  We address the ultimate charge detection scheme with a quantum point contact.
It is shown that a superposed input state is necessary to exploit the full
sensitivity of a quantum point contact detector. The coherence of the input
state provides an improvement in charge sensitivity, and this improvement is a
result of the fundamental property of the scattering matrix. Further, a
quantum-limited (maximally efficient) detection is possible by controlling the
interference between the two output waves. Our scheme provides the ultimate
sensitivity and efficiency of charge detection with a generic quantum point
contact.
