Abstract
A gapped ground state of a quantum spin system has a natural length scale set
by the gap. This length scale governs the decay of correlations. A common
intuition is that this length scale also controls the spatial relaxation
towards the ground state away from impurities or boundaries. The aim of this
article is to take a step towards a proof of this intuition. To make the
problem more tractable, we assume that there is a unique ground state that is
frustration-free and invertible (i.e. no long-range entanglement). Moreover, we
assume the property that we are aiming to prove for one specific kind of
boundary condition; namely open boundary conditions. With these assumptions we
can prove stretched exponential decay away from boundaries for any boundary
conditions or (large) perturbations and for all ground states of the perturbed
system. In particular, the perturbed system itself can certainly have
long-range entanglement.
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