Coherent Delocalization of Atomic Wave Packets in Driven Lattice   Potentials
  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. 
V.V. Ivanov, A. Alberti, M. Schioppo, G. Ferrari, M. Artoni, M. L.   Chiofalo, and G. M. Tino