  Matter-wave interferometry has been performed with helium atoms in high
Rydberg states. In the experiments the atoms were prepared in coherent
superpositions of Rydberg states with different electric dipole moments. Upon
the application of an inhomogeneous electric field, the different forces on
these internal state components resulted in the generation of coherent
superpositions of momentum states. Using a sequence of microwave and electric
field gradient pulses the internal Rydberg states were entangled with the
momentum states associated with the external motion of these matter waves.
Under these conditions matter-wave interference was observed by monitoring the
populations of the Rydberg states as the magnitudes and durations of the pulsed
electric field gradients were adjusted. The results of the experiments have
been compared to, and are in excellent quantitative agreement with, matter-wave
interference patterns calculated for the corresponding pulse sequences. For the
Rydberg states used, the spatial extent of the Rydberg electron wavefunction
was ~320 nm. Matter-wave interferometry with such giant atoms is of interest in
the exploration of the boundary between quantum and classical mechanics. The
results presented also open new possibilities for measurements of the
acceleration of Rydberg positronium or antihydrogen atoms in the Earth's
gravitational field.
