We report a measurement of the ensemble-averaged transverse spin relaxation time $\left(T_2^{\ast }\right)$ in bulk and few molecules of the organic semiconductor tris-(8-hydroxyquinolinolato aluminum) or ${\text{Alq}}_3$. This system exhibits two characteristic $T_2^{\ast }$ times: the longer of which is temperature independent and the shorter is temperature dependent, indicating that the latter is most likely limited by spin-phonon interaction. Based on the measured data, we infer that the single-particle $T_2$ time is probably long enough to meet Knill’s criterion for fault-tolerant quantum computing even at room temperature. ${\text{Alq}}_3$ is also an optically active organic, and we propose a simple optical scheme for spin qubit readout. Moreover, we found that the temperature-dependent $T_2^{\ast }$ time is considerably shorter in bulk ${\text{Alq}}_3$ powder than in few molecules confined in 1–2-nm-sized cavities. Because carriers in organic molecules are localized over individual molecules or atoms but the phonons are delocalized, we believe that this feature is caused by phonon bottleneck effect.

• Received 7 October 2008

DOI:https://doi.org/10.1103/PhysRevB.78.193306