Logical error rate
The rate of errors that still remain in a logical qubit after error correction. One of the most important measures of what a quantum computer can really do.
A physical qubit goes wrong roughly once in a thousand operations. Error correction bundles physical qubits to reduce errors, but never to zero. The logical error rate is the probability that the logical qubit fails in each cycle of correction.
What matters is whether the logical error rate falls as more physical qubits are added. If the physical qubits are not good enough, adding more makes things worse. In 2023 Google showed on real hardware, for the first time, that a larger code gave a slightly lower logical error rate[1]. Useful computations are thought to need rates below about one in a billion, still many orders of magnitude away.
Related terms
- Logical qubitA single, robust qubit made by bundling many physical qubits together and running error correction on them.
- Quantum error correctionSpreading the information of fragile qubits across many of them, so that errors can be detected and fixed.
- Fault-tolerant quantum computingRunning quantum computations so that, even though components fail at some rate, layered error correction lets arbitrarily long calculations finish correctly.