Low-Loss Superconducting Nanowire Circuits Using a Neon Focused Ion Beam

We present low-temperature measurements of low-loss superconducting nanowire-embedded resonators in the low-power limit relevant for quantum circuits. The superconducting resonators are embedded with superconducting nanowires with widths down to 20 nm using a neon focused ion beam. In the low-power limit, we demonstrate an internal quality factor up to $3.9\times {10}^5$ at 300 mK [implying a two-level-system-limited quality factor up to $2\times {10}^5$ at 10 mK], not only significantly higher than in similar devices but also matching the state of the art of conventional Josephson-junction-embedded resonators. We also show a high sensitivity of the nanowire to stray infrared photons, which is controllable by suitable precautions to minimize stray photons in the sample environment. Our results suggest that there are excellent prospects for superconducting-nanowire-based quantum circuits.

Figure 1

(a) The $S_{21}$ magnitude response of the nanowire-embedded resonator 3710_1qP. The red line is a fit to Eq. (1). (b) A false-color He FIB micrograph of a neon FIB-milled nanowire (3710_1qP) with dimensions of 27 nm by $1.2\mu \mathrm{m}$. The NbN is shown in blue, while the milled region is shown in red. (c),(d) Scanning electron micrographs of (c) the shorted end of a $\lambda /4$ resonator before milling by neon FIB. (d) The whole $\lambda /4$ resonator. (e) A photograph of the sample holder. In the center is a chip, which is wire bonded to a microwave printed circuit board; the dark material is ECCOSORB CR 117.

Figure 2

Resonant frequency of resonator 3710_1_qP as a function of the temperature. Red broken line: Variation arising from kinetic inductance changes described by MB theory. Blue dotted line: Variation arising from TLS losses. Green dash-dotted line: Fit to data including both MB and TLS effects. Inset: Normalized frequency shift as a function of the temperature for all resonators on chip 1.

Figure 3

(a) Loss tangent $({\delta }_{\mathrm{tot}}^i=1/Q_i)$ as a function of microwave drive for bare resonators. The colors correspond to different temperatures, and the hollow symbols indicate the use of an ECCOSORB-CR-117-lined sample box. The solid lines in all plots represent fits to Eq. (4). (b) Loss tangent for the resonator 4094_1qB with and without the ECCOSORB-lined box. (c) Loss tangent as a function of microwave drive for the nanowire-embedded resonators.