  Resistance thermometry provides a time-tested method for taking temperature
measurements. However, fundamental limits to resistance-based approaches has
produced considerable interest in developing photonic temperature sensors to
leverage advances in frequency metrology and to achieve greater mechanical and
environmental stability. Here we show that silicon-based optical ring resonator
devices can resolve temperature differences of 1 mK using the traditional
wavelength scanning methodology. An even lower noise floor of 80 microkelvin
for measuring temperature difference is achieved in the side-of-fringe,
constant power mode measurement.
