Logical qubit
An error-corrected qubit assembled from many physical ones, and the unit that actually predicts what a quantum computer can do.
A logical qubit is an error-corrected quantum bit built by encoding one unit of quantum information across many physical qubits, so that errors on individual physical qubits can be detected and corrected without destroying the encoded state. The number of physical qubits required per logical one depends on the hardware error rate and the code used, and it is frequently in the hundreds or thousands.
This is why physical qubit counts predict so little. A machine with a large physical count and a poor error rate may sustain no useful logical qubits at all, while a smaller machine with better fidelity may sustain several - which is why credible progress is reported as error-correction results rather than as device size.
For anyone outside the field the practical use of the number is as a clock. Resource estimates for breaking widely deployed public-key cryptography are expressed in logical qubits and in the operations they must sustain, which is what turns a research result into a deadline for post-quantum cryptography migration.
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