Quantum glossary

An amplitude is the number a quantum state assigns to each possible outcome; its size squared gives the probability of that outcome.

Amplitudes can be positive, negative, or complex. Because they can cancel as well as add, quantum systems show interference — the reason quantum algorithms can suppress wrong answers and reinforce right ones.

Lesson: Measurement and ProbabilityTopic: Quantum Information

The Bloch sphere is a picture of every possible state of a single qubit as a point on the surface of a sphere.

The north and south poles represent the definite outcomes 0 and 1. Every other point is a valid combination. Single-qubit gates correspond to rotations of the sphere. The picture works only for one qubit; it cannot show entanglement.

Lesson: What Is a Qubit?Topic: Qubits

A classical bit is a unit of information that is either 0 or 1 at any moment.

Ordinary computers store bits as voltages, magnetic domains, or pits on a disc. Reading a bit does not change it, and copying it is trivial — two things that are not generally true of quantum information.

Lesson: Classical vs. Quantum ComputingTopic: Quantum Information

Coherence is the property that lets a quantum system maintain well-defined relationships between its amplitudes, making interference possible.

Coherence times measure how long a qubit keeps that structure before the environment scrambles it. Longer coherence means more operations can be performed before the information degrades.

Topic: Quantum Hardware

Decoherence is the loss of quantum behaviour that happens when a system becomes correlated with its environment.

Stray heat, vibration, and electromagnetic noise all leak information out of a qubit. Decoherence is the central engineering obstacle in quantum hardware and the reason error correction is needed.

Lesson: Why Is Quantum Error Correction Necessary?Topic: Quantum Error Correction

Entanglement is a link between quantum systems such that the whole system has a definite description while its individual parts do not.

Measurements on entangled systems show correlations stronger than any classical shared-instruction model allows. The correlations appear only when results are compared over an ordinary channel, so entanglement alone cannot transmit a message.

Lesson: Quantum EntanglementTopic: Entanglement

Interference is the adding and cancelling of quantum amplitudes that steers a system toward some outcomes and away from others.

Quantum algorithms are largely exercises in arranging interference: paths leading to incorrect answers are made to cancel, while paths leading to the answer reinforce.

Lesson: How Does Quantum Computing Work?Topic: Quantum Algorithms

Measurement is the process that extracts a definite classical outcome from a quantum system, with probabilities set by its amplitudes.

Measurement generally disturbs the state: after it, the system is described by the outcome observed. This is why quantum results are gathered statistically over many repeated runs.

Lesson: Measurement and ProbabilityTopic: Quantum Measurement

A quantum circuit is a sequence of quantum gates applied to qubits, usually ending in measurement.

Circuits are the standard way to express quantum programs. Each horizontal line is a qubit and each symbol is an operation applied in time order.

Lesson: Quantum Gates and CircuitsTopic: Quantum Gates and Circuits

A quantum gate is a reversible operation that transforms the state of one or more qubits.

Common examples include the Hadamard gate, which creates an even combination of 0 and 1, and the CNOT gate, which can create entanglement between two qubits. A small set of gates is enough to approximate any quantum computation.

Lesson: Quantum Gates and CircuitsTopic: Quantum Gates and Circuits

Quantum information is information represented and processed using quantum systems, whose basic unit is the qubit.

The field studies how quantum properties such as superposition, interference, and entanglement change what can be computed, communicated, measured, and kept secret.

Lesson: What Is Quantum Information?Topic: Quantum Information

A quantum state is the complete description of a quantum system, giving the amplitude of every possible measurement outcome.

States can be written as vectors. Operations rotate them; measurement collapses them to an outcome. Two different states can give the same result for one measurement but differ for another.

Topic: Quantum Information

Quantum teleportation is a protocol that transfers a quantum state from one location to another using shared entanglement and two classical bits.

Nothing material travels and nothing outruns light: the classical message is required to complete the protocol. The original state is destroyed in the process, consistent with the no-cloning theorem.

Lesson: What Is Quantum Teleportation?Topic: Quantum Teleportation

A qubit is the basic unit of quantum information: a two-level quantum system that can hold a combination of the outcomes 0 and 1.

Qubits are built from superconducting circuits, trapped ions, neutral atoms, photons, and other physical systems. Whatever the hardware, the mathematics of a qubit is the same.

Lesson: What Is a Qubit?Topic: Qubits

Superposition is a quantum state formed by combining other states, with an amplitude attached to each.

A qubit in superposition is not secretly 0 or 1, and it is not in two places at once. It is a single state whose amplitudes determine the probabilities of each outcome and allow interference.

Lesson: Superposition ExplainedTopic: Superposition