FREE REFERENCE

Quantum Computing Glossary

Plain-English definitions for the terms you will meet across the three free courses. Linked from inline tooltips in every chapter.

A B C D E F G H L M N O P Q S V

A

Amplitude estimation algorithm: A quantum primitive that estimates the probability amplitude of a state with a quadratic speedup over classical sampling. Useful for Monte-Carlo-style pricing and risk.
Ansatz algorithm: A parameterised quantum circuit used in variational algorithms. A guess at the right shape of the answer, tuned by a classical optimizer.

Bell state concept: One of four specific maximally entangled two-qubit states. The standard recipe is a Hadamard on the first qubit followed by a CNOT with the first as control.
Bloch sphere concept: A 3D visualization of a single qubit's state. The north pole is 0, the south pole is 1, and every other surface point is a superposition.
Born rule concept: The rule that converts a qubit's amplitudes into measurement probabilities. The probability of an outcome is the square of its amplitude's magnitude.

CNOT gate gate: A two-qubit controlled-NOT. Flips the target qubit if and only if the control qubit is in state 1. The workhorse of entangling operations.

Decoherence concept: The process by which a qubit loses its quantum character through interaction with its environment. Sets a hard ceiling on how long a computation can run.

Entanglement concept: A correlation between two or more qubits that cannot be described by their individual states. The whole carries information the parts do not.
Error correction concept: Encoding logical information across many physical qubits so that local errors can be detected and reversed without measuring the protected state directly.

Fault-tolerant quantum computing era: An era beyond NISQ where logical qubits (made of many physical qubits with active error correction) run long algorithms reliably.

Gate fidelity metric: The probability that a quantum gate did what it was supposed to do. 99.9% fidelity sounds high but compounds badly over a deep circuit.
Grover's algorithm algorithm: A quantum search algorithm with a quadratic speedup over classical search. Useful but more modest than Shor.

Hadamard gate gate: A single-qubit gate that creates an equal superposition of 0 and 1 from a definite input. The gate that turns "definitely 0" into "50/50".
Harvest now, decrypt later security: Adversaries record encrypted traffic today, knowing they can decrypt it once large enough quantum computers arrive. Long-lived data is the worst case.

Logical qubit concept: A qubit constructed from many noisy physical qubits using a quantum error-correcting code. The unit that fault-tolerant algorithms operate on.

Measurement collapse concept: When a qubit is measured, the superposition is destroyed and the qubit takes a definite value. The act of measurement is part of the computation.
Mosca's theorem (security clock) security: If the time you need data to stay confidential plus the time it takes to migrate exceeds the time until a cryptographically relevant quantum computer arrives, you are already late.

NISQ era: Noisy Intermediate-Scale Quantum. Today's era: 50–1000+ qubits available but high error rates, no full error correction.
No-cloning theorem concept: You cannot copy an unknown quantum state. Not difficult, physically impossible. The reason quantum information behaves differently from classical.

OpenQASM tool: A vendor-neutral assembly-like language for quantum circuits. The format most transpilers consume and emit.

Pauli gates gate: The single-qubit X, Y, Z gates. X is the quantum NOT; Y and Z rotate the qubit on the Bloch sphere.
Post-quantum cryptography (PQC) security: Classical algorithms designed to resist quantum attack. NIST has standardized Kyber, Dilithium, and others; deployment is now the question.

QAOA algorithm: Quantum Approximate Optimization Algorithm. A hybrid algorithm for combinatorial optimization. Layers of parameterised gates tuned by a classical optimizer.
Qiskit tool: IBM's open-source quantum SDK. The most widely used framework for writing, transpiling, and running quantum circuits.
QPU concept: Quantum Processing Unit. The quantum analog of a GPU: a specialised accelerator that handles only the quantum-specific parts of a larger pipeline.
Quantum volume metric: A single-number benchmark for a quantum computer that captures qubit count, connectivity, gate fidelity, and compiler quality together.
Qubit concept: The quantum analogue of a classical bit. A qubit can be in a superposition of 0 and 1 until measured.
QUBO algorithm: Quadratic Unconstrained Binary Optimization. A standard mathematical form many real-world optimization problems map to, and that quantum solvers accept as input.

Shor's algorithm algorithm: A quantum algorithm that factors large integers in polynomial time. The reason quantum computing breaks RSA and ECC once large enough.
Superposition concept: A quantum state that is a weighted combination of basis states. A qubit in superposition is neither 0 nor 1 until measured.

VQE algorithm: Variational Quantum Eigensolver. A hybrid algorithm that finds the lowest-energy state of a molecule by alternating quantum sampling and classical optimization.