Theory of quantum systems

Abstract:

In these lectures, we discuss theoretical aspects of analogue quantum simulation, focussing on ultracold atoms in optical lattices as a specific example. We touch on the motivation for quantum simulation, before going into detail on the first-principles microscopic understanding of the system that allows us to use ultra-cold atoms as quantum simulators. This level of understanding and control crucially also exists for the dominant dissipative processes in experiments. We conclude by discussing the verification of quantum simulators, comparison with classical simulations, and the current role of many-body entanglement in this discussion.

Abstract:

The complex zeros of partition functions were originally investigated by Lee and Yang to explain the behavior of condensing gases. Since then, Lee-Yang zeros have become a powerful tool to describe phase transitions in interacting systems. Today, Lee-Yang zeros are no longer just a theoretical concept; they have been determined in recent experiments. In one approach, the Lee-Yang zeros are extracted from the high cumulants of thermodynamic observables at finite size. Here we employ this method to investigate a phase transition in a molecular zipper. From the energy fluctuations in small zippers, we can predict the temperature at which a phase transition occurs in the thermodynamic limit. Even when the system does not undergo a sharp transition, the Lee-Yang zeros carry important information about the large-deviation statistics and its symmetry properties. Our work suggests an interesting duality between fluctuations in small systems and their phase behavior in the thermodynamic limit. These predictions may be tested in future experiments.