Rate Function Modelling of Quantum Many-Body Adiabaticity

The quantum adiabatic theorem is a fundamental result in quantum mechanics, with a multitude of applications, both theoretical and practical. Here, we investigate the dynamics of adiabatic processes for quantum many-body systems %in detail by analysing the properties of observable-free, intensive quantities. In particular, we study the adiabatic rate function $f(T, \Delta \lambda)$ in dependence of the ramp time $T$, which gives us a complete characterization of the many-body adiabatic fidelity as a function of $T$ and the strength of the parameter displacement $\Delta \lambda$. $f(T, \Delta \lambda)$ quantifies the deviation from adiabaticity for a given process and therefore allows us to control and define the notion of adiabaticity in many-body systems. First we study $f(T, \Delta \lambda)$ for the 1D transverse field Ising model and the Luttinger liquid, both of which are quadratic systems and therefore allow us to look at the thermodynamic limit. For ramps across gapped phases, we relate $f(T, \Delta \lambda)$ to the transition probability of the system and for ramps across a gapless point, or gapless phase we relate it to the excitation density of the relevant quasiparticles. Then we investigate the XXZ model which allows us to see the qualitative features that survive when interactions are turned on. Several key results in the literature regarding the interplay of the thermodynamic and the adiabatic limit are obtained as inferences from the properties of $f(T, \Delta \lambda)$ in the large $T$ limit.