promoting access to White Rose research papers White Rose Research Online eprints@whiterose.ac.uk Universities of Leeds, Sheffield and York http://eprints.whiterose.ac.uk/ This is an author produced version of a paper published in Quantum Information Processing. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/10761/ Published paper Busch, Paul and Schmidt, Heinz-Jürgen (2010) Coexistence of qubit observables. Quantum Information Processing, 9 (2). pp. 143-169. http://dx.doi.org/10.1007/s11128-009-0109-x