Normally, electric current is the same in both directions, like water in a straight pipe. But at the boundary between a normal metal and a superconductor, this rule breaks down. Two waves on water, going around a wide obstacle from different sides, can arrive with different delays and meet ‘out of step’. Electrons and their mirror twins — holes — behave the same way inside the contact. Their waves overlap with a shift, and the current in one direction becomes stronger than in the other.
The effect resembles an interferometer: the slightest change in path alters the overall picture. Surprisingly, the asymmetry becomes more pronounced exactly where there is more disorder (entropy) in the material.
By measuring the current difference, we can peer into the depths of a superconductor, even when other signals are weak. The method works like a hypersensitive spectroscope, picking up echoes of the energy gap, crucial for the qubits of future quantum computers.
Thus, the simple geometry of the contact turns a superconductor into a ‘wave diode,’ adding a new facet to Bardeen’s theory and paving the way for reliable elements in quantum circuits.
Holograms on credit cards are created by the same principle of wave interference — only here, electron beams play the role of light.