  The Aharonov-Bohm effect is measured in a four-terminal open ring geometry
based on a Ga[Al]As heterostructure. Two quantum dots are embedded in the
structure, one in each of the two interfering paths. The number of electrons in
the two dots can be controlled independently. The transmission phase is
measured as electrons are added to or taken away from the individual quantum
dots. Although the measured phase shifts are in qualitative agreement with
theoretical predictions, the phase evolution exhibits unexpected dependence on
the magnetic field. For example, phase lapses are found only in certain ranges
of magnetic field.
