Quantum electrodynamic fluctuations of the macroscopic Josephson phase

TL;DR

This paper investigates how quantum electromagnetic fluctuations and quasiparticle tunneling influence the phase dynamics in a Josephson junction, revealing interference effects and potential observability of QED phenomena in superconducting systems.

Contribution

It introduces a novel model combining electromagnetic vacuum fluctuations and quasiparticle tunneling as independent baths affecting the Josephson phase.

Findings

Quantum fluctuations are enhanced by electromagnetic vacuum effects.

Interference between radiation and quasiparticle baths affects phase fluctuations.

The study suggests possible experimental observation of QED effects in superconducting circuits.

Abstract

We study the equilibrium dynamics of the relative phase in a superconducting Josephson link taking into account the quantum fluctuations of the electromagnetic vacuum. The photons act as a superohmic heat bath on the relative Cooper pair number and thus, indirectly, on the macroscopic phase difference \phi. This leads to an enhancement of the mean square <\phi^2> that adds to the spread due to the Coulomb interaction carried by the longitudinal electromagnetic field. We also include the interaction with the electronic degrees of freedom due to quasiparticle tunneling, which couple to the phase and only indirectly to the particle number. The simultaneous inclusion of both the radiation field fluctuations and quasiparticle tunneling leads to a novel type of particle--bath Hamiltonian in which the quantum particle couples through its position and momentum to two independent bosonic heat baths. We study the interplay between the two mechanisms in the present context and find interference contributions to the quantum fluctuations of the phase. We explore the observability of the QED effects discussed here.