Resources · Glossary

The words of quantum computing, in plain language.

Short definitions of the terms used across this site, from qubits and pulses to Merkle roots and post-quantum signatures.

Backend: The thing that actually runs a circuit: a simulator or a physical quantum computer.
Batch job: Work handed to a scheduler to run when resources are free, rather than run interactively.
Bell state: A maximally entangled state of two qubits, often the first test of a new device or toolchain.

Calibration: The routine of measuring a device and adjusting its control settings so its gates behave as intended.
Circuit: An ordered list of quantum gates and measurements applied to a set of qubits.
Coherence time: How long a qubit keeps its quantum information. T1 measures energy loss and T2 measures loss of phase.
Compilation: Turning a circuit into one a particular backend can run, using its gate set and connectivity. Also called transpilation.

Decoherence: The loss of quantum information to the environment.
Digital signature: A value made with a private key that anyone with the public key can check. A hash alone is not a signature.
Dilution refrigerator: A cryostat that cools superconducting devices to millikelvin temperatures.

Entanglement: A correlation between qubits that no classical description of the separate qubits can reproduce.
Evidence record: A file that ties a result to its circuit, backend, execution window, parameters and seeds. QCOS Ledger

Fault tolerance: Running a computation correctly even though the components underneath it make errors.
Fidelity: A measure of how close an actual operation or state is to the ideal one.
FPGA: A chip whose logic can be reconfigured. Quantum control electronics use them for timing-critical work.

Gate: An elementary operation on one or more qubits, the quantum counterpart of a logic gate.
GHZ state: An entangled state in which all qubits are correlated together. Widely used as a benchmark.

Hash: A short fixed-length fingerprint of data. Changing one byte changes the fingerprint.
Hybrid workflow: A computation that alternates between classical and quantum steps.

Logical qubit: A qubit encoded across many physical qubits so that errors can be detected and corrected.

Merkle root: A single hash that summarises many items in a tree, so that a change to any item changes the root.
Metrology: The science of measurement. In quantum hardware, characterising devices, pulses and noise.
ML-DSA: The module-lattice digital signature standard, published by NIST as FIPS 204. Post-quantum cryptography
ML-KEM: The module-lattice key-encapsulation standard, published by NIST as FIPS 203. Post-quantum cryptography

Neutral atoms: A qubit technology that holds uncharged atoms in optical traps and addresses them with lasers.
Noise: Unwanted effects that make a quantum operation differ from the ideal one.
NV centre: A defect in diamond made of a nitrogen atom next to a vacancy, used as a qubit and as a sensor.

Photonic qubit: A qubit carried by a property of light, such as its path or polarisation.
Post-quantum cryptography: Classical cryptographic schemes designed to resist attack by a large quantum computer. Overview
Pulse: The shaped microwave, laser or voltage signal that physically implements a gate. Pulse Certified

QAOA: The quantum approximate optimisation algorithm, a hybrid method for combinatorial problems.
QEC: Quantum error correction: protecting information by encoding it redundantly and measuring error syndromes. QEC stack
QPU: Quantum processing unit: the chip or atom array that holds the qubits.
Qubit: The basic unit of quantum information, which can be in a superposition of two states.

Scheduler: The system that allocates cluster resources to jobs. SLURM, PBS Pro and LSF are common examples. HPC & national centres
Seal: In QCOS records, a hash over the record’s contents. It shows whether the content changed, not who produced it.
Shot: One run of a circuit ending in a measurement. Statistics come from many shots.
SLH-DSA: The stateless hash-based digital signature standard, published by NIST as FIPS 205.
State-vector simulation: Classical simulation that stores every amplitude of the quantum state. Memory doubles with each added qubit.
Superconducting qubit: A qubit built from a superconducting circuit, operated at millikelvin temperatures.
Superposition: A quantum state that combines several basis states at once, until it is measured.
Surface code: An error-correcting code that arranges qubits on a two-dimensional grid and needs only neighbouring interactions.

Trapped ion: A qubit technology that holds charged atoms in electromagnetic fields and addresses them with lasers or microwaves.

VQE: The variational quantum eigensolver, a hybrid method for estimating low-energy states of a system.