Photon echo on a three-level system of discrete spectrum of a spherical quantum dot H. S. Nikoghosyan, S. I. Pokutnyi and G. Nikoghosyan Eur. Phys. J. Plus, 140 8 (2025) 731 Published online: 05 August 2025 DOI: 10.1140/epjp/s13360-025-06635-z