Quantum circuits with free fermions in disguise

TL;DR

This paper explores how certain quantum circuits can be constructed to exhibit free fermionic spectra, revealing new circuit geometries that produce free fermions in models previously not solvable by standard transformations.

Contribution

It introduces a method to construct free fermionic circuits from local Hamiltonian terms, including new geometries and proofs of free fermionic nature in specific models.

Findings

Certain circuit geometries produce free fermionic spectra.

Standard brickwork circuits are generally not free fermionic.

Symmetric circuit constructions can yield free fermionic behavior.

Abstract

Recently multiple families of spin chain models were found, which have a free fermionic spectrum,even though they are not solvable by a Jordan-Wigner transformation. Instead, the free fermions emerge as a result of a rather intricate construction. In this work we consider the quantum circuit formulation of the problem. We construct circuits using local unitary gates built from the terms in the local Hamiltonians of the respective models, and ask the question: which circuit geometries (sequence of gates) lead to a free fermionic spectrum? Our main example is the 4-fermion model of Fendley, where we construct free fermionic circuits with various geometries. In certain cases we prove the free fermionic nature, while for other geometries we confirm it numerically. Surprisingly, we find that many standard brickwork circuits are not free fermionic, but we identify certain symmetric constructions which are. We also consider a recent generalization of the 4-fermion model and obtain the factorization of its transfer matrix, and subsequently derive a free-fermionic circuit for this case as well.