Coherence and Raman sideband cooling of a single atom in an optical tweezer

TL;DR

This paper demonstrates effective Raman sideband cooling of a single atom in a tightly focused optical tweezer, overcoming decoherence from polarization effects, and achieving near-ground state vibrational levels.

Contribution

It identifies polarization-induced decoherence in optical tweezers and shows how magnetic bias fields can mitigate this, enabling near-ground state cooling of single atoms.

Findings

Achieved vibrational occupation numbers of 0.01 radially and 8 axially.

Demonstrated cooling close to the three-dimensional ground state.

Mitigated decoherence effects through magnetic field optimization.

Abstract

We investigate quantum control of a single atom in an optical tweezer trap created by a tightly focused optical beam. We show that longitudinal polarization components in the dipole trap arising from the breakdown of the paraxial approximation give rise to significant internal-state decoherence. We show that this effect can be mitigated by appropriate choice of magnetic bias field, enabling Raman sideband cooling of a single atom close to its three-dimensional ground state in an optical trap with a beam waist as small as $w=900$ nm. We achieve vibrational occupation numbers of $\bar{n}_r = 0.01$ and $\bar{n}_a = 8$ in the radial and axial directions of the trap, corresponding to an rms size of the atomic wavepacket of 24 nm and 270 nm, respectively. This represents a promising starting point for future hybrid quantum systems where atoms are placed in close proximity to surfaces.