We describe an experiment in which photon pairs from a pulsed parametric down-conversion (PDC) source were coupled into single-mode fibers with heralding efficiencies as high as 70%. Heralding efficiency or mode preparation efficiency is defined as the probability of finding a photon in a fiber in a definite state, given the detection of its twin. Heralding efficiencies were obtained for a range of down-conversion beam-size configurations. Analysis of spatial and spectral mode selection, and

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... ir mutual correlation, provides a practical guide for engineering PDC-produced single photons in a definite mode and spectral emission band. The spectrum of the heralded photons were measured for each beam configuration, to determine the interplay between transverse momentum and spectral entanglement on the preparation efficiency. Performance of photon-number resolving transition-edge sensors with integrated 1550 nm resonant cavities," IEEE Trans. Appl. Supercond. 15, 575-578 (2005). 19. The fiber-coupled bandwidth in this Type-I PDC experiment is significantly larger than that of the Type-II PDC setup in reference [11] . 20. Calculation according to the NIST program on the web page http://physics.nist.gov/Divisions/Div844/facilities/cprad/cprad.html 21. M. H. Rubin, "Transverse correlation in optical spontaneous parametric down-conversion," Phys. Rev. A 54, 5349-5360 (1996). 22. We calibrated the detector efficiency by using a mulitmode fiber in the same configuration of coupling lenses used with the single mode fiber after testing that the CUT collected all the correlated photons. 23. Fig. 6 . Transverse intensity profile of the heralded PDC photon by a fixed heralding single direction and wavelength, calculated exactly (a) and with the present approximation (b).