The Power of Entangled Measurements in Quantum Thermodynamics and Networks

Abstract

Characterizing the gaps between our classical world and quantum mechanics to identify the potential benefits that quantum can bring is the main drive behind the various quantum-related research done nowadays. This doctoral thesis explores two different areas where quantum effects can play an important role. The first part of the thesis is dedicated to quantum thermodynamics, where we are specifically considering different aspects of work. We start by looking at the work extraction protocol, which can be implemented on completely incoherent systems. Here we do not need actual quantum effects, but the use of quantum systems allows for much better control and isolation than what would be possible on macroscopic classical systems. But even quantum systems are still subject to various kinds of noise and we study their effect on optimal thermodynamic protocols. We then identify the issues that arise when considering work in coherent quantum processes and present proposals for reducing the quantum backaction, in particular by performing entangled measurements. The second part of the thesis explores quantum phenomena in various quantum networks. We start by considering the simplest non-trivial quantum network, the triangle scenario. Here we construct a measurement scheme using entangled sources and entangled projections that yields a purely quantum output distribution that does not allow for a local model. Trying to investigate characteristics that suggest quantum nonlocality, we analyze the set of fully symmetric distributions that can be reproduced by a classical model. We then scale up the number of involved qubits to consider bigger networks, and experimentally test the current limits of state-of-the-art quantum devices. Ultimately, we substantiate their power by their ability to produce nonlocal distributions by applying sophisticated entangled measurements on up to ten qubits.