The Josephson Effect and Josephson Junction
The British physicist Brian David Josephson postulated the existence of a current which could traverse two superconductors separated by an insulator of miniscule dimensions in 1962. His prediction has allowed the development of quantum mechanical devices such as the SQUID (superconducting quantum interference device) which is currently the most sensitive detector in existence. SQUIDS can measure fields as low as 5 T (5×10^−18 T). The Josephson junction is currently used to measure a “volt” as defined by the NIST standard.
The Josephson effect and equations can be related to incoming current, magnetic flux quantum, and the inverse of which is the Josephson constant. These principles are derived from the quantum mechanical flow of a single electron from each of the two superconductors. The electron is approximated as a wave function with time dependence.
The Josephson Junction can be broken down and analyzed as three separate effects. First, the DC Josephson effect. This involves direct current, which can pass through the insulator without the application of an external magnetic field. This quantum mechanical mechanism is called electron tunneling. The DC current transfer involves zero loss and is critical to ultra-efficient devices. The second effect is the AC Josephson effect, where the phase varies linearly as a function of time with amplitude and frequency. The last effect is called the inverse AC Josephson effect where DC and frequency can be converted perfectly.
The main function of the Josephson Junction can be found in single electron transistors which have gained recent publicity in their function of quantum supercomputers. The electron is allowed to pass through the super conductor-insulator-superconductor barrier but it may only do so by possessing specific levels of energy (quanta). These principles are governed by the quantum number (n) where it ranges from 1 to 4 generally. When the electron makes the jump, an electromagnetic signal is produced with a specific value. This is similar to current computers which make use of binary code, 1 and 0 values. In the case of the Josephson Junction the values have a wider range but each with a specific value. This allows for a significantly faster computational process to take place.
Josephson Junctions have become more popular lately due to the advances made in Type II superconductors which have operational temperatures greater than that of liquid nitrogen. This operational temperature is significantly easier to obtain than that of liquid helium which has a temperature near absolute zero.
The Josephson Junction has many successful applications and it has a bright future as material properties become more well controlled and processing techniques become better.
