A Josephson junction is a thin insulating barrier between two superconductors. Cooper pairs can tunnel through the barrier, producing a current with no applied voltage up to a critical current.
Why It Matters
Ordinary capacitors and inductors make harmonic oscillators with evenly spaced energy levels. A qubit needs two addressable levels separated from the rest. The Josephson junction supplies the nonlinearity that makes this possible.
In Circuit Language
The junction behaves like a nonlinear inductor whose energy depends on superconducting phase difference. This nonlinearity lets circuits such as transmons behave like artificial atoms.
Design Sensitivities
- Junction area affects critical current and Josephson energy.
- Oxide quality can introduce loss and noise.
- Fabrication variation changes device frequencies across a chip.