ZX-calculus Red State

The ZX-calculus is a rigorous graphical language for reasoning about linear maps between qubits, which are represented as string diagrams called ''ZX-diagrams''. A ZX-diagram consists of a set of generators called ''spiders'' that represent specific tensors. These are connected together to form a tensor network similar to Penrose graphical notation. Due to the symmetries of the spiders and the properties of the underlying category, topologically deforming a ZX-diagram (i.e. moving the generators without changing their connections) does not affect the linear map it represents. In addition to the equalities between ZX-diagrams that are generated by topological deformations, the calculus also has a set of graphical rewrite rules for transforming diagrams into one another. The ZX-calculus is ''universal'' in the sense that any linear map between qubits can be represented as a diagram, and different sets of graphical rewrite rules are complete for different families of linear maps. ZX-diagrams can be seen as a generalisation of quantum circuit notation.

History

The ZX-calculus was first introduced by Bob Coecke and Ross Duncan in 2008 as an extension of the categorical quantum mechanics school of reasoning. They introduced the fundamental concepts of spiders, strong complementarity and most of the standard rewrite rules.

In 2009 Duncan and Perdrix found the additional Euler decomposition rule for the Hadamard gate, which was used by Backens in 2013 to establish the first completeness result for the ZX-calculus. Namely that there exists a set of rewrite rules that suffice to prove all equalities between stabilizer ZX-diagrams, where phases are multiples of $\pi/2$, up to global scalars. This result was later refined to completeness including scalar factors.

Following an incompleteness result, in 2017, a completion of the ZX-calculus for the approximately universal $\pi/4$ fragment was found, in addition to two different completeness results for the universal ZX-calculus (where phases are allowed to take any real value).

Also in 2017 the book ''Picturing Quantum Processes'' was released, that builds quantum theory from the ground up, using the ZX-calculus. See also the 2019 book ''Categories for Quantum Theory''.

Informal introduction

ZX-diagrams consist of green and red nodes called ''spiders'', which are connected by ''wires''. Wires may curve and cross, arbitrarily many wires may connect to the same spider, and multiple wires can go between the same pair of nodes. There are also Hadamard nodes, usually denoted by a yellow box, which always connect to exactly two wires.

ZX-diagrams represent linear maps between qubits, similar to the way in which quantum circuits represent unitary maps between qubits. ZX-diagrams differ from quantum circuits in two main ways. The first is that ZX-diagrams do not have to conform to the rigid topological structure of circuits, and hence can be deformed arbitrarily. The second is that ZX-diagrams come equipped with a set of rewrite rules, collectively referred to as the ''ZX-calculus''. Using these rules, calculations can be performed in the graphical language itself.

Generators

The building blocks or generators of the ZX-calculus are graphical representations of specific states, unitary operators, linear isometries, and projections in the computational basis $, 0 \rangle, , 1 \rangle$ and the Hadamard-transformed basis $, + \rangle = \frac$ and $, - \rangle = \frac$. The colour green (or sometimes white) is used to represent the computational basis and the colour red (or sometimes grey) is used to represent the Hadamard-transformed basis. Each of these generators can furthermore be labelled by a phase, which is a real number from the interval controlled-NOT gate can be represented as follows:

Diagram rewriting

The following example of a quantum circuit constructs a GHZ-state. By translating it into a ZX-diagram, using the rules that "adjacent spiders of the same color merge", "Hadamard changes the color of spiders", and "parity-2 spiders are identities", it can be graphically reduced to a GHZ-state:

Any linear map between qubits can be represented as a ZX-diagram, i.e. ZX-diagrams are ''universal''. A given ZX-diagram can be transformed into another ZX-diagram using the rewrite rules of the ZX-calculus if and only if the two diagrams represent the same linear map, i.e. the ZX-calculus is sound and complete.

Formal definition

The category of ZX-diagrams is a dagger compact category">Category theory">category of ZX-diagrams is a symmetric monoidal structure (a tensor product), is compact closed (it has ''cups'' and ''caps'') and comes equipped with a Dagger category">dagger, such that all these structures suitably interact. The objects of the category are the natural numbers, with the tensor product given by addition (the category is a PROP