J. Niel de Beaudrap Vincent Danos Elham Kashefi Martin Rötteler Quadratic Form Expansions for Unitaries. 29-46 2008 TQC http://dx.doi.org/10.1007/978-3-540-89304-2_4 conf/tqc/2008 db/conf/tqc/tqc2008.html#BeaudrapDKR08