Quantum error correction (QEC) is a set of techniques that protect quantum information from errors due to decoherence, gate imperfections, and measurement noise. The key challenge is that measuring a qubit to check for errors collapses its state. QEC solves this by encoding one logical qubit across many physical qubits, and using ancilla qubits for syndrome measurements that reveal error information without disturbing the encoded data. The surface code is the most popular QEC code: it encodes 1 logical qubit in ~1000 physical qubits and can correct errors below a threshold of ~1%. Fault-tolerant quantum computing (FTQC) requires QEC to run algorithms with arbitrarily low error rates. Current NISQ devices do not have full QEC due to the overhead required. Progress toward FTQC is an active research area at IBM, Google, and Microsoft.
Related Terms
Decoherence
HardwareThe loss of quantum properties when a qubit interacts with its environment.
Fidelity
MetricsA measure (0 to 1) of how close an actual quantum operation or state is to the ideal target.
NISQ
HardwareNoisy Intermediate-Scale Quantum — devices with 50–1000 qubits without full error correction.
Ancilla Qubit
FundamentalsAn auxiliary qubit used as a helper in quantum computations, often for error detection or phase kickback.