High Impedance Quantum Circuits

Dr Ioan Pop

Karlsruhe Institute of Technology (Germany)

Ioan M. Pop was born in Transylvania, Romania, in 1983, and studied physics at Babeș-Bolyai University in Cluj-Napoca from 2002 to 2006. He received a Master’s diploma in material science in 2007, and a Ph.D. in physics in 2011 from the Institut NÉEL, Centre National de la RechercheScientifique (CNRS), and Université Joseph Fourier in Grenoble, France. During the following four years he has worked as a postdoctoral researcher at Yale University, USA. He has received several honors, including a fellowship from the French Ministry of Higher Education and Research, the 2012 Thesis Prize awarded by the French Nanoscience Foundation, and the Humboldt SofjaKovalevskaja Startup Award. Since 2015, Ioan Pop is a group leader in Karlsruhe Institute of Technology, Germany, working on high impedance superconducting quantum hardware for quantum computation and quantum detectors.


High Impedance Quantum Circuits
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) superinductorstrip[1]. I will argue that granular aluminium forms a compact effective junction array with high kinetic inductance and low nonlinearity[2], and it 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 and detectors, even though quasiparticle poisoning is a limiting factor[3] and should be addressed in future works.
[1] Grunhaupt, Spiecker et al. Nature Materials 18, 816-819 (2019)
[2] Maleeva et al. Nature Comm. 9, 3889 (2018)
[3] Grunhaupt et al., Phys. Rev. Lett. 121, 117001 (2018)