Observation of a quantum phase transition in the quantum Rabi model with a single trapped ion

TL;DR

This paper reports the first experimental observation of a quantum phase transition in the quantum Rabi model using a single trapped ion, demonstrating phase transition signatures in a minimal quantum system.

Contribution

It provides the first experimental demonstration of a quantum phase transition in the quantum Rabi model with a single trapped ion, without requiring many-body systems.

Findings

Observation of phase transition signatures in spin-up population

Measurement of phonon number as an order parameter

Evidence of quantum critical behavior in a minimal system

Abstract

Quantum phase transitions (QPTs) are usually associated with many-body systems with large degrees of freedom approaching the thermodynamic limit. In such systems, the many-body ground state shows abrupt changes at zero temperature when the control parameter of the Hamiltonian is scanned across a quantum critical point. Recently it has been realized that a QPT can also occur in a simple system composed of only a two-level atom and a single-mode bosonic field, described by the quantum Rabi model (QRM). Here we report the first experimental demonstration of a QPT in the QRM using a single trapped ion. We measure the average spin-up state population of the ion and the average phonon number in its spatial oscillation mode as two order parameters and observe the clear evidences of the phase transition via slow quench of the coupling between the ion and its spatial motion. An experimental probe of the phase transitions in a fundamental quantum optics model without imposing the thermodynamic limit opens up a new window for the controlled study of QPTs and quantum critical phenomena.