Towards quantum superpositions of a mirror.

We propose an experiment for creating quantum superposition states involving of the order of 10(14) atoms via the interaction of a single photon with a tiny mirror. This mirror, mounted on a high-quality mechanical oscillator, is part of a high-finesse optical cavity which forms one arm of a Michelson interferometer. By observing the interference of the photon only, one can study the creation and decoherence of superpositions involving the mirror.

Theory of an entanglement laser.

We consider the creation of polarization entangled light from parametric down-conversion driven by an intense pulsed pump field inside a cavity. The multiphoton states produced are close approximations to singlet states of two very large spins. A criterion is derived to quantify the entanglement of such states.

Multiphoton entanglement concentration and quantum cryptography.

Multiphoton states from parametric down-conversion can be entangled both in polarization and photon number. Maximal high-dimensional entanglement can be concentrated postselectively from these states via photon counting. This makes them natural candidates for quantum key distribution, where the presence of more than one photon per detection interval has up to now been considered undesirable.

Experimental quantum cloning of single photons.

Although perfect copying of unknown quantum systems is forbidden by the laws of quantum mechanics, approximate cloning is possible. A natural way of realizing quantum cloning of photons is by stimulated emission. In this context, the fundamental quantum limit to the quality of the clones is imposed by the unavoidable presence of spontaneous emission.

Experimental violation of a spin-1 bell inequality using maximally entangled four-photon states.

We demonstrate the experimental violation of a spin-1 Bell inequality. The spin-1 inequality is based on the Clauser, Horne, Shimony, and Holt formalism. For entangled spin-1 particles, the maximum quantum-mechanical prediction is 2.