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AIP conference proceedings

Radiative shifts in cavity QED and their use in quantum nondemolition measurement of small photon number fields

S. Haroche, M. Brune, J. M. Raimond

Published Jan 1, 1991• 0 citations
Publisher

Abstract

The coupling of atoms to high Q cavities results in strong modifications of the field and atomic frequencies. These effects are most conveniently analyzed in the ‘‘dressed atom’’ point of view. The vacuum Rabi splitting of the atom‐cavity resonance lines is now being observed in optical and microwave experiments where small samples of atoms resonantly interact with an empty cavity mode. Observation of this effect on a single atom coupled to a cavity vacuum is a realistic goal for experiments. Similar shifts are expected when the cavity mode is weakly excited. The detection of these shifts appears very promising for Quantum nondemolition measurements of very small photon numbers. We propose an experiment in which a microwave field stored in a high Q cavity is continuously monitored with the help of a non‐resonant beam of slow circular Rydberg atoms and we discuss possible applications of this method.

DOI
10.1063/1.40958

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The coupling of atoms to high Q cavities results in strong modifications of the field and atomic frequencies. These effects are most conveniently analyzed in the ‘‘dressed atom’’ point of view. The vacuum Rabi splitting of the atom‐cavity resonance lines is now being observed in optical and microwave experiments where small samples of atoms resonantly interact with an empty cavity mode. Observation of this effect on a single atom coupled to a cavity vacuum is a realistic goal for experiments. Similar shifts are expected when the cavity mode is weakly excited. The detection of these shifts appears very promising for Quantum nondemolition measurements of very small photon numbers. We propose an experiment in which a microwave field stored in a high Q cavity is continuously monitored with the help of a non‐resonant beam of slow circular Rydberg atoms and we discuss possible applications of this method.
A quantum-nondemolition method to measure the number of photons stored in a high-Q cavity, introduced by Brune et al. [Phys. Rev. Lett. 65, 976 (1990)], is described in detail. It is based on the detection of the dispersive phase shift produced by the field on the wave function of nonresonant atoms crossing the cavity. This shift can be measured by atomic interferometry, using the Ramsey separated-oscillatory-field method. The information acquired by detecting a sequence of atoms modifies the field step by step, until it eventually collapses into a Fock state. At the same time, the field phase undergoes a diffusive process as a result of the back action of the measurement on the photon-number conjugate variable. Once a Fock state has been generated, its evolution under weak perturbation can be continuously monitored, revealing quantum jumps between various photon numbers. When applied to an initial coherent field, the intermediate steps of the measuring sequence produce quantum superpositions of classical fields, known as ``Schr\"odinger cat states.'' Ways to prepare and detect these states in a cavity subjected to a weak relaxation process are discussed. The effects analyzed in this article could realistically be observed by using circular Rydberg atoms and very high-Q superconducting microwave cavities. The possibility of photon ``manipulation'' through nonresonant atom-field interactions opens a domain in cavity QED studies.
We describe a new generation of promising experiments in Cavity QED. Their goal is to manipulate weak quantum fields made of a few photons by coupling them adiabatically to atomic beams. By detecting the modification of the atomic trajectories, or the change of the atomic matter‐wave phase, information is acquired on the field photon number in a Quantum Nondemolition process. Various versions of the experiments are considered, which are reminiscent of the classic Stern‐Gerlach or Young double slit experiments. Preparation of ‘‘Schrödinger cats’’ of the field also becomes possible. The practical feasibility of these experiments is discussed.
Circular Rydberg atoms, detected by the very sensitive and state selective field ionization method, can be used to measure and manipulate quantum fields stored in a cavity. The method is based on an interferometric detection of the dispersive energy shifts experienced by these atoms when they interact with a slightly off‐resonant field mode sustained by a cavity which the atoms cross one at a time. These shifts give rise to a translation of the Ramsey fringe pattern observed in the field ionization signal of the atoms. The method consitutes a non‐destructive way of photon counting. In this experiment, non local correlations between the atom and the cavity field are created, which could be used to perform new types of Einstein‐Podolsky‐Rosen experiments. Non classical fields could also be generated, which would display some of the properties discussed by Schrödinger in his famous ‘‘cat paradox.’’ We present the theory of these experiments which until very recently would have been considered as mere ‘‘gedanken’’ ones and we describe the operation of a Rydberg atom interferometer which has already enabled us to detect subphoton fields and to measure vacuum field effects in a cavity.