Microwave dressed states and vacuum fluctuations in a superconducting condensate

Abstract Microwave dressed states are found to emerge within the superconducting condensate when coupled to a quantized electromagnetic field due to photon-Cooper pair entanglement. This renormalized energy separation exceeds the prediction of BCS theory, yielding an enhancement that depends on the number of photons and persists even in the presence of electromagnetic vacuum fluctuations. Our work introduces an equilibrium quantum model of microwave-enhanced superconductivity, representing a significant departure from the standard non-equilibrium Eliashberg theory. We further demonstrate that the superconducting condensate exerts a back-action on the electromagnetic field, suppressing electric field fluctuations, including those from the vacuum state. This result is consistent with Glauber and Lewenstein’s field quantization in dielectric media.