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Abstract said:Statistical mechanics is founded on the assumption that all accessible configurations of a system are equally likely. This requires dynamics that explore all states over time, known as ergodic dynamics. In isolated quantum systems, however, the occurrence of ergodic behavior has remained an outstanding question. Here, we demonstrate ergodic dynamics in a small quantum system consisting of only three superconducting qubits. The qubits undergo a sequence of rotations and interactions and we measure the evolution of the density matrix. Maps of the entanglement entropy show that the full system can act like a reservoir for individual qubits, increasing their entropy through entanglement. Surprisingly, these maps bear a strong resemblance to the phase space dynamics in the classical limit; classically chaotic motion coincides with higher entanglement entropy. We further show that in regions of high entropy the full multi-qubit system undergoes ergodic dynamics. Our work illustrates how controllable quantum systems can investigate fundamental questions in non-equilibrium thermodynamics.

*NB: For a more introductory version, phys.org ran a piece on this article last summer*

From my understanding entanglement is generally seen as purely a quantum phenomenon, while on the other hand chaos is generally seen as a purely classical phenomenon. The results of this research however suggest something quite different, namely a duality or correspondence between the two phenomena: a 'Quantum Entanglement Entropy/Classical Chaos' correspondence.

What makes this research even more fascinating (if a QEE/CC correspondence isn't already enough) is that this all somehow ties into concepts in non-equilibrium thermodynamics, implying these findings might feature naturally in a full mathematical formulation of that theory. This is speculative, but if any of this is true these findings could be potentially extremely relevant in general to biological physics (see [URL='https://www.physicsforums.com/insights/author/john-baez/']John Baez' blogposts on this[/URL]).