  We extract the phase coherence of a qubit defined by singlet and triplet
electronic states in a gated GaAs triple quantum dot, measuring on timescales
much shorter than the decorrelation time of the environmental noise. In this
non-ergodic regime, we observe that the coherence is boosted and several
dephasing times emerge, depending on how the phase stability is extracted. We
elucidate their mutual relations, and demonstrate that they reflect the noise
short-time dynamics.
