Comment on the ``Novel Isotope Effects Observed in Polarization Echo   Experiments in Glasses''
  The novel isotope and magnetic effects on two level system echoes in glasses observed by Nagel et al.(Phys.Rev.Lett. 92, 245511 (2004)) provide unique experimental insight into the nature of tunneling systems in amorphous solids. We show their observations are consistent with tunneling systems being extended systems having of order 100 moving units, as predicted by theory (Phys.Rev.Lett. 87, 195901 (2001)). The numerous nuclear quadrupole interactions within a tunneling center explain the magnitude of the magnetic and frequency dependence observed in deuterated glycerol. The extended nature of two level systems explains why a small residual effect can still be observed in regular glycerol even with its low abundance of quadrupolar nuclei. 
Vassiliy Lubchenko and Peter G. Wolynes