Decoherence and complementarity in multiple beam Ramsey interference

Summary form only given. The transition between quantum and classical systems is usually ascribed to decoherence effects, which become increasingly important for larger size systems and tend to obscure the quantum behavior. Decoherence effects also come into play in the famous thought ´Gedanken´ experiments on the wave-particle duality of matter. For example, in a double slit experiment with electrons, a wavepacket passes simultaneously through both slits and forms an interference pattern. However, if one attempts to keep track of the electron path, e.g., by scattering a photon from the electron, the resulting decoherence destroys the interference pattern and the particle behavior takes over. We report on the extension of such a which-path experiment to an arrangement with more than two interfering paths, which is realized using the interference of four internal states of the cesium atom. This two-qubits arrangement represents a more complex quantum system than a two-beam interferometer, which represents a single qubit system. While in a two-beam interference experiment the observation of a path will inevitable destroy the fringe signal, we demonstrate that the scattering of a photon on one of the path of a multiple path arrangement cannot only lead to a decrease of the contrast, but, under certain conditions, also lead to an increase of the fringe contrast.