The conventional understanding of heat transfer, where warmth naturally dissipates from hotter objects to cooler surroundings—like a cooling cup of coffee—may not hold true in the quantum universe. Here, heat flow can seemingly be reversed, prompting a re-evaluation of the second law of thermodynamics, a cornerstone of physics stating that heat energy invariably moves from hotter to colder regions.
A recent experiment involving a crotonic acid molecule, composed of carbon, hydrogen, and oxygen atoms, by Dawei Lu and his team at the Southern University of Science and Technology in China, suggests a potential breach of this fundamental principle. The researchers utilized the nuclei of four carbon atoms from the molecule, identifying them as qubits—the basic units for quantum computation capable of storing quantum information.
Normally, computational processes involving qubits are managed through deliberate pulses of electromagnetic radiation that manipulate their quantum states. However, in this particular study, the scientists manipulated this control mechanism to facilitate a flow of heat from a colder qubit to a hotter one, an outcome impossible in the macroscopic world without external energy input.
Unlike a cup of coffee, which would require additional energy to reverse its natural cooling process, the quantum environment offers alternative energy sources. The researchers harnessed a form of quantum information known as “coherence.” “By injecting and controlling this quantum information, we can reverse the direction of heat flow,” stated Lu. This finding was met with considerable excitement by the research team.
The observation that thermodynamic laws appear to break down at the quantum scale is not entirely unexpected. These laws were formulated in the 19th century, predating the formal development of quantum physics by approximately a century. To reconcile this discrepancy, Lu and his colleagues introduced the concept of “apparent temperature.” This metric, a modification of conventional temperature, incorporates an object’s quantum characteristics, such as coherence.
When considering apparent temperature, the researchers found that the second law of thermodynamics was satisfied. In this context, heat was observed to flow from a region of higher apparent temperature to one of lower apparent temperature.
Roberto Serra, a physicist at the Federal University of ABC in Brazil, commented that quantum properties like coherence can be viewed as a form of thermodynamic resource, akin to heat’s role in powering a steam engine. He posits that manipulating these microscopic quantum resources can lead to apparent violations of thermodynamic laws. “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,” Serra explained.
The research group now aims to refine their heat-reversal experiment into a functional protocol for managing heat among qubits. Such advancements could offer significant benefits beyond clarifying the intricate relationship between quantum information and heat. Improving methods for cooling qubits could enhance the performance of quantum computers, a development of considerable importance for the rapidly growing quantum computing industry.
As Serra noted, the operational capabilities of any computer, including conventional ones, are ultimately limited by their ability to manage heat buildup, highlighting the broad implications of this research.
Journal reference: Physical Review Letters