Glossary / Quantum computing
Logical qubit
A single, robust qubit made by bundling many physical qubits together and running error correction on them.
This is the qubit actually used for computation. Tens to thousands of physical qubits are grouped, and error correction runs continuously, so that together they behave as one logical qubit. Whether adding more physical qubits makes the logical qubit better, rather than worse, was the make-or-break question for the whole approach.
In 2024 Google showed on real hardware that each step up in code size roughly halved the logical error rate[1]. When comparing quantum computers, the number that matters is how many logical qubits they have and how good those are — not the raw physical qubit count.
Related terms
- QubitThe basic unit of information in a quantum computer. Rather than being either 0 or 1, it can be in a superposition of both.
- Quantum error correctionSpreading the information of fragile qubits across many of them, so that errors can be detected and fixed.
- Quantum LDPC codeA family of error-correcting codes that protect many logical qubits with few physical ones; each check looks at only a handful of qubits.