A Plain-English Glossary of Quantum Terms

Quantum technology comes with a lot of jargon. Bookmark this page and come back whenever you meet an unfamiliar term.

Quantum basics

Qubit: The basic unit of quantum information. Like a bit, it gives 0 or 1 when measured, but before that it can be in a blend of both.
Superposition: A qubit’s ability to be in a combination of 0 and 1 at the same time.
Entanglement: A link between qubits so that their measurement results are connected, even when they are far apart.
Interference: The way quantum possibilities can reinforce or cancel each other. Quantum algorithms use it to boost right answers.
Measurement: Reading a qubit. It ends the superposition and gives a plain 0 or 1.
Decoherence: The loss of a qubit’s quantum state due to disturbance from its surroundings.

Physical qubit: A real hardware qubit, such as a superconducting circuit or a trapped atom. Physical qubits make frequent errors.
Logical qubit: A reliable qubit made by combining many physical qubits with error correction.
Quantum error correction: Techniques that detect and fix qubit errors without destroying the information.
Fidelity: How accurately a quantum operation works, often given as a percentage. Higher is better.
Quantum advantage: When a quantum computer does a task that ordinary computers can’t do in a reasonable time.
Quantum annealer: A special-purpose quantum machine designed for optimisation problems.

Public-key cryptography: Encryption that uses a public key and a private key, such as RSA and elliptic curves. Vulnerable to large quantum computers.
Symmetric encryption: Encryption where both sides share the same key, such as AES. Not seriously threatened by quantum computers.
Shor’s algorithm: A quantum algorithm from 1994 that could break RSA and elliptic-curve cryptography.
Grover’s algorithm: A quantum search algorithm from 1996 that gives a square-root speed-up, handled by using longer symmetric keys.
Harvest now, decrypt later: Recording encrypted data today in order to decrypt it once quantum computers are powerful enough.
Post-quantum cryptography (PQC): New encryption methods, run on ordinary computers, designed to resist quantum attacks.
Hybrid key exchange: Using a classical and a post-quantum algorithm together, so both must be broken.
Quantum key distribution (QKD): Sharing encryption keys using single photons, so eavesdropping can be detected.
Quantum random number generator (QRNG): A device that creates random numbers from unpredictable quantum events.
Crypto-agility: The ability to switch encryption algorithms quickly, without rebuilding systems.
Cryptography Bill of Materials (CBOM): A standard, machine-readable list of where an organisation uses cryptography.

NIST: The US National Institute of Standards and Technology, which ran the public competition for post-quantum algorithms.
FIPS 203 (ML-KEM): The NIST standard for post-quantum key exchange, published in August 2024. Formerly called Kyber.
FIPS 204 (ML-DSA): The main NIST standard for post-quantum digital signatures, published in August 2024. Formerly called Dilithium.
FIPS 205 (SLH-DSA): A back-up NIST signature standard based only on hash functions. Formerly called SPHINCS+.
HQC: A key-exchange algorithm selected by NIST in March 2025 as a back-up to ML-KEM, based on different maths.
CNSA 2.0: The NSA’s set of approved algorithms and deadlines for US national security systems.
CRQC: A “cryptographically relevant quantum computer”: one powerful enough to break today’s public-key encryption.
Q-Day: The informal name for the day a CRQC exists. Its date is unknown.

check_circlePhysical qubits are noisy; logical qubits are reliable ones built with error correction.
check_circlePQC runs on ordinary computers; QKD and QRNG need quantum hardware.
check_circleML-KEM, ML-DSA and SLH-DSA are the first NIST post-quantum standards.