Extracting work from quantum measurement*

In classical heat engines, a thermal bath provides heat to the working agent. This uncontroled form of energy exchange is associated with entropy production as the reservoir acts randomly on the working agent.
In the quantum world, there is another source of randomness: quantum measurement. On top of producing entropy, quantum measurement also induces random changes of the measured system's energy just like a thermal reservoir: a genuinely quantum form of heat [1,2].
Building on this analogy, we present an engine fueled by quantum measurement instead of a thermal reservoir [3]. This engine involves a Qubit as a working agent, projective measurements of an observable of the Qubit, and a coherent field to extract work, and a cold thermal bath. As it involves only one bath, this engine is not limited by Carnot efficiency. In the Zeno limit of very frequent measurements, the engine reaches unit efficiency: the measurements becomes a deterministic source of work. For finite measurement frequencies, the efficiency is below one and instead, measurement provides heat.
[1] J. J. Alonso et al., Phys. Rev. Lett. 116, 080403 (2016)
[2] C. Elouard et al, npj Quantum Information 3, 9 (2017)
[3] C. Elouard et al., Phys. Rev. Lett. 118, 260603 (2017)