  The dephasing of a qubit coupled with a point-contact detector is
theoretically studied. We calculate the time evolution of the reduced density
matrix of qubit by using the perturbation expansion. We show that the dephasing
rate is proportional to the temperature at zero bias-voltage, while it is
proportional to the bias-voltage when the bias-voltage is large. We also
evaluate the dephasing rate by using the real time renormalization group method
and show that the higher order processes of the particle-hole excitation
enhances dephasing of qubit.
