# Quantum technology glossary

Plain-language definitions of key terms in quantum computing, sensing, networking, cryptography and components. Updated quarterly.

Updated 2026-11-01. Also on the [web page](https://quantumcompanies.fyi/guides/glossary).

Definitions for buyers, engineers and investors. Terms are grouped by domain.

## Quantum computing — core concepts

| Term | Definition |
|---|---|
| **Qubit** | Quantum bit. Two-level quantum system (|0⟩, |1⟩) that can exist in superposition α|0⟩ + β|1⟩. Physical realizations: transmon, trapped ion, neutral atom, photon, spin, Majorana. |
| **Superposition** | Qubit state that is a linear combination of basis states. Enables quantum parallelism. Collapses to a basis state on measurement. |
| **Entanglement** | Non-classical correlation between qubits. Measuring one instantly determines the state of the other(s), regardless of distance. Resource for quantum algorithms and QKD. |
| **Gate** | Unitary operation on one or more qubits. Single-qubit: X, Y, Z, H, S, T, Rz. Two-qubit: CNOT, CZ, iSWAP, √iSWAP, MS (Mølmer-Sørensen). Universal set = arbitrary single-qubit + one entangling two-qubit gate. |
| **Circuit** | Sequence of gates applied to qubits. Depth = longest path of dependent gates. Width = number of qubits. |
| **Measurement** | Projective readout in computational basis (Z-basis). Returns 0 or 1 with probabilities |α|², |β|². Destroys superposition. |
| **Fidelity** | Measure of gate/state quality. Process fidelity: overlap between ideal and actual quantum channel. State fidelity: ⟨ψ_ideal\|ρ_actual\|ψ_ideal⟩. 1.0 = perfect. |
| **Coherence time** | T1 (energy relaxation), T2 (dephasing). Upper bound on circuit depth. Typical: superconducting 50–200 μs, trapped ion 1–10 s, neutral atom 1–10 s, photonic (photon lifetime). |
| **Crosstalk** | Unintended interaction between qubits during gates. Causes correlated errors. Mitigated by frequency allocation, echo sequences, compiler-aware scheduling. |
| **Connectivity** | Graph of which qubit pairs support native two-qubit gates. Heavy-hex (IBM), grid (Google), all-to-all (trapped ion), reconfigurable (neutral atom). Affects SWAP overhead. |
| **Quantum volume** | Benchmark metric (IBM). Largest square circuit (width = depth = n) that runs successfully. Accounts for qubit count, connectivity, gate fidelity, measurement error, compiler efficiency. |
| **NISQ** | Noisy Intermediate-Scale Quantum (Preskill 2018). 50–1000 qubits, no full error correction. Current generation. Algorithms: VQE, QAOA, quantum ML, error mitigation. |
| **FTQC** | Fault-Tolerant Quantum Computing. Logical qubits encoded in many physical qubits with active error correction. Threshold theorem: if physical error rate < threshold (~10⁻³–10⁻⁴), arbitrary-long computation possible. |
| **Logical qubit** | Error-corrected qubit encoded in multiple physical qubits. Surface code: d² physical qubits for distance d. Distance d corrects ⌊(d-1)/2⌋ errors. |
| **Error correction (QEC)** | Active syndrome measurement and correction. Surface code, color code, LDPC codes, bosonic codes (cat, GKP). Requires high fidelity, fast feed-forward, low-latency decoding. |
| **Error mitigation** | Post-processing to reduce noise impact without full QEC. Zero-noise extrapolation (ZNE), probabilistic error cancellation (PEC), Clifford data regression (CDR), measurement error mitigation (M3). |
| **VQE** | Variational Quantum Eigensolver. Hybrid quantum-classical algorithm for ground-state energy. Parameterized ansatz + classical optimizer. NISQ flagship for chemistry. |
| **QAOA** | Quantum Approximate Optimization Algorithm. Alternating problem/mixer Hamiltonians. p layers. Approximate combinatorial optimization (MaxCut, scheduling). |
| **Quantum advantage** | Solving a useful problem faster/better than best classical method. Not yet demonstrated commercially (2024). |
| **Quantum supremacy** | Solving a contrived sampling problem beyond classical reach. Demonstrated: Google Sycamore (2019), USTC Jiuzhang (2020, 2021), Xanadu Borealis (2022). |

## Modalities

| Term | Definition |
|---|---|
| **Superconducting (transmon)** | Josephson junction shunted by capacitor. Anharmonic oscillator. Fixed frequency or tunable (fluxonium). IBM, Google, Rigetti, IQM, OQC, Alice & Bob. |
| **Trapped ion** | Individual ions (Yb⁺, Ca⁺, Ba⁺) confined in Paul trap. Laser-driven gates. All-to-all connectivity. IonQ, Quantinuum, AQT, Oxford Ionics. |
| **Neutral atom** | Alkali/alkaline-earth atoms (Rb, Sr, Yb) in optical tweezers. Rydberg blockade for entanglement. Reconfigurable geometry. QuEra, Pasqal, Atom Computing, Infleqtion. |
| **Photonic** | Qubits encoded in photons (polarization, path, time-bin, continuous variable). Measurement-based QC. Room temp. Xanadu, PsiQuantum, QuiX, Quandela. |
| **Spin qubit** | Electron/hole spin in quantum dot (Si/SiGe, GaAs) or donor (P in Si). CMOS-compatible. Intel, Diraq, Quantum Motion, UNSW. |
| **Topological** | Non-Abelian anyons (Majorana zero modes) in nanowire/superconductor heterostructures. Intrinsic error protection. Microsoft, academic. |
| **Quantum annealing** | Adiabatic evolution of Ising Hamiltonian. Ground state = solution to optimization. D-Wave. Not universal gate model. |

## Performance metrics

| Term | Definition |
|---|---|
| **1q fidelity** | Single-qubit gate fidelity (randomized benchmarking). Target > 99.9%. |
| **2q fidelity** | Two-qubit gate fidelity (RB or cycle benchmarking). Target > 99% (superconducting), > 99.7% (trapped ion). |
| **Readout fidelity** | Probability of correct |0⟩/|1⟩ assignment. Target > 99%. |
| **SPAM** | State Preparation and Measurement error. Includes readout, initialization, measurement crosstalk. |
| **Gate time** | Duration of gate pulse. Superconducting: 20–50 ns (1q), 100–300 ns (2q). Trapped ion: 1–10 μs (1q), 10–100 μs (2q). |
| **Circuit layer operations per second (CLOPS)** | IBM metric: layers of random circuit executed per second. Includes compilation, queue, execution, readout. |
| **Azure Quantum Token / HQC / eHQC** | Azure Quantum Token (IonQ billing on Azure Quantum), Hardware Quantum Credit (Quantinuum), emulator HQC. Provider-specific billing units. Note: AQT also names Alpine Quantum Technologies, a trapped-ion hardware vendor. |

## Cloud & software

| Term | Definition |
|---|---|
| **Qiskit** | IBM open-source SDK. Terra (circuits), Aer (simulators), Runtime (managed execution), Primitives (Estimator, Sampler). |
| **Cirq** | Google open-source SDK. Focus on NISQ, hardware-aware compilation, noise models. |
| **PennyLane** | Xanadu open-source SDK. Differentiable programming, ML integration, device agnostic. |
| **Braket SDK** | AWS SDK. Unified interface to IonQ, Rigetti, IQM, QuEra, AQT, simulators. Hybrid Jobs, managed notebooks. |
| **Azure Quantum SDK** | Microsoft SDK. Access to IonQ, Quantinuum, Pasqal, Rigetti, Q# language, QIR. |
| **Q#** | Microsoft domain-specific language for quantum. Classical host + quantum operations. Compiles to QIR. |
| **OpenQASM 3** | Open Quantum Assembly Language. Standard IR for circuits. Classical control flow, subroutines, timing. |
| **QIR** | Quantum Intermediate Representation. LLVM-based. Language-agnostic, hardware-agnostic. |
| **CUDA-Q** | NVIDIA C++/Python platform. GPU-accelerated simulation, hybrid quantum-classical, multi-QPU. |
| **Hybrid job** | Parameterized circuit batch with classical optimization loop. Managed by cloud (Braket Hybrid Jobs, Qiskit Runtime, Azure Quantum Jobs). |
| **Primitive** | Qiskit Runtime abstraction: Estimator (expectation values), Sampler (quasi-probabilities). Versioned, hardware-optimized. |

## Quantum sensing

| Term | Definition |
|---|---|
| **NV center** | Nitrogen-vacancy defect in diamond. Electron spin S=1. Optically detected magnetic resonance (ODMR). Room-temp magnetometer. |
| **SERF / OPM** | Spin-Exchange Relaxation-Free / Optically Pumped Magnetometer. Alkali vapor (Rb, Cs, K). Femtotesla sensitivity. Requires μT shielding. |
| **SQUID** | Superconducting Quantum Interference Device. Josephson junction loop. Flux-to-voltage transduction. Requires cryogenics (4 K). |
| **Atom interferometry** | Matter-wave interferometer using cold atoms (Rb, Cs). Measures acceleration, rotation, gravity. Space-compatible. |
| **CSAC** | Chip-Scale Atomic Clock. Coherent Population Trapping (CPT) in MEMS vapor cell. 10⁻¹¹–10⁻¹² stability. < 20 cm³, < 120 mW. |
| **Quantum gravimeter** | Absolute gravity measurement via atom interferometry. µGal (10⁻⁸ m/s²) sensitivity. Drone/aircraft deployable. |

## Quantum networking & QKD

| Term | Definition |
|---|---|
| **QKD** | Quantum Key Distribution. BB84 (prepare-and-measure), E91 (entanglement-based), MDI-QKD (measurement-device-independent), CV-QKD (continuous variable). |
| **Quantum repeater** | Entanglement swapping + purification to extend distance beyond fiber loss limit (~100 km). Requires quantum memory. |
| **Quantum memory** | Stores photonic qubit (ensemble, rare-earth, NV, trapped ion). Efficiency, fidelity, storage time, multiplexing. |
| **QRNG** | Quantum Random Number Generator. Photon shot noise, vacuum fluctuations, phase noise. NIST SP 800-90B validated. |
| **Trusted node** | Classical relay in QKD network. Decrypts/re-encrypts keys. Security assumption: node is physically secure. |
| **Satellite QKD** | Free-space QKD via satellite (Micius, QEYSSat). Global key distribution. Daytime background noise challenge. |

## Post-quantum cryptography

| Term | Definition |
|---|---|
| **PQC** | Post-Quantum Cryptography. Classical algorithms resistant to quantum attacks. Lattice, hash, code, multivariate, isogeny. |
| **ML-KEM** | Module-Lattice-Based Key Encapsulation Mechanism (CRYSTALS-Kyber). FIPS 203. IND-CCA2 secure. |
| **ML-DSA** | Module-Lattice-Based Digital Signature (CRYSTALS-Dilithium). FIPS 204. EUF-CMA secure. |
| **SLH-DSA** | Stateless Hash-Based Digital Signature (SPHINCS+). FIPS 205. No structured hardness assumption. Larger sigs. |
| **CNSA 2.0** | Commercial National Security Algorithm Suite 2.0 (NSA). Mandates ML-KEM-1024 and ML-DSA-87 for NSS. New acquisitions must support it from January 1, 2027; mandatory December 31, 2031. |
| **Crypto-agility** | Ability to swap cryptographic algorithms without system redesign. Hybrid certificates, dual signatures, algorithm negotiation. |
| **Harvest now, decrypt later** | Adversary records encrypted traffic today, decrypts when CRQC exists. Motivates early PQC migration. |

## Components & supply chain

| Term | Definition |
|---|---|
| **Dilution refrigerator** | ³He/⁴He mixing chamber. Base temp 10–15 mK. Cooling power ~1 μW at 100 mK. Bluefors, Oxford Instruments, Cryomech, Leiden. |
| **Pulse-tube cooler** | Pre-cooling stage (4 K, 50 K). No moving parts at cold head. Sumitomo, Cryomech, Brooks. |
| **AWG** | Arbitrary Waveform Generator. 10–100 GS/s, 14–16 bit, multi-channel. Keysight, Tektronix, Zurich Instruments, Qblox, Quantum Machines. |
| **FPGA / RFSoC** | Xilinx Zynq UltraScale+ RFSoC, Versal. Integrated DAC/ADC + FPGA fabric. Sub-μs latency. |
| **Ultra-stable laser** | Linewidth < 1 Hz (diode + reference cavity). Toptica, Menlo Systems, Stable Laser Systems. |
| **Frequency comb** | Mode-locked laser spectrum. Optical clock reference, microwave generation. Menlo Systems, IMRA. |
| **UHV** | Ultra-High Vacuum (< 10⁻¹⁰ mbar). Ion pumps, NEG, titanium sublimation. Required for trapped ion, neutral atom. |
| **Optical tweezers** | Focused laser beams trapping individual atoms. AOD/SLM for dynamic reconfiguration. 1064 nm typical. |

## Acronyms

| Acronym | Expansion |
|---|---|
| AQT | Alpine Quantum Technologies (trapped-ion hardware vendor); also Azure Quantum Token (IonQ billing on Azure Quantum) |
| BB84 | Bennett-Brassard 1984 (QKD protocol) |
| BQP | Bounded-Error Quantum Polynomial Time (complexity class) |
| CNOT | Controlled-NOT (two-qubit gate) |
| CLOPS | Circuit Layer Operations Per Second |
| CV-QKD | Continuous-Variable QKD |
| CQC | Coherent Quantum Control / Cambridge Quantum Computing (Quantinuum) |
| CSAC | Chip-Scale Atomic Clock |
| CPT | Coherent Population Trapping |
| DD | Dynamical Decoupling |
| E91 | Ekert 1991 (entanglement-based QKD) |
| ECCN | Export Control Classification Number |
| EUF-CMA | Existential Unforgeability under Chosen Message Attack |
| FTQC | Fault-Tolerant Quantum Computing |
| GKP | Gottesman-Kitaev-Preskill (bosonic code) |
| HQC | Hardware Quantum Credit (Quantinuum) / Hamming Quasi-Cyclic (KEM candidate) |
| HSM | Hardware Security Module |
| IND-CCA2 | Indistinguishability under Adaptive Chosen Ciphertext Attack |
| ITAR | International Traffic in Arms Regulations |
| LDPC | Low-Density Parity-Check (code) |
| MDI-QKD | Measurement-Device-Independent QKD |
| MPS | Matrix Product State |
| NISQ | Noisy Intermediate-Scale Quantum |
| NSS | National Security Systems |
| ODMR | Optically Detected Magnetic Resonance |
| OPM | Optically Pumped Magnetometer |
| PEC | Probabilistic Error Cancellation |
| PQC | Post-Quantum Cryptography |
| QEC | Quantum Error Correction |
| QKD | Quantum Key Distribution |
| QPU | Quantum Processing Unit |
| QRNG | Quantum Random Number Generator |
| QVM | Quantum Virtual Machine (Rigetti simulator) |
| RB | Randomized Benchmarking |
| Rydberg | Highly excited atomic state (large dipole moment) |
| SERF | Spin-Exchange Relaxation-Free |
| SLH-DSA | Stateless Hash-Based Digital Signature Standard |
| SPAM | State Preparation and Measurement |
| SQUID | Superconducting Quantum Interference Device |
| SWAP | Qubit swap gate (3 CNOTs) |
| T1 / T2 | Relaxation / Dephasing time |
| UHV | Ultra-High Vacuum |
| VQE | Variational Quantum Eigensolver |
| ZNE | Zero-Noise Extrapolation |

## Sources

- [NIST PQC Standards](https://csrc.nist.gov/projects/post-quantum-cryptography) — fetched 2026-11-01.
- [IBM Quantum Glossary](https://quantum.cloud.ibm.com/docs/guides/glossary) — fetched 2026-11-01.
- [Qiskit documentation](https://www.ibm.com/quantum/qiskit) — fetched 2026-11-01.
- [IBM Quantum Learning](https://www.ibm.com/quantum/learning) — fetched 2026-11-01.
- [QED-C Glossary](https://quantumconsortium.org/glossary/) — fetched 2026-11-01.
- [IEEE Quantum Glossary](https://quantum.ieee.org/glossary) — fetched 2026-11-01.
- [NIST Quantum Information Glossary](https://www.nist.gov/quantum-information-glossary) — fetched 2026-11-01.

## Questions

### What is the difference between a qubit and a bit?

A classical bit is 0 or 1. A qubit can be in a superposition of 0 and 1 simultaneously, enabling quantum parallelism. Measurement collapses the superposition to 0 or 1 with probabilities determined by the quantum state.

### What does NISQ mean?

Noisy Intermediate-Scale Quantum. Devices with 50–1000 qubits that lack full error correction. Current generation (2024). NISQ algorithms (VQE, QAOA) are designed to work despite noise.

### What is quantum advantage vs quantum supremacy?

Quantum supremacy: a quantum device solves a contrived problem faster than any classical computer (Google 2019, USTC 2020, Xanadu 2022). Quantum advantage: a quantum device solves a useful, real-world problem faster or better than classical methods. Not yet achieved commercially.

### What is T1 and T2?

T1 (relaxation time): time for an excited qubit to decay to ground state. T2 (dephasing time): time for qubit superposition to lose phase coherence. T2 ≤ 2×T1. Longer is better. Typical superconducting: T1 ~100 μs, T2 ~100 μs. Trapped ion: T1 > 1 s, T2 ~1–10 s.