Precise characterization of individual Josephson junctions (JJs) is essential for developing uniform superconducting JJ arrays, particularly in precision metrology, coherent THz sources, and quantum computing. Conventional electrical approaches are often limited by noise, averaging requirements, and the lack of single-junction resolution in large arrays. Here, we introduce a noninvasive optical modulation technique enabling efficient and selective probing of individual JJ characteristics. By modulating the intensity of a light source at high frequencies (50 kHz), we induce controlled changes in JJ properties and extract I c with high precision using lock-in detection. This method achieves 1% accuracy with only 100 ms of integration, offering comparable precision with substantially reduced acquisition time. Importantly, it enables scalable, parallel superlocalized mapping of JJ arrays through beam steering or multiplexed illumination, allowing identification of local inhomogeneities and defects. Beyond JJs, this approach provides a general framework for spatially resolved probing of emerging quantum materials and devices.
Precise characterization of individual Josephson junctions (JJs) is essential for developing uniform superconducting JJ arrays, particularly in precision metrology, coherent THz sources, and quantum computing. Conventional electrical approaches are often limited by noise, averaging requirements, and the lack of single-junction resolution in large arrays. Here, we introduce a noninvasive optical modulation technique enabling efficient and selective probing of individual JJ characteristics. By modulating the intensity of a light source at high frequencies (50 kHz), we induce controlled changes in JJ properties and extract I c with high precision using lock-in detection. This method achieves 1% accuracy with only 100 ms of integration, offering comparable precision with substantially reduced acquisition time. Importantly, it enables scalable, parallel superlocalized mapping of JJ arrays through beam steering or multiplexed illumination, allowing identification of local inhomogeneities and defects. Beyond JJs, this approach provides a general framework for spatially resolved probing of emerging quantum materials and devices.
The Josephson junction (JJ) is the fundamental nonlinear building block of superconducting quantum technologies. Its macroscopic quantum tunneling physics underpins superconducting quantum computing, sensing, and communication, but scaling these platforms to utility-scale architectures places increasingly stringent demands on junction materials, interfaces, and fabrication. Practical quantum computing requires high reproducibility, low power consumption, tunability, a small device footprint, and resilience to noise and defects. This review surveys how advances in materials science, device characterization, and nanofabrication are addressing these challenges and redefining the figures of merit for next-generation JJs. This review also examines the evolution of fabrication strategies, from conventional multi-angle evaporation to foundry-compatible superconducting processes and the integration of emerging junction materials. Progress along these directions will determine how rapidly JJs move from laboratory-scale components to the foundation of industrial-scale quantum processors.
Gate-tunable Josephson junctions (JJs) are the backbone of superconducting classical and quantum computation. Typically, these systems exploit low-charge-concentration materials and present technological difficulties limiting their scalability. Surprisingly, electric field modulation of a supercurrent in metallic wires and JJs has been recently demonstrated. Here, we report the realization of titanium-based monolithic interferometers which allow tuning both JJs independently via voltage bias applied to capacitively coupled electrodes. Our experiments demonstrate full control of the amplitude of the switching current ( I s ) and of the superconducting phase across the single JJ in a wide range of temperatures. Astoundingly, by gate-biasing a single junction, the maximum achievable total I s is suppressed down to values much lower than the critical current of a single JJ. A theoretical model including gate-induced phase fluctuations on a single junction accounts for our experimental findings. This class of quantum interferometers could represent a breakthrough for several applications such as digital electronics, quantum computing, sensitive magnetometry, and single-photon detection.
Josephson junctions, as pivotal components of modern technologies such as superconducting quantum computing, owe their prominence to their unique nonlinear properties at low temperatures. Despite their extensive use in static configurations, the study of dynamic Josephson junctions, under space-time (ST) modulation, remains largely unexplored. This study investigates the interaction and transmission of light through arrays of space-time-modulated (STM) Josephson junctions. A comprehensive mathematical framework is presented to model the propagation of electric and magnetic fields within and beyond these structures. We demonstrate how such dynamic arrays enable unique four-dimensional light manipulation, leading to angular-frequency beam multiplexing through a seamless integration of frequency conversion and beam-splitting functionalities. These advancements open new horizons for electromagnetic field engineering, with far-reaching implications for superconducting quantum technologies, next-generation wireless communications, biomedical sensing, and radar systems.
Josephson junctions, as pivotal components of modern technologies such as superconducting quantum computing, owe their prominence to their unique nonlinear properties at low temperatures. Despite their extensive use in static configurations, the study of dynamic Josephson junctions, particularly under space-time modulation, remains largely unexplored. This study investigates the interaction and transmission of electromagnetic waves through arrays of space-time-modulated Josephson junctions. A comprehensive mathematical framework is presented to model the propagation of electric and magnetic fields within and beyond these structures. We demonstrate how such dynamic arrays enable groundbreaking four-dimensional light manipulation, achieving angular-frequency beam multiplexing through a seamless integration of frequency conversion and beam-splitting functionalities. These advancements open new horizons for electromagnetic field engineering, with far-reaching implications for superconducting quantum technologies, next-generation wireless communications, biomedical sensing, and radar systems.
Microscopic inhomogeneity within superconducting films is a critical bottleneck hindering the performance and scalability of quantum circuits. All-nitride Josephson Junctions (JJs) have attracted substantial attention for their potential to provide enhanced coherence times and enable higher temperature operation. However, their performance is often limited by local variations caused by polymorphism, impurities, and interface quality. This work diagnoses atomic-scale limitations preventing superconducting NbN/AlN/NbN JJs from reaching their full potential. Electrical measurements reveal suppressed critical current density and soft onset of quasiparticle current. However, inverse proportionality between resistance and junction area confirms homogenous barrier thickness. This isolates structural and chemical variations in electrodes and barrier as the source of performance limitation. The observed characteristics are attributed to complex materials problems: NbN polymorphism, phase coexistence, and oxygen impurities. Using advanced microscopy and machine learning integrated approach, nanoscale inclusions of epsilon-Nb2N2 are found to coexist within dominant delta-NbN electrodes. DC performance of JJs may be affected by these defects, leading to unresolved supercurrent and soft transition to normal state. By identifying specific atomic scale defects, tracing its origin to initial film nucleation, and linking to its detrimental electrical signature, this work establishes a material-to-device correlation and provides targeted strategy for phase engineering towards reproducible, high coherence and scalable quantum devices.
Sub-μm Josephson Junctions for Superconducting Quantum Devices
For high-performance superconducting quantum devices based on Josephson junctions (JJs), decreasing lateral sizes is of great importance. Fabrication of sub-μm JJs is challenging, due to nonflat surfaces with step heights of up to several 100 nm generated during the fabrication process. We have refined a fabrication process with significantly decreased film thicknesses, resulting in almost flat surfaces at intermediate steps during the JJ definition. In combination with a mix-and-match process, combining electron-beam lithography and conventional photolithography, we can fabricate JJs with lateral dimensions down to 0.023 μm2. We propose this refined process as an alternative to the commonly used chemical-mechanical polishing procedure. Transport measurements of JJs, having critical-current densities ranging from 50 to 104A/cm2, are presented at 4.2 K. Our JJ process yields excellent quality parameters, Rsg/RNup to ~ 50, Vmfrom 15 to 80 mV and Vgapup to 2.81 mV, and also allows the fabrication of high-quality, sub-μm wide, long JJs (LJJs) for the study of Josephson vortex behavior. The developed technique can also be used for similar multilayer processes and is very promising for fabricating sub- μm JJs for quantum devices such as SQUIDs, qubits, and SIS mixers.