  We develop a multi-photon adiabatic passage to realize a highly efficient
Rydberg excitation in a four-level ladder-type atomic system. The adiabatic
passage is based on the existence of a novel quasi-dark state in the cascade
excitation system where the frequencies of the lasers are appropriately chirped
with time. We also investigate the influence of the interatomic Rydberg
interaction on the passage and extend its application to the preparation of
anti-blockade Rydberg atom pairs in the Rydberg blockade regime.
