Are Atoms the Bricks of Molecules?

The celebrations of the International Year of the Periodic Table (IYPT2019) brought to public attention that the 118 atoms, included and systematized in the periodic table, are the building blocks of everything that exists in the universe. We can even go further in that line of thought, by including time, saying that everything that existed in the past, present or the future in the universe is made of atoms: The periodic table is a Window to Eternity. Quantum Mechanics changed the paradigm of looking at atomic and molecular structures by creating the concept of probabilistic charge distributions commonly known as the electronic cloud. But this electronic cloud cannot be reproduced from the superposition of atomic charge distributions except at nuclei and core electron levels. The valence electrons have their distributions strongly affected by constructive and destructive interference to a point where there will be no resemblance to their atomic counterparts. Can we recover the concept of atom from that distorted mess? How do we split the charge density of a bond between the two intervening atoms? What are lone pairs? Do they belong to an atom? What’s a ring (like in benzene)? We will try to bridge the gap between the quantum formulation and the traditional chemical view.info:eu-repo/semantics/publishedVersio

Geometries for universal quantum computation with matchgates

Matchgates are a group of two-qubit gates associated with free fermions. They are classically simulatable if restricted to act between nearest neighbors on a one-dimensional chain, but become universal for quantum computation with longer-range interactions. We describe various alternative geometries with nearest-neighbor interactions that result in universal quantum computation with matchgates only, including subtle departures from the chain. Our results pave the way for new quantum computer architectures that rely solely on the simple interactions associated with matchgates.Comment: 6 pages, 4 figures. Updated version includes an appendix extending one of the result

The reduction of the closest disentangled states

We study the closest disentangled state to a given entangled state in any system (multi-party with any dimension). We obtain the set of equations the closest disentangled state must satisfy, and show that its reduction is strongly related to the extremal condition of the local filtering on each party. Although the equations we obtain are not still tractable, we find some sufficient conditions for which the closest disentangled state has the same reduction as the given entangled state. Further, we suggest a prescription to obtain a tight upper bound of the relative entropy of entanglement in two-qubit systems.Comment: a crucial error was correcte

Portuguese science needs a shakeup

info:eu-repo/semantics/publishedVersio

Is Small Perfect? Size Limit to Defect Formation in Pyramidal Pt Nanocontacts

We report high resolution transmission electron microscopy and ab initio calculation results for the defect formation in Pt nanocontacts (NCs). Our results show that there is a size limit to the existence of twins (extended structural defects). Defects are always present but blocked away from the tip axes. The twins may act as scattering plane, influencing contact electron transmission for Pt NC at room temperature and Ag/Au NC at low temperature.Comment: 4 pages, 3 figure

Spin-orbit mode transfer via a classical analog of quantum teleportation

We translate the quantum teleportation protocol into a sequence of coherent operations involving three degrees of freedom of a classical laser beam. The protocol, which we demonstrate experimentally, transfers the polarisation state of the input beam to the transverse mode of the output beam. The role of quantum entanglement is played by a non-separable mode describing the path and transverse degrees of freedom. Our protocol illustrates the possibility of new optical applications based on this intriguing classical analogue of quantum entanglement.Comment: 5 pages, 7 figure