Logical qubit

A logical qubit is a qubit protected by quantum error correction: its quantum information is spread across many physical qubits so that errors can be detected and corrected faster than they accumulate. It is the unit that matters for large, useful computation, because raw physical qubits are too noisy to run deep algorithms. Error correction works only once physical error rates drop below a fault-tolerance threshold; above that threshold, adding qubits adds noise faster than correction removes it, and no amount of scale helps.

The number that decides a hardware roadmap is the overhead: how many physical qubits are needed per logical qubit, which today can run from dozens to many thousands depending on the modality and the target error rate. This is why physical qubit counts, the figure most often headlined, can be misleading: a machine with a thousand noisy physical qubits and an error rate above threshold has zero logical qubits and cannot run a useful fault-tolerant algorithm. Fault tolerance is the regime where logical error rates can be driven arbitrarily low by scaling; reaching it is the central milestone of most serious hardware programs.

For an investor, the logical-qubit lens reframes the entire technical claim. The questions are: how many logical qubits, at what logical error rate, with what physical-to-logical overhead, and how far below threshold the physical devices operate. A roadmap stated in logical qubits with honest overheads is fundable; a pitch that leans on physical qubit counts while staying quiet on error rates and fault tolerance is quoting the flattering number and hoping the diligence does not ask the next question. It will.