Researchers Achieve Infinite Nonreciprocity in Superconducting Diodes with Planar Josephson Junctions

Superconducting circuits promise revolutionary advances in speed and energy efficiency, but a lack of fundamental components currently hinders their widespread use. Razmik A. Hovhannisyan, Amirreza Lotfian, Taras Golod, and Vladimir M. Krasnov, all from Stockholm University, address this challenge by demonstrating a highly efficient superconducting diode. The team fabricated a diode from niobium, a conventional superconductor, and engineered its structure to create a significant asymmetry in electrical current flow. This design achieves near-perfect nonreciprocity, allowing microwave signals to pass easily in one direction while almost completely blocking them in the opposite direction, and importantly, the researchers observe this effect with 75 GHz radiation and nearly 100% efficiency, paving the way for faster, more energy-efficient electronics and potentially enabling wireless communication at sub-THz frequencies.

This work investigates diodes based on planar Josephson junctions fabricated from a conventional niobium superconductor, aiming to address this gap. Nonreciprocity in these diodes arises from the self-field effect induced by the geometrical asymmetry of the junction, a phenomenon crucial for creating directional current flow. By deliberately tuning the junction parameters, researchers achieve effectively infinite nonreciprocity, within experimental resolution, characterised by a complete suppression of the superconducting critical current in one direction while maintaining a significant current in the opposite direction. The researchers achieved rectification at 75 GHz using a specifically designed Josephson junction with asymmetric geometry and self-field effects. The diode demonstrates a clear rectification effect, converting alternating current microwave power into a measurable direct current voltage. The team fabricated diodes using a thin film of niobium, deposited using magnetron sputtering, then patterned electrodes with photolithography and reactive ion etching, and created planar Josephson junctions with focused ion beam etching.

The depth of this etching controls the critical current density, and notches were strategically created to enhance the self-field effect. Measurements performed at cryogenic temperatures using a closed-cycle optical cryostat and a superconducting solenoid revealed a significant self-field effect. The characteristics of the Josephson junction were determined by measuring current-voltage curves, demonstrating successful conversion of microwave power into a measurable direct current voltage. The critical current dependence on magnetic field was modeled using the sine-Gordon equation, incorporating the self-field induced by the asymmetric bias current.

The model confirms that the asymmetry in the junction geometry and the self-field effect are essential for achieving rectification. The research demonstrates that breaking the symmetry of the Josephson junction, through asymmetric geometry and the self-field effect, leads to non-reciprocal behaviour, allowing current to flow more easily in one direction than the other. This non-reciprocal behaviour results in the rectification of the microwave signal.

Superconducting Diodes Demonstrate Nonreciprocal Current Flow

Superconducting electronics promise significant advances in speed and power efficiency for future communication systems, but practical implementation requires scalable counterparts to conventional semiconductor components. Researchers have now demonstrated a novel diode based on planar Josephson junctions fabricated from niobium, achieving a breakthrough in nonreciprocal current flow. This nonreciprocity arises from a carefully engineered self-field effect induced by the geometrical asymmetry of the junction, allowing for complete suppression of critical current in one direction while maintaining substantial current flow in the opposite direction. Through careful optimization of device geometry and junction parameters, they investigated the limits of achievable transport nonreciprocity, demonstrating effectively infinite nonreciprocity within experimental resolution.

Importantly, this research extends beyond simple current rectification to demonstrate threshold-free rectification of 75 GHz microwave radiation, indicating near-ideal optical nonreciprocity. This achievement represents the first reported high-frequency optical rectification in a superconducting diode, opening new avenues for ultrafast superconducting electronics and laying the groundwork for wireless sub-THz signal processing crucial for emerging 6G communication technologies. The ability to control current flow at these frequencies promises substantial improvements in bandwidth, noise performance, and overall efficiency compared to current semiconductor-based systems. This breakthrough establishes a pathway towards a complete suite of THz superconducting components, including digital devices and local oscillators, essential for realizing the full potential of future wireless communication networks.

Superconducting Diode Enables Infinite Nonreciprocity

This research demonstrates the creation of a superconducting diode based on planar Josephson junctions, exhibiting effectively infinite nonreciprocity, meaning it allows current to flow easily in one direction while almost completely blocking it in the opposite direction. The diodes achieve this through geometrical asymmetry within the junction, leveraging the self-field effect to control current flow without any need for an external magnetic field. The findings represent a significant step towards overcoming a key limitation in superconducting circuits, the lack of simple, efficient, and scalable diode components. While the presented devices demonstrate promising performance, further research is needed to optimize fabrication processes and fully characterize device performance across a wider range of frequencies and temperatures. Future work could focus on integrating these diodes into more complex circuits and exploring their potential for applications such as passive signal routers and high-frequency communication systems, potentially contributing to the development of 6G wireless technologies.