Spectroscopic Characterization of the Quantum Linear-Zigzag Transition in Trapped Ions

TL;DR

This paper experimentally investigates the quantum linear-zigzag transition in trapped ions, demonstrating control over the critical point and soft mode spectroscopy, paving the way for entangled-state generation near quantum phase transitions.

Contribution

It reports the first spectroscopic characterization of the quantum linear-zigzag transition in trapped ions with control over transition bias and trap asymmetries.

Findings

Observation of a stable critical point in ion chains

Spectroscopic probing of the soft mode near transition

Control of transition bias via optical dipole forces

Abstract

While engineered quantum systems are a general route to the manipulation of multipartite quantum states, access in a physical system to a continuous quantum phase transition under sufficient control offers the possibility of an intrinsic source of entangled states. To this end we realize the quantum version of the linear-zigzag structural transition for arrays of up to five ground state-cooled ions held in a linear Paul trap and we demonstrate several of the control requirements towards entangled-state interferometry near the critical point. Using in-situ spectroscopy we probe the energy level structure and occupation of the soft mode associated with the structural transition, and show a stable critical point and majority ground state occupation crossing the transition. We resolve biases arising from trap electrode asymmetries that change the nature of the transition, show that they can be suppressed by varying the ion number, and demonstrate control of the transition bias using optical dipole forces.