  We study noise-enhanced frequency mixing in an underdamped micromechanical
torsional oscillator. The oscillator is electrostatically driven into
bistability by a strong, periodic voltage at frequency $\omega_d$. A second,
weak ac voltage is applied at a frequency $\omega$ close to $\omega_d$. Due to
nonlinearity in the system, vibrations occur at both $\omega$ and
$2\omega_d-\omega$. White noise is injected into the excitation, allowing the
system to occasionally overcome the activation barrier and switch between the
two states. At the primary drive frequency where the occupations of the two
states are approximately equal, we observe noise-induced enhancement of the
oscillation amplitudes at both $\omega$ and the down-converted frequency
$2\omega_d-\omega$, in agreement with theoretical predictions. Such enhancement
occurs as a result of the noise-induced interstate transitions becoming
synchronous with the beating between the two driving frequencies.
