Coherence of a qubit stored in Zeeman levels of a single optically trapped atom

TL;DR

This paper experimentally investigates the coherence properties of a qubit stored in Zeeman substates of a single optically trapped Rb-87 atom, achieving significant coherence times through active magnetic stabilization and detailed analysis of dephasing mechanisms.

Contribution

It provides new measurements of atomic spin coherence times in a single trapped atom without a magnetic guiding field and analyzes the effects of optical trapping on coherence.

Findings

Transverse dephasing time T2* of 75-150 microseconds

Longitudinal relaxation time T1 exceeding 0.5 milliseconds

Residual position- and state-dependent effects limit coherence

Abstract

We experimentally investigate the coherence properties of a qubit stored in the Zeeman substates of the 5S1/2, F=1 hyperfine ground level of a single optically trapped Rb-87 atom. Larmor precession of a single atomic spin-1 system is observed by preparing the atom in a defined initial spin-state and then measuring the resulting state after a programmable period of free evolution. Additionally, by performing quantum state tomography, maximum knowledge about the spin coherence is gathered. By using an active magnetic field stabilization and without application of a magnetic guiding field we achieve transverse and longitudinal dephasing times of T2*=75..150 \mus and T1>0.5 ms respectively. We derive the light-shift distribution of a single atom in the approximately harmonic potential of a dipole trap and show that the measured atomic spin coherence is limited mainly by residual position- and state-dependent effects in the optical trapping potential. The improved understanding enables longer coherence times, an important prerequisite for future applications in long-distance quantum communication and computation with atoms in optical lattices or for a loophole-free test of Bell's inequality.