T1 and T2 Coherence Times
The two standard clocks for qubit decay: T1 measures energy relaxation, T2 measures how long a qubit holds a definite phase. T2 usually limits computation.
T1 and T2 are the two standard clocks for qubit decay. T1, energy relaxation, measures how long a qubit stays in its excited state before flopping to ground, how long a 1 stays a 1. T2, dephasing, measures how long the qubit holds a definite phase relationship in a superposition, how long it remembers what kind of '0 and 1 at once' it was.
T2 is always the stricter master (it can never exceed 2×T1) and usually the one that limits computation. The numbers vary wildly by platform: superconducting transmons live in the hundreds-of-microseconds range, trapped ions and neutral atoms can hold coherence for seconds to minutes.
What matters isn't the raw time but the ratio of coherence time to gate time, how many operations fit inside the qubit's attention span.