Abstract

In order to use quantum computers to answer questions in the natural sciences, you need to know the properties of the system under study (molecules, solids, etc). These include static properties, such as excited state energies, but many properties are dynamic: light absorption, AC conductivity, magnetic susceptibility. Experimentally, these are obtained by applying a field, and measuring the system’s response. These response functions are a fundamental aspect of physics; they represent the link between experimental observations and the underlying quantum many-body state. However, this link is often under-appreciated, as the Lehmann formalism for obtaining response functions in linear response has no direct link to experiment. By using a linear response framework, we restore this link by making the experiment an inextricable part of the quantum simulation. This method can be frequency-and momentum-selective, avoids limitations on operators that can be directly measured, and is ancilla-free. As prototypical examples of response functions, we demonstrate that both bosonic and fermionic Green’s functions can be obtained, and apply these ideas to the study of a charge-density- wave material on ibm_auckland.