Coherence Time (T1, T2)
How long a qubit holds its quantum state before decoherence destroys it. T1 is the energy relaxation time; T2 is the dephasing time. Longer is better.
Coherence time is a measure of how long a qubit can maintain its quantum state. It is typically expressed as two related quantities: T1 (the energy relaxation time, measuring how long before the qubit loses its energy state) and T2 (the dephasing time, measuring how long before the qubit loses its phase information).
Both T1 and T2 set hard limits on what can be computed. If a quantum algorithm requires more time to run than the qubit's coherence time, the computation will fail because the quantum information has been destroyed before the answer can be extracted.
Neutral atom and trapped ion systems generally have longer coherence times than superconducting qubits, measured in seconds rather than microseconds. The tradeoff is that their gate speeds are typically slower.