‘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 $\omega\tau$, where $\omega$ is the frequency of light and $\tau$ is the measurement time. This corresponds to the average absorbed energy of the order of $\hbar/\tau$. 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.