Observation of a dynamical topological phase transition

TL;DR

This paper reports the first experimental observation of a dynamical topological phase transition in a fermionic many-body system after a quench, using ultracold atoms and state tomography to track vortex dynamics and topological order changes.

Contribution

It provides the first direct observation of a dynamical topological phase transition in a non-equilibrium quantum system using ultracold atoms.

Findings

Vortices in reciprocal space appear, move, and annihilate during the dynamics.

The number of vortices acts as a dynamical topological order parameter.

The topological order parameter changes abruptly at the critical times of the DPT.

Abstract

Phase transitions are a fundamental concept in science describing diverse phenomena ranging from, e.g., the freezing of water to Bose-Einstein condensation. While the concept is well-established in equilibrium, similarly fundamental concepts for systems far from equilibrium are just being explored, such as the recently introduced dynamical phase transition (DPT). Here we report on the first observation of a DPT in the dynamics of a fermionic many-body state after a quench between two lattice Hamiltonians. With time-resolved state tomography in a system of ultracold atoms in optical lattices, we obtain full access to the evolution of the wave function. We observe the appearance, movement, and annihilation of vortices in reciprocal space. We identify their number as a dynamical topological order parameter, which suddenly changes its value at the critical times of the DPT. Our observation of a DPT is an important step towards a more comprehensive understanding of non-equilibrium dynamics in general.