What is a Josephson Junction?
A josephson junction, physically, consists of a very thin layer of insulating material (of the order of a few nanometers) separating two superconductors. It exhibits quantum mechanical behaviour at a macroscopic scale, and thus finds application in many quantum technologies.
How does it work?
A josephson junction's operation is explained by the Josephson effect.
-Phase:
In quantum mechanics, we can describe complex valued wavefunctions using their magnitude rho (usually, this value squared corresponds to the probability density at that point in space) and a relative phase factor e^i.phi, where phi is the phase. When we perform a measurement on a quantum system, the phase usually does not play a role indeciding the probability since it's complex magnitude is unity; the role of the phase is to control how the quantum wavefunction interacts with other quantum wavefunctions to produce interference effects.
-Superconductors
Superconductors are special materials through which current can flow even in the absence of any voltage difference. To explain superconductivity, physicists came up with BCS theory and Landau-Ginzburg theory to explain this phenomenon quantum mechanically. Experimental findings lead to the discovery of phenomenological equations, which form the core of the working of a josephson junction.
-BCS Theory
At very low temperatures (close to 0 Kelvin) in some crystals, electrons can overcome their electrostatic repulsion and become bound together to form a special quantum state called a cooper pair. This happens because the movement of one electron in a crystal lattice attracts the positively charged lattice points (ions), causing lattice vibrations which behave as a quantum state - called a "phonon".
The phonon vibrations respond more slowly than electron motion, the distortion created by one electron can attract another electron before the distortion relaxes, causing the two electrons to become a entangled into a bound quantum state called a cooper pair. This phenomenon forms the basis of superconductivity. In a superconductive material, millions of cooper pairs condense into a single coherent quantum state and move through a material with zero electrical resistance. This collective state is protected by an energy gap. The thermal vibrations that would normally disrupt electron flow are too weak to break the pairs apart, resulting in zero electrical resistance
-What happens in a Josephson Junction?
We describe each of the two superconductors by a macroscopic wavefunction. The difference in phases of the two superconductors determines the flow of Cooper pairs across the insulating barrier, in a phenomenon known as quantum tunnelling, where a quantum system can cross an obstruction (represented by a large potential barrier).
Now, if we apply a voltage difference across the two superconductors, we get a sinusoidally oscillating supercurrent with frequency proportional to the applied voltage; i.e, we get an AC current with the application of a DC voltage! This contradicts all classical electrical system behaviour and is used in SQUIDs (Superconducting quantum interference devices).
-How do SQUIDs work?
A SQUID consists of a superconducting loop interrupted by Josephson junctions. Magnetic flux threading the loop changes the quantum phase of the superconducting wavefunction around the loop. Because the Josephson current depends on phase difference, the junction currents interfere quantum mechanically. This makes the total current extremely sensitive to tiny magnetic fields. This concept has deep implications for the future of quantum sensing.