image: More asymmetric states can restore symmetry faster under the MBL evolution.
Credit: ©Science China Press
Why does hot water sometimes freeze faster than cold? This counterintuitive phenomenon, known as the Mpemba effect, has intrigued scientists for decades. In the classical world, it challenges common intuition about heat and cooling. In the quantum realm, it reappears in an even more surprising form: a more asymmetric quantum state can restore its subsystem symmetry faster than a less asymmetric one. This phenomenon is known as the quantum Mpemba effect (QME).
While the QME has been studied in integrable and chaotic quantum systems that eventually thermalize, one central mystery remained: can this counterintuitive phenomenon survive in systems that do not thermalize at all? MBL systems that resist thermalization due to disorder provide an ideal testing ground. Understanding whether and how the QME manifests in such systems could shed light on universal principles of nonequilibrium dynamics far beyond the reach of conventional thermodynamics.
In a new study published in Science Bulletin, researchers reveal that the quantum Mpemba effect indeed persists in the MBL regime, but through an entirely different mechanism. The team developed a theoretical framework for symmetry restoration in MBL systems and demonstrated that the subsystem symmetry can still be restored. Using the l-bit effective model and comprehensive numerical simulations, they confirmed that more symmetry-broken states can restore symmetry faster—an signature of the QME in the absence of thermalization.
The study also highlights striking contrasts between the QME in thermal and localized systems. In thermal systems, the QME can be viewed as a quantum analogue of the classical “hot water freezes faster” phenomenon. In localized systems, by contrast, the behavior resembles that of “syrup” that is hard to mix or flow—yet, remarkably, this “syrup” can still “freeze” and even do so faster when it starts hotter. This finding uncovers a new non-thermal mechanism of symmetry restoration and also provide a fresh qualitative indicator for the stability of many-body localized phases.
Beyond its conceptual significance, the work opens a new window on the nature of quantum relaxation and could inspire future experiments on quantum simulation and quantum computing platforms to explore counterintuitive dynamical phenomena in non-thermal systems.
The study, titled “Symmetry restoration and quantum Mpemba effect in many-body localization systems,” was conducted by researchers from Tsinghua University, the Institute of Physics, Chinese Academy of Sciences, and Sun Yat-sen University. It was supported by the National Natural Science Foundation of China, the Ministry of Science and Technology of China, and the New Cornerstone Science Foundation through the Xplorer Prize, among others.
Journal
Science Bulletin