Fig. 1: Compound clock scheme using dynamically decoupling interrogation.
From: Dynamical decoupling of laser phase noise in compound atomic clocks
a Schematic setup of a compound clock for operation beyond the laser coherence limit. The two clocks share a common local oscillator (LO) that is pre-stabilised to an ultrastable cavity. The frequency stability of the LO is transferred to interrogation lasers, e.g. by a frequency comb (FC). Using the spectroscopic sequences shown in b, clock 1 provides a coarse estimate (ϕ1) of the laser phase deviation to clock 2, which then refines this measurement. Their combined measured phase deviation (ϕtot) feeds back into a frequency shifter (Δν) to stabilise the LO frequency. Note that the measured phase and frequency deviations need to be scaled by the frequency ratio when transferred to a clock or LO operating at a different frequency, which has been omitted here for the sake of simplicity. b Pulse sequences of clocks 1 and 2 as a function of time t (example). After an initial π/2 excitation pulse (red), the interrogation sequence of clock 1 interleaves free-evolution times of duration Td or Td/2 (light grey) and ‘flip’ pulses (orange) of pulse area π − ϵ and phase φ = ±π/2 with respect to the initial pulse. It ends with a pulse of area ϵ/2 (magenta) and state read-out (blue). Clock 2 uses a two-pulse Ramsey sequence. It receives laser phase information (ϕ1) from clock 1 in time to adjust the phase of the second π/2 pulse such that the fringe centre is shifted to maximise the signal slope. The delay \({T}_{{\mathrm{i}}}-{T}_{{\mathrm{i}}}^{\prime}\) must be kept short to avoid excess phase noise (see “Methods” section). c–e Evolution of the atomic state in clock 1 on the Bloch sphere for constant laser detuning at times t1 through t3, as marked in b (example). g and e indicate the ground and excited state of the clock transition, respectively. After accumulating phase during the first dark time Td/2 (c, blue), a flip pulse nearly reverses this precession of the Bloch vector and maps it onto a small change of excitation probability (c, red). The process is repeated twice with dark time Td (d and e, red). Finally, another free-evolution time Td/2 (e, light red) and the final laser pulse with area ϵ/2 (e, green) are applied.
