Logical qubit
논리 큐비트
A group of error-prone qubits bundled together and used as if they were one reliable qubit.
Overview
The qubits actually built on a chip are called physical qubits. A physical qubit gets its value knocked off by even a small disturbance. So several of them are tied into a bundle, made to watch over one another, and fixed whenever one goes wrong, to protect a single piece of information. This bundle is a logical qubit.
Making one logical qubit takes a very large number of physical qubits. The less often physical qubits go wrong, the fewer are needed. So "how many usable logical qubits are there" is closer to a quantum computer's real ability than "how many qubits are there."
An everyday example
One person's memory can be wrong. But if several people each write down the same thing and check it against one another, the right content survives even if one or two get it wrong.
Why it matters
These days quantum companies put the number of logical qubits up front, along with the number of qubits. You have to read the two numbers separately to understand how much an announcement really weighs.