Efficient and pure femtosecond-pulse-length source of polarization-entangled photons by Weston, MM, Chrzanowski, HM, Wollmann, S, Boston, A, Ho, J, Shalm, LK, Verma, VB, Allman, MS, Nam, SW, Patel, RB, Slussarenko, S, Pryde, GJ