Crossing over from the quantum world to the classical world and back

The main goal of this program is to experimentally address the fundamental question: “What constitutes a quantum measurement?” Textbooks on quantum mechanics typically repeat the postulate made by Von Neumann that measurement occurs when a quantum state interacts with a classical measurement machine, and results in a probabilistic measurement outcome. Exactly how this postulate should be implemented has been at the center of debates ever since its formulation, and led to various theoretical proposals for an underlying dynamical description, in terms of either interactions with an unseen environment, or of fundamental perturbations to Schrödinger’s equation. We aim to approach this old problem in a modern, model-independent way, by providing experimental input coming from both the microscopic and macroscopic sides. At the quantum level, we aim to scale up the mass and/or displacements associated to quantum superposition states and measure their dynamics by highly controlled interactions with the surrounding. Coming from the classical side, we will cool down mechanical resonators that are in thermal equilibrium with their environment at a well-defined temperature, but weakly coupled to the thermal bath. We aim to improve these experiments so that we can measure the slight deviations in temperature between the resonator and its bath, that are expected to arise from changes in entanglement entropy during measurement. Both approaches will result in experimental constraint on various theoretical proposals for the dynamics of quantum measurement, and on a sharp experimental demarcation of what constitutes a quantum measurement.