Classifying quantum phases with the Kirby torus trick
Classifying phases of local quantum systems is a general problem that includes special cases such as free fermions, commuting projectors, and others. An important distinction in this classification should be made between classifying periodic and aperiodic systems. A related distinction is that between homotopy invariants (invariants which remain constant so long as certain general properties such as locality, gap, and others hold) and locally computable invariants (properties of the system that cannot change from one region to another without producing a gapless edge between them). We attack this problem using a technique inspired by Kirby's "torus trick" in topology. We use this trick to reproduce results for free fermions (in particular, using the trick to reduce the aperiodic classification to the simpler problem of periodic classification). We also show that a similar trick works for interacting phases which are nontrivial but lack anyons; these results include symmetry protected phases. A key part of this work is an attempt to classify quantum cellular automata (QCA).