  We consider how to forecast progress in the domain of quantum computing. For
this purpose we collect a dataset of quantum computer systems to date, scored
on their physical qubits and gate error rate, and we define an index combining
both metrics, the generalized logical qubit. We study the relationship between
physical qubits and gate error rate, and tentatively conclude that they are
positively correlated (albeit with some room for doubt), indicating a frontier
of development that trades-off between them. We also apply a log-linear
regression on the metrics to provide a tentative upper bound on how much
progress can be expected over time. Within the (generally optimistic)
assumptions of our model, including the key assumption that exponential
progress in qubit count and gate fidelity will continue, we estimate that that
proof-of-concept fault-tolerant computation based on superconductor technology
is unlikely (<5% confidence) to be exhibited before 2026, and that quantum
devices capable of factoring RSA-2048 are unlikely (<5% confidence) to exist
before 2039. It is of course possible that these milestones will in fact be
reached earlier, but that this would require faster progress than has yet been
seen.
