Quantum Coherence Reduces Friction in Quantum Heat Engines

Physics 14, s133
Predictions indicate that introducing quantum coherence into quantum heat engines can significantly reduce the friction in these systems.

Without the ability of quantum states to maintain their entanglement and superposition under external forcing, quantum technologies that outperform their classical counterparts would be a pipe dream. But while researchers know that this quantum coherence is key to the operation of quantum sensors and quantum information systems, for example, it is less clear how it might affect thermodynamic devices, such as quantum heat engines. Now, Hiroyasu Tajima of the University of Electro-Communications and Ken Funo of the RIKEN Cluster for Pioneering Research, both in Japan, have taken a step toward answering that question by looking at how quantum coherence influences “friction” in a quantum heat engine [1]. Tajima and Funo say that the results could help researchers build future quantum heat engines that are more powerful and efficient than current classical engines. These quantum engines might be used to cool down quantum systems or to transport energy in nanoscale devices.

A heat engine generates power from the heat flow between hot and cold reservoirs. First, the engine connects to the hot reservoir, absorbing heat, then it connects to the cold reservoir, releasing that heat. Then the cycle repeats. The engine’s efficiency depends on the rate of this process, with faster cycles dissipating more energy through friction. A classical engine could achieve the maximum efficiency only if its cycles were infinitely slow.

Tajima and Funo predict that the prospects are better for a quantum heat engine, however. Here, the quantum equivalent of friction—an entropy increase associated with the absorption and release of heat—can be completely eliminated if the engine contains enough quantum coherence. This “quantum lubrication” leads to “superconducting,” dissipationless flow of heat during fast engine cycles, allowing the engine to achieve its maximum efficiency under these operating conditions—something that isn’t possible for classical engines.

–Katherine Wright

Katherine Wright is the Deputy Editor of Physics Magazine.


  1. H. Tajima and K. Funo, “Superconducting-like heat current: Effective cancellation of current-dissipation trade-off by quantum coherence,” Phys. Rev. Lett. 127, 190604 (2021).

Subject Areas

Quantum Physics

Related Articles

Trapped Ions Go the Distance
Atomic and Molecular Physics

Trapped Ions Go the Distance

Researchers have achieved long-distance entanglement between two calcium ions, each of which lies in a different building, showing that trapped ions could be used to create quantum networks. Read More »

Quantum Circuit Tackles “Diabolical” Photochemical Process
Chemical Physics

Quantum Circuit Tackles “Diabolical” Photochemical Process

A quantum device shows promise for simulating molecular dynamics in a difficult-to-model photochemical process that is relevant to vision. Read More »

Twinkling of a Shrinking Droplet Reveals Hidden Complexity
Atomic and Molecular Physics

Twinkling of a Shrinking Droplet Reveals Hidden Complexity

Captivating patterns found in the light scattered by an evaporating water droplet could be used to infer the properties of the droplet as it shrinks. Read More »

More Articles