Pumping Chirality in Three Dimensions

TL;DR

This paper introduces a bosonization approach to analyze a 3-fermion quantum cellular automaton (QCA) related to pumping multiple copies of a $p+ip$ state, revealing its nontrivial topological properties and connections to Chern-Simons theory.

Contribution

It provides a simple form for the 3-fermion QCA using bosonization, relates it to $p+ip$ state pumping, and explores higher-dimensional generalizations and topological implications.

Findings

QCA is nontrivial despite shallow circuit nature.

Pumping $n$ $p+ip$ states corresponds to a non-trivial winding number.

Higher-dimensional generalizations are conjectured to be nontrivial QCAs.

Abstract

Using bosonization, which maps fermions coupled to a ${\mathbb{Z}}_2$ gauge field to a qubit system, we give a simple form for the non-trivial 3-fermion quantum cellular automaton (QCA) as a unitary operator realizing a phase depending on the framing of flux loops, building off work by Shirley et al. We relate this framing dependent phase to a pump of $8$ copies of a $p+ip$ state through the system. We give a resolution of an apparent paradox, namely that the pump is a shallow depth circuit (albeit with tails), while the QCA is nontrivial. We discuss also the pump of fewer copies of a $p+ip$ state, and describe its action on topologically degenerate ground states. One consequence of our results is that a pump of $n$ $p+ip$ states generated by a free Fermi evolution is a free fermion unitary characterized by a non-trivial winding number $n$ as a map from the third homotopy group of the Brilliouin Zone $3$-torus to that of $SU(N_ b)$, where $N_b$ is the number of bands. Using our simplified form of the QCA, we give higher dimensional generalizations that we conjecture are also nontrivial QCAs, and we discuss the relation to Chern-Simons theory.