Qubit

A qubit (quantum bit) is the basic unit of quantum information, a two-level quantum system that generalizes the classical bit. In superposition, a qubit occupies a weighted combination of the states 0 and 1 until it is measured, when it yields one classical bit. Through entanglement, several qubits can be correlated so strongly that none of them has an independent description.

#Physical qubits and error rates

Hardware realizes qubits in superconducting circuits, trapped ions, neutral atoms, and photons, among other platforms. All are noisy: as of early 2026, two-qubit gate error rates on leading devices are around 1 in 1,000, the regime John Preskill labeled NISQ (noisy intermediate-scale quantum). Long computations such as Shor's algorithm need error rates many orders of magnitude lower than any physical qubit achieves, so cryptographically relevant work requires combining many physical qubits into an error-corrected logical qubit. Google reported the first error-corrected memory operating below the surface code threshold in 2024 (Willow result), a milestone on the path to a fault-tolerant quantum computer.