  Quantum measurement of a solid-state qubit by a mesoscopic detector is of
fundamental interest in quantum physics and an essential issue in quantum
computing. In this work, by employing a unified quantum master equation
approach constructed in our recent publications, we study the
measurement-induced relaxation and dephasing of the coupled-quantum-dot states
measured by a quantum-point-contact. Our treatment pays particular attention on
the detailed-balance relation, which is a consequence of properly accounting
for the energy exchange between the qubit and detector during the measurement
process. As a result, our theory is applicable to measurement at arbitrary
voltage and temperature. Both numerical and analytical results for the qubit
relaxation and dephasing are carried out, and new features are highlighted in
concern with their possible relevance to future experiments.
