A Josephson junction consists of two superconducting electrodes separated by a very thin insulating barrier. Although the barrier normally blocks electrons, Cooper pairs (the paired electrons responsible for superconductivity) can quantum‑tunnel through it without resistance. This tunneling gives rise to two hallmark effects:
- DC Josephson Effect
Even with zero applied voltage, a supercurrent (a current of Cooper pairs) can flow across the junction. The maximum current that can pass without developing a voltage—the critical current—depends on the junction’s properties and the phase difference of the superconducting wavefunctions on either side. - AC Josephson Effect
When a constant voltage is applied across the junction, the supercurrent oscillates at a frequency directly proportional to that voltage. These microwave‑frequency oscillations enable precise voltage–frequency conversion and form the basis of superconducting qubit control and readout.
In superconducting quantum circuits—the leading platform for many quantum computers—Josephson junctions provide:
- Nonlinear inductance: Unlike ordinary inductors, their energy–current relationship is inherently nonlinear, allowing the creation of discrete energy levels in circuit “artificial atoms” (qubits).
- Tunable coupling: By embedding junctions in loops (SQUIDs), one can adjust their effective inductance—and thus qubit frequencies or interactions—using external magnetic flux.
- Ultralow dissipation: Operating at millikelvin temperatures, they introduce virtually no energy loss, preserving fragile quantum states.
Together, these properties make Josephson junctions the building blocks of superconducting qubits, parametric amplifiers, and other critical elements in today’s most advanced quantum processors.