Speaker
Description
In this talk, I will present incoherent and coherent frequency ratio measurements between optical atomic clocks, and the use of these measurements to constrain models of ultralight scalar dark matter. I will begin with a brief summary of the NIST Al$^+$ quantum-logic clocks, which use quantum-logic gates with a co-trapped second ion species for preparation and readout of the Al$^+$ state [1] and have achieved a fractional inaccuracy of $9.4 \times 10^{-19}$ [2]. We have performed incoherent frequency ratio measurements between an Al$^+$ ion clock, a Sr lattice clock, and a Yb lattice clock with total uncertainty below $10^{-17}$ and analyzed the results to place constraints on the coupling of ultralight scalar dark matter candidates to Standard Model particles and fields [3].
The sensitivity of these measurements is limited by the quantum-projection noise of the single Al$^+$ ion. Quantum-projection noise is minimized when the spectroscopic probe time is equal to the lifetime of the excited clock state, however for many clocks frequency noise of the clock laser limits the probe time to be much shorter. Coherent frequency ratio measurements, in which the atoms of one clock serve as the phase reference to probe the atoms of the other clock, can overcome this limitation.
I will proceed to describe coherent frequency ratio measurements between two independent Al$^+$ clocks, which reach the fundamental stability limit set by quantum projection noise with a probe time near the excited state lifetime, using the correlation spectroscopy technique [4]. Using a new technique called differential spectroscopy, we have recently performed coherent frequency ratio measurements between an Al$^+$ clock and a Yb clock with a 4 s coherence time, corresponding to a quality factor $Q \approx 1.4 \times 10^{16}$ [5].
[1] P. O. Schmidt et al., Science 309, 749 (2005)
[2] S. M. Brewer et al., Phys. Rev. Lett. 123, 033201 (2019)
[3] BACON collaboration, Nature 591, 564 (2021)
[4] E. R. Clements et al., Phys Rev. Lett. 125, 243602 (2020)
[5] M. E. Kim et al., arXiv:2109.09540 (2021)
