  Optical nonreciprocity is important in photonic information processing to
route the optical signal or prevent the reverse flow of noise. By adopting the
strong nonlinearity associated with a few atoms in a strongly coupled cavity
QED system and an asymmetric cavity configuration, we experimentally
demonstrate the nonreciprocal transmission between two counterpropagating light
fields with extremely low power. This nonreciprocity can even occur on a
few-photon level due to the high optical nonlinearity of the system. The
working power can be flexibly tuned by changing the effective number of atoms
strongly coupled to the cavity. The idea and result can be applied to optical
chips as optical diodes by using fiber-based cavity QED systems. Our work opens
up new perspectives for realizing optical nonreciprocity on a few-photon level
based on the nonlinearities of atoms strongly coupled to an optical cavity.
