Down with T2, long live T3!

One of the main metrics used to describe a quantum computer is qubit coherence time, typically denoted as $T_2$. There is just one problem: $T_2$ is pretty useless at predicting the effect of qubit decoherence on computational errors! In this post, I explain why that is, and argue for adopting a new coherence measure more fit for the task: $T_{3}$.