XI International Symposium on Photon Echo and Coherent Spectroscopy (PECS - 2017): prefaceK.R. Karimullin, A.A. Kalachev, A.V. Naumov, S.V. Sazonov and V.V. SamartsevEPJ Web Conf., 161 (2017) 00001DOI: https://doi.org/10.1051/epjconf/201716100001