  We present a fiber-based continuous-variable quantum key distribution system.
In the scheme, a quantum signal of two non-orthogonal weak optical coherent
states is sent through a fiber-based quantum channel. The receiver
simultaneously measures conjugate quadratures of the light using two homodyne
detectors. From the measured Q-function of the transmitted signal, we estimate
the attenuation and the excess noise caused by the channel. The estimated
excess noise originating from the channel and the channel attenuation including
the quantum efficiency of the detection setup is investigated with respect to
the detection of effective entanglement. The local oscillator is considered in
the verification. We witness effective entanglement with a channel length of up
to 2km.
