High Kinetic Inductance NbN Nanowire Superinductance

Decoherence due to charge fluctuations is a still outstanding problem in many device architectures for quantum information processing and quantum metrology. Charge fluctuations can be suppressed by embedding the circuit in a high-impedance microwave environment - a superinductance - i.e. a non-dissipative, inductive circuit element with impedance greater than the quantum resistance (6.5 kΩ) and low losses at the frequency of the circuit (several GHz). This requirement cannot be met using the ordinary, geometric inductance of a wire due to its unavoidable shunt capacitance.

Superinductance was realized using a Josephson-junction array. An attractive alternative to arrays is offered by the high kinetic inductance of strongly disordered superconducting thin films. We have fabricated NbN nanowires with widths down to 40 nm and thickness typically 20 nm, implementing a superinductance with impedance Z = 6.8 kΩ. We demonstrate internal quality factors Q_i = 25,000 at single-photon excitation, significantly higher than values reported in devices with similar materials and geometries. Moreover, we show that the dominant dissipation in our nanowires is not an intrinsic property of the disordered films, but can instead be understood within the well-studied framework of two-level systems.