  Thermal decoherence is a major obstacle to the realization of quantum
coherence for massive mechanical oscillators. Although optical trapping has
been used to reduce the thermal decoherence rate for such oscillators, it also
increases the rate by subjecting the oscillator to stochastic forces resulting
from the frequency fluctuations of the optical field, thereby setting a
fundamental limit on the reduction. This is analogous to the noise penalty in
an active feedback system. Here, we directly measure the rethermalizaton
process for an initially cooled and optically trapped suspended mirror, and
identify the current limiting decoherence rate as due to the optical trap. Our
experimental study of the trap-induced decoherence rate will enable future
advances in the probing of fundamental quantum mechanics in the bad cavity
regime, such as testing of deformed commutators.
