Quantum coherent control of Gaussian multipartite entanglement

Quantum information has reached a stage where real-world applications stimulate an intense research for the implementation of reliable and practical protocols for quantum communication and information processing. The implementation of such protocols, though, requires distributing quantum correlations (entanglement) among a number of degrees of freedom (modes) increasing with the complexity of the task to achieve. In the large-number-of-modes regime, the most promising example is probably one-way quantum computation in which the computation is achieved by applying local measurements to a set of modes initially in a cluster state [1]. However the generation of multipartite entangled states requires experimental configurations whose complexity increases with the number of the modes involved by means of optical devices. In contrast, a practical source should be compact, scalable, and permit to master the quantum properties of the generated states even when the number of modes is very large. We introduce a general approach for the generation of arbitrary Gaussian multipartite entangled states which is based on the use of naturally multimode parametric down-conversion processes, either in the spatial or in the temporal domain, either for single pass devices or for cavity devices. The advantage of this scheme relies on the fact that the generation of such quantum states can be easily controlled by an experimentally accessible parameter. In general the dynamics of parametric interactions in the low-gain regime is described by a linear operator that couples the different relevant modes.