  Photonic-crystal-based integrated optical systems have been used for a broad
range of sensing applications with great success. This has been motivated by
several advantages such as high sensitivity, miniaturization, remote sensing,
selectivity and stability. Many photonic crystal sensors have been proposed
with various fabrication designs that result in improved optical properties. In
parallel, integrated optical systems are being pursued as a platform for
photonic quantum information processing using linear optics and Fock states.
Here we propose a novel integrated Fock state optical sensor architecture that
can be used for force, refractive index and possibly local temperature
detection. In this scheme, two coupled cavities behave as an "effective beam
splitter". The sensor works based on fourth order interference (the
Hong-Ou-Mandel effect) and requires a sequence of single photon pulses and
consequently has low pulse power. Changes in the parameter to be measured
induce variations in the effective beam splitter reflectivity and result in
changes to the visibility of interference. We demonstrate this generic scheme
in coupled L3 photonic crystal cavities as an example and find that this
system, which only relies on photon coincidence detection and does not need any
spectral resolution, can estimate forces as small as $10^{-7}$ Newtons and can
measure one part per million change in refractive index using a very low input
power of $10^{-10}$W. Thus linear optical quantum photonic architectures can
achieve comparable sensor performance to semiclassical devices.

Quantum Physics;Optics