Physicists Create a Thermometer for Measuring ‘Quantumness’

original version of This is the story appeared Quanta Magazine.

If there is one law of physics that seems easy to grasp, it is the second law of thermodynamics: heat flows spontaneously from hotter bodies to colder bodies. But now, gently and almost casually, Alexandre de Oliveira Jr Just showed me that I really didn’t understand it.

Take this hot cup of coffee and this jug of cold milk, the Brazilian physicist said as we sat in a café in Copenhagen. Bring them into contact and sure enough, heat will flow from the hot object to the cold one, just as the 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 drive heat flow in reverse: from cold to cold.

That doesn’t really mean the second law fails, he adds as his coffee cools reassuringly. It is true that Clausius’ expression is the “classical limit” of the more complete formulation claimed by quantum physics.

Physicists began to appreciate the nuances of this situation more than two decades ago and have been exploring the quantum mechanical version of the second law ever since. Now, postdoctoral researcher De Oliveira and colleagues at the Technical University of Denmark showed The kind of “phenomenal heat flow” enabled at the quantum scale may have convenient and intelligent uses.

This, they say, can serve as a simple method of detecting “quantity”—for example, that an object is in a quantum “superposition” of multiple possible observable states, or that two such objects are involved in interdependent states—without destroying that subtle quantum phenomenon. Such a diagnostic tool can be used to confirm that a quantum computer is truly using quantum resources to perform calculations. It may even help understand the quantum aspects of gravity, one of the stretch goals of modern physics. All that is needed, the researchers say, is to connect a quantum system to a second system that can store information about it, and to a heat sink: a body capable of absorbing large amounts of energy. With this setup, you can increase the heat transfer to the heat sink, more than what would be classically allowed. By measuring how hot the sink is, you can detect the presence of superposition or entanglement in the quantum system.