  The fragile nature of quantum circuits is a major bottleneck to scalable
quantum applications. Operating at cryogenic temperatures, quantum circuits are
highly vulnerable to amplifier backaction and external noise. Non-reciprocal
microwave devices such as circulators and isolators are used for this purpose.
These devices have a considerable footprint in cryostats, limiting the
scalability of quantum circuits. We present a compact microwave diode
architecture, which exploits the non-linearity of a superconducting flux qubit.
At the qubit degeneracy point we experimentally demonstrate a significant
difference between the power levels transmitted in opposite directions. The
observations align with the proposed theoretical model. At -99 dBm input power,
and near the qubit-resonator avoided crossing region, we report the
transmission rectification ratio exceeding 90% for a 50 MHz wide frequency
range from 6.81 GHz to 6.86 GHz, and over 60% for the 250 MHz range from 6.67
GHz to 6.91 GHz. The presented architecture is compact, and easily scalable
towards multiple readout channels, potentially opening up diverse opportunities
in quantum information, microwave read-out and optomechanics.
