  We show that a noninvasive,``negative-result measurement'' can be realized in
quantum dot systems. The measurement process is studied by applying the
Schr\"odinger equation to the whole system (including the detector). We
demonstrate that the possibility of observing a particular state out of
coherent superposition leads to collapse of the corresponding nondiagonal
density-matrix elements of the measured system. No additional reduction
postulate is needed. Experimental consequences of the collapse time and the
relativistic requirement are discussed for mesoscopic and optical systems.
