Shared interrogation
One 848 nm laser probes both ionsNoise common to the laser can largely cancel when the two responses are compared.
Lu⁺Clock A

One lutetium-176 ion
Independent disturbance checks

Lu⁺Clock B

One lutetium-176 ion
Independent disturbance checks

Measured comparison
Frequency difference consistent with zero
Comparison time
200 hours
Statistical uncertainty
±5.7 × 10⁻¹⁹
Systematic uncertainty of difference
±1.0 × 10⁻¹⁹
Each clock’s evaluated systematic uncertainty
near 1 × 10⁻¹⁹
Illustrative gravity check
5 mm
Δf/f ≈ gh/c² ≈ 5.5 × 10⁻¹⁹

A five-millimetre height mismatch near Earth would shift the clocks on roughly the same scale as the statistical uncertainty. This is a calculation, not an additional measured result.

What agreement establishes: no resolved difference after known corrections and finite averaging. It does not prove that either clock is perfectly correct.