Back heat shows that the laws of thermodynamics may require a quantum update

A forgotten cup of coffee gradually cools as its heat flows into the cooler surrounding air, but in the quantum realm it appears that 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 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 the crotonic acid molecule, which contains carbon, hydrogen and oxygen atoms. The scientists 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 calculations, scientists normally control the quantum states of qubits using bursts of electromagnetic radiation, but in this case the team used this control to allow heat to flow from a cooler, lower-temperature qubit to a hotter one.

This would never happen spontaneously to something in our macroscopic world like a cup of coffee because it would require additional energy to support the reverse 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 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, a modification of the conventional temperature that takes into account some quantum properties of the object, such as coherence, and saw that the second law of thermodynamics was again fulfilled, and heat flowed from a higher apparent temperature to a lower one.

Roberto Serra at the Federal University of ABC in Brazil, says that quantum properties such as coherence can be considered a type of thermodynamic resource analogous to how, for example, heat is the resource used to operate a steam engine. He says that when these quantum, microscopic sources are manipulated, the laws of thermodynamics can apparently be broken. “But the usual laws of thermodynamics were developed with the understanding that we don’t have access to these microscopic states. That’s just an obvious violation because we have to write new laws because we have this access,” Serra says.

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

topics:

• quantum computing/
• quantum physics