Implementation of radio frequency reflectometry in a home-built millikelvin scanning tunneling microscope

We present a simple homemade solution enabling in situ radio frequency (RF) reflectometry measurements with a millikelvin scanning tunneling microscope (mk-STM). The additions described below were made using RF best practices, following similar detection schemes commonly employed in the quantum information science community. Using a Niobium STM tip to form a superconductor–insulator–normal metal tunnel junction, the evolution of coherence peaks at the SC-gap edge is carefully measured to characterize the RF losses and electron temperature. We further identify impedance matching as a crucial factor to achieve high sensitivity in the reflectometry by tuning the tip–sample capacitance as a function of approach distance. As a demonstration of this capability, we measure a 50 × 50 nm2 area of island features that have been condensed onto the surface of a gold single crystal. Position-dependent reflectometry losses allow us to image island sizes down to a total surface area of 5 nm2, given our current sensitivity.