Josephson effect


In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. The effect is named after the British physicist Brian Josephson, who predicted in 1962 the mathematical relationships for the current and voltage across the weak link. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mechanics are observable at ordinary, rather than atomic, scale. The Josephson effect has many practical applications because it exhibits a precise relationship between different physical measures, such as voltage and frequency, facilitating highly accurate measurements.
The Josephson effect produces a current, known as a supercurrent, that flows continuously without any voltage applied, across a device known as a Josephson junction. This consists of two or more superconductors coupled by a weak link. The weak link can be a thin insulating barrier, a short section of non-superconducting metal, or a physical constriction that weakens the superconductivity at the point of contact.
Josephson junctions have important applications in quantum-mechanical circuits, such as SQUIDs, superconducting qubits, and RSFQ digital electronics. The NIST standard for one volt is achieved by an array of 20,208 Josephson junctions in series.

History

The DC Josephson effect had been seen in experiments prior to 1962, but had been attributed to "super-shorts" or breaches in the insulating barrier leading to the direct conduction of electrons between the superconductors.
In 1962, Brian Josephson became interested in superconducting tunneling. He was then 23 years old and a second-year graduate student of Brian Pippard at the Mond Laboratory of the University of Cambridge. That year, Josephson took a many-body theory course with Philip W. Anderson, a Bell Labs employee on sabbatical leave for the 1961–1962 academic year. The course introduced
Josephson to the idea of broken symmetry in superconductors, and he "was fascinated by the idea of broken symmetry, and wondered whether there could be any way of observing it experimentally". Josephson studied the experiments by Ivar Giaever and Hans Meissner, and theoretical work by Robert Parmenter. Pippard initially believed that the tunneling effect was possible but that it would be too small to be noticeable, but Josephson did not agree, especially after Anderson introduced him to a preprint of "Superconductive Tunneling" by Marvin L. Cohen, Leopoldo Máximo Falicov, and James Charles Phillips about the superconductor-barrier-normal metal system.
Josephson and his colleagues were initially unsure about the validity of Josephson's calculations. Anderson later remembered:

We were all—Josephson, Pippard and myself, as well as various other people who also habitually sat at the Mond tea and participated in the discussions of the next few weeks—very much puzzled by the meaning of the fact that the current depends on the phase.

After further review, they concluded that Josephson's results were valid. Josephson then submitted "Possible new effects in superconductive tunnelling" to Physics Letters in June 1962. The newer journal Physics Letters was chosen instead of the better established Physical Review Letters due to their uncertainty about the results. John Bardeen, by then already Nobel Prize winner, was initially publicly skeptical of Josephson's theory in 1962, but came to accept it after further experiments and theoretical clarifications. See also:.
In January 1963, Anderson and his Bell Labs colleague John Rowell submitted the first paper to Physical Review Letters to claim the experimental observation of Josephson's effect "Probable Observation of the Josephson Superconducting Tunneling Effect". These authors were awarded patents on the effects that were never enforced, but never challenged.
Before Josephson's prediction, it was only known that single electrons can flow through an insulating barrier, by means of quantum tunneling. Josephson was the first to predict the tunneling of superconducting Cooper pairs. For this work, Josephson received the Nobel Prize in Physics in 1973. Bardeen was one of the nominators.
John Clarke, also a student of Pippard, says his work was heavily inspired by Brian Josepshon. In 1985, John Clarke's team, including Michel Devoret and John M. Martinis cooled a Josephson junction below 50 mK and demonstrated its macroscopic quantum behaviour described by a single phase. Using microwave pulses, they demonstrated that at zero bias the energy was quantized. This discovery was later used to develop superconducting qubits. Clarke, Devoret and Martinis were awarded the Nobel Prize in Physics in 2025 for this discovery.

Applications

Types of Josephson junction include the φ Josephson junction, long Josephson junction, and superconducting tunnel junction. Other uses include:
A Josephson junction consists of a barrier material sandwiched between two superconducting electrodes, where the barrier acts as a weak link between the superconductors. Depending on its nature, this weak link can be an insulating layer, a normal metal, or a narrow constriction, giving rise to different classes of Josephson junctions. Over the years, a wide variety of materials have been explored for both the electrodes and the barrier in order to optimize junction performance for specific applications.
Material selection in Josephson junctions is governed by several factors, including dielectric loss, structural and chemical stability, aging behavior, superconducting transition temperature, ease of fabrication, interface uniformity and roughness, and the presence of two-level system|two-level systems . Among the many material systems investigated, aluminum-based Al|AlOx|Al junctions have emerged as the state of the art for many superconducting qubit architectures due to their reproducibility and low microwave loss.

Electrode materials

The choice of electrode material depends on the intended application such as transmon qubits, SQUIDs, or detectors as well as fabrication compatibility. A key requirement for superconducting electrodes is a sufficiently high superconducting transition temperature, which sets the superconducting energy gap and directly influences the current voltage characteristics of the Josephson junction. Aluminum is the most widely used electrode material in state-of-the-art qubits due to its ease of fabrication and the high quality of its native oxide barrier. Niobium and tantalum are also commonly employed, offering higher transition temperatures and improved robustness, while materials such as niobium nitride provide even higher Tc values for specialized high-frequency or detector applications.
Aluminium is the most widely used electrode material in state-of-the-art superconducting qubits due to its ease of fabrication and compatibility with high-quality tunnel barriers.
Niobium is also widely used, with pure Nb exhibiting a superconducting transition temperature of approximately 9.3 K. Nb/Al–AlOx/Nb junctions are employed in many superconducting qubits and integrated circuits. However, Nb is sensitive to atmospheric oxygen and readily forms a native oxide, which can reduce the effective superconducting transition temperature; disordered Nb films have reported Tc values around 5.7 K.
Tantalum has recently emerged as a promising electrode material for low-loss quantum circuits. The superconducting α-Ta phase has a transition temperature of approximately 4.4 K and has been used in conjunction with Nb in Josephson junctions and superconducting resonators. In contrast, the β-Ta phase becomes superconducting only below 1 K. The crystalline phase and electrical properties of Ta films depend strongly on growth conditions and substrate choice, requiring careful control of deposition parameters.
Niobium nitride has attracted increasing interest due to its high superconducting transition temperature, which can reach values up to approximately 17 K in the cubic δ-NbN phase. NbN thin films have been grown using several deposition techniques, including sputtering and molecular beam epitaxy, with high-Tc films reported at nanometer-scale thicknesses. However, the short coherence length of NbN places stringent requirements on interface quality and uniformity in Josephson junction trilayers. Niobium nitride crystallizes primarily in cubic rock-salt and hexagonal wurtzite-related structures, with superconducting properties strongly dependent on crystal phase. The cubic δ-NbN phase exhibits the highest superconducting transition temperature, typically in the range of 11–17 K, whereas the hexagonal phases show significantly lower Tc values, often below 1 K. Control of growth parameters, particularly substrate temperature, enables stabilization of the cubic phase, making NbN attractive for high-Tc Josephson junctions and superconducting devices.