Physicists rewrite thermodynamics for the quantum age — new theory changes the picture
Researchers at the University of Basel have introduced a new way to apply thermodynamic principles to very small quantum systems. Their results were published recently in Physical Review Letters.
Historical context
Thermodynamics began in the late 18th century. In 1798 Benjamin Thompson (Count Rumford) observed during experiments with cannon-boring in Munich that heat is not a material substance but can be produced indefinitely by mechanical friction. He placed heated gun barrels in water and timed how long it took the water to boil; experiments like these helped shape 19th-century thermodynamics.
Thermodynamics became central to the Industrial Revolution by explaining how heat can be converted into useful work in devices such as steam engines. Today the main laws of thermodynamics are foundational in the physical sciences: total energy (including heat and work) is conserved in a closed system, and entropy, a measure of disorder, cannot decrease.
The quantum problem
Those classical laws work broadly, but applying them to the smallest quantum systems presents problems. In microscopic quantum systems the distinction between work (macroscopically useful energy) and heat (disordered microscopic motion) becomes unclear.
A team at the University of Basel, led by Professor Patrick P. Potts, found a new consistent way to define thermodynamic quantities for certain quantum systems (see Physical Review Letters). Doctoral student Aaron Daniel explains that in such systems "everything is microscopic," so separating work from heat is no longer straightforward.
Laser light in a cavity resonator
As an illustrative example the team studied cavity resonators, where incoming laser light bounces between two mirrors and then partially exits the cavity. Laser light differs from ordinary incandescent or LED light because its electromagnetic waves oscillate in perfect synchrony (coherence). When laser light passes through a cavity containing atoms, that coherence can be disrupted to varying degrees. The light leaving the cavity can therefore be partially or completely incoherent, corresponding to more disordered, heat-like motion.
Max Schrauwen, a student on the project, says that "the coherence of the light in such a laser–cavity system was the starting point of our calculations."
The new approach
The researchers first defined "work" in the context of laser light operationally — for example, as the ability to charge a so-called quantum battery, which requires coherent light able to collectively drive a set of atoms into an excited state.
A simplistic view would label all incoming coherent laser light as capable of producing work and all outgoing partially incoherent light as heat. However, even partially incoherent light can in principle produce useful work, albeit less efficiently than fully coherent light. The Basel team therefore explored a finer distinction: treating the coherent portion of the outgoing light as work and the incoherent portion as heat.
They found that when work is defined this way, both main laws of thermodynamics are satisfied for the systems considered, making the approach internally consistent. As Aaron Daniel notes, "in the future, we can use our formalism to investigate more subtle problems in quantum thermodynamics."
Implications
This formalism is important for quantum technologies—such as quantum networks—and for studying the transition from classical to quantum behavior in macroscopic systems. By providing a consistent way to separate work and heat in certain quantum settings, the new theory offers a practical tool for analyzing energy flows at the quantum scale.
Reference: results published in Physical Review Letters (University of Basel research team led by Patrick P. Potts).
