Logical Qubit
An error-corrected qubit: information spread across many imperfect physical qubits so errors can be caught faster than they pile up. The metric that actually matters.
A logical qubit is an error-corrected qubit: information encoded redundantly across many imperfect physical qubits so that errors can be detected and fixed faster than they accumulate. It's the difference between a qubit you have and a qubit you can trust.
The exchange rate is brutal. Depending on the error-correction code and the quality of the hardware, one logical qubit can cost anywhere from a dozen to several hundred physical qubits. That's why headline qubit counts are mostly noise; a machine with 100 excellent physical qubits can beat one with 1,000 mediocre ones once you ask for logical performance.
When Quantinuum or Atom Computing announce error-correction milestones, logical qubit count and logical error rate are the numbers to read first. Everything else is marketing in a lab coat.