A complementarity experiment with an interferometer at the quantum-classical boundary

To illustrate the quantum mechanical principle of complementarity, Bohr(1) described an interferometer with a microscopic slit that records the partic le's path. Recoil of the quantum slit causes it to become entangled with th e particle, resulting in a kind of Einstein-Podolsky-Rosen pair(2). As the motion of the slit can be observed, the ambiguity of the particle's traject ory 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 va rious aspects of complementarity have been proposed(3-9) and realized(10-18 ), none has addressed the quantum- classical limit in the design of the int erferometer. Here we report an experimental investigation of complementarit y using an interferometer in which the properties of one of the beam-splitt ing elements can be tuned continuously from being effectively microscopic t o macroscopic. Following a recent proposal(19), we use an atomic double-pul se Ramsey interferometer(20), in which microwave pulses act as beam-splitte rs for the quantum states of the atoms. One of the pulses is a coherent fie ld stored in a cavity, comprising a small, adjustable mean photon number. T he visibility of the interference fringes in the final atomic state probabi lity increases with this photon number, illustrating the quantum to classic al transition.