The Josephson Junction
A thin insulating barrier between two superconductors carries a lossless supercurrent set by phase difference, giving superconducting qubits their nonlinearity.
The physics
A Josephson junction is two superconductors separated by a barrier a nanometer or two thick, usually aluminum with an aluminum-oxide layer. Cooper pairs tunnel across coherently. The two Josephson relations govern it: the supercurrent I = I_c sin(delta), where delta is the phase difference across the junction and I_c the critical current; and the voltage V = (hbar/2e) d(delta)/dt.
Combining these, the junction acts as a nonlinear inductor whose inductance depends on the current flowing through it. It is this nonlinearity, not available from ordinary capacitors or inductors, that spaces a circuit's energy levels unequally and turns an oscillator into an addressable qubit.
Energy scale
The Josephson energy E_J = (hbar I_c)/(2e) sets the depth of the potential. Together with the charging energy E_C = e^2/(2C) of the shunt capacitance, it fixes both the qubit frequency and its anharmonicity. Fabricating junctions with reproducible I_c is central to yielding many qubits at target frequencies.
SQUIDs
Two junctions in a loop form a Superconducting Quantum Interference Device. Magnetic flux through the loop tunes the effective critical current and thus the qubit frequency, enabling flux-tunable transmons and flux-activated two-qubit gates. The price is added sensitivity to flux noise.
Fabrication
- Dolan-bridge or Manhattan double-angle evaporation of aluminum
- Controlled thermal oxidation to set barrier thickness and I_c
- Junction aging and drift that shift frequencies after fabrication
Junction quality, oxide cleanliness, and reproducibility are among the strongest levers on qubit coherence and yield, making the junction the heart of the whole modality.