A quantitative witness for Greenberger-Horne-Zeilinger entanglement

Along with the vast progress in experimental quantum technologies there is an increasing demand for the quantification of entanglement between three or more quantum systems. Theory still does not provide adequate tools for this purpose. The objective is, besides the quest for exact results, to develop operational methods that allow for efficient entanglement quantification. Here we put forward an analytical approach that serves both these goals. We provide a simple procedure to quantify Greenberger-Horne-Zeilinger-type multipartite entanglement in arbitrary three-qubit states. For two qubits this method is equivalent to Wootters' seminal result for the concurrence. It establishes a close link between entanglement quantification and entanglement detection by witnesses, and can be generalised both to higher dimensions and to more than three parties.

The Forbidden Quantum Adder

Quantum information provides fundamentally different computational resources than classical information. We prove that there is no unitary protocol able to add unknown quantum states belonging to different Hilbert spaces. This is an inherent restriction of quantum physics that is related to the impossibility of copying an arbitrary quantum state, i.e., the no-cloning theorem. Moreover, we demonstrate that a quantum adder, in absence of an ancillary system, is also forbidden for a known orthonormal basis. This allows us to propose an approximate quantum adder that could be implemented in the lab. Finally, we discuss the distinct character of the forbidden quantum adder for quantum states and the allowed quantum adder for density matrices.

European Newspapers’ Digital Transition: New Products and New Audiences

The adaptation of traditional newspapers to new digital media and its interface, far from being a mere technical transformation, has contributed to a gradual change in the media themselves and their audiences. With a sample including the top general information pay newspaper in each of the 28 countries of the European Union, this research has carried out an analysis using 17 indicators divided in 4 categories. The aim is to identify the transformations that the implementation of digital media have brought to the top European newspapers. In general terms, the results show that most dailies have managed to keep their leadership also in online environment. Moreover, an emerging group of global media is growing up, based in preexisting national media. Digital and mobile media have contributed to the appearance of new consumption habits as well, where users read more superficially and sporadically. The audience uses several formats at a time, and digital devices already bring the biggest amount of users to many media. The Internet-created new information windows –search engines, social networks, etc. –are also contributing to the change in professional work routines.

Quantum Emulation of Gravitational Waves

Gravitational waves, as predicted by Einstein's general relativity theory, appear as ripples in the fabric of spacetime traveling at the speed of light. We prove that the propagation of small amplitude gravitational waves in a curved spacetime is equivalent to the propagation of a subspace of electromagnetic states. We use this result to propose the use of entangled photons to emulate the evolution of gravitational waves in curved spacetimes by means of experimental electromagnetic setups featuring metamaterials.