Strontium optical lattice clocks and their uncertainty

Short answer: Strontium optical lattice clocks probe a narrow atomic transition in atoms held in a laser-made lattice, and they are among the most precise measuring devices reported. A 2024 arXiv preprint reports a total systematic uncertainty of 8.1 x 10^-19 in fractional frequency, which its authors call the lowest of any clock to date, and a 2024 Metrologia paper reports closed-loop operation of a strontium clock with a total uncertainty of 4.4 x 10^-18. These are laboratory measurements by the groups themselves.

What the clocks do

The preprint explains that the clock relies on interrogating the ultra-narrow transition between the 1S0 and 3P0 states in fermionic strontium atoms trapped in an optical lattice. The Metrologia abstract describes the same transition, probed with a cavity-stabilized 698 nm laser. The laser frequency is locked to the atomic transition, and the figure of merit is how well that lock can be characterized, expressed as a fractional frequency uncertainty.

2024 preprint: an uncertainty of 8.1 x 10^-19

The authors report an optical lattice clock with a total systematic uncertainty of 8.1 x 10^-19 in fractional frequency units, which they describe as the lowest uncertainty of any clock to date. The atoms are a dilute ensemble of fermionic strontium in a vertically oriented, shallow, one-dimensional lattice. They say earlier imaging spectroscopy demonstrated record atomic coherence time and measurement precision through precise control of collisional shifts and the lattice light shift. In this work they revise the black body radiation shift correction by evaluating the lifetime of the 5s4d 3D1 state, which required precise characterization and control of many-body effects in its decay. They also measure the second-order Zeeman coefficient on the least magnetically sensitive clock transition, and report that all other systematic effects have uncertainties below 1 x 10^-19. This is a preprint, and the record claim is the authors' own.

2024 Metrologia: closed-loop operation

The Metrologia paper reports closed-loop operation of an optical lattice clock based on strontium-87. It obtains a Fourier-limited Rabi spectrum with a 0.6 Hz linewidth. Two transitions from the plus and minus 9/2 ground states are interrogated alternately, and the clock laser is stabilized to the center of the two. Using interleaved measurement, the authors report a single-clock frequency instability of 4.5 x 10^-16 divided by the square root of the averaging time. A second similar setup was built for an asynchronous comparison between two clocks, where they measure a stability of 2.1 x 10^-18 at 47,000 seconds. After calibrating systematic effects, they evaluate the total uncertainty of their first clock as 4.4 x 10^-18.

A 2019 evaluation

A Metrologia paper on the JILA SrI clock reports an improved systematic evaluation, with more than a twenty-fold reduction in systematic uncertainty over its 2013 evaluation and a sevenfold improvement in stability, per its abstract. The specific values did not render when this post was written, so they are not quoted here.

Limits

The figures come from different groups with different setups, and the abstracts do not compare them under shared conditions. The ytterbium clocks suggested by the topic title are not covered by these sources. Uncertainty at the 10^-19 level is a statement about the evaluation of systematic shifts in one apparatus, and it does not by itself describe a deployed timekeeping service.

FAQ

What is an optical lattice clock?

A clock that probes an ultra-narrow transition in atoms trapped in an optical lattice, per the abstracts.

How low was the reported uncertainty?

8.1 x 10^-19 in a 2024 preprint, which the authors call the lowest of any clock to date.

What did the Metrologia paper report?

Closed-loop operation with a 0.6 Hz Rabi linewidth, an evaluated total uncertainty of 4.4 x 10^-18, and a two-clock stability of 2.1 x 10^-18 at 47,000 seconds.

What limits these clocks?

Systematic shifts such as black body radiation, lattice light and Zeeman shifts, which the papers characterize.

Is this quantum computing?

No. It is quantum metrology, which shares atom control techniques with some quantum computers.