What it is
The Mpemba effect is the counter-intuitive phenomenon in which, under certain conditions, a hotter system cools faster than a colder one, and its observation in quantum systems has spurred interest in the conditions that enable it. The authors consider weakly open many-body quantum systems, dominated by their own unitary evolution and only weakly coupled to their surroundings, started in different thermal states, and ask when the initially farther state relaxes faster to the non-equilibrium steady state. They claim that the number of conserved quantities in the unitary part plays a crucial role: the effect is possible only when the Hamiltonian commutes with other extensive operators or is integrable. When energy is the only approximately conserved quantity, dissipation pushes the dynamics along a single-parameter family of thermal states; with several conserved quantities, the dynamics drift in the multi-dimensional space of generalized Gibbs ensembles, whose distance to the steady state is less trivial. Numerical results for large system sizes, using tensor networks and free-fermion techniques, support the claim.
Why it matters
It gives a rule for where to look: in weakly open systems, extra conserved quantities appear necessary for the quantum Mpemba effect, though, the preprint notes, not sufficient. In the preprint's simulations, chaotic chains with energy as the only approximately conserved quantity relaxed smoothly, without their trajectories crossing, while the integrable chains showed the effect, and the preprint suggests its conclusions may facilitate experimental verification with cold-atom quantum simulators, for example using atom loss.
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Filed underQuantum many-body systems, Advanced Thermodynamics and Statistical Mechanics, Quantum and electron transport phenomena
