QDev Seminar: Lukas Grunhaupt – University of Copenhagen

Lukas Grunhaupt

Karlsruhe Institute of Technology

Granular aluminum superinductors

Superconducting materials with low microwave losses and high kinetic inductance are a valuable resource in quantum circuit design, enabling the realization of so-called superinductors. These circuit elements can provide electromagnetic environments with characteristic impedance larger than the resistance quantum RQ = 6.5 kΩ. To implement superinductors, a promising alternative to the predominantly used mesoscopic Josephson junction arrays is granular aluminum (grAl). Its microstructure consists of pure aluminum grains embedded in an AlOx matrix, effectively forming a compact self-assembled Josephson junction network.  Using microwave resonators, we show that grAl strips can reach kinetic inductances up to nH/square, while their microwave frequency losses are as low as state of the art superinductor implementations. We identify excess quasiparticles as a limiting loss mechanism in superconducting circuits employing grAl, and find quasiparticle relaxation times in the range of seconds, orders of magnitude longer than previously observed. To test a grAl superinductor we use it in a superconducting fluxonium quantum bit, which consists of a small Josephson junction shunted by a superinductor. The measured coherence times of our grAl fluxonium are comparable to fluxonium implementations using Josephson junction array superinductors, and demonstrate that granular aluminum is a viable material for superconducting quantum circuits.