Coherence times (T1 and T2)

T1 (relaxation) is the characteristic time for an excited qubit to decay to its ground state. T2 (dephasing) is the characteristic time over which superposition phase stays coherent; T2 ≤ 2·T1. The useful intuition is a budget: your circuit's wall-clock duration must be a small fraction of T1/T2, which is why gate speed matters as much as gate count.

Both are measured by standard pulse sequences (inversion recovery for T1; Ramsey or Hahn-echo experiments for T2 — echo values are typically longer than Ramsey values). Reported values may be per-qubit bests, medians, or means, measured at a specific calibration moment.

Cross-technology comparison is close to meaningless in isolation: trapped ions boast coherence times of seconds to minutes but execute gates ~1000× slower than superconducting qubits with T1 in the hundreds of microseconds. The dimensionless figure that matters is roughly coherence time ÷ gate duration — how many operations fit in the budget. Also check whether a T2 figure is Ramsey or echo-based, and whether it is a median or a hero qubit.