Loss Mechanisms and Quasiparticle Dynamics in Superconducting Microwave Resonators Made of Thin-Film Granular Aluminum

Abstract

Superconducting high kinetic inductance elements constitute a valuable resource for quantum circuit design and millimeter-wave detection. Granular aluminum (grAl) in the superconducting regime is a particularly interesting material since it has already shown a kinetic inductance in the range of nH/□ and its deposition is compatible with conventional Al/AlOx/Al Josephson junction fabrication. We characterize microwave resonators fabricated from grAl with a room temperature resistivity of 4×10^{3} μΩ cm, which is a factor of 3 below the superconductor to insulator transition, showing a kinetic inductance fraction close to unity. The measured internal quality factors are on the order of Q_{i}=10^{5} in the single photon regime, and we demonstrate that nonequilibrium quasiparticles (QPs) constitute the dominant loss mechanism. We extract QP relaxation times in the range of 1 s and we observe QP bursts every ∼20 s. The current level of coherence of grAl resonators makes them attractive for integration in quantum devices, while it also evidences the need to reduce the density of nonequilibrium QPs.

Decoherence benchmarking of superconducting qubits

“…Quasiparticle fluctuations have been extensively studied [41], where the magnitude of frequency shifts scales with the kinetic inductance. Therefore, while they can be of order 100 kHz in disordered superconductors [45], they are much smaller in elemental superconductors. In fact, recent experiments [41] showed that the quasiparticles in aluminium produced an un-measurably small frequency shift; instead, the quasiparticles' influence was revealed only by examining correlated amplitude and frequency noise.…”

Optical conductivity of granular aluminum films near the Mott metal-to-insulator transition

“…This behavior departs from the predictions of the BCS Mattis-Bardeen theory [5]. The enhanced low frequency dissipation has detrimental consequences on the quality factor of high impedance resonators that could be used in QC [6].…”

Rabi oscillations in a superconducting nanowire circuit

“…The inter-grain tunnel barriers lead to a sheet resistance of up to few kΩ which can be controlled during sample fabrication by adjusting the oxygen partial pressure during the film growth. Due to its low electrical loss, AlO x was recently proven to be very useful for implementing high kinetic inductances in quantum circuits [4] operated in the single photon regime [5]. In general, a wire made from AlO x can be seen as an alternating series of conducting grains separated by insulating barriers.…”