  The Kondo effect is the many-body screening of a local spin by a cloud of
electrons at very low temperature. It has been proposed as an explanation of
the zero-bias anomaly in quantum point contacts where interactions drive a
spontaneous charge localization. However, the Kondo origin of this anomaly
remains under debate, and additional experimental evidence is necessary. Here
we report on the first phase-sensitive measurement of the zero-bias anomaly in
quantum point contacts using a scanning gate microscope to create an electronic
interferometer. We observe an abrupt shift of the interference fringes by half
a period in the bias range of the zero-bias anomaly, a behavior which cannot be
reproduced by single-particle models. We instead relate it to the phase shift
experienced by electrons scattering off a Kondo system. Our experiment
therefore provides new evidence of this many-body effect in quantum point
contacts.
