  Highly excited atoms with an electron moved into a level with large principal
quantum number are fascinating hydrogen-like objects. The giant extension of
these Rydberg atoms leads to huge interaction effects. Monitoring these
interactions has provided novel insights into molecular and condensed matter
physics problems on a single quantum level. Excitons, the fundamental optical
excitations in semiconductors consisting of a negatively charged electron and a
positively charged hole, are the condensed matter analogues of hydrogen. Highly
excited excitons with extensions similar to Rydberg atoms are attractive
because they may be placed and moved in a crystal with high precision using
microscopic potential landscapes. Their interaction may allow formation of
ordered exciton phases or sensing of elementary excitations in the surrounding,
also on a quantum level. Here we demonstrate the existence of Rydberg excitons
in cuprous oxide, Cu2O, with principal quantum numbers as large as n=25 . These
states have giant wave function extensions of more than 2 micrometers, compared
to about a nanometer for the ground state. The strong dipole-dipole interaction
is evidenced by a blockade effect, where the presence of an exciton prevents
excitation of a further exciton in its vicinity.
