Spontaneous emission of Schr\"odinger cats in a waveguide at ultrastrong coupling

TL;DR

This paper explores how ultrastrong light-matter coupling in Josephson circuits causes spontaneous emission to produce broad Schr"odinger cat states, revealing complex many-body quantum dynamics beyond standard quantum optics.

Contribution

It introduces a new description of spontaneous emission in the ultrastrong coupling regime, showing the generation of spectrally broad Schr"odinger cats and their entanglement dynamics.

Findings

Emission produces spectrally broad Schr"odinger cats

Cats remain entangled with the emitter during emission

Quantum information is transferred to the emitted cats after decoherence

Abstract

Josephson circuits provide a realistic physical setup where the light-matter fine structure constant can become of order one, allowing to reach a regime dominated by non-perturbative effects beyond standard quantum optics. Simple processes, such as spontaneous emission, thus acquire a many-body character, that can be tackled using a new description of the time-dependent state vector in terms of quantum-superposed coherent states. We find that spontaneous atomic decay at ultrastrong coupling leads to the emission of spectrally broad Schr\"odinger cats rather than of monochromatic single photons. These cats states remain partially entangled with the emitter at intermediate stages of the dynamics, even after emission, due to a large separation in time scales between fast energy relaxation and exponentially slow decoherence. Once decoherence of the qubit is finally established, quantum information is completely transfered to the state of the emitted cat.