Granular Aluminum – A Versatile Material for Superconducting Detectors and Quantum Circuits

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Granular aluminum (grAl) is an intriguing superconducting material, which has been receiving increasing attention in the superconducting quantum bits (qubits) and detectors communities. Among its key features are a tunable kinetic inductance up to nH/sq, amenable nonlinearity, and low microwave frequency losses [1,2,3]. Furthermore, quasiparticle relaxation times on the order of ~s have been observed [1]. In elucidating the sources of excess quasiparticles, the role of ionizing radiation has recently come to the forefront, and abatement of quasiparticle bursts remains an open challenge [4]. Besides showcasing the key features of grAl, we will present our results on a fluxonium qubit, where granular aluminum strips realize a so-called superinductor with an inductance of 225 nH at an impedance > 6.6 kΩ [5]. We will also discuss a transmon-type qubit in which a grAl volume of 10-3 μm3 realizes the non-linearity [6]. Thanks to their impedance in the kΩ range, grAl strips also play an important role in providing the high-impedance environment for the measurement of Bloch-oscillations, the dual of Shapiro steps – potentially enabling a current standard traceable to fundamental constants.
[1] L. Grünhaupt et. al., Phys. Rev. Lett. 121, 117001 (2018)
[2] N. Maleeva et al., Nat. Commun. 9, 3889 (2018) 
[3] F. Valenti et al., Phys. Rev. Applied 11, 054087 (2019) 
[4] L. Cardani & F. Valenti et al., Nat. Commun. 12, 2733 (2021) 
[5] L. Grünhaupt & M. Spiecker et al., Nature Mat. 18, 816-819 (2019)
[6] P. Winkel et al., Phys. Rev. X 10, 031032 (2020)

Granular aluminum (grAl) is an intriguing superconducting material, which has been receiving increasing attention in the superconducting quantum bits (qubits) and detectors communities. Among its key features are a tunable kinetic inductance up to nH/sq, amenable nonlinearity, and low microwave frequency losses [1,2,3]. Furthermore, quasiparticle relaxation times on the order of ~s have been observed [1]. In elucidating the sources of excess quasiparticles, the role of ionizing radiation has recently come to the forefront, and abatement of quasiparticle bursts remains an open challenge [4]. Besides showcasing the key features of grAl, we will present our results on a fluxonium qubit, where granular aluminum strips realize a so-called superinductor with an inductance of 225 nH at an impedance > 6.6 kΩ [5]. We will also discuss a transmon-type qubit in which 

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