Quantum Circuits in Additive Hilbert Space

TL;DR

This paper introduces an additive Hilbert space framework for quantum circuits, simplifying the representation of certain primitives and aiding circuit optimization, with a new diagrammatic interpretation.

Contribution

It presents a novel additive formalism for quantum computation that complements the tensor product approach, enabling more efficient synthesis and optimization of quantum circuits.

Findings

Reversible classical circuits are elegantly represented in additive space.

High-level multi-controlled gates can be synthesized from low-level decompositions.

The formalism allows for circuit-like diagrams and a new interpretation of quantum computation.

Abstract

Representations of quantum computations are almost always based on a tensor product $\otimes$-structure. This coincides with what we are able to execute in our experiments, as well as what we observe in Nature, but it makes certain familiar quantum primitives convoluted. Reversible classical circuits, diagonal operations or controlled unitaries all have very elegant and simple matrix representations that cannot be expressed succinctly as a circuit in a simple gate set, complicating quantum algorithm design and circuit optimization. We propose a new additive presentation of quantum computation to address this. We show how conventional circuits can be expressed in the additive space and how they can be recovered. In particular in our formalism we are able to synthesize high-level multi-controlled primitives from low-level circuit decompositions, making it an invaluable tool for circuit optimization. Our formulation also accepts a circuit-like diagrammatic representation and proposes a novel and simple interpretation of quantum computation.