  A measurement consists in coupling a system to a probe and reading the output
of the probe to gather information about the system. The weaker the coupling,
the smaller the back-action on the system, but also the less information
conveyed. If the system undergoes a second measurement, the statistics of the
first output can be conditioned on the value of the second one. This procedure
is known as postselection. A postselected weak measurement of an observable can
give a large average output of the probe when the postselected state is nearly
orthogonal to the initial state of the system. This large value is an
interference effect in the readout of the probe, which is initially in a
coherent superposition of readout states (also known as pointer states).
Usually, the weak interaction between system and probe is considered
instantaneous, so that the dynamics of the probe can be neglected. However, for
a weak measurement in solid-state devices, an interaction of finite duration is
likely needed.
  Here, we show how this finite duration generates a contribution of the
dynamical phase to the readout statistics. Furthermore, we derive interpolation
formulas that are able to describe the statistics of the weak measurement for
the whole range of pre- and postselected states. Phase-space averages appear in
the expansion, suggesting an interpretation in terms of non-positive
probabilities. Decoherence in the probe is also accounted for and it is pointed
out the existence of a regime of intermediate coupling strength in which
coherent oscillations can be observed in the probability of the readout.
