How to project onto an arbitrary single-photon wave packet

Detecting two photons with one molecule

“…To construct the minimal absorber atom or molecule or multi-level system that can absorb two photons sequentially and produce classical outputs signaling the final state of the absorber, we consider the five level system of Figure 1 for efficient photon transduction. Some recent efforts for physically based fundamental models for photo detection assemble all parts of the process into a single fully coupled evolution problem [13,14,[18][19][20][21][22][23][24]. Minimal noise amplification of the absorbed photon signal has been shown to be optimally done with continuous quantum measurement [13,14,23].…”

“…In this way, we can calculate the evolution of the complete system with the elements of a single density matrix, instead of the multiple generalized density matrices in eq. (24). All other discrete-continuum couplings are at position x = 0.…”

“…One motivation for this work comes from recent theory efforts to find fundamental (i.e., device-independent) limits to photo detection [13,14,[18][19][20][21][22][23][24]. For reasons fully explained in [23] we construct a Hamiltonian here for the full detection process, including the crucial amplification step.…”