logical qubit

A logical qubit is not a piece of hardware. A review of the field puts it plainly: it is a space of multi-qubit, entangled states contained in a number of physical qubits, designed to allow correction of a particular set of errors that can occur on those physical qubits. One unit of quantum information is spread across many pieces of hardware at once, and deliberately never entrusted to just one of them.

The reason to accept that overhead is a result the Nature paper on surface code error correction states plainly: combining multiple physical qubits into a logical qubit suppresses the logical error rate exponentially as more qubits are added, provided the physical error rate sits below a critical threshold. Past that point, spending more hardware buys steadily better reliability rather than steadily more noise, which is the behaviour a scalable machine needs.

So a logical qubit is the unit a program is actually written in, and the number worth watching. It is also the reason qubit counts can be confusing. A machine advertising a hundred qubits usually means physical ones: Google's Willow processor holds 105, and the below-threshold demonstration combined 101 of them into a single logical qubit, one that outlived its own best physical qubit by a factor of 2.4.