Aristotle knew that hot water freezes faster than cold water. This surprising observation remained little studied until 1963, when Erasto B. Mpemba, a Tanzanian high-school student, accidentally found the same when he was making ice cream at school. The phenomenon noticed by Aristotle and Mpemba, nowadays known as "Mpemba effect", seems to be very counterintuitive and in disagreement with some well-established laws of physics. However, these laws, as well as our intuition, are only valid close to thermal equilibrium, while Mpemba effect is a genuine out-of-equilibrium phenomenon, impossible to occur if we stay near equilibrium. Since its rediscovery by Mpemba, this effect has been the subject of many experimental, numerical and analytical investigations involving, among others, polymers, spin glasses and cold atoms. Despite the huge research activity, the conditions to observe this phenomenon and the mechanism that explains why it occurs are still under debate.
In a new paper published in Nature Communications, SISSA researchers led by Pasquale Calabrese, find a quantum version of Mpemba effect. The setup they studied is very common in out-of-equilibrium physics, and it is called global quench. Filiberto Ares and Sara Murciano, two young authors of the research, explain: “We take a many-body quantum system (a spin chain) in a state that breaks a particular symmetry and we suddenly change one of its parameters such that the system evolves in time in a way that the broken symmetry is dynamically restored. Surprisingly, we find that, for a large variety of spin chains, the more the symmetry is initially broken, the faster it is restored. Since this phenomenon occurs at zero temperature and it is driven by entanglement, the characteristic feature of quantum mechanics, we dub it quantum Mpemba effect. To study it, we introduce a new quantity that measures how much a symmetry is broken in a part of the system: the entanglement asymmetry. With it, we can investigate quantitatively the quantum Mpemba effect, not only theoretically with paper and pen as we have done, but also experimentally in the laboratory. Symmetry breaking is crucial in modern physics, for instance to explain the phases of matter or the generation of mass”. The entanglement asymmetry opens a new way to understand and unveil the physical phenomena related to symmetry breaking using the tools developed to study quantum entanglement. So far, it has shown its potential by giving a bit of… quantumness to the connection between Aristotle and ice-creams.
Image by Colin Behrens from Pixabay