Qubit count
Qubit count is the register size — and by itself it ranks nothing. Usable capacity depends on connectivity, fidelity, and your circuit's shape; counts across different execution models are not comparable at all.
What is qubit count?
The headline number every announcement leads with. Distinctions that matter: physical qubits (hardware elements) vs logical qubits (error-corrected aggregates of many physical qubits); programmable qubits vs total fabricated elements (some chips include untunable or reserved elements); and array sites vs loaded atoms on neutral-atom machines.
How is it measured?
Counting is direct, but vendors choose what to count: IonQ leads with "algorithmic qubits" (#AQ, a benchmark-derived capability number), annealing vendors count annealing qubits with sparse couplers, and analog machines count atoms that are not gate-model qubits at all.
Logical-qubit overheads: Surface codes: towards practical large-scale quantum computation (Fowler et al., 2012).When are two values NOT comparable?
Gate-model, annealing, and analog counts live on different axes — 5,000+ annealing qubits and 156 gate-model qubits solve different problems and cannot be ranked against each other; QPU137 lists non-gate machines separately for exactly this reason. Within the gate model, 127 qubits at degree-3 connectivity and 36 at all-to-all suit different circuit shapes: compile your own circuit against both topologies instead of comparing counts.
See it in the data: the sourced catalog · compare two processors · lesson: reading hardware specs