QUROPE Quantum Information Processing and Communication in Europe

We review quantum information processing with cold neutral particles, that is, atoms or polar molecules. First, we analyze the best suited degrees of freedom of these particles for storing quantum information, and then we discuss both single- and two-qubit gate implementations. We focus our discussion mainly on collisional quantum gates, which are best suited for atom-chip-like devices, as well as on gate proposals conceived for optical lattices.

In recent years, substantial progress has been made in exploringand exploiting the analogy between classical light and matter waves for fundamental investigations and applications. Extending this analogy to quantum matter-wave optics is promoted by the nonlinearities intrinsic to interacting particles and is a stepping stone towards non-classical states. In light optics, twin-photon beams are a key element for generating the non-local correlations and entanglement required for applications such as precision metrology and quantum communication.

Hanbury Brown and Twiss correlations, i.e. correlations in far-field intensity fluctuations, yield fundamental information on the nature of light sources, as highlighted after the discovery of photon bunching. Drawing on the analogy between photons and atoms, comparable observations have been made studying expanding Bose gases. We have used two-point density correlations to study how matter-wave coherence is established when crossing the Bose-Einstein condensation threshold.

The strength of classical correlations is subject to certain constraints, commonly known as Bell inequalities. Violation of these inequalities is the manifestation of non-locality displayed in particular by quantum mechanics, meaning that quantum mechanics can out-perform classical physics at tasks associated with such Bell inequalities. Interestingly, however, there exist situations in which this is not the case.

 In this paper, we introduce a quantum generalization of classical kinetic Ising models (KIM), described by a certain class of quantum many-body master equations. Similarly to KIMs with detailed balance that are equivalent to certain Hamiltonian systems, our models reduce to a set of Hamiltonian systems determining the dynamics of the elements of the many-body density matrix. The ground states of these Hamiltonians are well described by the matrix product, or pair entangled projected states.

A universal quantum simulator is a controlled quantum device that reproduces the dynamics of any other many-particle quantum system with short-range interactions. This dynamics can refer to both coherent Hamiltonian and dissipative open-system evolution. Here we propose that laser-excited Rydberg atoms in large-spacing optical or magnetic lattices provide an efficient implementation of a universal quantum simulator for spin models involving n-body interactions, including such of higher order.