Diverging equilibration times in long-range quantum spin models

TL;DR

This paper demonstrates that in long-range quantum Ising models with slow decaying interactions, certain observables do not equilibrate within accessible timescales, especially as system size grows large, highlighting a divergence in equilibration times.

Contribution

The study analytically shows that some observables in long-range quantum spin models do not equilibrate on observable timescales for large systems, revealing a divergence in equilibration times.

Findings

Expectation values remain close to initial values for large N

Equilibration times grow beyond experimental timescales

Long-range interactions cause slow or absent equilibration

Abstract

The approach to equilibrium is studied for long-range quantum Ising models where the interaction strength decays like r^{-\alpha} at large distances r with an exponent $\alpha$ not exceeding the lattice dimension. For a large class of observables and initial states, the time evolution of expectation values can be calculated. We prove analytically that, at a given instant of time t and for sufficiently large system size N, the expectation value of some observable <A>(t) will practically be unchanged from its initial value <A>(0). This finding implies that, for large enough N, equilibration effectively occurs on a time scale beyond the experimentally accessible one and will not be observed in practice.