# Fabrication and characterization of aluminum SQUID transmission lines

## TL;DR

This paper presents a novel fabrication method for low-loss, flux-tunable aluminum SQUID transmission lines using ALD dielectric layers, enabling integration with superconducting qubits and potential applications in amplifiers.

## Contribution

The authors introduce a simplified fabrication process for SQUID transmission lines that does not require complex multilayer techniques and allows for flux tunability and band-engineering.

## Key findings

- Achieved low-loss, flux-tunable transmission lines with high-quality dielectric.
- Demonstrated systematic characterization of microscopic parameters.
- Showed potential for integration with superconducting qubits and applications in amplifiers.

## Abstract

We report on the fabrication and characterization of 50 Ohms, flux-tunable, low-loss, SQUID-based transmission lines. The fabrication process relies on the deposition of a thin dielectric layer (few tens of nanometers) via Atomic Layer Deposition (ALD) on top of a SQUID array, the whole structure is then covered by a non-superconducting 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 from dielectric losses. This fabrication method offers several advantages. First, the SQUID array fabrication does not rely on a Niobium tri-layer process but on a simpler double angle evaporation technique. Second, ALD provides high quality dielectric leading to low-loss devices. Further, 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 incertitude inherent to any fabrication process. Finally, the unit cell being 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.