A Josephson junction is a weak link between two superconducting electrodes, typically a thin insulating barrier. It is the key nonlinear circuit element that enables superconducting qubits by creating the discrete energy levels needed for quantum computation.
Why it matters. Without the Josephson junction, a superconducting circuit would behave as a simple harmonic oscillator with equally spaced energy levels, making it impossible to isolate a two-level system for use as a qubit. The Josephson junction introduces anharmonicity: the energy spacing between levels becomes unequal, allowing the two lowest levels to be addressed as a qubit without exciting higher states. The critical current of the junction, the quality of its interfaces, and its fabrication reproducibility are among the most important factors determining qubit coherence and gate fidelity. Variations in Josephson junction parameters across a chip contribute to frequency disorder, which complicates scaling.
How it connects. The control electronics must be matched to the qubit frequencies and anharmonicities determined by the Josephson junction parameters. Qblox RF modules cover a wide frequency range (2 to 18.5 GHz) to accommodate the variation in qubit frequencies that arises from junction fabrication spread, and the QCM's flux control capabilities allow real-time tuning of junction-based qubits to their optimal operating points. Learn more about the QCM-RF II.
