Researchers in China have demonstrated heat flowing from cold to hot in a quantum system, potentially requiring updates to the second law of thermodynamics. Using a molecule as qubits, the team manipulated quantum information to achieve this reversal. The finding highlights differences between classical and quantum physics.
In a study published in Physical Review Letters, Dawei Lu and his colleagues at the Southern University of Science and Technology in China explored heat dynamics in the quantum realm. They used a molecule of crotonic acid, employing the nuclei of four carbon atoms as qubits—the fundamental units of quantum computers. By controlling the quantum states of these qubits with electromagnetic radiation, the researchers reversed the typical direction of heat flow, directing it from a colder qubit to a hotter one.
This reversal contradicts the second law of thermodynamics, which holds that heat naturally moves from hot to cold objects, as seen in everyday scenarios like a cooling cup of coffee. However, in quantum systems, such behavior becomes possible through 'coherence,' a form of quantum information that acts as an energy source. 'By injecting and controlling this quantum information, we can reverse the direction of heat flow,' Lu explained. The team expressed excitement over the result.
The laws of thermodynamics originated in the 19th century, predating quantum physics by about a century. To reconcile the observation, the researchers introduced the concept of 'apparent temperature,' which incorporates quantum properties like coherence. Under this measure, heat flowed from higher to lower apparent temperatures, restoring consistency with the second law.
Roberto Serra from the Federal University of ABC in Brazil commented that coherence functions as a thermodynamic resource, similar to heat in classical engines. He noted that traditional thermodynamics assumes no access to microscopic quantum states, leading to an apparent violation. 'This is just an apparent violation because we have to write new laws considering that we have this access,' Serra said.
Looking ahead, Lu's team aims to develop practical protocols for managing heat in qubits. Such advances could enhance quantum computing by improving cooling methods, addressing a key challenge in the field where excess heat limits performance, much like in conventional computers.