Quantum and classical field scattered on a single two-level system
S. A. Gunin, A. Yu. Dmitriev, Andrei Vladimirovich Vasenin, Konstantin S. Tikhonov, Gleb Fedorov, O. V. Astafiev · Physical Review A · 2023
In many problems, the scattering amplitudes of a weak coherent pulse are almost equivalent to the ones of a single propagating photon. We thoroughly compare the scattering of (i) a short microwave coherent pulse from a rf generator and (ii) a vacuum-photon coherent superposition from the two-level emitter, both directed to a single two-level system---the probe. To do that, we use two superconducting qubits to implement emitter and probe, both strongly coupled to the same waveguide. However, with the use of a magnetic circulator we couple the field from the emitter to the probe without reverse backaction, thereby working with a cascaded atomic system implemented in a waveguide-QED setup. By measuring the dynamics of the scattered field, we find a certain discrepancy between two cases, confirmed by analytical and numerical study. Particularly, we find the optimal amplitude ${\mathrm{\ensuremath{\Omega}}}_{*}$ of classical pulse mimicking the superposition from the emitter, for which the difference becomes very small (but nonvanishing) and is almost unavailable to measure in practice.