Josephson effect

Some metals carry power very easily.

We put a tiny gap between two of these metals. This gap acts like a small wall. Even with a wall, power flows right through. This is called the Josephson effect.

It happens because of how tiny parts of matter work. This effect helps us measure power very well. It can even help build fast computers. It is a very special way to use power!

The Josephson effect is a special way that power flows. It happens when two superconductors are close together. Superconductors are metals that carry power very easily.

To make this work, we put a tiny barrier between them. This barrier is called a weak link. It might be a thin layer of insulation. It could also be a small piece of normal metal.

Even with this barrier, a special current flows. This is called a supercurrent. It flows without any voltage being applied.

A scientist named Brian Josephson predicted this in 1962. He found a math link between voltage and frequency. This link is very precise. Because of this, we use it to measure things. For example, the NIST uses it to define one volt. They use a chip with 20,208 junctions in a row.

This effect also helps us build new technology. It is used in SQUIDs. These are tools that measure magnetic fields. It also helps make qubits. Qubits are parts used in quantum computers. These computers use the tiny rules of science to work.

The Josephson effect is a special way that electricity behaves in certain materials. It happens when two superconductors are placed very close to each other. Superconductors are materials that can carry electricity with almost no resistance. Between these two superconductors, scientists place a tiny barrier called a weak link. This effect is a macroscopic quantum phenomenon. This means we can see the rules of quantum mechanics in objects we can actually observe.

To make a Josephson junction, you must create a specific path for electricity. The weak link can be made in a few different ways. It might be a very thin layer of insulation. This is called a superconductor-insulator-superconductor junction. It could also be a short piece of normal metal. This is known as an S-N-S junction. Sometimes, it is just a narrow constriction that weakens the superconductivity at a contact point.

A British physicist named Brian Josephson discovered this effect. He was a 23-year-old graduate student in 1962. He worked at the Mond Laboratory at the University of Cambridge. He studied under a scientist named Brian Pippard. Josephson was fascinated by the idea of broken symmetry. He predicted how current and voltage would act across the weak link. For this important work, he won the Nobel Prize in Physics in 1973.

This effect is very useful for making precise measurements. It creates a supercurrent that flows without any voltage being applied. This allows for a steady link between voltage and frequency. For example, the NIST uses an array of 20,208 Josephson junctions in a series. This array helps them define exactly what one volt is. Scientists also use these junctions to build SQUIDs. These are tools that can measure magnetic fields very sensitively.

Today, this science helps us build amazing new technology. Josephson junctions are used to make superconducting qubits. These are the building blocks for quantum computers. These computers use the tiny rules of physics to solve hard jobs. Different metals are used to make these parts work well. Aluminum is often used because it is easy to work with. Other metals like niobium or tantalum are also used for special tasks.

The Josephson effect is a unique phenomenon in physics. It occurs when two superconductors are placed in close proximity. A small barrier, known as a weak link, separates them. This effect is a macroscopic quantum phenomenon. This means quantum mechanical effects are visible at an ordinary scale. We can observe these effects in objects larger than single atoms.

To understand the mechanism, we must look at the Josephson junction (JJ). This device consists of two or more superconductors coupled by a weak link. A supercurrent flows continuously across the junction without any voltage being applied. There are three main types of these weak links. The first is a thin insulating barrier, called a superconductor–insulator–superconductor (S-I-S) junction. The second is a short section of non-superconducting metal, called an S-N-S junction. The third is a physical constriction that weakens superconductivity at the contact point, called an S-c-S junction.

Different materials are used to build these junctions. The choice of electrode material depends on the specific application. Aluminum (Al) is the most common material for state-of-the-art superconducting qubits. This is because it is easy to fabricate and has high-quality native oxide barriers. Niobium (Nb) is also widely used because it has a high superconducting transition temperature. Pure niobium has a transition temperature of approximately 9.3 K. Tantalum (Ta) is a newer, promising material for low-loss quantum circuits. Some specialized devices use niobium nitride (NbN). This material can reach transition temperatures as high as 17 K in certain phases.

History shows how this discovery changed physics. In 1962, a 23-year-old graduate student named Brian Josephson predicted the effect. He was studying at the Mond Laboratory at the University of Cambridge. He was a student of Brian Pippard. Josephson was fascinated by the concept of broken symmetry. He predicted mathematical relationships for current and voltage across the weak link. He was the first to predict that Cooper pairs could tunnel through a barrier. For this discovery, Josephson received the Nobel Prize in Physics in 1973.

Experimental proof followed the theory. In January 1963, Philip W. Anderson and John Rowell submitted a paper claiming the first observation of the effect. While some scientists were initially skeptical, experiments confirmed the theory. In 1985, a team including John Clarke, Michel Devoret, and John M. Martinis cooled a junction below 50 mK. They demonstrated macroscopic quantum behavior using microwave pulses. They showed that energy was quantized at zero bias. This specific discovery led to the development of superconducting qubits. Clarke, Devoret, and Martinis were awarded the Nobel Prize in Physics in 2025 for this work.

The Josephson effect is vital for precision metrology. It provides a reproducible conversion between frequency and voltage. This allows for highly accurate measurements of electrical units. For example, the National Institute of Standards and Technology (NIST) uses a specific array to define the volt. This array consists of 20,208 Josephson junctions connected in series.

This creates a standard representation of one volt based on the caesium standard of frequency.

Practical applications are found in many advanced technologies. SQUIDs, or superconducting quantum interference devices, are very sensitive magnetometers. They operate using the Josephson effect to detect tiny magnetic fields. The effect is also used in RSFQ digital electronics. In these systems, junction switching emits a single magnetic flux quantum. The presence or absence of this flux represents binary 1 and 0. Furthermore, superconducting tunnel junction detectors are used in specialized cameras.

Finally, the effect is a cornerstone of modern quantum computing. Superconducting quantum computing uses Josephson junctions as nonlinear inductive elements. These are used in qubits like the transmon or flux qubit. In these systems, phase and charge act as conjugate variables. By controlling these variables, scientists can build complex quantum-mechanical circuits. This connects the tiny world of quantum mechanics to the large world of digital technology.