Physicists built a thermometer to measure quantum
The original version from This story appeared 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 bodies to colder bodies. But now, gently and almost casually, Alexander de Oliveira Jr. showed me that I had not understood it at all.
Take this cup of hot coffee and this pitcher of cold milk, said the Brazilian physicist while we were sitting in a cafe in Copenhagen. Put them in contact and sure enough, heat will flow from a hot body to a cold body, 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 direct heat flow in reverse: from cold to hot.
That doesn’t really mean the second law fails, he added, as his coffee is reassuringly cool. Only Clausius’ expression of the “classical limit” is the more complete formulation that quantum physics requires.
Physicists began to understand the subtleties of this situation more than two decades ago, and have been investigating 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 a type of “anomalous heat flow” activated at the quantum scale can have convenient and intelligent applications.
They say this could serve as an easy way to detect “quantum” — for example, sensing that an object in a quantum “superspace” is visible from multiple states, or that two such objects are entangled with states that depend on each other — without destroying those subtle quantum phenomena. Such a diagnostic tool can be used to ensure that a quantum computer is actually using quantum resources to perform calculations. It may even help to understand the quantum aspects of gravity, one of the thrusting goals of modern physics. According to the researchers, all that is needed is to connect one 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 these settings, you can increase the heat transfer to the heatsink, exceeding the classical limit. By simply measuring how hot the sink is, you can detect entanglement in a quantum system.
