Materials in superconducting quantum bits

Jeffrey S. Kline, Haohua Wang, Seongshik Oh +2 more

We have found that crystalline Josephson junctions have problems with the control of critical current density that decrease the circuit yield.We present a superconducting quantum bit circuit designed to accommodate a factor of five variation in critical current density from one fabrication run to the next.The new design enables the evaluation of advanced tunnel barrier materials for superconducting quantum bits.Using this circuit design, we compare the performance of Josephson phase qubits fabricated with MgO and Al 2 O 3 advanced crystalline tunnel barriers to AlO x amorphous tunnel barrier qubits.

Eisuke Abe, Shuhei Tamate

Sina Khorasani

In this letter, it is proposed that cryogenic quantum bits can operate based on the nonlinearity due to the quantum capacitance of two-dimensional Dirac materials, and in particular graphene. The anharmonicity of a typical superconducting quantum bit is calculated, and the sensitivity of quantum bit frequency and anharmonicity with respect to temperature are found. Reasonable estimates reveal that a careful fabrication process can reveal expected properties, putting the context of quantum computing hardware into new perspectives.

John L. Clarke, Frank K. Wilhelm

Michel Devoret

Lukas Grünhaupt

We investigate superconducting granular aluminum (grAl) as a promising material for quantum circuits. We show that grAl strips can reach kinetic inductances up to nH/sq, while maintaining small microwave frequency losses. We identify excess quasiparticles as a limiting loss mechanism, and find quasiparticle relaxation times on the order of seconds. By fabricating a fluxonium quantum bit with a grAl superinductor, we demonstrate that grAl is a viable material for superconducting quantum circuits.