Observation of a Transition Between Dynamical Phases in a Quantum Degenerate Fermi Gas

TL;DR

This study demonstrates a dynamical phase transition in a cold-atom quantum simulator of the Heisenberg model, showing a switch from rapid magnetization decay to long-lived magnetization as interaction strength crosses a critical threshold.

Contribution

First experimental observation of a non-equilibrium dynamical phase transition in a quantum simulator emulating long-range interacting spins.

Findings

Magnetization decays quickly below critical interaction strength.

Magnetization remains long-lived above the transition point.

Energy gap protects against dephasing in the long-lived phase.

Abstract

A proposed paradigm for out-of-equilibrium quantum systems is that an analogue of quantum phase transitions exists between parameter regimes of qualitatively distinct time-dependent behavior. Here, we present evidence of such a transition between dynamical phases in a cold-atom quantum simulator of the collective Heisenberg model. Our simulator encodes spin in the hyperfine states of ultracold fermionic potassium. Atoms are pinned in a network of single-particle modes, whose spatial extent emulates the long-range interactions of traditional quantum magnets. We find that below a critical interaction strength, magnetization of an initially polarized fermionic gas decays quickly, while above the transition point, the magnetization becomes long-lived, due to an energy gap that protects against dephasing by the inhomogeneous axial field. Our quantum simulation reveals a non-equilibrium transition predicted to exist but not yet directly observed in quenched s-wave superconductors.