  We investigate the effect of the charge state measurement of the Kondo
singlet for a quantum dot transistor via a capacitively coupled quantum point
contact detector. By employing the variational ansatz for the singlet ground
state of the quantum dot combined with the density matrix formulation for the
coupled system, we show that the coherent Kondo singlet is destroyed by the
phase-sensitive as well as the magnitude-sensitive detection in the
transmission/reflection coefficients at the quantum point contact. We argue
that the phase-sensitive component of the decoherence rate may explain the
anomalous features observed in a recent experiment by Avinun-Kalish {\em et
al.} (Phys. Rev. Lett. {\bf 92}, 156801 (2004)). We also discuss the
correlations of the shot noise at the quantum point contact detector and the
decoherence in the quantum dot.
