NQCP Seminar - Andrea Trioni

Superinductors in Quantum Circuits: Geometric Inductance Coils and Insulating Josephson Junction Chains

In a superconducting circuit, quantum dynamics are governed by the interplay between charge and phase, two conjugate variables whose fluctuations are controlled by the circuit's characteristic impedance Z_C. When Z_C exceeds the resistance quantum R_Q = h/(2e)^2 ≃ 6.5 kΩ, charge fluctuations are suppressed, the phase delocalizes, and the circuit enters a regime of dual Josephson physics. Realizing this condition requires a superinductor, a high-impedance, low-loss inductive element with sufficiently small parasitic capacitance that Z_C = \sqrt{L/C} > R_Q.

This talk addresses both a realization of geometric superinductors and the use of junction chains to probe a quantum phase transition.

First, it is shown that planar aluminum coils, exploiting mutual inductive coupling between adjacent turns and substrate engineering, can exceed R_Q at GHz resonance frequencies without relying on kinetic inductance materials, a possibility long thought to be out of reach. These elements display low loss, high linearity, and strong reproducibility, with the additional possibility of magnetic coupling to external circuits.

Second, one-dimensional Josephson junction chains are studied across the superconductor-insulator transition. Devices on both sides of the transition are characterized in DC, revealing either a supercurrent branch or Coulomb blockade, confirming that the transition is directly observable. Whether an analogous crossover is imprinted on the microwave mode spectrum, and how disorder and phase-slip events reshape it, is then investigated, with implications for both many-body quantum simulation and the practical design of high-impedance quantum hardware.