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Hein S.M.,TU Berlin | Schulze F.,TU Berlin | Carmele A.,Institute For Quantenoptik Und Quanteninformation | Knorr A.,TU Berlin
Physical Review Letters | Year: 2014

In a solid-state platform for quantum information science, the biexciton cascade is an important source of entangled photons. However, the entanglement is usually reduced considerably by the fine-structure splitting of the exciton levels. We show how to counteract this loss of entanglement by applying optical feedback. Substantial control and enhancement of photon entanglement can be achieved by coherently feeding back a part of the emitted signal, e.g., by a mirror, and by tuning the feedback phase and delay time. We present full quantum-mechanical calculations, which include the external photon mode continuum, and discuss the mechanisms leading to the above effects. © 2014 American Physical Society.

Northup T.E.,University of Innsbruck | Blatt R.,Institute For Quantenoptik Und Quanteninformation
Nature Photonics | Year: 2014

Optical communication channels have redefined the scope and applications of classical computing; similarly, photonic transfer of quantum information promises to open new horizons for quantum computing. The implementation of light-matter interfaces that preserve quantum information is technologically challenging, but key building blocks for such devices have recently been demonstrated by several research groups. Here, we outline the theoretical framework for information transfer between the nodes of a quantum network, review the current experimental state of the art and discuss the prospects for hybrid systems currently in development. © 2014 Macmillan Publishers Limited. All rights reserved.

Hauke P.,Institute For Quantenoptik Und Quanteninformation | Hauke P.,University of Innsbruck | Lewenstein M.,Institute Of Ciencies Fotoniques | Lewenstein M.,Catalan Institution for Research and Advanced Studies | Eckardt A.,Max Planck Institute For Physik Komplexer Systeme
Physical Review Letters | Year: 2014

We propose a simple scheme for tomography of band-insulating states in one- and two-dimensional optical lattices with two sublattice states. In particular, the scheme maps out the Berry curvature in the entire Brillouin zone and extracts topological invariants such as the Chern number. The measurement relies on observing - via time-of-flight imaging - the time evolution of the momentum distribution following a sudden quench in the band structure. We consider two examples of experimental relevance: the Harper model with π flux and the Haldane model on a honeycomb lattice. Moreover, we illustrate the performance of the scheme in the presence of a parabolic trap, noise, and finite measurement resolution. © 2014 American Physical Society.

Jungnitsch B.,Institute For Quantenoptik Und Quanteninformation | Moroder T.,Institute For Quantenoptik Und Quanteninformation | Guhne O.,Institute For Quantenoptik Und Quanteninformation | Guhne O.,University of Siegen
Physical Review Letters | Year: 2011

We present an approach to characterize genuine multiparticle entanglement by using appropriate approximations in the space of quantum states. This leads to a criterion for entanglement which can easily be calculated by using semidefinite programing and improves all existing approaches significantly. Experimentally, it can also be evaluated when only some observables are measured. Furthermore, it results in a computable entanglement monotone for genuine multiparticle entanglement. Based on this, we develop an analytical approach for the entanglement detection in cluster states, leading to an exponential improvement compared with existing schemes. © 2011 American Physical Society.

Brownnutt M.,University of Innsbruck | Brownnutt M.,University of Hong Kong | Kumph M.,University of Innsbruck | Rabl P.,Vienna University of Technology | And 2 more authors.
Reviews of Modern Physics | Year: 2015

Electric-field noise near surfaces is a common problem in diverse areas of physics and a limiting factor for many precision measurements. There are multiple mechanisms by which such noise is generated, many of which are poorly understood. Laser-cooled, trapped ions provide one of the most sensitive systems to probe electric-field noise at MHz frequencies and over a distance range 30-3000 μm from a surface. Over recent years numerous experiments have reported spectral densities of electric-field noise inferred from ion heating-rate measurements and several different theoretical explanations for the observed noise characteristics have been proposed. This paper provides an extensive summary and critical review of electric-field noise measurements in ion traps and compares these experimental findings with known and conjectured mechanisms for the origin of this noise. This reveals that the presence of multiple noise sources, as well as the different scalings added by geometrical considerations, complicates the interpretation of these results. It is thus the purpose of this review to assess which conclusions can be reasonably drawn from the existing data, and which important questions are still open. In so doing it provides a framework for future investigations of surface-noise processes. © 2015 American Physical Society.

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