  We present a scheme for tuning two quantum point contacts as a
quantum-limited charge detector. Based on the scattering matrix approach, we
analyze a general condition of quantum-limited detection with a single-channel
quantum detector possessing time-reversal symmetry. From this analysis we find
that quantum-limited detection can be easily realized with two quantum point
contacts connected in series, which is not possible if only a single quantum
point contact is used. We also discuss the sensitivity enhancement due to
multiple reflections of the two point contacts.
