Quantum mechanical complementarity probed in a closed-loop Aharonov-Bohm interferometer

The complementarity principle(1) demands that a particle reveals wave-like properties only when the different paths that it can take are indistinguishable(2,3). The complementarity has been demonstrated in optics with pairs of correlated photons(4,5) and in two-path solid-state interferometers with phase-coherent electrons(6). In the latter experiment, a charge detector embedded near one path of a two-path electron interferometer provided which-path information(3). Here, we report on electron dephasing in an Aharonov-Bohm ring interferometer(7) via a charge detector adjacent to the ring. In contrast to the two-path interferometer, charge detection in the ring does not always provide path information. The interference was suppressed only when path information could be acquired, even if only in principle. This confirms that dephasing is not always induced by 'disturbing' the interfering particle through the interferometer environment interactions: path information of the particle must be available too. Our experiment suggests that acquisition of which-path information is more fundamental than the back-action in understanding quantum mechanical complementarity.