Implementation of a Transmon Qubit Using Superconducting Granular Aluminum

Abstract

The high kinetic inductance offered by granular aluminum (grAl) has recently been employed for linear inductors in superconducting high-impedance qubits and kinetic inductance detectors. Because of its large critical current density compared to typical Josephson junctions, its resilience to external magnetic fields, and its low dissipation, grAl may also provide a robust source of nonlinearity for strongly driven quantum circuits, topological superconductivity, and hybrid systems. Having said that, can the grAl nonlinearity be sufficient to build a qubit? Here we show that a small grAl volume (10 × 200 × 500 nm 3) shunted by a thin film aluminum capacitor results in a microwave oscillator with anharmonicity α two orders of magnitude larger than its spectral linewidth Γ 01 , effectively forming a transmon qubit. With increasing drive power, we observe several multiphoton transitions starting from the ground state, from which we extract α ¼ 2π × 4.48 MHz. Resonance fluorescence measurements of the j0i → j1i transition yield an intrinsic qubit linewidth γ ¼ 2π × 10 kHz, corresponding to a lifetime of 16 μs, as confirmed by pulsed time-domain measurements. This linewidth remains below 2π × 150 kHz for in-plane magnetic fields up to ∼70 mT.

Engineering high-coherence superconducting qubits

“…Finally, a tunnel-junction-free, charge-insensitive qubit such as a transmon can be realized using granular aluminum as a nonlinear element. 187…”

Multiphoton resonance in a driven Kerr oscillator in presence of high-order nonlinearities

“…For our purposes, it is more convenient to use the quasienergy (n) = ( n1 + n3 )/2 as an independent continuous variable. Also, in the stationary case, it is possible to express the nondiagonal density matrix elements ρ 13 n from ( 8) and to substitute them into (7). The resulting equations for P r ( ), r = 1, 3 in the domain of quasienergies…”

Initial Design of a W-Band Superconducting Kinetic Inductance Qubit

“…Superconducting kinetic inductance qubits operating in the W-band (75-110 GHz) have the potential to lift these bottlenecks. The nonlinear inductance of superconducting nanowires has been used in designs and experiments involving cavity-based parametric amplifiers [8,9] and qubits [10,11,12] in the sub-10 GHz regime and as a parametric amplifier in the W-band [13], but thus far, a viable superconducting qubit has not been demonstrated in the W-band.…”