Ultrahigh-impedance suspended Josephson circuits for quantum computing, simulations, and metrology

Chains of Josephson junctions probably have the largest kinetic inductance per-unit-length, which can exceed the geometric one by about 10⁴, primarily limited by quantum phase-slip fluctuations. However, the total inductance is also limited by the stray capacitance, which grows linearly with the chain length. This stray capacitance is unnecessarily large in most circuits because of the high dielectric constant of silicon or sapphire substrates. By releasing Josephson chains off the substrate, we can combine the maximal per-unit-length inductance with the minimal stray capacitance, thereby obtaining the highest impedance electromagnetic structures available today. As a first demonstration, we created a superconducting quasicharge qubit [1] (blochnium), a dual of transmon, made of a weak junction shunted by such a large inductance (hyperinductance) whose impedance reaches over 30 × R_Q (200 kΩ). In the second demonstration, we fabricated suspended "telegraph" transmission lines, composed of 5,000+ junctions, whose wave impedance exceeds 5 × R_Q (32.5 kΩ). These lines are a unique resource in exploring DC current metrology via Bloch oscillations, as well as in analog quantum simulations of many strongly-correlated 1D systems.

[1] I. Pechenezhskiy et al., Nature 585, 368 (2020)