Reducing Frequency Ratio Measurement Instability with Differential Spectroscopy

Differential spectroscopy is a protocol for performing interspecies frequency ratio measurements with an instability beyond the limit set by laser noise.  This protocol allows for probe durations exceeding the limit set by laser coherence for one or both clocks ^[1]. In this talk, we present an experimental demonstration of differential spectroscopy between a ¹⁷¹Yb lattice clock and a ²⁷ Al⁺ quantum-logic clock. An Er/Yb-glass free-space optical frequency comb is used to transfer the laser phase of the ¹⁷¹Yb local oscillator at 518 THz to the ²⁷Al⁺ local oscillator at 1.12 PHz. Synchronous Ramsey interrogations are performed and the common-mode laser phase is first measured by the ¹⁷¹Yb clock and subsequently fed-forward to the ²⁷Al⁺ clock before the conclusion of its Ramsey sequence, enabling operation of the ²⁷Al⁺ clock beyond its laser coherence time. The ²⁷Al⁺ probe duration is further extended to an integer multiple of the ¹⁷¹Yb probe duration using a second ¹⁷¹Yb lattice clock to make zero dead time phase measurements ^[2].  We achieve fractional measurement instabilities below 2 × 10^-16/τ^1/2 with a probe duration of 1.7 s. This measurement instability will allow comparisons with a fractional ratio resolution of 10^-18 in under 12 hours.

[1] D. B. Hume et al., Phys. Rev. A, 93, 032138, (2016).

[2] M. Schioppo et al., Nature Photonics, 11, 48-52, (2017)

This work was supported by the Defense Advanced Research Projects Agency, the National Institute of Standards and Technology, and the Office of Naval Research.