Qubit
A qubit (quantum bit) is the basic unit of quantum information. Unlike a classical bit (strictly 0 or 1), a qubit can exist in a superposition of both states simultaneously, enabling quantum computers to explore exponentially many states in parallel.
Properties
- Superposition: A qubit can represent 0, 1, or any linear combination until measured.
- Entanglement: Qubits can be correlated so that the state of one instantly determines the state of another.
- Decoherence: Qubits are fragile — temperature, electromagnetic noise, and vibration collapse quantum states into classical behaviour, introducing errors.
Hardware Implementations
| Type | Used by | Characteristics |
|---|---|---|
| Superconducting | google-quantum-ai (Willow), rigetti-computing | Fast; error-threshold crossed (Google Willow, 2024) |
| Trapped-ion | quantinuum, ionq | Slow but very high fidelity (99.99% for IonQ) |
| Neutral-atom | QuEra | Laser-trapped; flexible connectivity |
Accuracy: 2-Qubit Gate Fidelity
The standard accuracy metric is 2-qubit gate fidelity — accuracy after a computation passes through two quantum logic gates. ionq leads at 99.99%; rigetti-computing’s production system achieves 99% (drury-2026-rigetti-2-qubit-fidelity).
See quantum-error-correction, quantum-computing.
Sources: cottier-2026-quantum-computing-breakthroughs, drury-2026-rigetti-2-qubit-fidelity, babbush-neven-2026-quantum-vulnerabilities-cryptocurrency