  We report cooling of a single neutral atom to its three-dimensional
vibrational ground state in an optical tweezer. After employing Raman sideband
cooling for tens of milliseconds, we measure via sideband spectroscopy a
three-dimensional ground-state occupation of ~90%. We further observe coherent
control of the spin and motional state of the trapped atom. Our demonstration
shows that an optical tweezer, formed simply by a tightly focused beam of
light, creates sufficient confinement for efficient sideband cooling. This
source of ground-state neutral atoms will be instrumental in numerous quantum
simulation and logic applications that require a versatile platform for storing
and manipulating ultracold single neutral atoms. For example, these results
will improve current optical tweezer experiments studying atom-photon coupling
and Rydberg quantum logic gates, and could provide new opportunities such as
rapid production of single dipolar molecules or quantum simulation in tweezer
arrays.
