DEPHASING IN ELECTRON INTERFERENCE BY A WHICH-PATH DETECTOR

Wave-particle duality, as manifest in the two-slit experiment, provide s perhaps the most vivid illustration of Bohr's complementarity princi ple: wave-like behaviour (interference) occurs only when the different possible paths a particle can take are indistinguishable, even in pri nciple(1). The introduction of a which path (welcher Weg) detector for determining the actual path taken by the particle inevitably involved coupling the particle to a measuring environment, which in turn resul ts in dephasing (suppression of interference). In other words, simulta neous observations of wave and particle behaviour is prohibited. Such a manifestation of the complementarity principle was demonstrated rece ntly using a pair of correlated photons, with measurement of one photo n being used to determine the path taken by the other and so prevent s ingle-photon interference(2). Here we report the dephasing effects of a which-path detector on electrons traversing a double-path interferom eter. We find that by varying the sensitivity of the detector we can a ffect the visibility of the oscillatory interference signal, thereby v erifying the complementarity principle for fermions.