Time-and-frequency gated photon coincidence counting; a novel   multidimensional spectroscopy tool
  Coherent multidimensional optical spectroscopy techniques are broadly applied across the electromagnetic spectrum ranging from NMR to the UV. These reveal properties of matter through correlation plots of signal fields generated in response to sequences of short pulses with variable delays. Here we discuss a new class of multidimensional techniques obtained by time-and-frequency resolved photon coincidence counting measurements of N photons which constitutes a 2N dimensional spectrum. A compact description of these signals is developed based on time ordered superoperators rather than the normally ordered ordinary operators used in Glauber's photon counting formalism.The independent control of the time and frequency gate parameters reveals details of matter dynamics not available otherwise. Application to an anharmonic oscillator model with fluctuating energy and anharmonicity demonstrates the power of these signals. 
Konstantin E. Dorfman and Shaul Mukamel
quant-ph physics.chem-ph physics.optics