title: A complementarity experiment with an interferometer at the quantum-classical boundary.
abstract: To illustrate the quantum mechanical principle of complementarity, Bohr described an interferometer with a microscopic slit that records the particle's path. Recoil of the quantum slit causes it to become entangled with the particle, resulting in a kind of Einstein-Podolsky-Rosen pair. As the motion of the slit can be observed, the ambiguity of the particle's trajectory is lifted, suppressing interference effects. In contrast, the state of a sufficiently massive slit does not depend on the particle's path; hence, interference fringes are visible. Although many experiments illustrating various aspects of complementarity have been proposed and realized, none has addressed the quantum-classical limit in the design of the interferometer. Here we report an experimental investigation of complementarity using an interferometer in which the properties of one of the beam-splitting elements can be tuned continuously from being effectively microscopic to macroscopic. Following a recent proposal, we use an atomic double-pulse Ramsey interferometer, in which microwave pulses act as beam-splitters for the quantum states of the atoms. One of the pulses is a coherent field stored in a cavity, comprising a small, adjustable mean photon number. The visibility of the interference fringes in the final atomic state probability increases with this photon number, illustrating the quantum to classical transition.
authors.affiliation: Laboratoire Kastler Brossel, Département de Physique, Ecole Normale Supérieure, Paris, France.
authors.forename: P
authors.initials: P
authors.lastname: Bertet
authors.forename: S
authors.initials: S
authors.lastname: Osnaghi
authors.forename: A
authors.initials: A
authors.lastname: Rauschenbeutel
authors.forename: G
authors.initials: G
authors.lastname: Nogues
authors.forename: A
authors.initials: A
authors.lastname: Auffeves
authors.forename: M
authors.initials: M
authors.lastname: Brune
authors.forename: J M
authors.initials: JM
authors.lastname: Raimond
authors.forename: S
authors.initials: S
authors.lastname: Haroche