  The use of quantum bits (qubits) in cryptography holds the promise of secure
cryptographic quantum key distribution schemes. Unfortunately, the implemented
schemes can be totally insecure. We provide a thorough investigation of
security issues for practical quantum key distribution, taking into account
channel losses, a realistic detection process, and modifications of the
``qubits'' sent from the sender to the receiver. We first show that even
quantum key distribution with perfect qubits cannot be achieved over long
distances when fixed channel losses and fixed dark count errors are taken into
account. Then we show that existing experimental schemes (based on
``weak-pulse'') are usually totally insecure. Finally we show that parametric
downconversion offers enhanced performance compared to its weak coherent pulse
counterpart.
