11.6 Quantum Volume

How powerful is a gate- and circuit-based quantum computer? How much progress is being made with one qubit technology versus another? When we say we can find a solution on a ‘‘powerful enough’’ quantum computer, what does that mean? When will we know we have arrived?

While it is certainly useful to know how well a given qubit is doing in terms of decoherence and error rates, it tells you nothing about the system in total and how well the components are working together. You may have one or two spectacular, connected, and low error rate qubits, but other aspects of your system may make it unusable for executing useful algorithms.

Implementing hundreds of very bad qubits does not give you an advantage in the circuit model over having far fewer but excellent qubits with good control and measurement. Therefore, we need a whole-system, or ‘‘holistic,’’ metric that can tell us at a glance the relative...