#QEC is a multi-faceted challenge spanning the quantum and classical computing worlds.  To implement QEC, quantum hardware companies need qubits with:  1. Physical qubit error rates below the ‘[[QEC threshold]]’  2. A low physical to logical qubit ratio, called the ‘[[QEC overhead]]’ Using a QEC correcting code to identify the error, the QEC threshold is the maximum physical error rate the code can withstand before failing. If a device’s physical error rates are below the threshold, the redundancy suppresses errors. If they exceed the threshold, the additional physical qubits compound errors.  The QEC threshold is around 99% fidelity (i.e. an error rate of 10-2) for two-qubit gates, meaning QEC becomes possible with error rates below this value.  However, since the overhead approaches infinity at the QEC threshold, we must be well below the QEC threshold for the overhead to be practical. Therefore, the practical QEC threshold is widely regarded as about 99.9% physical two-qubit gate fidelity (i.e. an error rate of 10-3), also known as ‘the three nines’.  Below the QEC threshold, there are diminishing returns for reducing physical error rates. For example, a quantum computer with:  - 99.9% fidelity versus a 99.2% fidelity quantum computer would use 100x fewer qubits.  - 99.99% fidelity versus a 99.9% fidelity quantum computer would use 4x fewer qubits.  - 99.999% versus a 99.99% quantum computer would use 2.25x fewer qubits.  - 99.9999% versus a 99.999% quantum computer would use 1.78x fewer qubits.