We report on the fabrication and characterization of flux-tunable, low-loss, superconducting-quantum-interference-device- (SQUID) based transmission lines whose impedance is close to $50\mathrm{\Omega }$. The fabrication process relies on the deposition of a thin dielectric layer (few tens of nanometers) by atomic layer deposition on top of a SQUID array. The whole structure is covered by a nonsuperconducting metallic top-ground plane. We present experimental results from five different samples. We systematically characterize their microscopic parameters by measuring the propagating phase in these structures. We also investigate losses and discriminate conductor losses from dielectric losses. This fabrication method offers several advantages. First, the SQUID-array fabrication relies not on a niobium-trilayer process but on a simpler, double-angle evaporation technique. Second, atomic layer deposition provides a high-quality dielectric, leading to low-loss devices. Furthermore, the SQUID-array fabrication is based on a standard, all-aluminum process, allowing direct integration with superconducting qubits. Moreover, our devices are in situ flux tunable, allowing mitigation of uncertainty inherent in any fabrication process. Finally, because the unit cell is a single SQUID (no extra ground capacitance is needed), it is straightforward to modulate the size of the unit cell periodically, allowing band engineering. This fabrication process can be directly applied to traveling-wave parametric amplifiers.

• Received 23 July 2019
• Revised 6 October 2019

DOI:https://doi.org/10.1103/PhysRevApplied.12.064017