MEASUREMENT OF A WEAK VALUE

''Weak measurements'' are measurements in which the coupling between t he measuring device and the observable to be measured is so weak that the eigenvalues of the observable are not resolved. Under certain circ umstances the corresponding eigenfunctions can be made to interfere, p roducing a measurement result which is outside the allowed range of th e observable's eigenvalues. We present the first measurement of this s o-called ''weak value'' using an optical experiment. In our experiment , the small displacement between the two orthogonally polarized compon ents of a laser beam passed through a birefringence crystal is measure d. We use a numerical simulation to show that this phenomenon may be p ractical for detecting and amplifying small effects.