Non-Invasive Readout of the Kinetic Inductance of Superconducting Nanostructures

The energy landscape of multiply connected superconducting structures is
ruled by fluxoid quantization due to the implied single-valuedness of the
complex wave function. The transitions and interaction between these energy
states, each defined by a specific phase winding number, are governed by
classical and/or quantum phase slips. Understanding these events requires the
ability to probe, non-invasively, the state of the ring. Here, we employ a
niobium resonator to examine the superconducting properties of an aluminum
loop. By applying a magnetic field, adjusting temperature, and altering the
loop's dimensions via focused ion beam milling, we correlate resonance
frequency shifts with changes in the loop's kinetic inductance. This parameter
is a unique indicator of the superconducting condensate's state, facilitating
the detection of phase slips in nanodevices and providing insights into their
dynamics. Our method presents a proof-of-principle spectroscopic technique with
promising potential for investigating the Cooper pair density in inductively
coupled superconducting nanostructures.