The algebra of complex 2 x 2 matrices and a general closed Baker-Campbell-Hausdorff formula

We derive a closed formula for the Baker-Campbell-Hausdorff series expansion in the case of complex 2 x 2 matrices. For arbitrary matrices A and B, and a matrix Z such that exp Z = exp A exp B, our result expresses Z as a linear combination of A and B, their commutator [A, B], and the identity matrix I. The coefficients in this linear combination are functions of the traces and determinants of A and B, and the trace of their product. The derivation proceeds purely via algebraic manipulations of the given matrices and their products, making use of relations developed here, based on the CayleyHamilton theorem, as well as a characterization of the consequences of [A, B] and/or its determinant being zero or otherwise. As a corollary of our main result we also derive a closed formula for the Zassenhaus expansion. We apply our results to several special cases, most notably the parametrization of the product of two SU(2) matrices and a verification of the recent result of VanBrunt and Visser (2015 J. Phys. A: Math. Theor. 48 225207) for complex 2 x 2 matrices, in this latter case deriving also the related Zassenhaus formula which turns out to be quite simple. We then show that this simple formula should be valid for all matrices and operators.