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

Abstract

We experimentally investigate the coherence properties of a qubit stored in the Zeeman substates of the ${5}^{\phantom{\rule{0.16em}{0ex}}2}{S}_{1/2},\phantom{\rule{0.222222em}{0ex}}F=1$ hyperfine ground level of a single optically trapped ${}^{87}$Rb 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 ${T}_{2}^{*}=75\ensuremath{-}150\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{s}$ and ${T}_{1}>0.5\phantom{\rule{0.28em}{0ex}}\mathrm{ms},$ 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.