Josephson Junctions for Superconducting Electronics and Qubits

Josephson junctions are the basic building blocks of many superconducting electronic and quantum circuits. A Josephson junction consists of two superconducting electrodes separated by a very thin nonsuperconducting region, through which Cooper pairs can tunnel while maintaining quantum phase coherence. The resulting Josephson effect provides a direct connection between superconducting phase, electrical current, and voltage, making Josephson junctions useful for both classical superconducting electronics and superconducting quantum devices.

Our research explores new materials, device structures, and physical mechanisms in Josephson junctions, with the goal of achieving improved performance and new functionality. Rather than being limited to conventional junction designs, we are interested in how new material combinations and transport mechanisms can extend the capabilities of Josephson devices. Our current work ranges from high-performance junctions for superconducting digital electronics, to magnetic and π junctions that combine superconductivity with emerging forms of magnetism, to low-defect junction structures for superconducting qubits.

High-Performance Josephson Junctions for Superconducting Electronics 

Superconducting digital electronics offers a fundamentally different approach to computation from conventional semiconductor electronics. In Single Flux Quantum (SFQ) circuits, information is represented and processed using quantized magnetic-flux pulses generated by Josephson junctions. Variants such as Rapid Single Flux Quantum (RSFQ) logic can operate at very high clock frequencies while taking advantage of the fast switching and low energy consumption of superconducting devices. These technologies are being explored for high-speed digital electronics, cryogenic computing, and control and interface electronics operating at low temperatures.

Josephson junctions for these applications place demanding requirements on device performance. High critical-current density is particularly attractive because it allows junction dimensions to be reduced while maintaining the critical current required for circuit operation. Low noise and clean electrical characteristics are also important for reliable high-speed switching. In addition, improved thermal stability can allow Josephson junctions to withstand more demanding fabrication and integration processes, potentially expanding the range of materials and technologies with which superconducting circuits can be integrated.

Our research takes an exploratory approach to these challenges. We investigate new materials, physical mechanisms, and junction structures that may offer advantages beyond conventional Josephson junction technologies. We are particularly interested in understanding how the microscopic transport mechanism and the properties of the barrier and interfaces determine critical-current density, characteristic voltage, noise, thermal stability, and device scaling. The goal is to identify new approaches that enable smaller, faster, and more robust Josephson junctions for SFQ, RSFQ, and other superconducting electronic technologies.

Magnetic and π Josephson Junctions 

Josephson junctions also provide an unusual platform for combining superconductivity with magnetism. When magnetic interactions are introduced into a Josephson junction, the superconducting order parameter and the magnetic state can interact in ways that have no direct counterpart in conventional electronic devices. One important example is the π Josephson junction, in which the ground-state phase difference across the junction is shifted by π rather than being zero. This intrinsic phase shift can be used as a built-in phase-bias element and can introduce new functions into superconducting circuits.

Our work explores magnetic and π Josephson junctions using emerging forms of magnetism and new classes of magnetic materials. In addition to conventional ferromagnetic systems, we are interested in antiferromagnetic and other unconventional magnetic states that provide new ways of controlling spin, magnetic order, and superconducting phase. These systems offer an opportunity to study the interaction between superconductivity and magnetism at the nanoscale and to search for Josephson effects that are difficult or impossible to realize in conventional junctions.

This direction also forms a natural bridge between our research in spintronics and superconducting electronics. We are interested in how magnetic order, spin-dependent transport, and superconducting phase can be coupled within a single device. Such junctions may provide controllable 0 and π states, intrinsic phase bias, nonvolatile functionality, or other new circuit elements. Beyond the underlying physics, these properties could ultimately be useful for superconducting logic, cryogenic memory, and other superconducting circuits in which magnetic information and superconducting phase are integrated.

Josephson Junctions for Qubits

Josephson junctions also play a central role in superconducting quantum circuits. The junction provides the nonlinear inductance needed to create discrete, anharmonic energy levels in a superconducting circuit, allowing the lowest energy states to be used as a quantum two-level system. Josephson junctions therefore form the essential nonlinear element in widely used superconducting qubits and are closely connected to the performance and coherence of these devices.

One of the continuing challenges in superconducting qubits is decoherence caused by microscopic defects and material losses. Structural disorder, interfaces, surfaces, and defects within or near the junction can produce unwanted two-level systems and other loss channels. These microscopic degrees of freedom can interact with the qubit, introduce noise and energy loss, and ultimately limit the time over which quantum information remains coherent. Improving the materials quality of the junction and its surrounding interfaces is therefore an important route toward improving qubit performance.

We investigate alternative Josephson-junction materials, interfaces, and device structures with the goal of reducing these microscopic sources of decoherence. Rather than being restricted to conventional junction structures, we explore approaches that may provide cleaner interfaces, lower defect densities, and reduced two-level-system loss. By understanding how junction materials and fabrication affect electrical quality and microscopic defects, we aim to develop low-loss Josephson junctions that could support longer coherence times and improved superconducting qubit performance.