ticks per second. Going from the microwave to the optical makes it possible for Ye's clock to be tens of thousands of times as precise.

Frequency and its inverse, time, are the most accurately measured quantities. Historically, improvements in the accuracy of clocks have enabled advances in navigation, communication, and science. Since 1967, the definition of the International System (SI) second has been based on the frequency of a microwave transition in cesium, and present day cesium atomic clocks have a fractional uncertainty near 10-16. Recently, a new type of atomic clock based on optical transitions has been developed, with a current fractional uncertainty near 10-18 (approximately one second divided by the age of the universe), and they are rapidly improving. This talk presents a brief summary of the development of optical atomic clocks, with a focus on the Al+ quantum-logic clock developed at NIST. We discuss the current state-of-the-art in optical clock performance, and describe new applications in sensing and fundamental physics. Future directions in optical atomic clock research are also considered.

Since 1967, the Second has been defined by the CGPM as 9,192,631,770 periods of oscillation corresponding to a transition between two hyperfine levels in a Cesium atom with zero external fields. Key to this definition was the fact that as experimentalists found new and more precise ways to measure this frequency, the definition of the second would become more and more accurate with time. However, in the last 30 years, new technologies based on tunable lasers addressing optical transitions in atoms, ions, and molecules have offered an entirely new approach to defining the Second with significantly higher precision and accuracy. Here, I will show that by trapping thousands of atoms inside a specially engineered optical lattice one can create an extremely accurate frequency standard with 2 orders of magnitude improvement over current Cs standards. Furthermore, I will explain that standards based on this technology are fundamentally more stable than the primary standard by 3 orders of magnitude. Leapfrogging the currently held accuracy records of ion clocks, this work documents the first optical lattice clock to best all other atomic clock implementations, a mere 8 years after the first proof of principle experiments. In this thesis I will describe how we have overcome a number of important systematic uncertainties to achieve these results. This revolution in accuracy and precision opens the door for new experiments utilizing the clock as a probe of quantum many-body physics. To this end, a new apparatus has been designed that combines the unprecedented precision of clock experiments with the amazing progress attained in quantum gas experiments.

The advancement of optical atomic clocks in the past two decades has motivated many potential applications in navigation, communication, and science. Accurate optical clocks that outperform the current cesium time standard also trigger a discussion about a possible redefinition of the SI second. The 27Al+ quantum logic clocks developed at the National Institute of Standards and Technology (NIST) were the first to achieve the clock fractional frequency uncertainty below 10-17 in 2010. To date, a few research groups around the world have demonstrated optical clocks based on various atomic transitions with fractional frequency uncertainty in the 10-18 range. The accuracy of the previous 27Al+ clocks was limited by the second order Doppler (time dilation) shift and the blackbody shift. Since 2010, the third 27Al+ optical clock is being developed at NIST to achieve a higher accuracy. The frequency uncertainties dominant in the previous clocks are controlled and reduced in the current optical clock. The new design of the ion trap system has reduced significantly both the blackbody radiation shift uncertainty and the time dilation shift uncertainty due to micromotion, while the time dilation shift uncertainty due to the secular motion has been reduced by more than an order of magnitude by operating the optical clock near the three dimensional zero-point energy. Despite those previously dominant uncertainties, several other systematic effects are being evaluated and some other efforts are being made to achieve a total uncertainty towards 1.0 x 10-18. In this thesis, I will document the detail of the construction and evaluation of the current 27Al+ optical clock developed at NIST.

8half a century ago and have provided the definition of the second since 1967. The international atomic time (TAI) is currently kept by an ensemble of Cs clocks with a fractional uncertainty of Δν/ν0 ≈ 10 –15 , and some Cs fountain clocks 9,10 operate at even smaller uncertainties. Although Cs atomic clocks operate at the microwave transition of ν0 ≈ 9.2 GHz, using optical transition frequencies of several hundred terahertz should improve the fractional uncertainty Δν/ν0 by 4–5 orders of magnitude, as most measurement uncertainties Δν are independent of ν0 (except the Doppler shift Δ νD = (v/c)ν0, where v is the atomic velocity and c is the speed of light). The invention of the laser and the subsequent improvement of laser frequency stabilities 11 has allowed the realization of optical clocks to become a reality 2,3,12

Metrology Timing standards around the world define the second using atomic clocks, specifically the microwave frequencies emitted from trapped atoms. Optical clocks, which are based on optical transitions of atoms, operate at much higher frequency and have been shown to exhibit better stability. Nakamura et al. demonstrate a framework that carries the improved stability of the optical domain over to microwaves (see the Perspective by Curtis). In addition to contributing to the eventual redefinition of the second based on optical clocks, this work will also lead to improvements in microwave-based technologies such as astronomical imaging and geodesy through very long baseline interferometry, radar, communications, and navigation systems. Science , this issue p. [889][1]; see also p. [825][2] [1]: /lookup/doi/10.1126/science.abb2473 [2]: /lookup/doi/10.1126/science.abb8629