[1] Middlesex Univ, Dept Comp Sci, London N17 8HR, England
[2] Univ London, Dept Comp Sci & Informat Syst, London WC1E 7HU, England
[3] Univ Waterloo, Inst Quantum Comp, Waterloo, ON N2L 3G1, Canada
[4] Univ Guelph, Dept Math & Stat, Guelph, ON N1G 2W1, Canada