  We report on the first experimental realization of optimal linear-optical
controlled phase gates for arbitrary phases. The realized scheme is entirely
flexible in that the phase shift can be tuned to any given value. All such
controlled phase gates are optimal in the sense that they operate at the
maximum possible success probabilities that are achievable within the framework
of any postselected linear-optical implementation. The quantum gate is
implemented using bulk optical elements and polarization encoding of qubit
states. We have experimentally explored the remarkable observation that the
optimum success probability is not monotone in the phase.
