  Non-equilibrium photon correlations of coherently excited single quantum
systems can reveal their internal quantum dynamics and provide spectroscopic
access. Here we propose and discuss the fundamentals of a coherent photon
coincidence spectroscopy based on the application of laser pulses with variable
delay and the detection of an time-averaged two-photon coincidence rate. For
demonstration, two simple but important cases, i.e., an exciton - biexciton in
a quantum dot and two coupled quantum emitters, are investigated based on
quantum dynamics simulations demonstrating that this nonlinear spectroscopy
reveals information specific to the particular single quantum system.

Quantum Physics