Excited-State Phase Diagram of a Ferromagnetic Quantum Gas

TL;DR

This paper experimentally maps the excited-state phase diagram of a ferromagnetic quantum gas, demonstrating how quantum many-body systems can be structured through order parameters in excited states.

Contribution

It presents the first experimental determination of an excited-state phase diagram, including excitation energy as a control parameter.

Findings

Successful mapping of excited-state phase diagram

Identification of order parameters in excited states

Demonstration of structured Hilbert space in quantum gases

Abstract

The ground-state phases of a quantum many-body system are characterized by an order parameter, which changes abruptly at quantum phase transitions when an external control parameter is varied. Interestingly, these concepts may be extended to excited states, for which it is possible to define equivalent excited-state quantum phase transitions. However, the experimental mapping of a phase diagram of excited quantum states has not yet been demonstrated. Here we present the experimental determination of the excited-state phase diagram of an atomic ferromagnetic quantum gas, where, crucially, the excitation energy is one of the control parameters. The obtained phase diagram exemplifies how the extensive Hilbert state of quantum many-body systems can be structured by the measurement of well-defined order parameters.