- We present a scheme for realization of quantum-mechanical weak values of observables using entangled photons produced in parametric down-conversion. We consider the case when the signal and idler modes are respectively in a coherent state and vacuum. We use a low-efficiency detector to detect the photons in the idler mode. This weak detection leads to a large displacement and fluctuations in the signal field's quantum state which can be studied by monitoring the photon number and quadrature distributions. This also demonstrates the effect of weak measurement of one partner on its twin. 2007 The American Physical Society.