Glossary

Accuracy

How close the quantum method's result is to the baseline's, as a percentage with an error bar. 100% means the quantum result equals the baseline. How it is computed differs by workflow and is stated on the workflow page. See Accuracy, fidelity and noise.

Active space

In chemistry workflows, the small set of electrons and orbitals the quantum method handles while the rest of the molecule is treated classically. A larger active space is more accurate and needs more qubits (two per orbital). The chemistry workflows accept up to 6 orbitals, which is 12 qubits.

Baseline

The classical answer a workflow's quantum result is compared with. Every workflow declares one method as its baseline, and Qsolve runs it on the same input. Qsolve cannot prove the baseline is the best classical method that exists. See Reading your results.

Chemical accuracy

An energy error of 1.6 millihartree (mHa) or less, the usual threshold for a chemistry result to be useful. The chemistry workflows show errors in mHa so you can compare.

Circuit

A sequence of quantum gates applied to qubits, followed by measurement. Its depth is the number of layers of gates, and its two-qubit gates are the noisiest operations. Every circuit a run sends is recorded on the run page. See Run records.

Credit

The unit of spending. 1 credit equals 1 US dollar. New accounts receive 100 after confirming their email. See Credits and pricing.

Error mitigation

Classical post-processing that reduces the effect of hardware noise, such as readout correction, post-selection and zero-noise extrapolation. The workflow reports mitigated figures, and Qsolve measures the raw figures beside them.

Fidelity

How close the output distribution of a quantum run is to what a perfect, noise-free machine would produce for the same circuit. 100% is identical. Qsolve measures the raw fidelity from the recorded circuits and counts and shows the mitigated fidelity next to it.

Hartree and millihartree

The unit of energy in chemistry workflows, written Ha. A millihartree (mHa) is one thousandth of that.

Noise model

A simulation of a specific quantum device's published error rates, run on a classical computer. A "noise-model estimate" in a track record shows how a workflow behaves under that device's noise without using the device. Real devices can be noisier than their models. See Machines and targets.

Post-selection

Discarding shots that fail a validity check, for example a bit string with the wrong number of ones when the problem requires exactly k. The share that remains is shown as Shots kept.

QAOA

Quantum approximate optimisation algorithm. A short circuit that alternates between favouring low-cost answers and mixing possibilities, with its angles tuned by a classical optimiser. Used by generic-qubo, max-cut and portfolio-optimization.

Qubit

The basic unit of a quantum computer, the counterpart of a bit. Each workflow states the largest number of qubits it accepts under Limits.

QUBO

Quadratic unconstrained binary optimisation: choose a 0 or 1 for each variable to minimise x'Qx for a matrix Q. Many scheduling, routing, selection and partitioning problems can be written this way. The generic-qubo workflow accepts any such matrix.

Seed

A number that fixes the workflow's random choices. The same workflow version, input, settings and seed give the same result on a simulator. See Reproducibility.

Shots

How many times a circuit is run and measured. More shots reduce sampling noise and cost more.

Target

The machine a run executes on: the CPU simulator, the GPU simulator or a quantum computer. See Machines and targets.

Track record

The accuracy, fidelity and cost of a workflow's benchmark cases, shown on its page. Simulator results count towards the headline accuracy. Hardware results are shown separately.

Transpilation

Rewriting a circuit into the gates a specific chip supports and mapping its qubits to physical ones. It usually makes the circuit longer. The Circuits card can show a circuit "As written" and "As run on the chip".

VQE

Variational quantum eigensolver. A circuit whose parameters a classical optimiser tunes to find the lowest energy of a molecule. The chemistry workflows train the circuit on a noise-free simulator and measure the final energy on the target.

Workflow

A packaged algorithm for one kind of problem, with an input form, declared methods, benchmark cases and a version. See What is Qsolve.

ZNE

Zero-noise extrapolation. A mitigation technique that runs a circuit at several amplified noise levels (for example by repeating gates) and extrapolates back to zero noise. The chemistry workflows use it on quantum devices.