title: Quantum state preparation and conditional coherence.
abstract: It is well known that spontaneous parametric down-conversion can be used to probabilistically prepare single-photon states. We have performed an experiment in which arbitrary superpositions of zero- and one-photon states can be prepared by appropriate postselection. The optical phase, which is meaningful only for superpositions of photon number, is related to the relative phase between the zero- and one-photon states. Whereas the light from spontaneous parametric down-conversion has an undefined phase, we show that this technique collapses one beam to a state of well-defined optical phase when a measurement succeeds on the other beam.
authors.affiliation: Department of Physics, University of Toronto, 60 St. George Street, Toronto ON M5S 1A7, Canada.
authors.forename: K J
authors.initials: KJ
authors.lastname: Resch
authors.forename: J S
authors.initials: JS
authors.lastname: Lundeen
authors.forename: A M
authors.initials: AM
authors.lastname: Steinberg