Quantum volume
IBM's metric that folds in both qubit count and qubit quality — an attempt to make comparisons between machines meaningful.
Counting qubits says as little as counting megahertz. Quantum volume is measured by running random circuits that are as wide as they are deep and seeing how large they can get before the result drowns in noise. The metric therefore captures error rate, coherence time and how the qubits are connected, all in one number.
The scale is exponential: a machine handling circuits of size 6×6 has volume 64. The metric has seen some uptake but is criticised for favouring IBM's own architecture, and other manufacturers prefer measures of their own. No industry-wide benchmark exists yet, which makes marketing figures hard to compare.