Quantum Glossary
Plain-language definitions across quantum science, computing, psychology, consciousness and AI. Search as you type.
Qubit
The basic unit of quantum information: any two-level quantum system (electron spin, photon polarization, superconducting circuit) that can hold superpositions of 0 and 1.
Superposition
A quantum state expressed as a weighted combination of basis states. The weights (amplitudes) determine measurement probabilities. It does not mean 'being in two places at once' in any classical sense.
Entanglement
A relationship between quantum systems whose joint state can't be described by describing each system separately. Produces correlations no classical model can match; cannot transmit information faster than light.
Measurement
A physical interaction that yields a definite outcome from a quantum state, with probabilities given by the Born rule. 'Observation' means interaction with apparatus — not human awareness.
Decoherence
Loss of quantum behavior through entanglement with the environment. Explains why everyday objects look classical, and why quantum computers need extreme isolation.
Interference
Quantum amplitudes can add or cancel like waves. Algorithms are choreographed so wrong answers cancel and right answers reinforce.
Quantum gate
A reversible operation on qubits — the quantum analogue of a logic gate. Common gates: Hadamard, Pauli-X, CNOT, phase gates.
Quantum circuit
A sequence of gates applied to qubits, ending in measurement. The standard model for describing quantum algorithms.
Quantum error correction
Encoding one logical qubit across many physical qubits so errors can be detected and corrected without destroying the quantum information.
Logical qubit
An error-corrected qubit built from many physical qubits; the building block of fault-tolerant quantum computers.
Quantum advantage
A demonstration that a quantum device performs a specific task beyond the practical reach of classical computers. Task-specific — not general superiority.
Shor's algorithm
A quantum algorithm that factors large numbers efficiently — the reason future large-scale quantum computers threaten today's public-key encryption.
Grover's algorithm
A quantum search algorithm giving a quadratic speedup for unstructured search problems.
Bloch sphere
A geometric picture of a single qubit's state as a point on a sphere, with |0⟩ and |1⟩ at the poles.
Wave function
The mathematical object encoding everything knowable about a quantum system's state.
Heisenberg uncertainty
A structural limit: certain pairs of properties (position/momentum) cannot both be sharply defined. Not a statement about clumsy instruments.
Quantum field theory
The framework combining quantum mechanics and special relativity; particles are excitations of underlying fields.
NISQ
'Noisy Intermediate-Scale Quantum' — today's era of quantum devices: useful for research, too noisy for most practical applications.
Quantum machine learning
Research into using quantum computers for ML tasks. Promising directions; no broad proven advantage yet.
Neuroplasticity
The brain's capacity to reorganize connections through experience and practice — the biological basis of learning and habit change.
Cognitive bias
A systematic, predictable deviation from rational judgment — e.g., confirmation bias, anchoring, loss aversion.
Implementation intention
An 'if situation X, then I will do Y' plan — one of the best-supported techniques in behavior-change research.
Metacognition
Thinking about your own thinking: monitoring, evaluating and steering your mental processes.
The hard problem
David Chalmers' term for the question of why physical processes are accompanied by subjective experience at all.
Global workspace theory
A leading scientific model proposing consciousness arises when information is broadcast widely across brain networks.
Large language model (LLM)
An AI system trained on text to predict tokens, capable of fluent generation and reasoning-like behavior — and of confident errors.
Hallucination (AI)
Plausible-sounding but false AI output, such as invented citations or facts. Always verify high-stakes answers.
Alignment
The field of making AI systems reliably pursue intended goals and human values as capability grows.
Brain–computer interface
A system creating a direct communication path between neural activity and external devices; today mostly clinical and experimental.
Digital twin
A computational model of a person, object or system used for simulation. Applied to humans, raises major privacy and accuracy questions.