3,245 research outputs found
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
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
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