Physicists Create a Thermometer for Measuring ‘Quantumness’

The original version to This story Featured in Quanta Magazine.

If there’s one law of physics that seems easy to understand, it’s the second law of thermodynamics: heat flows spontaneously from hotter objects to cooler objects. But now, gently and almost casually, Alexandre de Oliveira Jr. has just explained to me that I didn’t really understand it at all.

“Take this hot cup of coffee, and this cold jug of milk,” the Brazilian physicist said as we sat in a café in Copenhagen. Put them in contact, and heat is sure to flow from the hot object to the cold object, just as German scientist Rudolf Clausius first formally stated in 1850. However, in some cases, de Oliveira explained, physicists have learned that the laws of quantum mechanics can push the flow of heat in the opposite direction: from cold to hot.

As his coffee cooled reassuringly, he added, “This does not mean the second law has failed.” It is just that Clausius’s expression is the “classical limit” of a more complete formulation required by quantum physics.

Physicists began to appreciate the precision of this situation more than two decades ago, and have been exploring the quantum mechanical version of the second law ever since. Now, de Oliveira, a postdoctoral researcher at the Technical University of Denmark, and his colleagues have shown that this type of “anomalous heat flow” enabled at the quantum level could have a relevant and innovative use.

They say it could serve as an easy way to detect “quantum” — for example, sensing that an object is in a quantum “superposition” of multiple possible observable states, or that two such objects are entangled, with correlated states — without destroying those subtle quantum phenomena. Such a diagnostic tool could be used to confirm that a quantum computer is truly using quantum resources to perform calculations. It may also be useful for sensing the quantum aspects of the force of gravity, one of the stretch goals of modern physics. All that’s needed, the researchers say, is to connect the quantum system to a second system that can store information about it, and to a heat sink: an object capable of absorbing a lot of energy. With this setting, you can enhance the heat transfer to the heat sink, beyond what is classically permissible. Simply by measuring how hot the aquarium is, you can then detect the presence of superposition or entanglement in the quantum system.