Optical Lattice Clock
A clock that loses less than a second over the age of the universe.
Realizes the most accurate clock humanity has ever built. Probes a narrow electronic transition in a cold atom — Sr at 698 nm or Yb at 578 nm — whose frequency drifts by parts in 10¹⁸ over a day. Will likely become the next definition of the SI second.
Laser-cool roughly 10,000 strontium atoms to a microkelvin and trap them in a 3-D 'optical lattice' — a standing wave formed by retroreflected lasers, with each atom held at a node where light intensity is uniform. The trap depth is tuned to a 'magic wavelength' where the atomic transition frequency is unaffected by the trap itself. A second 'clock laser' is locked to the resonance of all the atoms simultaneously. The clock laser's frequency is the time signal.
In plain terms
Picture a giant egg carton made of laser beams. In each cup sits one atom. Every atom is the world's most precise pendulum — and there are thousands of them all swinging together. By listening to all of them at once, scientists hear time itself with stunning sharpness.
Also called: strontium clock, Yb clock.
Why it matters
- Most accurate measurement humans can make of any physical quantity.
- Already used to test general relativity in centimeter-scale altitude differences.
- Will redefine the SI second around 2030 (recommended by the Consultative Committee for Time and Frequency).
- Networks of clocks across continents act as detectors of dark-matter clouds and gravitational waves.
Where it lives
- JILA / NIST Sr clock (Boulder, Colorado)
- RIKEN Sr clock (Japan)
- PTB Yb⁺ ion clock (Germany)
- European Space Agency ACES on the International Space Station
Key numbers
- Atom species: ⁸⁷Sr or ¹⁷¹Yb
- Clock laser λ: 698 nm (Sr) / 578 nm (Yb)
- Atom temperature: ~1 µK
- Fractional accuracy: ~1 × 10⁻¹⁸
Surprising facts
- Today's best optical lattice clocks are accurate to about 1 part in 10¹⁸ — which means if two of them had been started at the Big Bang, they would still agree today within a fraction of a second.
- Move the clock up by 1 centimeter and it ticks faster — gravitational time dilation, predicted by Einstein, is now measurable on a tabletop.
- The atomic transition the clock listens to is so narrow it has a Q-factor of about 10¹⁸ — about ten billion times sharper than the best radio receivers.
- It takes about 10⁻¹⁸ seconds of time error per day for these clocks to drift. That's the time it takes light to cross an atomic nucleus.
Open questions
- Gravitational redshift across 1 millimeter: A JILA experiment in 2021 measured the gravitational time dilation predicted by general relativity across a cloud of strontium atoms only one millimeter tall — a height difference so small it was previously thought unmeasurable. The fractional frequency difference of 10⁻¹⁹ between the top and bottom of the atomic cloud matched Einstein's prediction, but also showed that quantum mechanics and general relativity can be simultaneously probed at the millimeter scale, a regime with no established theoretical framework.
- Clock comparisons reveal unexplained frequency offsets: When two independently operated optical lattice clocks of the same species are compared with all known systematic shifts accounted for, they occasionally show persistent frequency disagreements at the 10⁻¹⁷ level that cannot be attributed to any identified effect. These residual offsets, observed in international comparisons between PTB, NIST, and SYRTE, drive ongoing searches for new systematic effects and motivate increasingly rigorous cross-laboratory audits.
A short history
Hidetoshi Katori at the University of Tokyo proposed the magic-wavelength optical lattice clock in 2003. The first working version, also Katori's, ran in 2005. NIST/JILA, RIKEN, PTB, and SYRTE have driven the accuracy down by more than three orders of magnitude since. The international comparison campaigns of the 2020s set the stage for an SI redefinition planned for the late 2020s.
Sources
- Katori, Nat. Photonics (2011) — magic-wavelength clock review (https://www.nature.com/articles/nphoton.2011.34)
- Bothwell et al., Nature (2022) — gravitational redshift in a millimeter-scale Sr cloud (https://www.nature.com/articles/s41586-021-04349-7)
- BIPM CCTF roadmap to redefining the second (https://www.bipm.org/en/committees/cc/cctf)