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Glossary

Quantum glossary

Qubit

Also: quantum bit

The basic unit of quantum information: a two-level system whose state is a normalised combination of |0⟩ and |1⟩.

A qubit is better pictured as a direction than a switch. It holds a continuum of possible states but yields just one classical bit when measured — so n qubits do not store 2ⁿ readable numbers.

|ψ⟩ = α|0⟩ + β|1⟩, |α|² + |β|² = 1

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Related terms

  • Bloch sphere — A sphere on which every pure state of a single qubit is one point, and every single-qubit gate is a rotation.
  • Superposition — A quantum state that is a weighted combination of basis states, with complex amplitudes.
  • Amplitude — The complex number attached to each possible outcome of a quantum state; its squared magnitude is that outcome’s probability.