Conditional homodyne detection and time asymmetric fluctuations of light

We study temporal fluctuations of the light emitted by an open system (photoemissive source) by investigating the two-time correlations of the output fields. The main focus of our interest is on cavity QED systems, where a single or many atoms placed inside an optical cavity are driven by laser light. Such systems have two output channels: emission though the cavity mirrors and atomic spontaneous fluorescence. Intensity-intensity (Hanbury Brown and Twiss) and intensity-field-amplitude (conditional homodyne detection) correlations are considered for the light emitted in the same channel and in two different channels.; We obtain analytical solutions for the two-time correlations in the weak excitation limit. Correlations for the same output channel are found to be time-symmetric, while cross-correlations for different channels show time-asymmetric behavior. Beyond the weak-field limit we find time-asymmetric correlations for conditional homodyne detection in the same channel. Numerical simulations verify the analytical results both for a single and many-atom cavity QED system. A post-conditioned quantum trajectory formalism is developed to obtain the negative time delay conditional homodyne correlations. An expression analogous to the quantum regression formula is obtained.; In thermal equilibrium, where detailed balance holds, microscopic reversibility guarantees time symmetry for the correlations. Away from thermal equilibrium time asymmetry is permitted, indicating the breakdown of detailed balance. In the case of single-output-channel conditional homodyne detection, time asymmetry not only indicates the breakdown of detailed balance, but is also a direct probe of non-Gaussian fluctuations.