Colloquium: Manipulating quantum entanglement with atoms and photons in a cavity

After they have interacted, quantum particles generally behave as a single nonseparable entangled system. The concept of entanglement plays an essenti al role in quantum physics. We have performed entanglement experiments with Rydberg atoms and microwave photons in a cavity and tested quantum mechani cs in situations of increasing complexity. Entanglement resulted either fro m a resonant exchange of energy between atoms and the cavity field or from dispersive energy shifts affecting atoms and photons when they were not res onant. With two entangled particles (two atoms or one atom and a photon), w e have realized new versions of the Einstein-Podolsky-Rosen situation. The detection of one particle projected the other, at a distance, in a correlat ed state. This process could be viewed as an elementary measurement, one pa rticle being a "meter" measuring the other. We have performed a "quantum no ndemolition" measurement of a single photon, which we detected repeatedly w ithout destroying it. Entanglement is also essential to understand decohere nce, the process accounting for the classical appearance of the macroscopic world. A mesoscopic superposition of states ("Schrodinger cat") gets rapid ly entangled with its environment, losing its quantum coherence. We have pr epared a Schrodinger cat made of a few photons and studied the dynamics of its decoherence, in an experiment which constitutes a glimpse at the quantu m/classical boundary. We have also investigated entanglement as a resource for the processing of quantum information. By using quantum two-state syste ms (qubits) instead of classical bits of information, one can perform logic al operations exploiting quantum interferences and taking advantage of the properties of entanglement. Manipulating as qubits atoms and photons in a c avity, we have operated a quantum gate and applied it to the generation of a complex three-particle entangled state. We finally discuss the perspectiv es opened by these experiments for further fundamental studies.