Magnetic Fields Turn Superconducting Circuits into Quantum Diodes

Superconducting circuits are essential for advanced technologies like quantum computing and ultra-sensitive sensors, but they often struggle to operate in strong magnetic fields. A new study reveals that these circuits can not only withstand such fields but actually improve, thanks to an unexpected diode effect that emerges when magnetic fields are applied. This breakthrough could enhance devices used in everything from dark matter detectors to medical imaging, making them more powerful and versatile.

The researchers found that when a magnetic field is applied parallel to the circuit plane, the circuits develop a pronounced asymmetry in their response to magnetic flux. Specifically, the characteristic tuning arcs of the circuits become skewed, with one side extending further than the other. This skewing indicates that the nano-constrictions within the circuits act as Josephson diodes, where current flows more easily in one direction than the other. The effect is demonstrated in niobium-based circuits with integrated nano-constriction quantum interferometers, tested in magnetic fields up to 300 millitesla.

To achieve this, the team used a setup combining a single-current-source vector magnet with in-situ sample rotation, ensuring minimal noise and precise field alignment. They characterized the microwave circuits by measuring the transmission parameter S21 as a function of bias flux and in-plane magnetic field. The circuits were fabricated from a 100 nm thick niobium film on a silicon substrate, with nano-constrictions patterned using a focused neon-ion beam. The experimental data, including resonance frequency shifts and linewidth changes, were analyzed to model the current-phase relations of the constrictions.

The data show that the sweetspot resonance frequency decreases with increasing magnetic field, from about 10.233 GHz at zero field to lower values at 300 mT, as seen in Figure 3b. Simultaneously, the flux responsivity, a key figure of merit for sensing applications, increases by more than a factor of five, from approximately 25 MHz per flux quantum at zero field to around 130 MHz per flux quantum at high fields, as shown in Figure 3d. The Kerr anharmonicity, which probes circuit nonlinearities, becomes bimodal in the diode state, differing by up to a factor of four between the two sides of the skewed flux arcs, as illustrated in Figure 5d. This bimodality confirms the diode effect and rules out alternative explanations like chip rotation.

These have significant for practical applications. The enhanced flux responsivity and tuning range make these circuits promising for hybrid quantum systems operating in magnetic fields, such as spin resonance spectrometers, microwave quantum magnonics, and dark matter axion detectors. The diode effect itself could be harnessed for superconducting electronics and spintronics, offering new functionalities in quantum circuits. Moreover, the simple macroscopic model developed for the Josephson diodes provides a tool for designing future devices with tailored properties, potentially at lower magnetic fields.

However, the study has limitations. The diode effect is attributed to inhomogeneities in the nano-constrictions, likely due to fabrication damage, which may vary between devices and affect reproducibility. The model assumes linear gradients in critical current density and magnetic flux per height, but real constrictions could have more complex profiles. Additionally, the experiments were conducted at 2.8 K, and performance at lower temperatures, typical for quantum applications, needs further investigation. The exact mechanism behind the premature switching currents, which are consistently lower than the theoretical critical currents, also remains not fully understood.