Josephson Junction
Definition
Two superconductors separated by a whisper-thin insulator — quantum tunneling lets current flow with no voltage, the heart of quantum computers.
Technical definition
Cooper pairs tunnel through the barrier with current I = I_c sin(φ). The junction acts as a nonlinear inductor — the building block of superconducting qubits.
Example
IBM's and Google's quantum processors use Josephson junctions as qubits, chilled to about 0.01 K — colder than deep space.
Common confusion
A Josephson junction is not just a tiny wire. Its quantum behavior — discrete energy levels from tunneling — is what makes qubits possible; an ordinary resistor can't do that.