Error Correction

Noise and Decoherence: What Actually Goes Wrong on Real Qubits

Every tutorial up to here pretended qubits are perfect. They aren't. This tutorial covers the four main noise processes every quantum dev should know cold — relaxation, dephasing, depolarization, and readout error — with their Kraus operator forms, their T₁/T₂ signatures, and a runnable Qiskit experiment that measures them on a real device.

intermediate · ~22 min · prereq: Gates & Circuits track

The Surface Code and Willow: What Below-Threshold Actually Means

Google's Willow chip (December 2024) was the first demonstration of a quantum error-correcting code with errors that decrease as you add qubits — the 'below threshold' result the field had chased for 30 years. This tutorial explains what the surface code is, why the threshold theorem matters, and what Willow's numbers imply for the path to fault-tolerant quantum computing.

advanced · ~25 min · prereq: Tutorial 18: Noise and Decoherence