Superconducting Qubits
Superconducting qubits are engineered nonlinear microwave circuits cooled to millikelvin temperatures, the leading solid-state modality.
Circuits as atoms
A superconducting qubit is an electrical circuit built from capacitors, inductors, and Josephson junctions, patterned on a chip and cooled below the superconducting transition. At millikelvin temperatures thermal noise is small compared with the microwave energy quantum, so the circuit behaves as a quantum oscillator with discrete energy levels.
A plain LC oscillator has equally spaced levels, so a drive that excites 0 to 1 also excites 1 to 2, and you cannot address a single transition. The Josephson junction supplies nonlinearity (anharmonicity), making the 0 to 1 gap differ from the 1 to 2 gap. That gap difference lets a shaped microwave pulse drive one transition while leaving the rest alone.
Families
- Charge qubits and the transmon, which suppress charge noise with a large shunt capacitor
- Flux qubits and fluxonium, biased by magnetic flux through a loop
- Phase qubits, an early current-biased design now largely historical
Why they lead
Superconducting circuits are made with lithography borrowed from the semiconductor industry, operate with commercial microwave electronics, and reach gate times in the tens of nanoseconds, far faster than trapped ions. Coherence times have improved from nanoseconds in the 1990s to hundreds of microseconds, a gain of roughly six orders of magnitude driven by materials cleanup and better geometries.
The costs are a dilution refrigerator, dense cryogenic wiring, and sensitivity to material defects called two-level systems that live in oxides and interfaces. Frequency crowding and crosstalk grow as chips scale. Despite this, superconducting processors have hosted some of the largest publicly benchmarked quantum devices.