OBSERVATION AND MEASUREMENT OF AN OPTICAL AHARONOV-ALBERT-VAIDMAN EFFECT

The notion of the weak value of a quantum mechanical observable has re cently been introduced by Aharonov ct al., hereafter referred to as AA V. Their analysis led to the controversial result that the weak values for an observable can lie well outside its associated eigenvalue spec trum. We have performed an experiment similar to one proposed by Knigh t ct al. and demonstrated that weak values can be observed and that th ey exhibit the properties described by AAV within the regime of validi ty defined by Duck et al. It is shown that these properties are produc ed by interference phenomena that result from the loss of 'welcher Weg ' information. We have also identified an intrinsic 'weak energy' whic h appears in the equation of motion for the weak value of an observabl e and have shown that it can be non-vanishing only for systems which a re both preselected and postselected. It is noted that such a 'weak sp in-rotation coupling energy' can be associated with our experiment and that it is manifested in our experimental data. We suggest that prese lection and postselection measurement techniques could be useful for t he creation and study of physical phenomena that might be otherwise un observable.