Josephson’s prediction: phase could drive a zero-voltage current
What was new about a supercurrent through an insulating barrier?
Sandbox Physics ·
Nobel contribution: 1973-2
01
Couple two superconductors
02
Set a relative phase
03
Predict pair current
04
Test current, voltage and magnetic interference
A superconducting phase difference is a collective circuit variable. Its response requires more than a single-electron transmission probability.
The prediction concerned coherent pair transfer
Josephson predicted in 1962 that two weakly coupled superconductors could carry a phase-dependent supercurrent without a dc voltage. The electrodes possess collective phase coherence, so the coupling has consequences beyond quasiparticle tunneling.
Experiments soon tested zero-voltage current, magnetic interference and voltage–frequency relations. The 1973 award recognized the theoretical prediction as its own contribution.
In the simplest sinusoidal junction, phase sets the pair current. A voltage causes phase to advance in time. A steady voltage can consequently produce an oscillating supercurrent, with frequency set by the pair charge and Planck’s constant.
Two junctions in a superconducting loop constrain their phase differences through magnetic flux. Their currents combine constructively or destructively, producing a periodic flux response. Inductance, junction asymmetry and noise modify the ideal pattern.
The SQUID Lab examines loop interference, the switching Lab examines escape from a biased state, and the Transmon Lab examines bound energy levels. They share Josephson coupling but have different geometries, controls and evidence. None substitutes for all the others.