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What is a qubit?
A qubit is the basic unit of information in a quantum computer: measured, it gives 0 or 1, but before measurement it can be in a superposition of both.
In a classical computer the smallest piece of information is the bit, which is always either 0 or 1. Its quantum counterpart is the qubit (“quantum bit”). A measured qubit also gives 0 or 1; the difference lies in its state before measurement.
Before measurement a qubit can be in a combination of the states |0⟩ and |1⟩, a superposition. This combination is described by two complex numbers (amplitudes) that determine how likely each result is.
Physically, a qubit can be any small system with two levels: IBM and Google use superconducting circuits, Quantinuum and IonQ trapped ions, QuEra and Pasqal neutral atoms, and Quandela photons.
Try it yourself
The lesson Bits and qubits: the smallest piece of information lets you try this concept in the browser, with no installation.
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Related concepts
- Superposition · Superposition is the quantum state in which a qubit, before it is measured, holds the states 0 and 1 together with definite amplitudes.
- Quantum measurement · Quantum measurement ends a qubit’s superposition and returns 0 or 1, with the result chosen at random according to the amplitudes.
- Quantum gate · A quantum gate is a basic operation that reversibly changes the state of one or more qubits; quantum programs are sequences of gates.