Atom Interferometers and Atomic Coherence D. E. Pritchard, M. S. Chapman, T. D. Hammond, D. A. Kokorowski, A. Lenef, R. A. Rubenstein, E. T. Smith Massachusetts Institute of Technology, Cambridge Massachusetts 02139 J. Schmiedmayer Institut f ur Experimentalphysik, Universita Èt Innsbruck, A-6020 Innsbruck, Austria Abstract Atom interferometers are powerful tools for the study of fundamental issues in quantum mechanics. This paper describes the use of our atom interferometer [1] for an experimental realization of Feyn- man's gedanken experiment in which the observation of photons scattered off of particles emerging from a double slit is used to obtain which path information. This determination, in principal, of the particle's path, destroys any interference effects downstream. The interference can be regained by ob- serving only those particles which scatter a photon into a small range of final directions. 1. Introduction This brief review discusses our application of atom interferometry to the fundamental issue of loss of atomic coherence. In addition, we will demonstrate a method of recovery for this ``lostº coherence. The essential components of our experiment are three nanofabricated amplitude gratings, of period 200 nm, arranged in a Mach-Zehnder geometry (Fig. 1). An atomic beam incident upon the first grating is split into several diffracted orders, with a divergence of approxi- mately 1 mrad. Two of these orders are redirected at the second grating and made to over- lap at the third, where a transverse interference pattern is obtained. Observation of the phase and contrast of this interference pattern is a sensitive probe of any interactions the atoms experience inside the interferometer. Further details concerning our apparatus and other experiments are reviewed in [2], from which the following material is condensed. This review is available via the web at the URL http://coffee.mit.edu/. 2. Coherence Loss due to Photon Scattering ±± Discussion One motivating question driving our interest in coherence phenomena is: ``What limits do the size and complexity of particles place on the ability of their center of mass motion to exhibit interference effects?º From their origins in electron and neutron experiments, matter wave optics and interferometry have now been extended to atomic and molecular sys- tems ±± systems characterized by many degenerate and non-degenerate internal quantum states. Might there be limits on the manipulation of ever larger and more complex particles? The principle that a system can be in a coherent superposition of different states and exhibit interference effects is a fundamental element of quantum mechanics. Immediately, Fortschr. Phys. 46 (1998) 6±±8, 801±±808