Quantum computing terminology
Here is a comprehensive list of quantum commuting terminology, categorized for clarity.
I. Fundamental Physics & Concepts
These are the core quantum mechanical principles that enable quantum computing.
- Qubit (Quantum Bit): The fundamental unit of quantum information, analogous to a classical bit. Unlike a classical bit (0 or 1), a qubit can be in a superposition of both 0 and 1.
- Superposition: The ability of a quantum system to be in multiple states at the same time. A qubit is in a superposition of |0⟩ and |1⟩ until it is measured.
- Entanglement: A profound quantum connection between two or more qubits where the state of one cannot be described independently of the state of the other(s). Measuring one entangled qubit instantly influences the state of the other, no matter the distance.
- Quantum State: The complete description of a quantum system (e.g., a qubit or a set of qubits), represented by a state vector (e.g., |ψ⟩, pronounced “psi”).
- Coherence: The property that allows qubits to maintain their quantum state (superposition and entanglement). The loss of coherence is called decoherence.
- Decoherence: The process by which a quantum system loses its quantum properties and becomes classical due to interaction with its external environment. This is a major engineering challenge.
- Measurement: The act of observing a quantum system, which causes its wavefunction to collapse into a single, definite classical state (e.g., either 0 or 1).
- Wavefunction Collapse: The phenomenon where a quantum system in superposition randomly settles into one of its possible definite states upon measurement.
II. Quantum Hardware & Technologies
The physical implementations of qubits and the machines that house them.
- Quantum Processing Unit (QPU): The core processor of a quantum computer, which contains the qubits.
- Qubit Modalities (Types of Qubits):
- Superconducting Qubits: Use superconducting electrical circuits to create artificial atoms. (Used by Google, IBM, Rigetti).
- Trapped Ions: Use individual atoms suspended in a vacuum by electromagnetic fields. (Used by IonQ, Honeywell).
- Photonic Qubits: Use particles of light (photons) to represent quantum information. (Used by Xanadu, PsiQuantum).
- Semiconductor Spin Qubits: Use the spin of an electron or nucleus in a semiconductor material, like silicon. (Pursued by Intel, academic labs).
- Topological Qubits: A theoretical approach that encodes information in non-local properties that are highly resistant to decoherence. (Pursued by Microsoft).
- Cryogenics: The technology required to cool superconducting qubits to temperatures near absolute zero (~10-15 millikelvin).
- Dilution Refrigerator: The extremely powerful refrigerator used to achieve the cryogenic temperatures for superconducting qubits.
- Fidelity: A measure of the accuracy of a quantum operation. High fidelity means the quantum computer is performing as expected with minimal error.
- Quantum Volume (QV): A holistic metric invented by IBM to measure the performance of a quantum computer, considering the number of qubits, connectivity, and error rates.
- NISQ (Noisy Intermediate-Scale Quantum): The current era of quantum computing, characterized by quantum processors with 50-1000 qubits that are “noisy” (prone to errors) and not yet fault-tolerant.
III. Quantum Algorithms & Software
The programs and tools used to run computations on quantum hardware.
- Quantum Algorithm: A step-by-step procedure, designed to run on a quantum computer, to solve a specific problem (e.g., Shor’s algorithm, Grover’s algorithm).
- Quantum Circuit: A model for quantum computation where a sequence of quantum gates is applied to a set of qubits.
- Quantum Gate (Logic Gate): The basic operation in a quantum circuit. It manipulates the state of one or more qubits.
- Single-Qubit Gates: e.g., Pauli-X (quantum NOT), Hadamard (creates superposition).
- Two-Qubit Gates: e.g., CNOT (controlled-NOT), which is essential for creating entanglement.
- Quantum Supremacy / Quantum Advantage: The milestone where a quantum computer solves a problem that is practically impossible for any classical computer to solve in a reasonable time. “Advantage” is now the preferred term for practical applications.
- Quantum Error Correction (QEC): A set of techniques to protect quantum information from errors due to decoherence and other quantum noise. This typically requires many physical qubits to create one stable logical qubit.
- Fault-Tolerant Quantum Computing (FTQC): The ultimate goal of quantum computing, where QEC is so effective that computations of arbitrary length can be performed reliably.
- Quantum Simulation: Using a quantum computer to simulate and understand other quantum systems (e.g., molecules, materials), which is classically very difficult.
- Variational Quantum Algorithm (VQA): A class of hybrid quantum-classical algorithms where a quantum computer is used to prepare and measure a quantum state, and a classical computer is used to optimize the parameters of that state.
- Quantum Software Development Kit (SDK): A programming framework for developing quantum applications (e.g., Qiskit (IBM), Cirq (Google), Braket (Amazon)).
IV. Quantum Networking & Communication
Terms related to connecting quantum computers and secure communication.
- Quantum Key Distribution (QKD): A secure communication method that uses quantum mechanics to enable two parties to produce a shared random secret key, which can then be used to encrypt and decrypt messages. Its security is guaranteed by the laws of physics.
- Quantum Repeater: A device that is needed to extend the range of quantum communication (e.g., for a future quantum internet) by performing entanglement swapping.
- Entanglement Swapping: A technique to entangle two quantum particles that have never directly interacted.
- Quantum Internet: A theoretical network that would connect quantum processors to distribute quantum information and entanglement over long distances.
V. Companies & Key Players
A non-exhaustive list of prominent organizations in the field.
- IBM: A leader in superconducting quantum computers, known for its public-facing
IBM Quantumplatform. - Google Quantum AI: Achieved a landmark quantum supremacy experiment in 2019 and is advancing superconducting technology.
- IonQ: A leading company focused on trapped-ion quantum computers.
- Rigetti Computing: Develops superconducting quantum integrated circuits and offers cloud access.
- D-Wave Systems: The first company to sell quantum computers, specializing in quantum annealers for optimization problems (a different model than the gate-based model described above).
- Honeywell Quantum Solutions (now part of Quantinuum): Known for high-fidelity trapped-ion systems.
- Quantinuum: The merged company of Honeywell Quantum Solutions and Cambridge Quantum, a leader in software and trapped-ion hardware.
- PsiQuantum: Working on building a large-scale, fault-tolerant quantum computer using photonic qubits.
- Xanadu: Developing photonic quantum computers and known for its open-source software
PennyLane. - Microsoft Azure Quantum: Offers a cloud platform for accessing various quantum hardware and software, and is researching topological qubits.
VI. Advanced & Theoretical Concepts
For when you want to dive deeper.
- Bell State: A specific, maximally entangled state of two qubits. The foundation for many quantum protocols.
- Bloch Sphere: A geometrical representation of the pure state space of a single qubit.
- No-Cloning Theorem: A fundamental theorem stating that it is impossible to create an identical copy of an arbitrary unknown quantum state.
- Quantum Teleportation: A protocol for transferring an unknown quantum state from one location to another, using a pair of entangled qubits and classical communication.
- Quantum Fourier Transform (QFT): The quantum analogue of the discrete Fourier transform, a key component of many quantum algorithms, including Shor’s algorithm.
- Topological Quantum Computing: A theoretical approach that uses non-abelian anyons (quasiparticles) to create qubits whose states are topologically protected from local errors.
Foundational Concepts
- Bit – Classical unit of information (0 or 1).
- Qubit (Quantum bit) – Quantum version of a bit. Can exist in state |0⟩, |1⟩, or any superposition α|0⟩ + β|1⟩ where |α|² + |β|² = 1.
- Superposition – Ability of a qubit to be in multiple states (0 and 1) at the same time until measured.
- Measurement – Collapses a qubit from superposition into a definite classical state (|0⟩ or |1⟩) with probabilities given by the amplitudes squared.
- Entanglement – Quantum correlation between qubits such that the state of one instantly determines the state of the other, no matter the distance (Einstein called it “spooky action at a distance”).
- Interference – Quantum waves can add constructively (amplify correct answers) or destructively (cancel wrong answers) in algorithms.
Quantum Gates & Circuits
- Quantum gate – Unitary operation that changes qubit states (analogous to classical logic gates).
- H (Hadamard) – Creates superposition from |0⟩ or |1⟩
- X (Pauli-X) – Quantum NOT gate, flips |0⟩ ↔ |1⟩
- Z (Pauli-Z) – Phase flip
- Y (Pauli-Y) – Both bit and phase flip
- CNOT (Controlled-NOT) – Flips second qubit if first is |1⟩; creates entanglement
- Toffoli (CCNOT) – Classical reversible AND gate
- Rotation gates (Rx, Ry, Rz) – Rotate state on Bloch sphere by arbitrary angle
- Quantum circuit – Sequence of quantum gates applied to qubits, followed by measurement.
- Universality – Any quantum computation can be built from a small set of universal gates (e.g., H, phase, CNOT, π/8 gate).
Key Quantum Algorithms
- Deutsch–Jozsa – First quantum algorithm showing exponential speedup for a specific problem.
- Shor’s algorithm – Factors large numbers exponentially faster than classical → threatens RSA encryption.
- Grover’s algorithm – Provides quadratic speedup for unstructured search (e.g., finding item in unsorted database).
- VQE (Variational Quantum Eigensolver) – Hybrid algorithm for finding ground states of molecules (chemistry).
- QAOA (Quantum Approximate Optimization Algorithm) – For combinatorial optimization problems.
- Quantum Fourier Transform (QFT) – Core subroutine in Shor’s and many other algorithms.
Practical & Hardware Terms
- Coherence time – How long a qubit maintains its quantum state before decohering.
- Decoherence – Loss of quantum properties due to interaction with environment.
- Quantum volume – Metric that combines qubit count, gate fidelity, connectivity, etc.
- Gate fidelity – How accurately a quantum gate performs its intended operation.
- T1, T2 – Relaxation (T1) and dephasing (T2) times; measures of coherence.
- Error correction / Quantum error correction (QEC) – Uses multiple physical qubits to create one fault-tolerant logical qubit (e.g., surface code, color code).
- Logical qubit – Error-corrected qubit that behaves almost perfectly.
- FTQC (Fault-Tolerant Quantum Computing) – Quantum computing with error rates low enough for large algorithms.
- NISQ (Noisy Intermediate-Scale Quantum) – Current era (50–1000 qubits) where noise is still too high for full error correction.
Paradigms & Platforms
- Gate-based (circuit model) – Most common (IBM, Google, Rigetti, IonQ).
- Adiabatic quantum computing – D-Wave’s approach; slowly evolves system to minimize energy.
- Quantum annealing – Specialized form of adiabatic computing for optimization.
- Topological quantum computing – Uses anyons (Microsoft’s approach); inherently protected from noise.
- Photonic quantum computing – Uses photons (Xanadu, PsiQuantum).
- Trapped-ion – Ions held by electromagnetic fields (IonQ, Honeywell/Quantinuum).
- Superconducting qubits – Josephson junctions at mK temperatures (IBM, Google, Rigetti).
Complexity & Advantage
- BPP, BQP – Classical and quantum polynomial-time classes.
- Quantum advantage / supremacy – Demonstrated when a quantum computer solves a problem infeasible for any classical computer (Google 2019, USTC Jiuzhang 2020, Xanadu 2022, etc.).
- Quantum speedup types
- Exponential (Shor, quantum simulation)
- Quadratic (Grover)
- Potential but unproven (many NISQ heuristics)
Quick reference table of common notation
| Symbol | Meaning |
|---|---|
| ψ⟩ | |
| ⟨ψ | |
| 0⟩, | |
| +⟩, | |
| Φ⁺⟩, | |
| U | Unitary operator (reversible) |
| ρ (rho) | Density matrix (mixed states) |
This covers ~95% of the terms you’ll encounter in papers, talks, and roadmapping discussions as of 2025. Let me know if you want deeper explanation of any specific term or category!
I. Fundamental Concepts
These terms describe the physics that makes quantum computing possible.
- Qubit (Quantum Bit): The basic unit of quantum information. Unlike a classical bit (which is 0 or 1), a qubit can represent 0, 1, or a complex combination of both simultaneously.
- Superposition: The ability of a quantum system to exist in multiple states at the same time until it is measured.
- Analogy: A spinning coin is in a state of superposition (both heads and tails) until it lands (measurement).
- Entanglement: A phenomenon where two qubits become linked. Changing the state of one instantly affects the other, no matter the distance between them. Einstein famously called this “spooky action at a distance.”
- Interference: A method used to control quantum states. It amplifies the “correct” answers (constructive interference) and cancels out the “wrong” answers (destructive interference) during a calculation.
- Measurement (Collapse): The act of observing a qubit. This forces the qubit out of superposition, making it “collapse” into a single definite state (0 or 1).
II. Hardware & Architecture
Terms related to the physical machines and how they are built.
- QPU (Quantum Processing Unit): The “brain” of the quantum computer, analogous to a CPU in a classical computer.
- Superconducting Qubits: The most common approach (used by Google, IBM). These qubits are made from superconducting circuits that must be kept at temperatures near absolute zero.
- Trapped Ions: An approach that uses electromagnetic fields to suspend individual charged atoms (ions) in space to use them as qubits (used by IonQ).
- Dilution Refrigerator: The massive cooling system (often looking like a golden chandelier) used to keep the quantum chip at near absolute zero to prevent errors.
- Photonic Quantum Computing: A method that uses particles of light (photons) as qubits. These can theoretically operate at room temperature.
III. Performance & Metrics
How we measure the power and status of quantum computers.
- Quantum Supremacy: The milestone when a quantum computer performs a calculation that is practically impossible for the best classical supercomputer to solve in a reasonable timeframe.
- Quantum Advantage: A more practical milestone where a quantum computer solves a useful real-world problem better, faster, or cheaper than a classical computer.
- Quantum Volume: A metric (popularized by IBM) that measures the overall power of a quantum computer, taking into account not just the number of qubits, but also their quality (error rate) and connectivity.
- NISQ (Noisy Intermediate-Scale Quantum): The current era of quantum computing. We have functional quantum computers (50–100+ qubits), but they are “noisy” (prone to errors) and not yet fully fault-tolerant.
IV. Algorithms & Error Correction
Terms related to software and reliability.
- Shor’s Algorithm: A famous quantum algorithm that can factor large numbers exponentially faster than classical computers. It theoretically poses a threat to current encryption methods (RSA).
- Grover’s Algorithm: A quantum algorithm used for searching unsorted databases. It is significantly faster than classical search methods.
- Decoherence: The loss of quantum information. This happens when qubits interact with their environment (noise, heat, vibration), causing the system to lose its quantum behavior.
- Physical vs. Logical Qubit:
- Physical Qubit: The actual hardware qubit (which is error-prone).
- Logical Qubit: A group of many physical qubits working together to act as one error-free, reliable qubit.
