According to Bohr's complementarity principle, a particle possesses wave-likeproperties only when the different paths the particle may take areindistinguishable. In a canonical example of a two-path interferometer with awhich-path detector, observation of interference and obtaining which-pathinformation are mutually exclusive. Such duality has been demonstrated inoptics with a pair of correlated photons and in solid-state devices withphase-coherent electrons. In the latter case, which-path information wasprovided by a charge detector embedded near one path of a two-path electroninterferometer. Note that suppression of interference can always be understoodeither as obtaining path information or as unavoidable back action by thedetector. The present study reports on dephasing of an Aharonov-Bohm (AB) ringinterferometer via a coupled charge detector adjacent to the ring. In contrastto the two-path interferometer, charge detection in the ring does not alwaysprovide path information. Indeed, we found that the interference was suppressedonly when path information could be acquired, even if only in principle. Thisdemonstrates that dephasing does not always take place by coupling the`environment' to the interfering particle: path information of the particlemust be available too. Moreover, this is valid regardless of the strength ofenvironment-interferometer coupling, which refutes the general notion of theeffect of strong interaction with the environment. In other words, it verifiesthat an acquisition of which-path information is more fundamental than theback-action in understanding quantum mechanical complementarity.
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