We report on recent progress in the observation of time-dependent matter wa
ve optics with very cold neutrons (VCN). We have performed an experiment wh
ere a VCN beam was amplitude modulated by a high-frequency chopper. The res
ulting diffraction in time was measured with a special time-of-flight spect
rometer that achieving an energy resolution of 22 neV. The measured spectra
of different cases are discussed qualitatively. (C) 2000 Elsevier Science
B.V. All rights reserved.