In analogy to squeezing of light, noise in a classical oscillator can be squeezed to reduce amplitude uncertainty. While this can be achieved to some extent in a harmonic oscillator parametrically driven at 2${\mathrm{\omega }}_0$, true amplitude squeezing is possible in anharmonic oscillators, either by driving at 2${\mathrm{\omega }}_0$ or allowing amplitude-dependent dephasing. These techniques can reduce the uncertainty in measurements of the frequency of an oscillator; for example, the thermal uncertainty in the relativistic frequency shift in single ion mass spectroscopy can be reduced by more than a factor of 5.

• Received 21 January 1992

DOI:https://doi.org/10.1103/PhysRevLett.68.2859