Nature 453, 203–206 (2008)

The authors report that in a recent extension of this Letter, considering different concentrations, they were not able to reproduce the very weak signal of the ground-state EPR transition (the red curves termed ‘1/2’ in Figs 3 and 4). The authors report that the signal is hardly observable (with intensity lower than that of the cavity background); but the more important S = 3/2 transitions (blue curves in Figs 3 and 4), which give rise to well-defined oscillations with several periods and high signal intensity, are reproducible. The extension of the work — about the concentration dependence of decoherence — will be published1 but leads to the same major conclusion as in the original manuscript. The authors thank J. Du and colleagues for independently bringing the mentioned problem to their attention.