High kinetic inductance microwave resonators made by He-Beam assisted deposition of tungsten nanowires

We evaluate the performance of hybrid microwave resonators made by combining sputtered Nb thin films with Tungsten nanowires grown with a He-beam induced deposition technique. Depending on growth conditions, the nanowires have a typical width w ∈ [ 35 − 75 ] nm and thickness t ∈ [ 5 − 40 ] nm. We observe a high normal state resistance R ◻ ∈ [ 65 − 150 ] Ω / ◻ which together with a critical temperature T c ∈ [ 4 − 6 ] K ensures a high kinetic inductance making the resonator strongly nonlinear. Both lumped and coplanar waveguide resonators were fabricated and measured at low temperature exhibiting internal quality factors up to 3990 at 4.5 GHz in the few photon regime. Analyzing the wire length, temperature, and microwave power dependence, we extracted a kinetic inductance for the W nanowire of L K ≈ 15 pH / ◻, which is 250 times higher than the geometrical inductance, and a Kerr non-linearity as high as K W , He / 2 π = 200 ± 120 Hz / photon at 4.5 GHz. The nanowires made with the helium focused ion beam are thus versatile objects to engineer compact, high impedance, superconducting environments with a mask and resist free direct write process.