qu an t - ph / 9 70 90 22 10 S ep 1 99 7 1 Dephasing due to Which Path Detector

We study dephasing of electrons induced by a which path detector and thus verify Bohr's complementarity principle for fermions. We utilize a double path interferometer with two slits, with one slit being replaced by a coherent quantum dot (QD). A short one dimensional channel, in the form of a quantum point contact (QPC), in close proximity to the QD, serves as a which path detector. We find that by varying the properties of the QPC detector we affect the visibility of the interference, inducing thus dephasing. We develop a simple model to explain the dephasing due to the nearby detector and find good agreement with the experiment. Bohr's complementarity principle excludes the possibility of observation of multiple paths interference simultaneously with which path (WP) determination [1]. Wave-like behavior, or interference, is possible only when the different possible paths a particle can take are indistinguishable, even in principle, therefore, their wave functions add coherently and thus interfere. Determining the actual path, by coupling to a measuring environment results in dephasing, namely, suppression of interference. Such principles have been demonstrated recently using parametric down conversion of photons where one photon had been used to determine the path of the second, thus preventing single photon interference [2]. Mesoscopic systems [3] can be used as ideal tools to study the interplay between interference and dephasing. Observation of quantum interference in such systems requires the absence of dephasing, prevalent due to interaction between the interfering electrons and the environment (other electrons, phonons, etc.) [4]. Nano fabrication and low temperature techniques allow observation of a variety of coherent effects, such as Aharonov Bohm (AB) interference, weak localization, resonant tunneling and conductance quantization [3]. In the present work we study a new dephasing mechanism induced by an artificial and controllable WP detector. We utilize in the experiment an electronic double path interferometer [5, 6], fabricated in the plane of a high mobility two dimensional electron gas (2DEG). The two paths are defined by two slits electrons can pass through. One slit is in the form of a quantum dot (QD) [7] and the other is a quantum point contact (QPC). We use a coherent QD for one slit in order to allow the electronic partial wave, while staying coherent, to dwell long enough near the detector as it goes through this slit, so it can be detected more easily. Nearby the QD, but electrically separated from it, …