  We discuss the conditional preparation of single photons via parametric
down-conversion. This technique is commonly used as a single photon source in
modern quantum optics experiments. A significant problem facing this technique
is the inability of present day photo-detectors to resolve photon number. This
results in mixing with higher photon number terms. To overcome this several
techniques have been proposed, including multi-port detection and time-division
multiplexing. These techniques help approximate number resolving detection even
when using non-number resolving detectors. In this paper we focus on 2-port
detection, the simplest such scheme. We show that by making the 2-port device
asymmetric the fidelity of prepared photons can be improved.
