  The phenomenon that a quantum particle propagating in a detector, such as a
Wilson cloud chamber, leaves a track close to a classical trajectory is
analyzed. We introduce an idealized quantum-mechanical model of a charged
particle that is periodically illuminated by pulses of laser light resulting in
repeated indirect measurements of the approximate position of the particle. For
this model we present a mathematically rigorous analysis of the appearance of
particle tracks, assuming that the Hamiltonian of the particle is quadratic in
the position- and momentum operators, as for a freely moving particle or a
harmonic oscillator.
