physical qubit

A physical qubit is the hardware itself: one trapped ion, one superconducting circuit, whichever technology a machine is built from. It is the thing an engineer can point at. Every physical qubit is imperfect, and the rate at which it fails is what researchers call its physical error rate.

The distinction matters because the qubits quoted in a headline are often not the qubits doing the useful work. Error correction combines multiple physical qubits into a logical qubit, so that the physical errors are suppressed enough for a computation to finish with a usable answer. Physical qubits are therefore the raw material and the source of the noise, not the units a program is written in.

The ratio between the two is the central engineering problem. In the Nature paper reporting a surface code operating below the error threshold, the best logical qubit was built from 101 of the processor's physical qubits, and the authors calculate that reaching a logical error rate of one in a million would take 1,457 of them for a single logical qubit. The whole processor held 105.