Resonant symmetry lifting in a parametrically modulated oscillator
  We study a parametrically modulated oscillator that has two stable states of vibrations at half the modulation frequency $\omega_F$. Fluctuations of the oscillator lead to interstate switching. A comparatively weak additional field can strongly affect the switching rates, because it changes the switching activation energies. The change is linear in the field amplitude. When the additional field frequency $\omega_d$ is $\omega_F/2$, the field makes the populations of the vibrational states different thus lifting the states symmetry. If $\omega_d$ differs from $\omega_F/2$, the field modulates the state populations at the difference frequency, leading to fluctuation-mediated wave mixing. For an underdamped oscillator, the change of the activation energy displays characteristic resonant peaks as a function of frequency. 
D. Ryvkine and M. I. Dykman