1. Laboratoire Kastler Brossel, Département de Physique, Ecole Normale Supérieure, 24 rue Lhomond, F-75231, Paris Cedex 05, France

Correspondence to: S. Haroche Correspondence and requests for materials should be addressed to S.H. (e-mail: Email: haroche@lkb.ens.fr).

Abstract

To illustrate the quantum mechanical principle of complementarity, Bohr¹ 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 pair². 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^{3, 4, 5, 6, 7, 8, 9} and realized^{10, 11, 12, 13, 14, 15, 16, 17, 18}, 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 proposal¹⁹, we use an atomic double-pulse Ramsey interferometer²⁰, 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.