Bloch oscillations. Coherent control of wave packet dynamics in superlattices

Bloch oscillations in semiconductor superlattices have recently been observed using optical techniques. The main point which makes these quantum beats of the Wannier-Stark ladder to Bloch oscillations is a true spatial oscillation of the wave packet. We present a novel technique which measures the spatial amplitude directly: The macroscopic dipole field of the wave packet is detected by small shifts of the Wannier-Stark ladder peaks. The observed signals are in good agreement with theoretical estimates.