Experimentally simulating the dynamics of quantum light and matter at ultrastrong coupling

TL;DR

This paper demonstrates a digital quantum simulation of ultrastrong light-matter interactions using a circuit QED chip, revealing complex entanglement and photon dynamics, and paving the way for exploring quantum phase transitions.

Contribution

It introduces a high-fidelity digital simulation method for USC dynamics in solid-state systems, surpassing previous spectroscopic approaches.

Findings

Simulated USC dynamics with up to 90 Trotter steps.

Observed Schrödinger-cat like entanglement.

Detected large photon number build-up.

Abstract

The quantum Rabi model describing the fundamental interaction between light and matter is a cornerstone of quantum physics. It predicts exotic phenomena like quantum phase transitions and ground-state entanglement in the ultrastrong-coupling (USC) regime, where coupling strengths are comparable to subsystem energies. Despite progress in many experimental platforms, the few experiments reaching USC have been limited to spectroscopy: demonstrating USC dynamics remains an outstanding challenge. Here, we employ a circuit QED chip with moderate coupling between a resonator and transmon qubit to realise accurate digital quantum simulation of USC dynamics. We advance the state of the art in solid-state digital quantum simulation by using up to 90 second-order Trotter steps and probing both subsystems in a combined Hilbert space dimension $\sim80$, demonstrating the Schr\"odinger-cat like entanglement and build-up of large photon numbers characteristic of deep USC. This work opens the door to exploring extreme USC regimes, quantum phase transitions and many-body effects in the Dicke model.