XI International Symposium on Photon Echo and Coherent Spectroscopy (PECS - 2017): preface K.R. Karimullin, A.A. Kalachev, A.V. Naumov, S.V. Sazonov and V.V. Samartsev EPJ Web Conf., 161 (2017) 00001 Published online: 31 October 2017 DOI: 10.1051/epjconf/201716100001