  Measurements of the transmission phase in transport through a quantum dot
embedded in an Aharonov-Bohm interferometer show systematic sequences of phase
lapses separated by Coulomb peaks. Using a two-level quantum dot as an example
we show that this phenomenon can be accounted for by the combined effect of
asymmetric dot-lead coupling and interaction-induced "population switching" of
the levels, rendering this behavior generic. In addition, we use the notion of
spectral shift function to analyze the relationship between transmission phase
lapses and the Friedel sum rule.
