Low-noise coherent microwave source based on a Shapiro step

Coherent microwave sources are one of the primary tools to investigate quantum devices and circuits, integration of sources at cryogenic temperatures is very tempting but really challenging due to the coherence requirements and power dissipated by such systems. While Shapiro steps in Josephson junctions have been used to produce ultra-stable voltage references; it can also be used to realize a coherent tone by current driving such a junction in the Shapiro regime. Thanks to a tailored environment made of a spiral inductor, we experimentally build a phase locked loop between a capacitively shunted Josephson junction and this environment, providing a low-noise coherent microwave source. We demonstrate a coherent tone of 28 pW with a phase noise of -116 dBc/Hz. Our device may lead to new tools for scaling cryogenic control systems.