Measurement-Controlled Engines - Investigating the role of system-meter coupling time quantum information engines

Abstract

Nanoscale devices that transform energy into useful work are becoming ubiquitous. A critical task is to control energy transduction at the nanoscale. In this context, quantum measurement and the associated information acquisition can be leveraged to guide and enhance work output through feedback control. This thesis explores a quantum information engine as a prototype energy-transducing device controlled by measurement. This engine harnesses information transfer between a working medium, modelled as a two-level system, and a meter, modelled as a quantum harmonic oscillator. However, this information transfer is not instantaneous; it depends on the coupling time, which is the time required to correlate the system and the meter. This measurement time sets a lower bound on the cycle time of the quantum information engine, making information acquisition a crucial resource for the process. We investigate the cost of quantum measurement, in particular the energetic cost of coupling and decoupling the system and the meter in finite-time operations. Furthermore, we analyse possible schemes of extracting useful work: the ergotropy, or maximum work extraction under unitary transformations, and the excess work by stimulated emission. In both cases, the information about the system is exploited by conditioning the act of extracting work on the measurement outcome. Heat and work flows are analysed as functions of the system and meter temperatures to show that the quantum information engine can operate in different regimes: as a heat engine, a heat valve, a refrigerator and a “true” information engine by extracting work and cooling a colder bath. We show that the quantum information engine performance in terms of power output for very short measurement times, the Zeno limit, is small. To increase the power we need to increase the measurement time which, however, comes with a higher cost of measurement. We carefully analyse the work output-cost relation in different operating regimes of the quantum information engine to find optimal conditions for net work output and high engine performance.