Ice cream-inspired physics: Team uncovers a quantum Mpemba effect, with a host of 'cool' implications

The Mpemba effect is best known as a perplexing phenomenon, where hot water freezes faster than cold water. Observations of the counter-intuitive effect date back to Aristotle who, over 2,000 years ago, noted that the Greeks of Pontus were exploiting the effect in their fishing practices.

The Mpemba effect has also stoked the curiosity of other great minds throughout history, such as René Descartes and Francis Bacon. It continues to be the subject of numerous broadsheet articles and pops up regularly as a curious focus in various settings, such as in cooking competition MasterChef, where contestants have tried capitalising on the effect to deliver frozen delicacies more quickly than seems possible in dessert challenges.

And now, we can say that this strange effect is much more ubiquitous than we previously expected as the Trinity QuSys team, led by Prof. John Goold from the School of Physics, has just published a fascinating research paper in the journal Physical Review Letters. The paper outlines their breakthrough in understanding the effect in the very different -- and extremely complex -- world of quantum physics.

Prof. Goold said: "The 'Mpemba effect' gets its name from Erasto Mpemba who, as a school kid in 1963, was making ice cream in his home economics class in Tanzania. Mpemba did not wait for his hot ice cream mixture to cool before putting it directly in the fridge and was unsurprisingly puzzled to find that it froze before all the colder samples of his classmates.

"He pointed this out to his teacher, who ridiculed him for not knowing his physics -- Newton's law of cooling, for example, tells us that the rate at which an object cools is proportional to the temperature difference between the object and its surroundings. However, Mpemba convinced a visiting professor -- Denis Osoborne from the University of Dar es Salaam -- to test what he had seen and the pair published a paper that indeed evidenced the strange effect."

While the Mpemba effect is still not wholly understood -- its presence is hotly debated at the macroscopic scale -- it is much more apparent on the microscopic scale, where physicists use the theory of quantum mechanics to describe nature.

The quantum Mpemba effect has recently become a trending topic, but myriad questions hung in the air; for example, how does the quantum effect relate to the original effect? And can we construct a thermodynamic framework to understand the phenomenon better?

The QuSys research group's breakthrough answers some of the key questions.

Prof. Goold said: "We are experts in the interface between non-equilibrium thermodynamics and quantum theory and, as such, we have the right toolbox to tackle these questions. Our work essentially provides a recipe to generate the Mpemba effect in quantum systems, where a physical transformation that effectively 'heats' the quantum system can be performed. This transformation of the quantum system then paradoxically allows it to relax or 'cool' exponentially faster by exploiting unique features in quantum dynamics."

Using the toolkit of non-equilibrium quantum thermodynamics, the team has successfully bridged the gap between Aristotle's observations from two millennia ago and our modern understanding of quantum mechanics.

And it now opens the door to many research and applications-related questions.

Prof. Goold added: "While we first took this project on out of intellectual curiosity it forced us to ask several fundamental questions about the relationship between the laws of thermodynamics that describe cooling, and the quantum mechanics, which describe reality at the fundamental level. We are currently developing a geometrical approach to the problem, which will hopefully allow us to understand different types of Mpemba effect in the same mathematical framework.

"What you actually have in this really 'cool' Mpemba effect is a way to speed up cooling -- and the cooling of quantum systems is absolutely vital for applications in quantum technologies. With that in mind I am sure some of the tools we are developing to investigate this fundamental effect will be of paramount importance for understanding things like heat flows, and how to minimise dissipation in future technologies."