Coherence Preserving Leakage Detection and Cooling in Alkaline Earth Atoms

Abstract

Optically trapped atoms in arrays of optical tweezers have emerged as a powerful platform for quantum information processing given the recent demonstrations of high-fidelity quantum logic gates and on-demand reconfigurable geometry. Both in gate operations and atomic transport, additional errors will occur due to leakage out of the computation space, atomic motional heating, or loss of an atom out of a trap completely. In this work, we address these error channels in a unified manner through laser fluorescence that can detect and cool the atom without disturbing the quantum information encoded therein. As only the electrons in the atom couple directly to the laser field, such quantum nondemolition (QND) processes are made possible by encoding quantum information in the nuclear spin of alkaline earth-like atoms and avoiding the effects of the hyperfine interaction which couples it to the electrons. By detuning a fluorescence laser off-resonantly from the $\mathrm{^1S_0} \rightarrow \mathrm{^1P_1}$ transition, far compared to the (small) hyperfine spitting, optical pumping between nuclear states falls off rapidly with detuning, scaling as $~1/\Delta^4$. In contrast, Rayleigh scattering falls off as $~1/\Delta^2$. We also consider a resonant leakage detection protocol off the $^1\mathrm{P}_1$ line. This is achieved by disabling the hyperfine coupling via a strong AC stark effect and canceling the residual lightshifts via dressing. The same scheme can be used to recool the atoms towards the vibrational ground state for the quantum information encoded in the ground state of alkaline earth atoms while preserving the coherence. These advances could significantly improve the prospect of neutral atoms for fault-tolerant quantum computation.