Thermodynamic control by frequent quantum measurements

Extreme measurements Erez et al . enter uncharted territory with their prediction of the behaviour of a hitherto unexplored quantum mechanical setting in which neither the second law of thermodynamics, nor the common notion that heat always flows from hotter to colder ensembles, can be relied upon. In work compared in the accompanying News and Views piece to a quantum evocation of Maxwell's demon, the processes of quantum measurement appear to control thermodynamical behaviour. The system considered consists of two energy levels surrounded by a 'heat bath' that can supply or soak up any amount of heat. In such two-level quantum systems, the act of making a measurement can cause their relaxation to either slow down (the Zeno effect, where a continuously observed unstable particle never decays), or to speed up (the anti-Zeno effect). The latter effect is associated with a decrease in entropy and temperature of the system and its bath, while the former results in heating and higher entropy. That type of behaviour breaks the standard thermodynamical rules. In practical terms, these anomalies may offer the possibility of very fast control of heat and entropy in quantum systems. This paper predicts a trend in a purely quantum mechanical setting. It is known that measurements of two-level quantum systems can cause their relaxation to either speed-up (the anti-Zeno effect) or slow-down (the Zeno effect). But this paper finds that the former effect is associated with a decrease in the entropy and temperature of the system and the bath, while the latter effect results in heating and higher entropy. This behaviour is contrary to standard thermodynamical rules. Heat flow between a large thermal ‘bath’ and a smaller system brings them progressively closer to thermal equilibrium while increasing their entropy 1 . Fluctuations involving a small fraction of a statistical ensemble of systems interacting with the bath result in deviations from this trend. In this respect, quantum and classical thermodynamics are in agreement 1 , 2 , 3 , 4 , 5 . Here we predict a different trend in a purely quantum mechanical setting: disturbances of thermal equilibrium between two-level systems (TLSs) and a bath 6 , caused by frequent, brief quantum non-demolition 7 , 8 , 9 , 10 measurements of the TLS energy states. By making the measurements increasingly frequent, we encounter first the anti-Zeno regime and then the Zeno regime (namely where the TLSs’ relaxation respectively speeds up and