Extended Data Fig. 2: An optimized zero-dead-time clock.

(a) The timing sequence for a ZDT clock with a variable delay between the π/2-pulses. Note that the length of each step is not to scale. (b) Displayed is the sensitivity function for a zero- dead-time sequence in which the Ramsey pulses of the two atomic systems are precisely overlapped (red) or offset from each other by 27.73% (green). The sensitivity functions have been vertically offset from each other for visual clarity. (c) The optimal delay is determined by calculating the one-second Dick instability for a variable delay between the two pulses. These calculations assume the OLO noise is flicker-frequency and at the level of 1.5 × 10-16. The blue line demonstrates that the Dick instability exhibits a sharply defined minimum. We emphasize that the Dick instability reported here would be added in quadrature with other sources of instability, such as atomic detection noise and quantum-projection noise, which under current conditions would limit the total clock instability at a level at least an order of magnitude higher than the calculated Dick instability. The red and green dots represent the conditions with overlapping Ramsey pulses and optimized timing, respectively.