The most accurate clocks interrogate a single trapped ion. One atom, one laser, exquisite control. The price is measurement time: a single quantum absorber yields limited statistics per interrogation cycle.
The authors (arXiv:2603.23446) scale up to 10 strontium ions while maintaining a fractional frequency uncertainty of 5.3 × 10⁻¹⁹ — below one part in 10¹⁸. The key enabling technique: ion-resolved state detection, which identifies which specific ions are in which states. This allows position-dependent shifts to be measured and minimized for each ion individually, suppressing residual systematic effects below the 10⁻²⁰ level.
With 8-10 ions, measurement time drops by a factor of 4.8 compared to single-ion operation. A comparison with an ytterbium single-ion clock yields a frequency ratio measured to 2.9 × 10⁻¹⁸ combined uncertainty.
The through-claim: the single-ion clock's accuracy was never about single-ion physics — it was about control. The systematics that degraded multi-ion clocks were position-dependent shifts that couldn't be corrected without knowing each ion's state individually. Ion-resolved detection removes that obstacle. More ions means more statistics without sacrificing accuracy, because the systematic corrections are now per-ion, not ensemble-averaged.