Josephson junction
/ JOH-zef-sun JUNK-shun /
Two ponds, each calm and full, are separated by a thin earthen wall. You might expect nothing to cross such a barrier — but in the quantum world the water can seep through the wall even without going over it, a ghostly leak called tunnelling. If both ponds are superconductors, something stranger still happens: a current of paired electrons flows across the gap entirely on its own, with no voltage pushing it.
A Josephson junction is exactly this arrangement: two superconductors separated by a barrier so thin that Cooper pairs tunnel across it coherently, keeping their shared quantum rhythm. Brian Josephson predicted in 1962 that a supercurrent would flow across such a gap with zero voltage, its size governed by the difference in quantum phase between the two sides. Apply a steady voltage instead and the current oscillates at a frequency set purely by that voltage — a precise bridge between voltage and frequency.
This matters because the junction turns the abstract quantum phase of a superconductor into something you can measure and control. It underpins SQUIDs, the most sensitive magnetic detectors ever built; it defines the international standard of the volt; and it forms the basic switch in many superconducting quantum computers. A common misunderstanding is to picture the pairs being pushed across by a voltage — in the headline Josephson effect they cross with no voltage at all, driven only by the phase difference.
A SQUID is a small superconducting ring containing two Josephson junctions; it can sense magnetic fields a hundred billion times weaker than Earth's, enough to map the faint currents of a beating human heart from outside the body.
A SQUID built from Josephson junctions is the most sensitive magnetometer ever made.
The relation between applied voltage and oscillation frequency depends only on fundamental constants, so a Josephson junction reproduces exactly the same voltage anywhere in the universe — which is why it now defines the volt itself.