’Interaction-free’ measurement: possibilities and limitations
The so-called ’interaction-free’ measurement is a very interesting quantum effect that allows discovering the presence of an opaque object in a given spatial domain, with the probability that the object absorbs a photon being, in principle, as low as desired. This probability is bounded from below only by a value of the order of 1/ωτ, where ω is the frequency of light and τ is the measurement time. This corresponds to the average absorbed energy of the order of \hbar/τ. The ’interaction-free’ technique can also be used to measure the coordinate of an
object but only under the condition that the object is prepared in a special ’discretized’ quantum state. Such is, for instance, the state of a ponderomotive meter of electromagnetic energy, which, in principle, enables the interaction-free’ measurement
of the energy contained in an electromagnetic cavity. Estimations show that with modern experimental equipment and with the help of ’interaction-free’ measurement, single atoms can be registered inside optical cavities.