Backwards heat shows laws of thermodynamics may need a quantum update

A forgotten cup of coffee will gradually cool down as its heat flows into the cooler surrounding air, but in the quantum realm, it appears this experience can be turned on its head. As a result, we may need to update the second law of thermodynamics, a fundamental principle of physics that states heat energy always flows from hot to cold.

Dawei Lu at the Southern University of Science and Technology in China and his colleagues have seemingly broken this law with a molecule of crotonic acid, which contains atoms of carbon, hydrogen and oxygen. The researchers used the nuclei of four of its carbon atoms as qubits, which are the basic building blocks of quantum computers and can store quantum information. When used in computation, researchers normally control the quantum states of the qubits with bursts of electromagnetic radiation, but in this case, the team leveraged this control to make heat flow from a colder, lower-temperature qubit towards a hotter one instead.

This would never spontaneously happen to something in our macroscopic world, like a cup of coffee, as it would require extra energy to fuel the backwards flow. But in the quantum setting, other forms of fuel are available – in this case, a form of quantum information called “coherence”. “By injecting and controlling this quantum information, we can reverse the direction of heat flow,” says Lu. “We were excited.”

The fact that the laws of thermodynamics break down in the quantum realm is perhaps unsurprising, as they were laid down in the 19th century, about 100 years before the formalisation of quantum physics. To fix this issue, Lu and his colleagues calculated each qubit’s “apparent temperature,” which is a modification of conventional temperature that accounts for some of an object’s quantum properties, like coherence, and saw the second law of thermodynamics become satisfied again and heat flow from a higher apparent temperature to a lower one.

Roberto Serra at the Federal University of ABC in Brazil says that quantum properties like coherence can be considered a type of thermodynamic resource analogous to how, for example, heat is a resource that is used to make a steam engine work. He says that when these quantum, microscopic resources are manipulated, thermodynamics laws can be apparently broken. “But the usual laws of thermodynamics were developed thinking that we do not have access to these microscopic states. This is just an apparent violation because we have to write new laws considering that we have this access,” says Serra.

The researchers now want to turn their heat-reversal experiment into a more practical protocol for controlling heat among qubits, says Lu. Beyond uncovering fundamental connections between quantum information and heat, finding new practical ways to cool qubits could improve quantum computers. This could be of great importance for the burgeoning quantum computing industry as, ultimately, even conventional computers can only work as well as they can avoid heating up, says Serra.