Quantum computing glossary: 30 key terms explained in plain English

[BadBunny]

Quantum computing news is full of jargon, and most articles assume you already know what the words mean. This thread is a plain English glossary of the terms that come up most often on this forum and in the press. Each definition is kept short and as accurate as possible without the maths. Where a term is often misused, the definition says so

The basics

1. Qubit: the basic unit of quantum information. Like a bit, it gives 0 or 1 when measured, but before measurement its state can be a combination of both
2. Superposition: a qubit state that combines 0 and 1 with particular amplitudes. It is not the same as the qubit secretly being one or the other
3. Amplitude: a number attached to each possible outcome. Amplitudes can be positive, negative or complex, which is what makes interference possible
4. Measurement: reading a qubit. It gives a definite 0 or 1, with probabilities set by the amplitudes, and destroys the superposition
5. Entanglement: a link between qubits where their measurement results are correlated in ways no classical system can match. It does not allow faster than light messaging
6. Interference: amplitudes adding up or cancelling out. Quantum algorithms work by arranging interference so wrong answers cancel and right answers are boosted
7. Quantum gate: an operation on one or more qubits, the quantum version of a logic gate
8. Quantum circuit: a sequence of gates followed by measurements. Circuit depth is roughly how many steps it has
9. Bloch sphere: a common picture of a single qubit's state as a point on a sphere. Handy for one qubit, useless for many

Hardware and errors

10. Decoherence: a qubit losing its quantum state through unwanted contact with its surroundings, such as heat, vibration or stray fields
11. Coherence time: how long a qubit keeps its state before decoherence ruins it
12. Fidelity: how close an operation comes to the ideal. A two qubit gate fidelity of 99.9 percent means roughly one error in a thousand operations
13. NISQ: noisy intermediate scale quantum, a term coined by John Preskill in 2018 for today's error prone machines with tens to hundreds of qubits
14. Physical qubit: an individual hardware qubit, such as a superconducting circuit or a trapped ion
15. Logical qubit: a more reliable qubit built from many physical qubits using error correction. Logical qubit counts matter far more than physical ones for useful work
16. Quantum error correction: spreading information across many physical qubits so errors can be detected and fixed without destroying the computation
17. Error detection: spotting that an error happened without being able to fix it. Some headline results use detection rather than full correction, which is worth checking
18. Threshold: the physical error rate below which adding more qubits to an error correcting code makes logical errors smaller rather than larger
19. Surface code: a widely studied error correcting code that arranges qubits in a grid. Google's below threshold results in 2024 used it
20. Fault tolerance: a machine that keeps errors under control throughout a long computation. This is the goal of most company roadmaps
21. Magic states and T gates: an ingredient needed for fully universal error corrected computing. They are expensive to produce, so T gate counts are often quoted

Types of machine

22. Gate based quantum computer: a general purpose machine that runs circuits of gates. IBM, Google, Quantinuum and most others build these
23. Quantum annealer: a machine designed for optimisation problems, such as those built by D-Wave. Its qubit counts are not comparable with gate based machines
24. Qubit modalities: the physical technology used. The main ones are superconducting circuits, trapped ions, neutral atoms, photons, silicon spins and the topological approach Microsoft is pursuing
25. Cat qubit: a superconducting qubit design, used by Alice and Bob and Amazon, that resists one type of error by design

Algorithms and security

26. Quantum advantage: a quantum computer doing a task faster or better than the best classical method. Claims are often challenged by improved classical algorithms
27. Shor's algorithm: a quantum algorithm from 1994 that can factor large numbers and solve related problems efficiently. A large enough fault tolerant machine running it would break RSA and elliptic curve cryptography
28. Grover's algorithm: a quantum search method with a quadratic speedup. It weakens symmetric encryption like AES, which is why AES-256 is recommended, but it does not break it
29. Q-Day: the day a quantum computer can break today's public key encryption. Nobody knows when, or whether it will be announced
30. Post quantum cryptography: classical encryption designed to resist quantum attacks. NIST published the first standards, ML-KEM, ML-DSA and SLH-DSA, in August 2024. It is different from quantum key distribution, which uses quantum physics to share keys over special links

If you spot a mistake or want a term added, reply below and the list will be updated. The goal is a reference page that members can link to whenever a news story throws a new word at them

Last updated: 4 October 2026

Sources and further reading:
https://csrc.nist.gov/Projects/Post-Quantum-Cryptography
https://www.nist.gov/news-events/news/2024/08/nist-releases-first-3-finalized-post-quantum-encryption-standards
https://quantum.cloud.ibm.com/learning
https://quantum.country
https://www.ncsc.gov.uk/guidance/pqc-migration-timelines

[Karen73]

Useful list, thanks. Could you add qudit, for systems that use more than two levels instead of just 0 and 1? A few companies are starting to mention them

[ShadowKernel]

Number 17 deserves a bold font. A lot of logical qubit headlines this year have quietly relied on error detection, and the difference matters. Detection tells you a run went wrong so you can throw it away, while correction lets the computation carry on

People comparing logical qubit counts across companies should read the small print first. Otherwise it is apples and oranges