Optimal Work Extraction and Thermodynamics of Quantum Measurements and Correlations

We analyze the role of indirect quantum measurements in work extraction from quantum systems in nonequilibrium states. In particular, we focus on the work that can be obtained by exploiting the correlations shared between the system of interest and an additional ancilla, where measurement backaction introduces a nontrivial thermodynamic tradeoff. We present optimal state-dependent protocols for extracting work from both classical and quantum correlations, the latter being measured by discord. Our quantitative analysis establishes that, while the work content of classical correlations can be fully extracted by performing local operations on the system of interest, accessing work related to quantum discord requires a specific driving protocol that includes interaction between system and ancilla.

Figure 1

Starting from ${\rho }_{SA}$, work can be extracted from $S$ either by a direct isothermal protocol [path (a)], or by first performing a measurement on $A$, and then applying an outcome dependent isothermal protocol [path (b)].

Figure 2

The maximum amount of extractable work from $S$ and $A$ [path (a)] can be enhanced using quantum measurements [path (b)]. However, the work content of quantum discord is in general lost in this case. Optimal feedback enhanced work extraction can then be achieved by extracting it before the measurement is performed [step (c)].