Sub-micrometer epitaxial Josephson junctions for quantum circuits

TL;DR

This paper introduces a fabrication process for sub-micrometer epitaxial Josephson junctions, demonstrating their potential for quantum circuits through improved qubit performance linked to high subgap resistance.

Contribution

It presents a novel high-temperature via process for creating small epitaxial Josephson junctions with sapphire barriers, and evaluates their microwave loss properties in quantum circuits.

Findings

Junctions as small as 0.8 μm were fabricated.

Al top electrodes yield higher low-temperature subgap resistance.

Higher subgap resistance correlates with better qubit performance.

Abstract

We present a fabrication scheme and testing results for epitaxial sub-micrometer Josephson junctions. The junctions are made using a high-temperature (1170 K) "via process" yielding junctions as small as 0.8 mu m in diameter by use of optical lithography. Sapphire (Al2O3) tunnel-barriers are grown on an epitaxial Re/Ti multilayer base-electrode. We have fabricated devices with both Re and Al top electrodes. While room-temperature (295 K) resistance versus area data are favorable for both types of top electrodes, the low-temperature (50 mK) data show that junctions with the Al top electrode have a much higher subgap resistance. The microwave loss properties of the junctions have been measured by use of superconducting Josephson junction qubits. The results show that high subgap resistance correlates to improved qubit performance.