  We study the back action on a coupled multiple-qubit system induced by a
quantum cavity probe in a non-demolition quantum measurement scheme. The exact
quantum state stochastic Schr\"{o}dinger equation is derived to systematically
investigate the dynamics of quantum entanglement of the multiple-qubit system
inside the cavity probe. Although quantum entanglement cannot be directly
measured through experiments, the back action on quantum entanglement in the
multi-qubit system is witnessed by applying the exact quantum state stochastic
equations. Our results demonstrate that the analysis on the sensitivity of the
quantum measurement should include not only the standard quantum limit of the
canonical operators, but also the back action on the quantum entanglement. Our
new method proposed a theoretical approach to investigate the effects of the
measurement back action on the multi-qubit systems induced by the cavity probe.
