Odd thermodynamic limit for the Loschmidt echo

TL;DR

This paper demonstrates that in antiferromagnetic quantum spin rings, the Loschmidt echo reveals a fundamental odd-even effect related to topological frustration, enabling distinction between systems with different numbers of elements through out-of-equilibrium experiments.

Contribution

It introduces a novel out-of-equilibrium protocol using the Loschmidt echo to distinguish frustrated and non-frustrated antiferromagnetic spin chains based on their spectral properties.

Findings

Loschmidt echo differs for even and odd chain sites

Topological frustration affects quantum thermodynamics observables

Spectral differences lead to measurable out-of-equilibrium signatures

Abstract

Is it possible to readily distinguish a system made by an Avogadro's number of identical elements and one with a single additional one? Usually, the answer to this question is negative but, in this work, we show that in antiferromagnetic quantum spin rings a simple out-of-equilibrium experiment can do so, yielding two qualitatively and quantitatively different outcomes depending on whether the system includes an even or an odd number of elements. We consider a local quantum-quench setup and calculate a generating function of the work done, namely, the Loschmidt echo, showing that it displays different features depending on the presence or absence of topological frustration, which is triggered by the even/oddness in the number of the chain sites. We employ the prototypical quantum Ising chain to illustrate this phenomenology, which we argue being generic for antiferromagnetic spin chains, as it stems primarily from the different low energy spectra of frustrated and non frustrated chains. Our results thus prove that these well-known spectral differences lead indeed to distinct observable characteristics and open the way to harvest them in quantum thermodynamics protocols.