  The transformation cycle and associated inequality are suggested for the
basic demonstration of the wavefunction reduction in a mesoscopic qubit in
measurements with quantum-limited detectors. Violation of the inequality would
show directly that the qubit state changes in a way dictated by the
probabilistic nature of the wavefunction and inconsistent with the dynamics of
the Schr\"{o}dinger equation: the qubit tunnels through an infinitely large
barrier. Estimates show that the transformation cycle is within the reach of
current experiments with superconducting qubits.
