Physics / mechanism
Superconducting qubits exploit Josephson junctions — tunnel junctions between two superconductors separated by a thin insulator — to create artificial two-level quantum systems. Below ~20 mK, Cooper-pair tunnelling produces nonlinear inductance, making energy levels anharmonic and addressable at microwave frequencies (4–8 GHz). Key qubit types: transmon (dominant; anharmonicity ~200–300 MHz), fluxonium (longer T1, up to ~1 ms demonstrated). Gate fidelities: single-qubit >99.9%, two-qubit ~99.5% (IBM Heron, Google Willow, 2024). Coherence times T1/T2 now routinely 100–500 µs on best devices. Scaling bottleneck is cryogenic wiring fan-out and materials-limited coherence (TLS defects at interfaces, substrate, and junction oxide).
Competitive landscape
The primary qubit modality competitors are trapped ions (higher fidelity, slower gates, room-temperature laser control), photonic qubits (room-temp, hard to make deterministic gates), neutral atoms (fast reconfigurable arrays, early-stage error correction), and spin qubits in silicon (CMOS-compatible, short coherence currently). Superconducting qubits lead on gate speed (~10–100 ns) and ecosystem maturity.
| Modality | Gate speed | Coherence | CMOS integration |
|---|---|---|---|
| Superconducting | ~10–100 ns | 100–500 µs | Partial (via fab) |
| Trapped ion | ~1–10 µs | Seconds | No |
| Spin (Si) | ~1–100 ns | ~1 ms | High potential |
Companies using
Connected ideas
Sources
Frontier (open questions)
- To be added.