X-entangled biphotons: Schmidt number for 2D model D.B. Horoshko, G. Patera, A. Gatti and M.I. Kolobov Eur. Phys. J. D, 66 9 (2012) 239 Published online: 12 September 2012 DOI: 10.1140/epjd/e2012-30099-y