Complementarity is a property of a pair of observables being ‘maximally distinct’ from each other and, in this chapter, we analyse this property in categorical terms as a pair of interacting Frobenius structures. Complementary observables play a central role in quantum information theory, and we will see how they can be used to understand the structure of the Deutsch—Jozsa algorithm. We show that complementarity is closely linked to the theory of Hopf algebras. We discuss how many-qubit gates can be modelled using only complementary Frobenius structures, such as controlled negation, controlled phase gates and arbitrary single qubit gates. This leads to the ZX calculus, a sound and complete way to handle quantum computations using only equations in the graphical calculus.
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