Limitations on practical quantum cryptography

We provide limits to practical quantum key distribution, taking into accoun t channel losses, a realistic detection process, and imperfections in the " qubits" sent from the sender to the receiver. As we show, even quantum key distribution with perfect qubits might not be achievable over long distance s when the other imperfections are taken into account. Furthermore, existin g experimental schemes (based on weak pulses) currently do not offer uncond itional security for the reported distances and signal strength. Finally we show that parametric down-conversion offers enhanced performance compared to its weak coherent pulse counterpart.