A note on the correspondence between qubit quantum operations and special relativity

Abstract

We exploit a well-known isomorphism between complex hermitian $2\\times 2$ matrices and $\\mathbb{R}^4$, which yields a convenient real vector representation of qubit states. Because these do not need to be normalized we find that they map into a Minkowskian cone in $\\mathbb{E}^{(3,1)}$, whose vertical cross-sections are nothing but Bloch spheres. Pure states are represented by light-like vectors, unitary operations correspond to special orthogonal transforms about the axis of the cone, positive operations correspond to pure Lorentz boosts. We formalize the equivalence between the generalized measurement formalism on qubit states and the Lorentz transformations of special relativity, or more precisely elements of the restricted Lorentz group together with future-directed null boosts. The note ends with an informal discussion of the equivalence and some of its possible consequences.