We measured the optical decoherence times $T_2$, or, equivalently, the homogeneous line width, in an Er-doped optical fiber at low temperature as a function of external magnetic field and temperature using two-pulse photon echoes. The decoherence times were up to 230 ns at fields above 3 T. The magnitude of the line narrowing induced by a magnetic field of 3 T is 2.5 MHz, which is anomalously large compared to that typical for oxide crystals with similar ${\mathrm{Er}}^{3+}$ concentration. This is interpreted as evidence for dephasing by coupled spin-elastic tunneling modes where the normal glass tunneling modes acquire a magnetic character by coupling to the ${\mathrm{Er}}^{3+}$ spin.

• Received 26 October 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.033602