03 May 2021


Prof. C. Schönenberger

Nano- & Quantum Physics Seminar

High Impedance Quantum Circuits

Prof. Dr. Ioan Pop, Karlsruhe Institute of Technology (KIT), Germany


Superconducting quantum information processing machines are predominantly based on microwave circuits with relatively low characteristic impedance, about 100 Ohm, and small anharmonicity, which can limit their coherence and logic gate fidelity. A promising alternative are circuits based on so-called superinductors, with characteristic impedances exceeding the resistance quantum R_Q = 6.4 kOhm. However, previous implementations of superinductors, consisting of mesoscopic Josephson junction arrays, can introduce unintended nonlinearity or parasitic resonant modes in the qubit vicinity, degrading its coherence. I will present a fluxonium qubit design based on a granular aluminum (grAl) superinductor strip [1]. I will argue that grAl forms a compact effective junction array with high kinetic inductance and amenable nonlinearity [2,3], which can be in-situ integrated with standard aluminum circuit processing. The measured qubit coherence time T_2^* = 30 µs illustrates the potential of grAl for applications ranging from protected qubit designs to quantum limited amplifiers, detectors and couplers for spin qubits, even though quasiparticle poisoning is a limiting factor [4] and should be addressed in future works [5].


[1] Grunhaupt, Spiecker et al. Nature Materials 18, 816-819 (2019)

[2] Maleeva et al. Nature Comm. 9, 3889 (2018)

[3] Winkel et al. Phys. Rev. X 10, 031032 (2020)

[4] Grunhaupt et al., Phys. Rev. Lett. 121, 117001 (2018)

[5] Cardani, Valenti et al. Nature Comm. (in production 2021)

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