Josephson Junction ------------------ The Joesphson junction is named after the 1973 physics Nobel Prize winner Brian Josephson who carried out extensive theoretical work on its properties. The junction itself consists of two superconducting metals separated by a thin insulating oxide layer usually only 10 to 20 angstroms, or equivalently about 30 atomic layers, in width. The remarkable properties of the junction are due to a superconducting current that can flow through the oxide layer even at zero voltage difference between the outer plates. The origin of this current depends crucially on the quantum mechanical principles underlying superconductivity. In a superconducting metal, electrons exert forces on the crystal lattice of the metal. This force displaces positive ions in the lattice and sets up a polarization which can in turn exert attractive forces on the original electrons. This net force is ordinarily much weaker than the usual Coulomb repulsion between electrons, but this Coulomb force is also suppressed by screening effects of positive ions in the lattice. At very low temperatures, in the absence of thermal fluctuations, the attractive force overcomes the repulsive force and electrons can actually bind with each other and form pairs called Cooper pairs. In these Cooper pairs, the intrinsic spin of each electron cancels the other's spin and so the Cooper pairs all have total spin zero. The Pauli exclusion principle which applies to each individual electron then no longer applies to the Cooper pair as a whole, and so at very low temperatures all the Cooper pairs settle down to the same macroscopic quantum state. So in a Joesphson junction, at low temperatures, on each side of the oxide layer there are two macroscopic quantum states each described by a wavefunction. However the wavefunction need not be continuous across the junction. In particular the magnitude of the wavefunction can be the same, signifying equal charge densities of electrons on either side and thus no voltage difference across the junction, but the phase of the wavefunction may be different. Upon solving Schroedinger's equation, which governs time dependence in quantum mechanics, one finds that a current flows across the junction whose magnitude is proportional to the sine of the phase difference. This current is the famous Josephson effect and it requires no voltage difference to maintain itself. The Josephson effect forms the basis for several extremely successful low temperature technologies that are in use today. One of their first applications was as extremely sensitive voltmeters. When a voltage is applied to a Josephson junction, the phase difference of the wavefunctions across the junction changes over time and this leads to an AC as opposed to DC Josephson current. When an alternating voltage is applied, for example using microwave radiation, this AC current rises in steps as the voltage is increased and the length of each step in voltage is proportional to the frequency of the Joesphson current. Since the frequency of the Josephson current can be measured extremely accurately, one can measure the voltages extremely accurately as well, often to the level of picovolts. Josephson junctions are also crucial in the operation of superconducting quantum interference devices (SQUIDs) which are basically superconducting loops containing one or more Joesphson junctions inside. When these SQUIDS are coupled to a superconducting flux transformer, they can measure small spatial fluctuations in the magnetic field piercing the current loop. One of the far ranging applications of this phenomenon includes the study of biomagnetism, especially the measurement of magnetic fields in the human brain. SQUIDS are also used in geophysical surveying, nondestructive testing, and nuclear magnetic resonance. Perhaps the most promising applications of Josephson junctions are as switching elements in digital circuits. At low temperatures, the zero voltage Josephson current can be increased up to a certain point until the junction suddenly switches to a voltage carrying state. This switching occurs on the order of picoseconds, much faster than in conventional semiconductor circuit switching elements, and moreover this switching occurs at very little power loss. If Josephson junctions could be used as logic and memory gates then these gates could be placed much closer to each other than usual because of the negligible heat dissipation involved in switching states. Allowing gates to be closer to each other minimizes the amount of time it takes electromagnetic signals to propagate between them and results in a much faster chip. However progress in this direction requires the development of high temperature superconductivity. Currently such gates have been operated at 77 Kelvin and efforts are underway to improve upon this result. Clearly, the story of the full technological promise of Josephson junctions has yet to be told.