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# Monday 1st April: STP Seminar – Gauge ambiguities imply Jaynes-Cummings physics remains valid in ultrastrong coupling QED

Title: Gauge ambiguities imply Jaynes-Cummings physics remains valid in ultrastrong coupling QED

Speaker: Dr. Adam Stokes (University of Manchester)

Abstract: In quantum electrodynamics (QED) each choice of gauge provides different physical definitions of light and matter as quantum subsystems. This gauge-relativity is most evident in the ultrastrong light-matter coupling regime where it can have important implications for effective models that are in common use. In particular, we show that when truncating the material system to two levels, each gauge gives a different description whose predictions vary significantly for ultrastrong-coupling. Quantum Rabi models (QRMs) are obtained through specific gauge choices, but so too is a Jaynes-Cummings model (JCM) without needing the rotating-wave approximation. Analysinga circuit QED setup, we find that this JCM provides more accurate predictions than the QRM for the ground state, and often for the first excited state as well. Thus, Jaynes-Cummings physics is not restricted to light-matter coupling below the ultrastrong limit. Next, we consider time-dependent couplings as required for numerous applications. We show that in the absence of an argument to choose a particular gauge when promoting the coupling parameter to a time-dependent function, the description that results is essentially ambiguous. For sufficiently strong and non-adiabatic interactions, the qualitative physical predictions of final subsystem properties, such as entanglement and photon number, depend on the gauge chosen. This occurs even when the coupling vanishes at the preparation and measurement stages of the protocol, at which times the subsystems are unique and experimentally addressable.

Time: Monday, 1 April 2019, 2:30 pm

Location: Lecture Room, 1st Floor, School of Theoretical Physics, DIAS, 10 Burlington Road, Dublin 4