  We propose an experimentally realizable nonreciprocal magnonic device at the
single-magnon level by exploiting magnon blockade in a magnon-based hybrid
system. The coherent qubit-magnon coupling, mediated by virtual photons in a
microwave cavity, leads to the energy-level anharmonicity of the composite
modes. In contrast, the corresponding dissipative counterpart, induced by
traveling microwaves in a waveguide, yields inhomogeneous broadenings of the
energy levels. As a result, the cooperative effects of these two kinds of
interactions give rise to the emergence of the direction-dependent magnon
blockade. We show that this can be demonstrated by studying the equal-time
second-order correlation function of the magnon mode. Our study opens an avenue
to engineer nonreciprocal magnonic devices in the quantum regime involving only
a small number of magnons.

Quantum Physics;Mesoscale and Nanoscale Physics