T1 and T2 are the two key timescales characterizing qubit quality. T1 (longitudinal relaxation time, or energy relaxation time) measures how long a qubit in the excited state |1⟩ takes to spontaneously decay to |0⟩. T2 (transverse relaxation time, or dephasing time) measures how long a superposition state maintains its phase coherence. T2 ≤ 2·T1 always. A qubit with T1 = 100 µs and T2 = 80 µs can maintain coherence for about 80 microseconds. Superconducting qubits (IBM, Google) have T1/T2 of 50–500 µs. Trapped-ion qubits (IonQ, Quantinuum) have much longer coherence times — seconds to minutes — but slower gate speeds. Circuit execution time must be much shorter than T2 to avoid significant decoherence errors.
Related Terms
Decoherence
HardwareThe loss of quantum properties when a qubit interacts with its environment.
Qubit
FundamentalsThe fundamental unit of quantum information — the quantum analog of a classical bit.
Fidelity
MetricsA measure (0 to 1) of how close an actual quantum operation or state is to the ideal target.
QPU
HardwareQuantum Processing Unit — the physical hardware chip that executes quantum circuits.