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What is quantum error correction?
Quantum error correction spreads one logical qubit over many physical qubits so that errors caused by noise can be found and fixed without measuring the qubits directly.
Qubits interact with their environment and make errors, and quantum information cannot be copied directly (the no-cloning theorem). Error-correcting codes therefore spread information over many qubits with entanglement and locate errors by measuring only parity information called the “syndrome”.
If the physical error rate is below a certain threshold, the logical error rate falls as the code grows. In 2024 Google showed on its Willow processor that the surface code works below this threshold, an important step towards fault-tolerant quantum computers.
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Related concepts
- Surface code · The surface code arranges qubits on a two-dimensional grid and uses only operations between neighbouring qubits; it is the most widely tested quantum error-correcting code today.
- Decoherence · Decoherence is a qubit gradually losing its superposition and entanglement, its quantum properties, through unwanted interaction with its environment.
- NISQ (noisy intermediate-scale quantum) · NISQ is the current era of quantum computing, named by John Preskill in 2018: devices with tens to a few hundred noisy qubits and no error correction.