  Laser-cooled atoms that are trapped and optically interfaced with light in
nanophotonic waveguides are a powerful platform for fundamental research in
quantum optics as well as for applications in quantum communication and quantum
information processing. Ever since the first realization of such a hybrid
quantum nanophotonic, heating rates of the atomic motion observed in various
experimental settings have typically been exceeding those in comparable
free-space optical microtraps by about three orders of magnitude. This
excessive heating is a roadblock for the implementation of certain protocols
and devices. Its origin has so far remained elusive and, at the typical
atom-surface separations of less than an optical wavelength encountered in
nanophotonic traps, numerous effects may potentially contribute to atom
heating. Here, we theoretically describe the effect of mechanical vibrations of
waveguides on guided light fields and provide a general theory of
particle-phonon interaction in nanophotonic traps. We test our theory by
applying it to the case of laser-cooled cesium atoms in nanofiber-based
two-color optical traps. We find excellent quantitative agreement between the
predicted heating rates and experimentally measured values. Our theory predicts
that, in this setting, the dominant heating process stems from the
optomechanical coupling of the optically trapped atoms to the continuum of
thermally occupied flexural mechanical modes of the waveguide structure. Beyond
unraveling the long-standing riddle of excessive heating in nanofiber-based
atom traps, we also study the dependence of the heating rates on the relevant
system parameters. Our findings allow us to propose several strategies for
minimizing the heating. Finally, our findings are also highly relevant for
optomechanics experiments with dielectric nanoparticles that are optically
trapped close to nanophotonic waveguides.
