Dr.jens.brede

Quantum error correction in a solid-state hybrid spin register

Nature 506, 7487 (2014). doi:10.1038/nature12919

Authors: G. Waldherr, Y. Wang, S. Zaiser, M. Jamali, T. Schulte-Herbrüggen, H. Abe, T. Ohshima, J. Isoya, J. F. Du, P. Neumann & J. Wrachtrup

Error correction is important in classical and quantum computation. Decoherence caused by the inevitable interaction of quantum bits with their environment leads to dephasing or even relaxation. Correction of the concomitant errors is therefore a fundamental requirement for scalable quantum computation. Although algorithms for error correction have been known for some time, experimental realizations are scarce. Here we show quantum error correction in a heterogeneous, solid-state spin system. We demonstrate that joint initialization, projective readout and fast local and non-local gate operations can all be achieved in diamond spin systems, even under ambient conditions. High-fidelity initialization of a whole spin register (99 per cent) and single-shot readout of multiple individual nuclear spins are achieved by using the ancillary electron spin of a nitrogen–vacancy defect. Implementation of a novel non-local gate generic to our electron–nuclear quantum register allows the preparation of entangled states of three nuclear spins, with fidelities exceeding 85 per cent. With these techniques, we demonstrate three-qubit phase-flip error correction. Using optimal control, all of the above operations achieve fidelities approaching those needed for fault-tolerant quantum operation, thus paving the way to large-scale quantum computation. Besides their use with diamond spin systems, our techniques can be used to improve scaling of quantum networks relying on phosphorus in silicon, quantum dots, silicon carbide or rare-earth ions in solids.

Sociology: Sexual violence rife on US campuses

Nature 506, 7487 (2014). doi:10.1038/506159b

Author: Stephen Shaffer

Gun violence may be relatively rare on university campuses in the United States (see Nature505, 150–152; 201410.1038/505150a), but other violent incidents are rampant.Acts of sexual violence, for example, are committed against an estimated 20–25% of females attending

Author(s): H. C. Herper, S. Bhandary, O. Eriksson, B. Sanyal, and B. Brena

The adsorption of FePc on Co(001) and c(2×2)O/Co(001) was studied by means of density functional theory calculations, taking into account the long range van der Waals dispersion forces. Several high symmetry adsorption sites were analyzed, together with two possible orientations of the molecules. Fo...

[Phys. Rev. B 89, 085411] Published Thu Feb 13, 2014

We study the behavior of skyrmions in thin films under the action of stochastic torques arising from thermal fluctuations. We find that the Brownian motion of skyrmions is described by a stochastic Thiele's equation and its corresponding Fokker-Planck equation. The resulting Fokker-Planck equation is recognized as the one for a high-friction Brownian particle which has been studied extensively in different physical contexts. It is shown that thermal fluctuations favor the skyrmion motion allowing a finite mobility even in presence of pinning traps. We calculate explicitly the mobility tensor of skyrmions in linear response to an electric current finding that it increases with temperature.

Time-dependent density-functional theory (TDDFT) is a formally exact approach to the time-dependent electronic many-body problem which is widely used for calculating excitation energies. We present a survey of the fundamental framework, practical aspects, and applications of TDDFT. This paper is mainly intended for non-experts (students or researchers in other areas) who would like to learn about the present state of TDDFT without going too deeply into formal details.

Whistle-blower breaks his silence

Nature 505, 7485 (2014). http://www.nature.com/doifinder/10.1038/505593a

Author: David Cyranoski

South Korean researcher reveals the fallout he faced from his tip-offs about former cloning fraudster Woo Suk Hwang.

Observation of Dirac monopoles in a synthetic magnetic field

Nature 505, 7485 (2014). doi:10.1038/nature12954

Authors: M. W. Ray, E. Ruokokoski, S. Kandel, M. Möttönen & D. S. Hall

Magnetic monopoles—particles that behave as isolated north or south magnetic poles—have been the subject of speculation since the first detailed observations of magnetism several hundred years ago. Numerous theoretical investigations and hitherto unsuccessful experimental searches have followed Dirac’s 1931 development of a theory of monopoles consistent with both quantum mechanics and the gauge invariance of the electromagnetic field. The existence of even a single Dirac magnetic monopole would have far-reaching physical consequences, most famously explaining the quantization of electric charge. Although analogues of magnetic monopoles have been found in exotic spin ices and other systems, there has been no direct experimental observation of Dirac monopoles within a medium described by a quantum field, such as superfluid helium-3 (refs 10, 11, 12, 13). Here we demonstrate the controlled creation of Dirac monopoles in the synthetic magnetic field produced by a spinor Bose–Einstein condensate. Monopoles are identified, in both experiments and matching numerical simulations, at the termini of vortex lines within the condensate. By directly imaging such a vortex line, the presence of a monopole may be discerned from the experimental data alone. These real-space images provide conclusive and long-awaited experimental evidence of the existence of Dirac monopoles. Our result provides an unprecedented opportunity to observe and manipulate these quantum mechanical entities in a controlled environment.

Peer production projects such as Wikipedia or open-source software development allow volunteers to collectively create knowledge based products. The inclusive nature of such projects poses difficult challenges for ensuring trustworthiness and combating vandalism. Prior studies in the area deal with descriptive aspects of peer production, failing to capture the idea that while contributors collaborate, they also compete for status in the community and for imposing their views on the product. In this paper we investigate collaborative authoring in Wikipedia where contributors append and overwrite previous contributions to a page. We assume that a contributors goal is to maximize ownership of content sections such that content owned (or originated) by her survived the most recent revision of the page. We model contributors interactions to increase their content ownership as a noncooperative game where a players utility is associated with content owned and cost is a function of effort expended. Our results capture several real life aspects of contributors interactions within peer production projects. We show that at the Nash equilibrium there is an inverse relationship between the effort required to make a contribution and the survival of a contributors content. In other words majority of the content that survives is necessarily contributed by experts who expend relatively less effort than non experts. An empirical analysis of Wikipedia articles provides support for our models predictions. Implications for research and practice are discussed in the context of trustworthy collaboration as well as vandalism.

We review our recent work on the simulation, description and prediction of spin-excitations in adatoms and dimers deposited on metallic surfaces. This work done together with Douglas L. Mills, is an extension of his seminal contribution (with Pascal Lederer) published 50 years ago on the spin-dynamics of transition metal impurities embedded in transition metal hosts [P. Lederer, D.L. Mills, Phys. Rev. {\bf 160}, 590 (1967)]. The main predictions of his model were verified experimentally with state of the art inelastic scanning tunneling spectroscopy on adatoms. Our formalism, presented in this review, is based on time-dependent density functional theory, combined with the Korringa-Kohn-Rostoker Green function method. Comparison to experiments is shown and discussed in detail. Our scheme enables the description and prediction of the main characteristics of these excitations, \emph{i.e.} their resonance frequency, their lifetime and their behavior upon application of external perturbations such as a magnetic field.

Author(s): Can-Li Song, Yi-Lin Wang, Ye-Ping Jiang, Zhi Li, Lili Wang, Ke He, Xi Chen, Jennifer E. Hoffman, Xu-Cun Ma, and Qi-Kun Xue

Scanning tunneling spectroscopy has been used to reveal signatures of a bosonic mode in the local quasiparticle density of states of superconducting FeSe films. The mode appears below Tc as a “dip-hump” feature at energy Ω∼4.7kBTc beyond the superconducting gap Δ. Spectra on strained regions of the ...

[Phys. Rev. Lett. 112, 057002] Published Thu Feb 06, 2014

Experimental advances allow for the inclusion of multiple probes to measure the transport properties of a sample surface. We develop a theory of dual-probe scanning tunnelling microscopy using a Green's Function formalism, and apply it to graphene. Sampling the local conduction properties at finite length scales yields real space conductance maps which show anisotropy for pristine graphene systems and quantum interference effects in the presence of isolated impurities. The spectral signatures of the Fourier transform of real space conductance maps include characteristics that can be related to different scattering processes. We compute the conductance maps of graphene systems with different edge geometries or height fluctuations to determine the effects of non-ideal graphene samples on dual-probe measurements.

Author(s): P. Löptien, L. Zhou, J. Wiebe, A. A. Khajetoorians, J. L. Mi, B. B. Iversen, Ph. Hofmann, and R. Wiesendanger

The electrostatic behavior of a prototypical three-dimensional topological insulator, Bi2Se3(111), is investigated by a scanning tunneling microscopy (STM) study of the distribution of Rb atoms adsorbed on the surface. The positively charged ions are screened by both free electrons residing in the t...

[Phys. Rev. B 89, 085401] Published Mon Feb 03, 2014

Abstract

Author(s): Matthias Menzel, André Kubetzka, Kirsten von Bergmann, and Roland Wiesendanger

The magnetic ground state of biatomic Fe chains on the reconstructed (5×1)-Ir(001) surface is a cycloidal 120° spin spiral. Spin-resolved scanning tunneling microscopy reveals a striking variation of magnetic field dependences among the chains, which we attribute to parity effects resulting from fin...

[Phys. Rev. Lett. 112, 047204] Published Wed Jan 29, 2014

Author(s): Gaël Reecht, Fabrice Scheurer, Virginie Speisser, Yannick J. Dappe, Fabrice Mathevet, and Guillaume Schull

The electroluminescence of a polythiophene wire suspended between a metallic surface and the tip of a scanning tunneling microscope is reported. Under positive sample voltage, the spectral and voltage dependencies of the emitted light are consistent with the fluorescence of the wire junction mediate...

[Phys. Rev. Lett. 112, 047403] Published Tue Jan 28, 2014

Article

Magnetic skyrmions—topologically stable swirls in the spin structure—have great potential for spintronics devices. Here, the authors find two bound skyrmions with a topological charge of two in bilayer-structured manganite, which can be electrically driven with an exceptionally low current density.

Nature Communications doi: 10.1038/ncomms4198

Authors: X. Z. Yu, Y. Tokunaga, Y. Kaneko, W. Z. Zhang, K. Kimoto, Y. Matsui, Y. Taguchi, Y. Tokura

Author(s): T. Frederiksen, M. Paulsson, and H. Ueba

A theory of action spectroscopy, i.e., a reaction rate or yield as a function of bias voltage, is presented for single-molecule reactions induced by the inelastic tunneling current with a scanning tunneling microscope. A formula for the reaction yield is derived using the adsorbate resonance model, ...

[Phys. Rev. B 89, 035427] Published Thu Jan 23, 2014

Author(s): Shi-Zeng Lin, Cristian D. Batista, and Avadh Saxena

A spin texture called skyrmion has been recently observed in certain chiral magnets without inversion symmetry. The observed skyrmions are extended objects with typical linear sizes of 10 to 100 nm that contain 103 to 105 spins and can be deformed in response to external perturbations. Weak deformat...

[Phys. Rev. B 89, 024415] Published Thu Jan 23, 2014

This month, Jean-Pierre Bourguignon began his tenure as president of the European Research Council (ERC). This is a daunting task for many reasons: The growing ERC budget requires that it be carefully positioned within a complex array of European funding opportunities; embedding the presidential role into the ERC executive agency in the European Commission's Directorate-General for Research and Innovation potentially reduces ERC's independence; and variable success rates for acquiring ERC funding are concerning, and research expectations will need to be managed. Author: Paul Boyle

Nature Chemistry 6, 91 (2014). doi:10.1038/nchem.1845

Author: C. Scott Hartley

The organic synthesis of graphene nanostructures requires exceptionally efficient chemistry and is made more challenging by difficulties in characterization and processing. Now, solution-dispersible graphene nanoribbons have been synthesized on the gram scale.

Nature advance online publication 22 January 2014. doi:10.1038/nature12949

Authors: Pau Carazo, Cedric K. W. Tan, Felicity Allen, Stuart Wigby & Tommaso Pizzari

To resolve the mechanisms that switch competition to cooperation is key to understanding biological organization. This is particularly relevant for intrasexual competition, which often leads to males harming females. Recent theory proposes that kin selection may modulate female harm by relaxing competition among male relatives. Here we experimentally manipulate the relatedness of groups of male Drosophila melanogaster competing over females to demonstrate that, as expected, within-group relatedness inhibits male competition and female harm. Females exposed to groups of three brothers unrelated to the female had higher lifetime reproductive success and slower reproductive ageing compared to females exposed to groups of three males unrelated to each other. Triplets of brothers also fought less with each other, courted females less intensively and lived longer than triplets of unrelated males. However, associations among brothers may be vulnerable to invasion by minorities of unrelated males: when two brothers were matched with an unrelated male, the unrelated male sired on average twice as many offspring as either brother. These results demonstrate that relatedness can profoundly affect fitness through its modulation of intrasexual competition, as flies plastically adjust sexual behaviour in a manner consistent with kin-selection theory.

We investigate cooling of a vibrational mode of a magnetic quantum dot by a spin-polarized tunneling charge current, exploiting the interaction between the magnetization and the vibration. The spin-polarized charge current polarizes the magnetic nanoisland, lowering its energy. Inevitable Ohmic energy losses due to the charge current flow will heat up the vibration. A small but finite coupling between the vibration and the local magnetic moment then permits an energy exchange, resulting in a lower energy, i.e., cooling, of the vibrational mode. We determine parameter regimes for the cooling of the vibration below $50\%$ of its initial value. Lowest final phonon temperature is observed for weak electron-phonon-coupling but similar magnetization-phonon coupling strength. The cooling rate, thereby, increases at first with the magnetization-phonon coupling and then saturates.

Article

Europium-based materials are interesting for magnetic devices because divalent europium carries a large spin-magnetic moment. Chikina et al . show that the non-magnetic Si-Rh-Si surface trilayer of the antiferromagnet EuRh 2 Si 2 reveals a surface state with large spin splitting controllable by temperature.

Nature Communications doi: 10.1038/ncomms4171

Authors: A. Chikina, M. Höppner, S. Seiro, K. Kummer, S. Danzenbächer, S. Patil, A. Generalov, M. Güttler, Yu. Kucherenko, E. V. Chulkov, Yu. M. Koroteev, K. Köpernik, C. Geibel, M. Shi, M. Radovic, C. Laubschat, D. V. Vyalikh