Phase Shift of a Weak Coherent Beam by a Single Atom

We report the direct measurement of a phase shift induced by single $^{87}$Rb atom on a focused coherent light field. The atom is localized in an optical tweezer between two confocal ashpheric lenses in one arm of a balanced Mach-Zehnder interferometer. In this arrangement, the light beam passes only once through the atom localization volume. We observe a maximum phase shift of 1$^\circ$ for near-resonant light passing through the interferometer.