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

It was recently shown that a single atom can efficiently scatter photons out of a focused coherent light beam [1, 2, 3]. The scattering probability is strongly dependent on a thermal motion of the atom and can be maximized if the atom is well localized at a focus. To achieve that, we implement a Raman sideband cooling technique that is commonly used in ion traps [4]. Our trap, formed by focused Gaussian light beam at 980nm, has characteristic frequencies of ντ = 55 kHz and νl = 7 kHz corresponding to transverse and longitudinal confinements. A single ⁸⁷Rb atom is loaded into the trap from an optical molasses. Two Raman beams couple the motional states of |F = 2〉 and |F = 1〉 manifolds with a Lamb-Dicke parameter η = 0.084 (Figure 1). The Raman beams are oriented such that momentum transfer occurs only along the strong confinement of the trap with ντ = 55 kHz. The cooling sequence consists of following steps: (1) initial preparation of the atom in |F = 2,mF = −2〉 Zeeman state, (2) Raman transfer between the motional states |F = 2,mF = −2,N〉 and |F = 1,mF = −1,N − 1〉. (3) recycling the atomic population back to |F = 2,mF = −2〉 state via an optical pulse resonant to |5P3/2, F = 2〉 state thus removing a phonon via spontaneous emission.