Pfaffian Circuits

Abstract: It remains an open question whether the apparent additional power of quantum computation derives inherently from quantum mechanics, or merely from the flexibility obtained by "lifting" Boolean functions to linear operators and evaluating their composition cleverly. Holographic algorithms provide a useful avenue for exploring this question. We describe a new, simplified construction of holographic algorithms in terms of Pfaffian circuits. Novel proofs of some key results are provided, and we extend the approach of [34] to nonsymmetric, odd, and homogenized signatures, circuits, and various models of execution flow. This shows our approach is as powerful as the matchgate approach. Holographic algorithms provide in general $O(n^{\omega_p})$ time algorithms, where $\omega_p$ is the order of Pfaffian evaluation in the ring of interest (with $1.19 \leq \omega_p \leq 3$ depending on the ring) and $n$ is the number of inclusions of variables into clauses. Our approach often requires just the evaluation of an $n \times n$ Pfaffian, and at most needs an additional two rows per gate, whereas the matchgate approach is quartic in the arity of the largest gate. We give examples (even before any change of basis) including efficient algorithms for certain lattice path problems and an $O(n^{\omega_p})$ algorithm for evaluation of Tutte polynomials of lattice path matroids. Finally we comment on some of the geometric considerations in analyzing Pfaffian circuits under arbitrary basis change. Connections are made to the sum-product algorithm, classical simulation of quantum computation, and SLOCC equivalent entangled states.