Publications in Physical Review Applied

Arrays of Josephson junctions can be tuned through anomalous metallic, quantum-critical, and insulating regimes. We introduce an alternative experimental probe, capturing microwave radiation across all three regimes, using a two-dimensional array of superconductor-semiconductor hybrid Josephson junctions as a model system. Our approach allows in situ calibration of the sample’s circuit parameters and provides isolation from measurement back-action effects. We measure the radiation temperature of the anomalous metal and find that it is hotter than both the quantum-critical and insulating regimes. We further show that the anomalous metallic regime is more susceptible to additional heating than other regimes, explaining its emergence in otherwise thermalized systems. Turning to the quantum-critical regime, we discover nonlinear scaling of radiative noise with applied bias, consistent with theoretical predictions of universal nonequilibrium behavior at quantum-critical points.

State-of-the-art transmon qubits rely on large capacitors, which systematically improve their coherence due to reduced surface-loss participation. However, this approach increases both the footprint and the parasitic cross-coupling and is ultimately limited by radiation losses – a potential roadblock for scaling up quantum processors to millions of qubits. In this work we present transmon qubits with sizes as low as 36×39μm2 with ≳100-nm-wide vacuum-gap capacitors that are micromachined from commercial silicon-on-insulator wafers and shadow evaporated with aluminum. We achieve a vacuum participation ratio up to 99.6% in an in-plane design that is compatible with standard coplanar circuits. Qubit relaxation-time measurements for small gaps with high zero-point electric field variance of up to 22 V/m reveal a double exponential decay indicating comparably strong qubit interaction with long-lived two-level systems. The exceptionally high selectivity of up to 20 dB to the superconductor-vacuum interface allows us to precisely back out the sub-single-photon dielectric loss tangent of aluminum oxide previously exposed to ambient conditions. In terms of future scaling potential, we achieve a ratio of qubit quality factor to a footprint area equal to 20μm-2, which is comparable with the highest T1 devices relying on larger geometries, a value that could improve substantially for lower surface-loss superconductors. © 2023 American Physical Society.

Superinductors have a characteristic impedance exceeding the resistance quantum RQ≈6.45kω, which leads to a suppression of ground-state charge fluctuations. Applications include the realization of hardware-protected qubits for fault-tolerant quantum computing, improved coupling to small-dipole-moment objects, and the definition of a new quantum-metrology standard for the ampere. In this work, we refute the widespread notion that superinductors can only be implemented based on kinetic inductance, i.e., using disordered superconductors or Josephson-junction arrays. We present the modeling, fabrication, and characterization of 104 planar aluminum-coil resonators with a characteristic impedance up to 30.9 kω at 5.6 GHz and a capacitance down to ≤1 fF, with low loss and a power handling reaching 108 intracavity photons. Geometric superinductors are free of uncontrolled tunneling events and offer high reproducibility, linearity, and the ability to couple magnetically – properties that significantly broaden the scope of future quantum circuits.