2. QEC: Surface Codes

Surface Codes & Fault Tolerance

How do we build a reliable computer from unreliable parts?

Quantum Error Correction (QEC): We spread the information of 1 Logical Qubit across many noisy Physical Qubits.

The Surface Code: The leading candidate for QEC. It uses a checkerboard pattern of data and measurement qubits to detect and correct errors without collapsing the state.

Threshold Theorem: If the physical error rate is below a certain threshold (~1%), we can make the logical error rate arbitrarily low by adding more physical qubits.