  Using optical pulses of variable duration up to 80 ps, we report on spin
coherence initialization and its subsequent detection in n-type singly-charged
quantum dots, subject to a transverse magnetic field, by pump-probe techniques.
We demonstrate experimentally and theoretically that the spin coherence
generation and readout efficiencies are determined by the ratio of laser pulse
duration to spin precession period: An increasing magnetic field suppresses the
spin coherence signals for a fixed duration of pump and/or probe pulses, and
this suppression occurs for smaller fields the longer the pulse duration is.
The reason for suppression is the varying spin orientation due to precession
during pulse action.
