What is a logical qubit?
A reliable qubit built from many physical qubits using error correction.
A logical qubit spreads one qubit’s worth of information across many physical qubits, so that errors on a few of them can be found and fixed. With today’s error rates, one good logical qubit takes somewhere from a few hundred to about a thousand physical ones, depending on the method.
Algorithms are written for logical qubits. Breaking RSA-2048 with Shor’s algorithm takes on the order of a thousand or more logical qubits, which is where estimates of hundreds of thousands to millions of physical qubits come from.
Related
- Physical qubitOne actual qubit on a chip or in a trap. Physical qubits make errors, roughly once every few hundred to few thousand operations today.
- Quantum error correctionMethods that protect quantum information by spreading it over many qubits and repeatedly checking for errors.
- Shor's algorithmA quantum algorithm, published by Peter Shor in 1994, that factors large numbers and breaks the encryption most of the internet uses for key exchange and signatures.