Introduction

Flux quantization

In superconductors, electrons condense into a quantum coherent state at low temperatures. The continuity of the macroscopic wavefunction of this condensate implies that a superconducting ring can only be threaded by a magnetic flux which is a multiple of the flux quantum, \(\Phi_0 = 2.068~\textrm{fWb}\).

The quantization of flux was predicted by Fritz London in 1948 and verified by Deaver, Fairbank, Doll, and Näubauer more than ten years later.

Josephson junctions

When two superconductors are separated by a sufficiently thin barrier, Cooper pairs can tunnel across in a quantum coherent manner. This effect was predicted by Brian Josephson in 1962, for which he later won the Nobel prize. The superconductor-insulator-superconductor sandwich in which it occurs is called a Josephson junction.

The foundation for most of the research conducted in the group is the AC Josephson effect, which states that a Josephson junction with a DC voltage bias \(V\) will emit photons of energy \(h\nu = 2eV\). The proportionality constant between (\nu\) and \(V\) is the inverse of the flux quantum and is called the Josephson constant, \(K_J = 2e/h = 483.6~\textrm{THz/V}\).