Quantum limited amplification with a nonlinear cavity detector

We consider the quantum measurement properties of a driven cavity with a Kerr-type nonlinearity which is used to amplify a dispersively coupled input signal. Focusing on an operating regime which is near a bifurcation point, we derive simple asymptotic expressions describing the cavity's noise and response. We show that the cavity's backaction and imprecision noise allow for quantum limited linear amplification and position detection only if one is able to utilize the sizeable correlations between these quantities. This is possible when one amplifies a non-resonant signal, but is not possible in QND qubit detection. We also consider the possibility of using the nonlinear cavity's backaction for cooling a mechanical mode.Comment: 15 pages, 6 figure

Entropy and Time

The emergence of a direction of time in statistical mechanics from an underlying time-reversal-invariant dynamics is explained by examining a simple model. The manner in which time-reversal symmetry is preserved and the role of initial conditions are emphasized. An extension of the model to finite temperatures is also discussed.Comment: 9 pages, 8eps figures. To appear in the theme issue of the American Journal of Physics on Statistical Physic