Transmission phase of a quantum dot: Testing the role of population switching

We propose a controlled experiment to clarify the physical mechanism causing phase lapses of the amplitude for electron transmission through nanoscale devices. Such lapses are generically observed in valleys between adjacent Coulomb--blockade peaks. The experiment involves two quantum dots embedded in the same arm of an Aharonov--Bohm interferometer. It offers a decisive test of "population switching", one of the leading contenders for an explanation of the phenomenon.