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Qubit

A qubit (quantum bit) is the fundamental unit of quantum information, capable of existing in a superposition of the classical states 0 and 1.

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What it means

Unlike a classical bit that must be either 0 or 1, a qubit's state is a linear combination (superposition) of both basis states, described by the state vector |ψ⟩ = α|0⟩ + β|1⟩, where α and β are complex probability amplitudes satisfying |α|² + |β|² = 1.The amplitudes can interfere — adding up or cancelling — and quantum algorithms are built on arranging that interference; a single measurement, however, still yields only one classical bit.Physical implementations include superconducting circuits, trapped ions, photon polarization, and electron spin.Upon measurement, the qubit collapses to either |0⟩ or |1⟩ with probabilities |α|² and |β|² respectively.

Everyday analogy

A qubit is like the polarization of a single photon: light polarized at 45° is not secretly horizontal or vertical, it points in one definite direction between them. A 0°/90° filter still lets each photon through only one way, 50/50 — just as measuring a qubit gives only 0 or 1.
Think of a qubit as a point on a globe: the north pole is |0⟩, the south pole is |1⟩, and every other point on the surface represents a superposition of the two — far richer than the simple heads-or-tails of a classical bit.

Common misconceptions

  • A qubit is NOT simply 0 and 1 at the same time — it exists in a specific quantum state described by complex amplitudes that determine measurement probabilities.
  • A qubit does NOT store infinite information — upon measurement, you only get a single classical bit of information (0 or 1).
  • A qubit is NOT a spinning coin that is secretly heads or tails. A coin always has a definite hidden side, and classical probabilities can only add up; a qubit's amplitudes can cancel (interference), which no hidden-coin picture can reproduce.

Key takeaways

  • A qubit is described by two complex amplitudes α and β with the normalization constraint |α|² + |β|² = 1.
  • Measurement collapses the qubit to a definite classical state, yielding 0 with probability |α|² or 1 with probability |β|².
  • Qubits are the building blocks of all quantum computation and quantum information protocols.

Check your understanding

A qubit is in the state |ψ⟩ = (1/√2)|0⟩ + (1/√2)|1⟩. What is the probability of measuring |0⟩?

  1. A.0
  2. B.1/4
  3. C.1/2
  4. D.1
Show the answer

Answer: C. 1/2

Why: The probability of measuring |0⟩ is |α|² = |1/√2|² = 1/2 = 50%.

Builds toward

Basis StateBloch SphereBorn RuleDirac (Bra-Ket) NotationHadamard GateQuantum MeasurementPauli-X GatePauli-Y Gate+41 more

Primary source: Schumacher, Quantum coding, Phys. Rev. A 51, 2738 (1995), doi:10.1103/PhysRevA.51.2738

Term and formalism per Schumacher 1995; state-vector treatment per Nielsen & Chuang (2010), doi:10.1017/CBO9780511976667.

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