“…The non-exponential shape of the efficiency curve shown in figure 4 indicates decoherence due to spectral diffusion [38,39] that might be mitigated by DD techniques [28,29,40,41,42]. To demonstrate that DD can indeed extend the coherence time we performed storage experiments with increasing number of sequences n s = 1, 2, 4, 8 while maintaining n p = 2 (the global number of pulses is then n = 2, 4, 8, 16).…”