Qubit

A qubit is a quantum system used to encode two-state information. Unlike a classical bit, it can occupy a superposition of its computational basis states.

Quantum superposition: A quantum state can combine multiple basis states with complex probability amplitudes. A qubit’s defining state can combine its two computational basis states.
Classical bit: A unit of classical information that takes one of two values, conventionally 0 or 1. A classical bit has a definite value, unlike a qubit before measurement.
Quantum computing: Computation that uses quantum states and operations to process information. Qubits are the information units manipulated by quantum computers.
Qudit: A quantum information unit whose state space has dimension greater than two. A qudit generalizes the two-level information unit represented by a qubit.
Quantum measurement: A physical interaction that produces an outcome governed by a quantum system’s state. Measuring a qubit yields a basis outcome, with probabilities set by its amplitudes.
Computational basis: The standard basis of quantum states used to represent binary values in quantum computation. Its two basis states provide the qubit’s conventional encodings of 0 and 1.
Quantum circuit: A model of quantum computation built from wires for quantum states and sequences of quantum gates. Circuit wires carry qubits through sequences of gates and measurements.
Quantum harmonic oscillator: A quantum system with equally spaced energy levels and an unbounded ladder of states. Oscillator-based hardware can encode qubits in selected states rather than a two-level system.
Bloch sphere: A geometric representation of pure states of a two-level quantum system as points on a sphere. It depicts every pure qubit state as a point on a sphere.
Quantum state: A mathematical description of a quantum system that determines probabilities for measurement outcomes. A qubit is a two-dimensional quantum system described by a quantum state.

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