Qubit

Technical Explanation

Mathematically, a qubit's state is |ψ⟩ = α|0⟩ + β|1⟩, where α and β are complex amplitudes with |α|² + |β|² = 1. Measurement collapses the superposition, yielding 0 with probability |α|² or 1 with probability |β|². Before measurement, both possibilities exist simultaneously.

Physical implementations include superconducting circuits (IBM, Google), trapped ions (IonQ, Honeywell), photonic systems, and neutral atoms. Each technology has tradeoffs in coherence time, gate fidelity, and scalability. Logical qubits (error-corrected from many physical qubits) are needed for reliable computation. Google's Sycamore ran 53 active qubits in 2019; Willow reached 105 qubits with below-threshold error correction in December 2024; IBM's Condor hit 1,121 physical qubits in December 2023. The best public hardware in mid-2026 is around 2,500 physical qubits — and none of it is fault-tolerant at scale, so nobody has thousands of logical qubits yet.