Vibrational ground state cooling of a neutral atom in a tightly focused optical dipole trap

Recent experiments have shown that an efficient interaction between a single trapped atom and light can be established by concentrating light field at the location of the atom by focusing [1-3]. However, to fully exploit the benefits of strong focusing one has to localize the atom at the maximum of the field strength [4]. The position uncertainty due to residual kinetic energy of the atom in the dipole trap (depth $\sim 1\mathrm{mK}$) after molasses cooling is significant (few 100 nm). It limits the interaction between a focused light mode and an atom already for moderate focusing strength [2]. To address this problem we implement a Raman Sideband cooling technique, similar to the one commonly used in ion traps [5], to cool a single $^{\mathrm{87}}$Rb atom to the ground state of the trap. We have cooled the atom along the transverse trap axis (trap frequency $\nu_{\tau}=55\,\mathrm{kHz}$), to a mean vibrational state $\bar{n_{\tau}}=0.55$ and investigate the impact on atom-light interfaces.\\[4pt] [1] M. K. Tey, et al., Nature Physics {\bf 4} 924 (2008)\\[0pt] [2] M. K. Tey et. al., New J. Phys. {\bf 11}, 043011 (2009)\\[0pt] [3] S.A. Aljunid et al., PRL {\bf 103}, 153601 (2009)\\[0pt] [4] C. Teo and V. Scarani Opt. Comm. {\bf 284} 4485-4490 (2011)\\[0pt] [5] C. Monroe et al., PRL {\bf 75}, 4011 (1995)