Tunneling of Cooper Pairs
In 1962, a 22-year-old Cambridge graduate student named Brian Josephson predicted that Cooper pairs could tunnel through a thin insulating barrier between two superconductors, maintaining phase coherence. This was remarkable — individual electrons tunnel easily, but bound pairs tunneling coherently was unexpected. The prediction was confirmed within a year and earned Josephson the 1973 Nobel Prize.
DC and AC Effects
The DC Josephson effect states that a supercurrent Is = Ic sin(φ) flows at zero voltage, where φ is the phase difference across the junction and Ic is the maximum (critical) current. When I exceeds Ic, the phase begins to evolve in time according to dφ/dt = 2eV/ℏ — the AC Josephson effect. This oscillating phase produces an alternating supercurrent at a frequency proportional to voltage, precisely 483.6 GHz per millivolt.
The SQUID
A DC SQUID places two Josephson junctions in a superconducting loop. Quantum interference between the two paths makes the critical current oscillate as a function of the magnetic flux threading the loop, with period Φ₀ = h/2e. By biasing near a steep part of this pattern, the SQUID converts tiny flux changes into large voltage signals. SQUIDs can detect magnetic fields of 10⁻¹⁵ T — sensitive enough to measure brain activity (magnetoencephalography).
Quantum Computing and Metrology
Josephson junctions are the building blocks of superconducting qubits — the leading quantum computing platform. The transmon qubit, used by IBM, Google, and others, is essentially a Josephson junction shunted by a capacitor, creating a nonlinear quantum oscillator. In metrology, Josephson junction arrays define the international voltage standard with parts-per-billion accuracy via the exact frequency-voltage relation.