  Quantum control of levitated dielectric particles is an emerging subject in
quantum optomechanics. A major challenge is to efficiently measure and
manipulate the particle's motion at the Heisenberg uncertainty limit. Here we
present a nanophotonic interface suited to address this problem. By optically
trapping a 150 nm silica particle and placing it in the near field of a
photonic crystal cavity, we achieve tunable single-photon optomechanical
coupling of up to $g_0/2\pi=9$ kHz, three orders of magnitude larger than
previously reported for levitated cavity optomechanical systems. Efficient
collection and guiding of light through the nanophotonic structure results in a
per-photon displacement sensitivity that is increased by two orders of
magnitude compared to conventional far-field detection. The demonstrated
performance shows a promising route for room temperature quantum optomechanics.
