Original version With this story appeared in Quanta Magazine.
If there is one law of physics that seems basic to understand, it is the second law of thermodynamics: heat flows spontaneously from warmer to colder bodies. But now, gently and almost casually, Alexsandre de Oliveira Jr. it just showed me that I don’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 cafe in Copenhagen. Just touch them and heat will flow from warm to frosty, as German scientist Rudolf Clausius first formally stated in 1850. But in some cases, de Oliveira explained, physicists have learned that the laws of quantum mechanics can direct heat flow in the opposite direction: from frosty to warm.
That doesn’t really mean the second law is failing, he added as his coffee cooled reassuringly. But the Clausius expression is the “classical limit” of the fuller formulation required by quantum physics.
Physicists began to appreciate the subtlety of this situation more than two decades ago, and since then they have been studying the quantum version of the second law. Now de Oliveira, a postdoctoral researcher at the Technical University of Denmark, and his colleagues they showed that the kind of “anomalous heat flow” that is possible on the quantum scale could have convenient and ingenious applications.
They argue that it could serve as an basic method of detecting “quantity” – for example, detecting that an object is in a quantum “superposition” of many possible observable states, or that two such objects are entangled with states that are interdependent – without destroying these exquisite quantum phenomena. Such a diagnostic tool could be used to check whether a quantum computer is actually using quantum resources to perform calculations. It may even assist sense the quantum aspects of gravity, one of the ambitious goals of state-of-the-art physics. Scientists say all they need to do is connect the quantum chip to another chip that can store information about it, and to a heat sink: a body that can absorb a lot of energy. With this configuration it is possible to escalate heat transfer to the heat sink beyond what would be allowed classically. By simply measuring how warm the sink is, one can then detect the presence of superposition or entanglement in the quantum system.