Localization effects induced by decoherence in superpositions of many-spin quantum states

TL;DR

This paper studies how decoherence causes localization in many-spin quantum states, limiting quantum information spread, using NMR quantum simulation and a phenomenological model to describe the observed effects.

Contribution

It demonstrates how local decoherence induces spatial localization in many-spin systems and introduces a phenomenological model to explain the limiting size of quantum state spreading.

Findings

Decoherence causes loss of spatial coherence in spin systems.

Adding perturbations halts quantum information spreading.

A dynamical equilibrium size emerges under perturbations.

Abstract

The spurious interaction of quantum systems with their environment known as decoherence leads, as a function of time, to a decay of coherence of superposition states. Since the interactions between system and environment are local, they can also cause a loss of spatial coherence: correlations between spatially distant parts of the system are lost and the equilibrium states can become localized. This effect limits the distance over which quantum information can be transmitted, e.g., along a spin chain. We investigate this issue in a nuclear magnetic resonance quantum simulator, where it is possible to monitor the spreading of quantum information in a three-dimensional network: states that are initially localized on individual spins (qubits) spread under the influence of a suitable Hamiltonian apparently without limits. If we add a perturbation to this Hamiltonian, the spreading stops and the system reaches a limiting size, which becomes smaller as the strength of the perturbation increases. This limiting size appears to represent a dynamical equilibrium. We present a phenomenological model to describe these results.