Tensors Masquerading as Matchgates: Relaxing Planarity Restrictions on Pfaffian Circuits

TL;DR

This paper explores relaxing planarity restrictions in Pfaffian circuits by enabling the SWAP gate to function as a Pfaffian cogate through basis transformations, expanding the expressiveness of holographic algorithms.

Contribution

It introduces a novel technique using hyperplane properties to incorporate SWAP gates into Pfaffian circuits, bypassing previous orbit closure limitations.

Findings

SWAP gate can be realized as a Pfaffian cogate with suitable bases

The set of Pfaffian (co)gates lies in a hyperplane

Multiple SWAP gates can be integrated into Pfaffian circuits

Abstract

Holographic algorithms, alternatively known as Pfaffian circuits, have received a great deal of attention for giving polynomial-time algorithms of $\#\mathsf{P}$-hard problems. Much work has been done to determine the extent of what this machinery can do and the expressiveness of these circuits. One aspect of interest is the fact that these circuits must be planar. Work has been done to try and relax the planarity conditions and extend these algorithms further. We show that an approach based on orbit closures does not work, but give a different technique for allowing the SWAP gate to be used in a Pfaffian circuit given a suitable basis and restricted type of graph. This is done by exploiting the fact that the set of Pfaffian (co)gates always lies in a hyperplane. We then give a variety of bases that can be chosen such that the SWAP gate acts like a Pfaffian cogate and discuss how many SWAP gates can be implemented in a Pfaffian circuit.