Quantum Nondemolition Measurement of a Single Photon

Synopsis

The interest in high precision measurements led to the discovery of a quantum nondemolition (QND) technique. The idea was to prevent the unavoidable disturbance due to the first measurement of the system and subsequent free motion, from demolishing the possibility of a second accurate measurement. Thus, a method of QND measurement of a number of photons in a cavity made of highly reflecting mirrors was found. Moreover, after a decade of work on experiments of this kind, non-destructive measurement of a single photon became possible. A Rydberg atom is sent through the cavity to reveal the presence or absence of a photon inside it. A phase of an atomic wavefunction that can be measured is shifted if the atom absorbs and reemits the photon passing the cavity. Ramsey interferometry and other methods of atomic physics that are essential for performing the experiment will be discussed in this talk.