  Non-reciprocity of signal transmission enhances capacity of communication
channels and protects transmission quality against possible signal
instabilities, thus becoming an important component ensuring coherent
information processing. However, non-reciprocal transmission requires breaking
time-reversal symmetry (TRS) which poses challenges of both practical and
fundamental character hindering the progress. Here we report a new scheme for
achieving broadband non-reciprocity using a specially engineered hybrid
microwave cavity. The TRS breaking is realized via strong coherent coupling
between a selected chiral mode in the microwave cavity and a single collective
spin excitation (magnon) in a ferromagnetic yttrium iron garnet (YIG) sphere.
The non-reciprocity in transmission is observed spanning nearly a 0.5 GHz
frequency band, which outperforms by two orders of magnitude the previously
achieved bandwidths. Our findings open new directions for robust coherent
information processing in a broad range of systems in both classical and
quantum regimes.
