Optical atomic clock
A clock that steers a laser to a narrow optical transition in atoms or ions, at hundreds of terahertz, and counts its cycles with a frequency comb. The best reach fractional uncertainties near 10⁻¹⁸, enough to sense a centimeter of height change.
A clock is an oscillator and a counter. An atomic clock steers its oscillator to a transition in an atom, whose frequency is set by nature and identical everywhere; the caesium microwave transition at 9 192 631 770 Hz defines the SI second. An optical clock does the same with a transition at optical frequency: about 429 THz (698 nm) in strontium, 518 THz (578 nm) in ytterbium, and 1.12 PHz (267 nm) in the aluminium ion. For the same absolute frequency uncertainty, the fractional uncertainty improves in proportion to the carrier frequency, and the strontium transition is 46 700 times higher in frequency than caesium's. Optical transitions can also be made extremely narrow; the strontium clock transition has a natural linewidth in the millihertz range.
Three parts make up the clock. The atoms are either thousands of neutral atoms held in an optical lattice, a standing wave of laser light tuned to a "magic" wavelength (813 nm for strontium) at which the lattice shifts both clock levels equally and so leaves the transition frequency unchanged, or a single ion held in an electromagnetic trap. The clock laser probes the transition; to resolve a line that narrow it must itself be narrow, so it is locked with Pound-Drever-Hall stabilization to an ultrastable optical cavity, reaching linewidths below 1 Hz, and the atomic signal then corrects the cavity's slow drift. The counter is an optical frequency comb referenced to the clock laser, which divides the optical frequency down to a microwave signal that electronics can count, and which lets clocks at different wavelengths be compared directly.
The best clocks now report systematic uncertainties near ; a clock in error by that fraction and running for the age of the universe, s, would be off by 0.44 s. General relativity makes clocks at different heights tick at different rates, by , which is per centimeter at the Earth's surface. Optical clocks therefore resolve height differences of about a centimeter, and one experiment measured the rate change across a millimeter-scale atomic sample. The effect cuts both ways: clocks in different laboratories can be compared only once the gravitational potential at each is known, and clock comparisons over phase-stabilized fiber links are now a tool of geodesy.
Because optical clocks outperform the caesium standard that defines the second by about two orders of magnitude, the international metrology bodies have published a roadmap for redefining the second in terms of optical transitions, with a decision foreseen around 2030. Work toward transportable and compact clocks, using integrated photonics and microcombs in place of laboratory optics, is aimed at taking the performance outside the metrology laboratory.
References: A. D. Ludlow, M. M. Boyd, J. Ye, E. Peik, P. O. Schmidt, Rev. Mod. Phys. 87, 637 (2015); T. Bothwell et al., Nature 602, 420 (2022); N. Dimarcq et al., Metrologia 61, 012001 (2024).