  To improve the performance of a quantum key distribution (QKD) system, high
speed, low dark count single photon detectors (or low noise homodyne detectors)
are required. However, in practice, a fast detector is usually noisy. Here, we
propose a "dual detectors" method to improve the performance of a practical QKD
system with realistic detectors: the legitimate receiver randomly uses either a
fast (but noisy) detector or a quiet (but slow) detector to measure the
incoming quantum signals. The measurement results from the quiet detector can
be used to bound eavesdropper's information, while the measurement results from
the fast detector are used to generate secure key. We apply this idea to
various QKD protocols. Simulation results demonstrate significant improvements
in both BB84 protocol with ideal single photon source and Gaussian-modulated
coherent states (GMCS) protocol; while for decoy-state BB84 protocol with weak
coherent source, the improvement is moderate. We also discuss various practical
issues in implementing the "dual detectors" scheme.
