Quantum Glossary
10 terms defined in plain English
Foundational
- Entanglement
- A quantum phenomenon where two qubits become correlated such that measuring one instantly determines the state of the other, regardless of the distance between them.
- Entanglement is a genuine physical phenomenon — not faster-than-light communication. It is a key resource for quantum algorithms, quantum cryptography, and quantum teleportation.
- Single Systems
- Qubit
- The basic unit of quantum information — analogous to a classical bit but capable of existing in a superposition of 0 and 1 simultaneously until measured.
- Unlike a classical bit which is always either 0 or 1, a qubit can be in a combination of both states at once. Physical qubits are implemented using superconducting circuits, trapped ions, photons, or other quantum systems. Measurement collapses the qubit to a definite 0 or 1.
- What is Quantum Computing?Single Systems
- Superposition
- A quantum property allowing a qubit to exist in a combination of 0 and 1 states simultaneously, enabling quantum computers to explore many possibilities at once.
- Superposition is not the same as being "both 0 and 1 at the same time" — it is a probability distribution that collapses to a definite value when measured. Quantum algorithms exploit superposition to process many inputs simultaneously.
- So You Want to Understand Quantum Mechanics...Single Systems
Algorithms
- Grover's Algorithm
- A quantum search algorithm that finds a marked item in an unsorted list of N items in O(√N) steps — a quadratic speedup over classical search.
- Grover's Algorithm | Understanding Quantum Information & Computation | Lesson 08
- Quantum Circuit
- A sequence of quantum gates applied to qubits to perform a computation — the standard way to describe a quantum algorithm.
- Grover's Algorithm | Understanding Quantum Information & Computation | Lesson 08Coding with Qiskit 1.x — Series Entry
- Quantum Gate
- A basic quantum operation that transforms the state of one or more qubits — the quantum equivalent of a classical logic gate.
- Quantum gates are reversible (unlike many classical gates) and are represented mathematically as unitary matrices. Common gates include the Hadamard (H), CNOT, and Pauli-X gates.
- Grover's Algorithm | Understanding Quantum Information & Computation | Lesson 08
Hardware
- Decoherence
- The loss of quantum properties in a qubit due to interaction with its environment — the primary source of errors in current quantum computers.
- Dynamic Circuits — Coding with Qiskit
- NISQ
- Noisy Intermediate-Scale Quantum — describes the current era of quantum computers with 50–1000 qubits that are too noisy for error correction but large enough for research.
- Dynamic Circuits — Coding with Qiskit
- Quantum Error Correction
- Techniques that protect quantum information from decoherence and other errors by encoding logical qubits across many physical qubits.
- Physical qubits are fragile — they lose their quantum state quickly through interaction with their environment (decoherence). Error correction is essential for fault-tolerant quantum computation but currently requires many physical qubits per logical qubit.
- Dynamic Circuits — Coding with Qiskit
Business
- Quantum Advantage
- The point at which a quantum computer solves a problem faster or more efficiently than any classical computer can for the same task.
- Quantum advantage has been demonstrated for narrow tasks (e.g. random circuit sampling) but has not yet been achieved for commercially useful problems. It is a spectrum, not a binary event.
- But what is quantum computing? (Grover's Algorithm)Quantum Computing Expert Explains One Concept in 5 Levels of Difficulty