Probing excited-state quantum phase transitions with trapped cold ions

TL;DR

This paper presents experimental protocols using a single trapped ion to observe excited-state quantum phase transitions, identifying specific states and observables, and analyzing their critical behaviors through controlled parameter changes.

Contribution

It introduces a practical method to realize and study ESQPTs in a trapped ion system, linking theoretical models with experimental parameters and simulations.

Findings

Identification of excited states between critical ESQPT energies in the ERM model.

Definition of ESQPT witness observables and analysis of their critical scaling.

Simulations showing state evolutions and open-system effects in realistic setups.

Abstract

We propose concrete protocols to realize quantum criticality due to excited-state quantum phase transitions (ESQPTs) experimentally in presumably the simplest and most resilient system involving a single trapped ion oscillating in a radio-frequency Paul trap. We identify a specific class of excited states of the Extended Rabi Model (ERM) Hamiltonian, which occur between two critical ESQPT energies of the model in its (anti)Jaynes-Cummings superradiant phase. Properties of these states motivate the definition of several ESQPT witness observables. We study their critical scaling behaviors as well as various distinct state evolutions by driving the system across the quantum criticalities by changing the qubit-phonon coupling strength linearly in time at different finite rates. A mapping of the theoretical control parameters of the ERM to the experimental parameters of a trapped ion setup is provided, and simulations are performed for values referencing existing state-of-the-art setups, addressing both unitary state evolutions as well as relevant open-system corrections.