Superstrong coupling in circuit quantum electrodynamics

TL;DR

This paper demonstrates a new superstrong coupling regime in circuit quantum electrodynamics where a single atom significantly modifies the vacuum modes, enabling exploration of many-body physics and quantum impurity models.

Contribution

It introduces the superstrong coupling regime in circuit QED, characterized by many vacuum modes hybridizing with a single atom, and provides experimental evidence and theoretical modeling.

Findings

Observation of atom-induced vacuum mode hybridization

Measurement of the transmon's spontaneous emission linewidth

Agreement with Caldeira-Leggett model predictions

Abstract

Vacuum fluctuations fundamentally affect an atom by inducing a finite excited state lifetime along with a Lamb shift of its transition frequency. Here we report the reverse effect: modification of vacuum modes by a single atom in circuit quantum electrodynamics. Our one-dimensional vacuum is a long section of a high wave impedance (comparable to resistance quantum) superconducting transmission line. It is directly wired to a transmon qubit circuit. Owing to the combination of high impedance and galvanic connection, the transmon's spontaneous emission linewidth can greatly exceed the discrete transmission line modes spacing. This condition defines a previously unexplored "superstrong" coupling regime of quantum electrodynamics where many frequency-resolved vacuum modes hybridize with a single atom. We establish this regime by observing the spontaneous emission line of the transmon, revealed through the mode-by-mode measurement of the vacuum's density of states. The linewidth as well as the atom-induced dispersive photon-photon interaction are accurately described by a physically transparent Caldeira-Leggett model, with the transmon's quartic non-linearity treated as a perturbation. Non-perturbative modification of vacuum, including inelastic scattering of single photons, can be enabled by the superstrong coupling regime upon replacing the transmon by more anharmonic qubits, with broad implications for simulating quantum impurity models of many-body physics.