through an interferometric procedure For unitary evolution(PrL 85, 2845(2000)) his definition is based on interferometry B=arglpoU(t) M Mirror b Beam-splitters, X Phase shift M Experiment: .] Du et al Phys. Rev. Lett. 91, 100403 (2003) M .M. Ericsson etal, phys. Rev lett94,050401(2005) U(t) ∞U.+ 0016 • through an interferometric procedure – For unitary evolution (PRL 85,2845(2000)) This definition is based on interferometry UM 0 arg[ ( )]   g = U tUi 0  UB UM UB 0 0 1 0 ( ) 0 1 0 0 i U t U e i      =  +          UM Mirror, UB Beam-splitters,  Phase shift Experiment: •J. Du et al. Phys. Rev. Lett. 91, 100403 (2003) •M. Ericsson etal., Phys. Rev. Lett. 94, 050401 (2005)