Abstract
Laser-cooled atoms coupled to nanophotonic structures constitute a powerful research platform for the exploration of new regimes of light-matter interaction. While the initialization of the atomic internal degrees of freedom in these systems has been achieved, a full preparation of the atomic quantum state also requires controlling the center of mass motion of the atoms at the quantum level. Obtaining such control is not straightforward, due to the close vicinity of the atoms to the photonic system that is at ambient temperature. Here, we demonstrate cooling of individual neutral Cesium atoms, that are optically interfaced with light in an optical nanofiber, preparing them close to their three-dimensional motional ground state. The atoms are localized less than 300nm away from the hot fiber surface. Ground-state preparation is achieved by performing degenerate Raman cooling, and the atomic temperature is inferred from the analysis of heterodyne fluorescence spectroscopy signals. Our cooling method can be implemented either with externally applied or guided light fields. Moreover, it relies on polarization gradients which naturally occur for strongly confined guided optical fields. Thus, this method can be implemented in any trap based on nanophotonic structures. Our results provide an ideal starting point for the study of novel effects such as light-induced self-organization, the measurement of novel optical forces, and the investigation of heat transfer at the nanoscale using quantum probes.
Highlights
Trapped laser-cooled atoms constitute a versatile experimental platform, with applications ranging from precision measurements [1,2], to quantum simulations of solid-state systems [3], to quantum communication [4]
We demonstrate cooling of individual neutral cesium atoms that are optically interfaced with light in an optical nanofiber, preparing them close to their three-dimensional motional ground state
Ground-state preparation is achieved by performing degenerate Raman cooling, and the atomic temperature is inferred from the analysis of heterodyne fluorescence spectroscopy signals
Summary
Zeeman substate jmFi, here shown for two states of the F 1⁄4 4 hyperfine manifold, has an associated set of motional states jni, here sketched for a simple 1D harmonic oscillator. The offset magnetic field Boff can be used to tune different states jmF; ni and jm0F; n0i (mF ≠ m0F) into resonance. The coupling between these states (green dashed arrows) originates from the fictitious magnetic field Bfict gradient. Because of the strong transverse confinement of the trapping light fields in the nanofiber, the atoms experience a strongly spatially varying vector ac Stark shift [14], known as a fictitious magnetic field [23]. The resulting fictitious magnetic field Bfict dominantly points along the x axis, and its magnitude varies linearly along y. Using our expression for Bfict, the Hamiltonian (2) can be rewritten as
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