Physicists Find a Way to Separate Useful Work From 'Waste Heat' in Quantum Machines

Researchers led by Professor Patrick Potts at the University of Basel in Switzerland have proposed a theoretical framework, published in Physical Review Letters, that reconciles thermodynamics, the 19th-century physics of how machines like steam engines convert and transfer energy, with quantum physics, which describes the behaviour of atoms and photons, or particles of light. The goal was a description of energy that holds up whether an entire system is treated fully quantum mechanically or in the "semi-classical limit," where only part of the system needs quantum treatment.

The team modelled an atom placed in a cavity between two mirrors, continuously fed photons by a laser while some light leaks out through the partially reflecting mirrors, "a textbook example of...a driven-dissipative system that continuously receives energy and simultaneously loses it to the environment," said postdoctoral researcher Marcelo Janovitch. Potts and colleagues had previously shown that photons leaving such a cavity should not automatically be counted as waste heat in a thermodynamic sense, because some of that escaping light can still do useful work on another quantum system.

The new study examined what happens to that heat-versus-work distinction as the system approaches the semi-classical limit, where the light is treated as a classical wave rather than as individual photons. The researchers showed mathematically that classifying part of the emitted light as useful work, rather than counting it all as heat, allows the theory to transition smoothly into the semi-classical limit; the conventional all-heat approach does not make that same consistent transition.

The calculations also showed that quantum effects can reduce fluctuations in the emitted light, a property the researchers say could be useful for quantum metrology, the science of taking especially precise measurements using quantum systems.