Magnetically induced Josephson nano-diodes in field-resilient superconducting microwave circuits
Benedikt Wilde, Mohamad Kazouini, Timo Kern, Kevin Uhl, Christoph Füger, Dieter Koelle, Reinhold Kleiner, Daniel Bothner
TL;DR
This study demonstrates magnetically robust Nb microwave circuits incorporating nano-constriction interferometers that exhibit a field-induced Josephson-diode effect in large in-plane fields ($B_\parallel$ up to ~300 mT). A macroscopic diode model explains the observed asymmetric flux-tuning arcs and reconstructs the diode current-phase relations, while Kerr anharmonicity measurements reveal a bimodal response consistent with diode CPRs. The results establish that inhomogeneous constrictions can function as intrinsic Josephson diodes, enhancing flux responsivity and potentially enabling diode-enabled three-wave mixing and high-field magnetometry in superconducting circuits. The findings provide design principles for high-field hybrid quantum systems and introduce a practical framework for studying and exploiting diode effects in microwave superconducting circuits.
Abstract
The development of nonlinear and frequency-tunable superconducting microwave circuits for operation in large magnetic fields is of high relevance for hybrid quantum systems such as spin resonance spectrometers, microwave quantum magnonics, dark matter axion detectors or flux-mediated optomechanics. With these exciting perspectives in mind, we investigate niobium-based circuits with integrated nano-constriction quantum interferometers in magnetic in-plane fields up to several hundred mT. Our experiments reveal an unexpected and pronounced field-induced asymmetry in the bias-flux response of the circuits, which is demonstrated to originate from a field-induced Josephson-diode effect within the nano-constrictions and which considerably enhances the circuit figures of merit in a magnetic field. An intuitive macroscopic Josephson-diode model attributes the effect to inhomogeneous constriction properties and provides us with the diode current-phase relation as a function of the in-plane field. Finally, we demonstrate that in the diode-state the circuit Kerr nonlinearity is bimodal in frequency, not only eliminating alternative explanations for the bias-flux-asymmetries but also being potentially useful for quantum circuit applications. Overall, our report underlines that niobium nano-constriction circuits belong to the most promising candidates for high-field hybrid quantum systems and reveals the untapped potential of combining Josephson nano-diodes with microwave quantum circuits.