The entropic cost of quantum generalized measurements

Landauer’s principle introduces a symmetry between computational and physical processes: erasure of information, a logically irreversible operation, must be underlain by an irreversible transformation dissipating energy. Monitoring micro- and nano-systems needs to enter into the energetic balance of their control; hence, finding the ultimate limits is instrumental to the development of future thermal machines operating at the quantum level. We report on the experimental investigation of a lower bound to the irreversible entropy associated to generalized quantum measurements on a quantum bit. We adopted a quantum photonics gate to implement a device interpolating from the weakly disturbing to the fully invasive and maximally informative regime. Our experiment prompted us to introduce a bound taking into account both the classical result of the measurement and the outcoming quantum state; unlike previous investigation, our entropic bound is based uniquely on measurable quantities. Our results highlight what insights the information-theoretic approach provides on building blocks of quantum information processors. A lower bound for the thermodynamic cost of quantum measurements can be found from non-thermodynamic quantities. Modern classical and quantum devices can be used to explore the fundamental limits of computation, such as the energy required to measure or erase information. Luca Mancino and collaborators in Italy and the United Kingdom have demonstrated the evaluation of a lower limit on irreversible thermodynamic entropy generated during a quantum measurement. In general, thermodynamic quantities are difficult to measure but the authors' bound is calculated using information obtained from standard quantum optical measurements. Mancino et al. find that although their bound does not constrain the entropy tightly it does reflect how the experimental parameters affect the entropic cost of the measurements. This demonstrates the potential for using information-theoretic quantities to gain insight into quantum thermodynamics.