Waveguide-QED-Based Measurement of a Reservoir Spectral Density

The spectral density (SD) function has a central role in the study of open quantum systems (OQSs). We show a spectroscopic method for measuring the SD, requiring neither the OQS to be initially excited nor its time evolution to be tracked in time, which is not limited to the weak-coupling regime. This is achieved through one-dimensional photon scattering along a waveguide for a zero-temperature reservoir coupled to a two-level OQS via the rotating wave approximation. We find that the SD profile associated with the OQSs own reservoir can be exactly mapped into a universal simple function of the photon's reflectance and transmittance. As such, it can be straightforwardly inferred from the photon reflection and transmission spectra. (host: H. Baranger)