  Whispering gallery mode biosensors allow selective unlabelled detection of
single proteins and, combined with quantum limited sensitivity, the possibility
for noninvasive realtime observation of motor molecule motion. However, to date
technical noise sources, most particularly low frequency laser noise, have
constrained such applications. Here we introduce a new technique for whispering
gallery mode sensing based on direct detection of back-scattered light. This
experimentally straightforward technique is immune to frequency noise in
principle, and further, acts to suppress thermorefractive noise. We demonstrate
27 dB of frequency noise suppression, eliminating frequency noise as a source
of sensitivity degradation and allowing an absolute frequency shift sensitivity
of 76 kHz. Our results open a new pathway towards single molecule biophysics
experiments and ultrasensitive biosensors.
