Backwards heat shows laws of thermodynamics may need a quantum update

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

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

This would never happen spontaneously to something in our macroscopic world, like a cup of coffee, because it would require additional energy to fuel the reflux. But in the quantum context, 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 not surprising, since they were established in the 19th century, about 100 years before quantum physics was formalized. To solve this problem, Lu and his colleagues calculated the “apparent temperature” of each qubit, which is a change in conventional temperature that explains certain quantum properties of an object, such as coherence, and saw the second law of thermodynamics satisfied again and heat flow from a higher apparent temperature to a lower temperature.

Roberto Serra, of ABC Federal University in Brazil, says quantum properties such as coherence can be seen as a type of thermodynamic resource analogous to the way, for example, heat is a resource used to operate a steam engine. He says that when these quantum and microscopic resources are manipulated, the laws of thermodynamics can apparently be broken. “But the usual laws of thermodynamics were developed thinking that we don’t have access to these microscopic states. This is only 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 inversion experiment into a more practical protocol for controlling heat between qubits, says Lu. Beyond discovering fundamental links between quantum information and heat, discovering new practical ways to cool qubits could improve quantum computers. This could be of great importance to the burgeoning quantum computing industry because, ultimately, even conventional computers can only perform as well as if they can avoid heating up, Serra says.