If there’s one law of physics that seems easy to grasp, it’s the second law of thermodynamics: Heat flows spontaneously from hotter bodies to colder ones. But now, gently and almost casually, Alexssandre de Oliveira Jr. has just shown me I didn’t truly 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. 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 stated formally in 1850. However, in some cases, de Oliveira explained, physicists have learned that the laws of quantum mechanics can drive heat flow the opposite way: from cold to hot.
This doesn’t really mean that the second law fails, he added as his coffee reassuringly cooled. It’s just that Clausius’ expression is the “classical limit” of a more complete formulation demanded by quantum physics.
Physicists began to appreciate the subtlety 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 colleagues have shown that the kind of “anomalous heat flow” that’s enabled at the quantum scale could have a convenient and ingenious use.
It can serve, they say, as an easy method for detecting “quantumness” — sensing, for instance, that an object is in a quantum “superposition” of multiple possible observable states, or that two such objects are entangled, with states that are interdependent — without destroying those delicate quantum phenomena. Such a diagnostic tool could be used to ensure that a quantum computer is truly using quantum resources to perform calculations. It might even help to sense 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 a quantum system to a second system that can store information about it, and to a heat sink: a body that’s able to absorb a lot of energy. With this setup, you can boost the transfer of heat to the heat sink, exceeding what would be permitted classically. Simply by measuring how hot the sink is, you could then detect the presence of superposition or entanglement in the quantum system.
Practical benefits aside, the research demonstrates a new aspect of a deep truth about thermodynamics: How heat and energy can be transformed and moved in physical systems is intimately bound up with information — what is or can be known about those systems. In this case, we “pay for” the anomalous heat flow by sacrificing stored information about the quantum system.
“I love the idea that thermodynamic quantities can signal quantum phenomena,” said the physicist Nicole Yunger Halpern of the University of Maryland. “The topic is fundamental and deep.”
Knowledge Is Power
“It is impossible for a self-acting machine, unaided by any external agency, to convey heat from one body to another at a higher temperature,” Rudolf Clausius wrote (in German) in 1850. It was the first expression of the second law of thermodynamics. Theo Schafgans/Public Domain
The connection between the second law of thermodynamics and information was first explored in the 19th century by the Scottish physicist James Clerk Maxwell. To Maxwell’s distress, Clausius’ second law seemed to imply that pockets of heat will dissipate throughout the universe until all temperature differences disappear. In the process, the total entropy of the universe — crudely, a measure of how disordered and featureless it is — will inexorably increase. Maxwell realized that this trend would eventually remove all possibility of harnessing heat flows to do useful work, and the universe would settle into a sterile equilibrium pervaded by a uniform buzz of thermal motion: a “heat death.” That forecast would be troubling enough to anyone. It was anathema to the devoutly Christian Maxwell. But in a letter to his friend Peter Guthrie Tait in 1867, Maxwell claimed to have found a way to “pick a hole” in the second law.
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