‘New’ Mpemba effect untangles old thermodynamics enigma
‘New’ Mpemba effect untangles old thermodynamics enigma lead image
The Mpemba effect — named for a Tanzanian physics student who first observed it while making ice cream in 1963 — refers to the thermodynamics phenomenon of how hot substances or materials, such as water, sometimes cool faster than cold ones. The enigma has intrigued physicists ever since, yet there is no definitive explanation for it.
Recent studies have examined quantum systems, interactions between such systems and their environments, and their thermalization dynamics, which describe the way their energy is evenly distributed to achieve equilibrium.
Stefano Longhi examined the phenomenon through the lens of quantum entanglement.
“While the Mpemba effect has been known in classical physics for some time, we’re now looking at it in the quantum world, where things get even weirder,” said Longhi. “What we’ve discovered is a new kind of quantum Mpemba effect that happens only if the system and its environment are entangled right from the start — that is, when they’re deeply connected in a uniquely quantum way. This kind of entanglement changes how the system relaxes and releases energy.”
To illustrate, Longhi examined the spontaneous emission of a two-level system coupled to a photonic waveguide at zero temperature. Only when the atom and photon field are initially entangled does the state exhibit time-reversed decay and noticeable energy emission delay.
“Sometimes, a system further from equilibrium actually relaxes faster than one closer to equilibrium purely because of these initial quantum correlations,” said Longhi. “This is a fundamentally new kind of Mpemba effect — it doesn’t rely on exotic environments or complex dynamics. It may open new ways to think about controlling how quantum systems relax and lead to novel methods and applications in quantum computers and thermal machines.”
Source: “Quantum Mpemba effect from initial system-reservoir entanglement,” by Stefano Longhi, APL Quantum (2025). The article can be accessed at https://doi.org/10.1063/5.0266143 .
