  Atomic wave packets loaded into a phase-modulated vertical optical-lattice
potential exhibit a coherent delocalization dynamics arising from intraband
transitions among Wannier-Stark levels. Wannier-Stark intraband transitions are
here observed by monitoring the in situ wave-packet extent. By varying the
modulation frequency, we find resonances at integer multiples of the Bloch
frequency. The resonances show a Fourier-limited width for interrogation times
up to 2 s. This can also be used to determine the gravity acceleration with ppm
resolution.
