01 July 2014
Thermodynamics for individual quantum systems

Schematic of thermal bath, quantum system and work storage

Framework for extracting the maximum amount of work from any system in

Thermodynamics is the fundamental set of laws describing heat, temperature and energy that governs, among other things, how much useful work can be extracted from a system. These laws are however only valid in the regime of many particles. Nevertheless, thermodynamic effects are becoming increasingly important at smaller scales, as ever smaller systems can be probed and manipulated. At the smallest scales, where systems behave according to quantum mechanics, an important question is to what extent thermodynamics remains valid.

The researchers have shown how to construct a framework, incorporating explicitly a thermal bath and a work storage system, such that the first and second laws of thermodynamics that govern the behavior of large numbers of particles apply also to individual ones. Within this framework they showed how the maximal amount of work compatible with the second law, that equal to the change in free energy of the system, can always be extracted via a simple protocol. In particular, this includes situations involving only a single copy of the system, without requiring the need to collectively process many copies simultaneously, and from any initial state, including those far from equilibrium and containing quantum coherences.

*Nature Communications*In a recent study, Paul Skrzypczyk in the Quantum Information Theory group led by ICREA Prof at ICFO Antonio Acín, in collaboration with Anthony Short and Sandu Popescu from the University of Bristol, UK, have presented a framework for extending thermodynamics to individual quantum systems, and shown how, within this framework, it is possible to extract the maximal amount of work possible from any system. This work was published in*Nature Communications*.Thermodynamics is the fundamental set of laws describing heat, temperature and energy that governs, among other things, how much useful work can be extracted from a system. These laws are however only valid in the regime of many particles. Nevertheless, thermodynamic effects are becoming increasingly important at smaller scales, as ever smaller systems can be probed and manipulated. At the smallest scales, where systems behave according to quantum mechanics, an important question is to what extent thermodynamics remains valid.

The researchers have shown how to construct a framework, incorporating explicitly a thermal bath and a work storage system, such that the first and second laws of thermodynamics that govern the behavior of large numbers of particles apply also to individual ones. Within this framework they showed how the maximal amount of work compatible with the second law, that equal to the change in free energy of the system, can always be extracted via a simple protocol. In particular, this includes situations involving only a single copy of the system, without requiring the need to collectively process many copies simultaneously, and from any initial state, including those far from equilibrium and containing quantum coherences.