Quantum Computing Glossary

Summary

Short, pragmatic definitions for terms used across these notes.

Qubit: Quantum bit; two-level quantum system used as a unit of information.
Superposition: State that is a linear combination of basis states with complex amplitudes.
Entanglement: Non-classical correlations between subsystems; cannot be factored into product states.
Gate: Unitary operation acting on one or more qubits.
Circuit depth: Number of sequential gate layers; proxy for algorithm time.
Logical qubit: Encoded qubit protected by quantum error correction.
Physical qubit: Actual hardware element used to implement a qubit (or part of one).
Stabilizer: Operator whose +1 eigenspace defines a quantum code’s valid states.
Syndrome: Measurement outcome pattern that indicates which error likely occurred.
NISQ: Noisy Intermediate-Scale Quantum; pre-fault-tolerant devices with tens–thousands of qubits.
Fault tolerance: Regime where increasing code size can make logical error rates arbitrarily small.
Bosonic code: Quantum error-correcting code that encodes information into oscillator modes.
Cat qubit: Bosonic qubit using superpositions of coherent states; can exhibit strongly biased noise.
QKD: Quantum Key Distribution; uses quantum states to establish shared keys with eavesdropping detection.
PQC: Post-Quantum Cryptography; classical crypto designed to resist quantum attacks.