The fundamental laws governing heat flow—specifically, that heat always moves from hotter to colder objects—may require revision at the quantum level. New research demonstrates the apparent reversal of this principle using a molecule of crotonic acid, potentially forcing a reassessment of the second law of thermodynamics.
Quantum Heat Flow Reversal
Researchers led by Dawei Lu at the Southern University of Science and Technology in China manipulated the quantum states of carbon atoms within a crotonic acid molecule (a compound containing carbon, hydrogen, and oxygen). These atoms functioned as qubits—the fundamental units of quantum computing—and were controlled using electromagnetic radiation. Instead of heat flowing from warmer to cooler qubits as expected, the team engineered a reverse flow, pushing heat from lower-temperature qubits toward hotter ones.
This outcome defies classical thermodynamics, where such a process would require external energy input. However, in the quantum realm, the team leveraged a resource called “coherence”—a form of quantum information—to effectively fuel this backwards heat transfer. According to Lu, “By injecting and controlling this quantum information, we can reverse the direction of heat flow.”
The Role of Apparent Temperature
The apparent violation of the second law is not necessarily a flaw in the law itself, but rather a limitation of its traditional formulation. The second law was established in the 19th century, predating the development of quantum physics. To reconcile this discrepancy, Lu and his colleagues calculated an “apparent temperature” for each qubit. This adjusted temperature accounts for quantum properties such as coherence, restoring the validity of the second law by ensuring heat flows from higher to lower apparent temperatures.
Quantum Resources and Thermodynamics
Roberto Serra at the Federal University of ABC in Brazil suggests that quantum properties like coherence should be considered a thermodynamic resource, similar to how heat drives a steam engine. Manipulating these microscopic resources allows for apparent breaches of traditional thermodynamics. “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 explains.
Implications for Quantum Computing
The research team aims to translate this heat-reversal experiment into a practical method for qubit thermal control. This has significant implications for quantum computing, where efficient heat management is crucial. Improved cooling strategies could enhance the stability and performance of qubits, and even inform the development of conventional computers, as overheating remains a fundamental limitation in all computing systems.
This research highlights the profound interplay between quantum information and thermodynamics, suggesting that our understanding of heat flow must evolve to accommodate the unique rules governing the quantum world.
