Clock transitions generated by defects in silica glass

Clock transitions (CTs) in spin systems, which occur at avoided level crossings, enhance quantum coherence lifetimes T2 because the transition becomes immune to the decohering effects of magnetic field fluctuations to first order. We present the first electron-spin resonance characterization of CTs in certain defect-rich silica glasses, noting coherence times up to 16 μs at the CTs. We find CT behavior at zero magnetic field in borosilicate and aluminosilicate glasses, but not in a variety of silica glasses lacking boron or aluminum. Annealing reduces or eliminates the zero-field signal. Since boron and aluminum have the same valence and are acceptors when substituted for silicon, we suggest the observed CT behavior could be generated by a spin-1 boron vacancy center within the borosilicate glass, and similarly, an aluminum vacancy center in the aluminosilicate glass.