Abstract

A new kind of quantum optics experiment in which atomic-wave interferometry is used to study quantum fields and measure their photon number is discussed^{[1, 2, 3]}. The essence of the method is that a non resonant field stored in a cavity behaves as a retarding medium for the matter wave of an atom traversing the cavity, producing on this wave a phase shift proportional to the photon number in the field. Recording the atomic interferences results in the reduction of the field into a Fock state with a well defined photon number. The process is a paradigm of a Quantum Nondemolition measurement. In the course of the field evolution, “Schrödinger cats” states, which are quantum superpositions of classical fields with different phases or amplitudes, are generated. This method can be implemented in various schemes, the interference affecting either the external degrees of freedom of the atom (spatial interferences) or its internal states (polarization interferences). In the most promising scheme,^[2−4] a microwave field stored in a high Q superconducting cavity is manipulated by performing Ramsey type interferometric measurements on a beam of Rydberg atoms crossing the cavity. The evolution of the field undergoing quantum jumps between states with different photon numbers can be detected. An experiment in progress to study these effects will be described, along with possible applications.

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