Theory of controlled dephasing of the Kondo singlet in a quantum dot

We present a theory of controlled dephasing for the Kondo singlet in a quantum dot (QD) via charge detection in a nearby quantum point contact (QPC) device. 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 scattering coefficients at the quantum point contact. We argue that the phase-sensitive component of the decoherence rate can be much larger than the magnitude-sensitive term in generic situation, which may explain the anomalous features observed in a recent experiment by Avinun-Kalish et al. [Phys. Rev. Lett. 92 (2004) 156801]. We also discuss the correlations of the shot noise at the quantum point contact detector and the decoherence in the quantum dot.