  Single photon detection is important for a wide range of low-light
applications, including quantum information processing, spectroscopy, and light
detection and ranging (LiDAR). A key challenge in these applications has been
to integrate single-photon detection capability into photonic circuits for the
realization of complex photonic microsystems. Short-wavelength ($\lambda$ < 1.1
$\mu$m) integrated photonics platforms that use silicon (Si) as photodetectors
offer the opportunity to achieve single-photon avalanche diodes (SPADs) that
operate at or near room temperature. Here, we report the first
waveguide-coupled Si SPAD. The device is monolithically integrated in a Si
photonic platform and operates in the visible spectrum. The device exhibited a
single photon detection efficiency of > 6% for wavelengths of 488 nm and 532 nm
with an excess voltage less than 20% of the breakdown voltage. The dark count
rate was below 100 kHz at room temperature, with the possibility of improving
by approximately 35% by reducing the temperature to -5$^{\circ}$C.
