Thermodynamics of measurements and isolated quantum systems

Quantum mechanics was invented over a century ago to explain weird experimental results seen in studies of tiny particles. Today, we know that quantum theory is the deepest layer of our reality, explaining how the fundamental particles were made of move and interact, and its the basis of modern marvels like computer circuits. But the theory itself is highly unintuitive, predicting bizarre things like particles existing in two places at once simultaneously. This clearly cant happen in the everyday world we inhabit, but the evidence shows that at the smallest scales the particles were made of do behave like this. Clearly, as systems grow bigger, they lose their quantumness in some way, until eventually, by the time theyre visible to us through microscopes, they behave normally, or classically. Whats also clear is that whenever we look directly at a particle, we never see this two-places-at-once effect. Somehow the act of measurement is also involved in making the quantum world disappear. How exactly quantum systems become non-quantum is, to this day, a big mystery in physics. In this new project, Dr. Tom Rivlin of Technische Universität Wien will investigate the problem from the modern perspective of quantum thermodynamics, a new and growing field within quantum theory that seeks to connect the quantum world with the well-known thermodynamics of concepts like energy, heat, work, entropy, and temperature. In particular, the project will build on recent work that describes quantum measurements as thermodynamic processes, like milk mixing with coffee in a latte. Just as how the coffee and milk reach an equilibrium where theyre fully mixed, the idea is that during a measurement, quantum systems reach an equilibrium with their surrounding environment, spreading information from one into the other. This concept of measurement as equilibration relies on the thermodynamic quantity called entropy, which measures the amount of disorder in a system. Entropy always increases in the everyday world we live in, and we expect the same to be true of this measurement process. As such, this project will study different aspects of entropy in the quantum world in its own right, using various methods to explore how quantum and classical versions of entropy to connect to each other. More broadly, the project will use mathematical calculations, computer simulations, and conceptual arguments to study the thermodynamics of quantum measurements, to resolve deep, long-standing conceptual issues and paradoxes around the quantum-to-classical transition, and bring the quantum world a little closer to our own.