The Road Ahead: Evaluating Claims Yourself
The road from today's noisy devices to fault-tolerant quantum computing is real but unscheduled: error correction is demonstrated at small scale, useful error-corrected computation is not. What you can take from this course is a permanent skill — an evaluation filter that separates proved theory, lab demonstrations, practical capability, and roadmap claims — plus the arithmetic habits to apply it to any headline without trusting anyone, including us.
Where are we on the road from NISQ to fault tolerance?
NISQ — noisy intermediate-scale quantum, Preskill's 2018 coinage — describes the current era: devices with tens to ~1,000 physical qubits whose gates err roughly once per 100 to 1,000 operations, with no error correction protecting the computation. Every result in this course's honest-evidence sections — small-molecule chemistry, QAOA experiments, sampling demonstrations — is a NISQ result: real physics, small scale, no proven commercial value yet.
The destination is fault tolerance: encoding each logical qubit (a reliable, error-corrected qubit) across many noisy physical qubits so that errors are detected and fixed faster than they accumulate — proved theory since the 1990s threshold theorems. Where the road actually stands: experiments have now demonstrated small numbers of logical qubits whose error rates improve as the code grows — the key sign error correction works as theorized. That is demonstrated at small scale. What does not exist anywhere: a machine running deep algorithms on many logical qubits — the regime chapter 6's RSA arithmetic requires (millions of physical qubits versus today's ~10^3; see qubit count). The gap between those sentences is measured in orders of magnitude, not quarters; see our fault-tolerance reality page for the sourced timeline of what has actually been shown.
Between here and there lies "early fault tolerance": machines with a handful to a few hundred logical qubits. Which applications those first machines unlock — quantum simulation is the strongest candidate, per chapter 2 — is the most consequential open question in the field, and anyone who answers it with confidence is selling something.
You’ve read the opening of chapter 10 — 8 more sections follow, with worked examples and circuits you can run on the page. A free account unlocks every chapter of every course (paid plans aren’t live yet — early readers get everything free).