QN Seminar 2013

Abstract 18 December 2013 Coherent Oscillations in a Si/SiGe Quantum Dot Hybrid Qubit Mark Eriksson, Universit of Wisconsin-Madison

I will discuss measurement and manipulation of the recently proposed quantum dot hybrid qubit, in these experiments formed in a Si/SiGe double quantum dot. X-rotations on the Bloch sphere are performed by pulsing a gate voltage so that the detuning of a double quantum dot makes the (1,2) and (2,1) occupation ground states degenerate [2]. The resulting rotations occur at approximately 5 GHz and reveal an experimentally measured visibility greater than 80%. Z-rotations on the Bloch sphere are performed by pulsing a gate voltage away from the (1,2)-(2,1) degeneracy point, resulting in oscillations at a rate of approximately 10 * GHz and a measured visibility greater than 85%. The T2 time at this detuning is greater than 15 ns, many times longer than the 100 ps gate operation time. Methods for future improvements of the oscillation visibility will be discussed. This work was supported in part by ARO (W911NF-12-0607), NSF (DMR-1206915), and the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government. [1] Z. Shi, et al., Phys. Rev. Lett. 108, 140503 (2012). [2] Teck Seng Koh, et al., Phys. Rev. Lett. 109, 250503 (2012).

Abstract 11 December 2013 Microscopic Origin of the 0.7-Anomaly in Quantum Point Contacts Stefan Ludwig, Ludwig-Maximilians-Universität, München

Quantum point contacts are the fundamental building blocks of semiconducting quantum circuits. The conductance of such a one-dimensional constriction can be controlled by gate voltages and is quantized in units of GQ = 2e2=h. However, the conductance also exhibits an unexpected shoulder at ' 0:7GQ, known as the ’0.7- anomaly’ [1], whose origin is still subject to controversial discussions. The most prominent proposed theoretical explanations exclude each other: one has evoked spontaneous spin polarization [2] and the other one the formation of a quasi-bound state leading to the Kondo effect [3,4]. In this talk I present an alternative approach based on our experiments and numerical calculations, performed in Jan von Delft’s group. We evoke no assumptions beyond experimental evidences. Our model offers a detailed microscopic explanation for both the 0.7-anomaly and the zero-bias peak accompanying it: their common origin is a pronounced maximum in the local density of states at the bottom of the lowest onedimensional subband of the point contact, which causes an anomalous enhancement of the back-scattering rate and the magnetic spin susceptibility. We find that the 0.7-anomaly is a Fermi-liquid feature while neither spontaneous spin polarization nor a bound state are needed for its explanation [5]. Characterization of Qubit Dephasing by Landau-Zener Interferometry If time permits, I will then present some fairly new data on a two-electron charge qubit defined in a double quantum dot. We have performed Landau-Zener interferometry and established it as a straightforward tool to fully characterize a qubit and its dephasing environment [6]. It is, e. g., possible to determine the characteristic time scales T2 (coherence time) and T? 2 (ensemble average dephasing) individually and without the need of pulsed gate measurements. References [1] Thomas, K. J. et al, Phys. Rev. Lett. 77, 135 (1996). [2] Reilly, D. J. et al., Phys. Rev. Lett. 89, 246801 (2002). [3] Cronenwett, S. M. et al., Phys. Rev. Lett. 88, 226805 (2002). [4] Meir, Y., et al., Phys. Rev. Lett. 89, 196802 (2002) [5] Bauer, F., et al., Nature 501, 73 (2013). [6] Forster, F. et al., arXiv:1309.5907 (2013).

Abstract 4 December 2013 Modern 3D optical and electron microscopy for mapping neuronal circuits Kevin Briggman, NINDS/NIH

The neuronal circuitry of the retina is the first stage in the processing of visual information by vertebrate brains. The retina transduces photons into chemical and electrical signals and is capable of encoding visual information across an extreme dynamic range from single photon responses in the dark to near saturating conditions on a bright sunny day. In addition, the retina encodes spatial and temporal patterns of light including the presence of edges, color opponency, and the direction of motion. However, how retinal neurons actually perform these sophisticated computations is not well understood. The major problem is that we simply do not comprehensively know the anatomical synaptic connectivity of neurons in the retina.

This is in large part due to the disparate length scales involved; the neurites (including dendrites and axons) of neurons can be as thin as 50 nm and yet extend over many millimeters. The tortuous trajectories of these processes in the nervous system necessitate the ability to acquire large volumes at high resolution in all 3 spatial dimensions. High lateral (x-y) resolutions of 5-10 nm are easily achieved in scanning electron microscopes, but the major challenge has been to repeatedly cut tens of thousands of thin sections (each at least <50 nm) from a block of tissue. Historically, tissue sectioning is performed manually using an ultramicrotome which is tedious, error-prone and is limited to, at best, 50 nm sections. We have therefore automated both image acquisition and sectioning by developing a serial block-face scanning electron microscopy (SBEM) technique. This method allows us to image large 3-dimensional EM datasets, typically hundreds of microns on a side, at nearly isotropic voxel resolutions of 12 x 12 x 25 nm3.

I will discuss the technical aspects of SBEM in relation to other current volume electron microscopy techniques and present how we have used this technique to reconstruct the connectivity of the mouse retina. In particular, I will stress the power of combining large-scale functional recordings of neurons with subsequent anatomical reconstruction of neuronal circuits. I will also discuss our efforts to automate the analysis of large (multi- terabyte) 3D EM datasets.

Abstract 27 November 2013 Quantum Annealing and the D-Wave Devices Matthias Troyer, ETH Zürich, and Microsoft Research

Quantum annealing - a finite version of the quantum adiabatic algorithm - combines the classical technology of slow thermal cooling with quantum mechanical tunneling, to try bring a physical system towards its ground state. The Canadian company D-Wave systems has recently built and sold programmable devices that are designed to use this effect to find solutions to optimization problems. I will present results of experiments designed to shed light on crucial questions about these controversial devices: are these devices quantum or classical? Are they faster than classical devices? What is their potential?

Abstract 13 November 2013 Josephson junction-based coherent caloritronic nanocircuits Francesco Giazotto, NEST, Instituto Nanoscienze-CN & Scuola Normale Superiore, Pisa, Italy

The Josephson effect [1] represents perhaps the prototype of macroscopic phase coherence and is at the basis of the most widespread interferometer, i.e., the superconducting quantum interference device (SQUID) [2]. Yet, in analogy to electric interference, Maki and Griffin [3] predicted in 1965 that thermal current flowing through a temperature-biased Josephson tunnel junction is a stationary periodic function of the quantum phase difference between the superconductors. The interplay between quasiparticles and Cooper pairs condensate is at the origin of such phase-dependent heat current, and is unique to Josephson junctions. In this scenario, a temperature-biased SQUID would allow heat currents to interfere [4, 5] thus implementing the thermal version of the electric Josephson interferometer. In this presentation we shall initially report the first experimental realization of a heat interferometer [6,7]. We investigate heat exchange between two normal electrodes kept at different and tunnel- coupled to each other through a thermal `modulator' [5] in the form of a DC-SQUID. Heat transport in the system is found to be phase dependent, in agreement with the original prediction. Besides offering remarkable insight into thermal transport in Josephson junctions, our results represent a significant step toward phase- coherent mastering of heat in solid-state nanocircuits, and pave the way to the design of novel-concept coherent caloritronic devices, for instance, heat transistors, thermal splitters and diodes [8] which exploit phase-dependent heat transfer peculiar to the Josephson effect. In this latter context, we shall also present the concept for a further development of a Josephson heat interferometer based on a double superconducting loop [9] which allows, in principle, enhanced control over heat transport.

We shall finally conclude presenting experimental results on the first prototypical quantum diffractor for thermal flux [10]. Specifically, thermal diffraction manifests itself with a peculiar modulation of the electron temperature in a small metallic electrode nearby-contacted to a Josephson junction when sweeping the magnetic flux Φ [11]. Remarkably, the observed temperature dependence exhibits Φ-symmetry and a clear reminiscence with a Fraunhofer-like modulation pattern, as expected fingerprints for a quantum diffraction phenomenon. Our results confirm a pristine prediction of quantum heat transport and, joined with double- junction heat interferometry demonstrated in [6], exemplify the complementary and conclusive proof of the existence of phase-dependent thermal currents in Josephson-coupled superconductors. This approach combined with well-known methods for phase-biasing superconducting circuits provides with a novel tool for mastering heat fluxes at the nanoscale.

References [1] B. D. Josephson, Phys. Lett. 1, 251 (1962) [2] J. Clarke and A. I. Braginski, The SQUID Handbook (Wiley-VCH, 2004) [3] K. Maki and A. Griffin, Phys. Rev. Lett. 15, 921 (1965) [4] G. D. Guttman, E. Ben-Jacob, and J. Bergman, Phys. Rev. B 57, 2717 (1998) [5] F. Giazotto and M. J. Martínez-Pérez, Appl. Phys. Lett. 101, 102601 (2012) [6] F. Giazotto and M. J. Martínez-Pérez, Nature 492, 401 (2012) [7] R. W. Simmonds, Nature 492, 358 (2012)

Abstract 6 November 2013 Carbon nanotube based quantum dot circuits in a cQED architecture Takis Kontos, LPA, ENS, Paris

The recent development of hybrid cQED allows one to study how cavity photons interact with a system driven out of equilibrium by fermionic reservoirs. We study here one of the simplest combination: a double quantum dot coupled to a single mode of the electromagnetic field. We are able to couple resonantly the charge levels of a carbon nanotube based double dot to cavity photons. We perform a microwave read out and of the charge states of this system which allows us to unveil features of the out of equilibrium charge dynamics, otherwise invisible in the DC current. We develop a theory explaining our measurements and extract relaxation rate, dephasing rate and photon number of the hybrid system. These findings open the path for manipulating other degrees of freedom e.g. the spin and/or the valley in nanotube based double dots using microwave light [1]. Finally, I will show how the developed cQED architecture can be used to investigate other types of mesoscopic systems. I will describe our recent experiment on the quantum RC circuit problem and our recent ideas for nanowires hosting Majorana fermions [2].

[1]J.J. Viennot et al., arXiv:1310.4363 [2]A. Cottet, T. Kontos and B. Douçot, arXiv:1307.4185

Abstract 23 October 2013 Heat-dissipation in atomic-scale junctions Juan Carlos Cuevas, Dept. of Theoretical Condensed Matter Physics, Universidad Autónoma deMadrid

Atomic and single-molecule junctions represent the ultimate limit to the miniaturization of electrical circuits [1]. They are also ideal platforms to test quantum transport theories that are required to describe charge and energy transfer in novel functional nanodevices. Recent work has successfully probed electric and thermoelectric phenomena in atomic-scale junctions. However, heat dissipation and heat transport in atomic- scale devices remain poorly characterized due to experimental challenges. In this talk, I will present our recent experimental and theoretical efforts to elucidate how heat dissipation takes place in metallic atomic-size contacts and single-molecule junctions [2]. In particular, I will describe how, by using novel scanning probes with integrated nanoscale thermocouples, we have been able to show that heating in the electrodes of molecular junctions, whose transmission characteristics are strongly dependent on energy, is asymmetric, i.e. unequal and dependent on both the bias polarity and the identity of majority charge carriers (electrons vs. holes). In contrast, atomic contacts whose transmission characteristics show weak energy dependence do not exhibit appreciable asymmetry. Our results prove unambiguously a central prediction of Landauer theory that has remained untested for decades despite its relevance to a range of nanoscale systems where transport is elastic. Moreover, the techniques developed in our work will enable the study of Peltier effects and other heat transport phenomena at the atomic scale.

References: [1] J.C. Cuevas and E. Scheer, Molecular Electronics: An Introduction to Theory and Experiment. (World Scientific, 2010). [2] W. Lee, K. Kim, W. Jeong, L. A. Zotti, F. Pauly, J.C. Cuevas, P. Reddy, Nature 498, 209 (2013).

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Abstract 16 October 2013 Majorana fermions in chains of magnetic atoms on a superconductor Stevan Nadj-Perge, Princeton University

Majorana fermions are zero energy excitations predicted to exist on the edges of topological superconductors. Due to their non-abelian nature, these excitations can potentially be used as qubits in a topological quantum computer.

I will present a novel experimental approach to realize Majorana fermions in a chain of magnetic atoms on the surface of an s-wave superconductor. Our experimental efforts are motivated by model calculations which show that such chains can support topological superconductivity with Majorana end modes. Surprisingly, even short chains consisting of tens of atoms can host well resolved Majorana modes under suitable conditions depending on the relative spin orientations of adjacent atoms [1]. We realize magnetic chains using self- assembled growth technique and probe their electronic structure using scanning tunneling microscopy. Results from spatially resolved spectroscopic mapping reveal zero energy modes at the chain ends. I will discuss the possible origin of the observed peaks in the context of Majorana bound states and other recently proposed theories. I will end my talk with a brief outlook for this line of research. [1] S. Nadj-Perge, I. K. Drozdov, B. A. Bernevig., Ali Yazdani, Phys. Rev. B 88, 020407(R) (2013).

Abstract 2 October 2013 Characterization of Hybrid Nanostructures using Electron Microscopy; from Atoms to 3d Solids M.A. van Huis, Debye Institute for Nanomaterials Science, Utrecht University

With the increasing structural complexity of hybrid nanostructures and nanoparticle solids, increasingly higher demands are put on electron microscopy methods to obtain a full structural characterization. In the first part of the talk, various electron tomography approaches (BF-TEM, HAADF-STEM, IBF-STEM) are used to obtain accurate 3D reconstructions of nanoparticle solids, in particular icosahedral supraballs of FexO/CoFe2O4 core/shell nanoparticles, and disordered PbSe/CdSe nanoparticle films. The part of the talk will focus on the atomic-scale characterization of nanoscale interfaces. Several hybrid nanostructures with applications for solar energy conversion are discussed, in particular Au-tipped CdS nanorods, CdSe tetrapods, and pentagonal silver nanowires coated with Cu2O semiconductor shells. The Cu2O shell follows the pentagonal morphology of the Ag nanowire, aided by a well-defined orientation relationship. In the case of the CdS/Au nanostructures, a spectacular transformation takes place upon electron irradiation, whereby the CdSe/Au nanorod-tip particles transform into AuS/Cd core-shell particles. The experimental results are complemented with force-field molecular dynamics (FF-MD) and quantum mechanical density functional theory (DFT) calculations to shed light on the energetics driving the formation and transformation of these remarkable nanostructures.

Abstract 25 September 2013 Rationally designed complex hierarchical microarchitectures Wim Noorduin, School of Engineering and Applied Sciences, Harvard University, Cambridge

Complex nano/microstructures are of fundamental interest, and the ability to program their form has practical ramifications in fields such as optics, catalysis and electronics. We developed microstructures in a dynamic reaction-diffusion system that allows us to rationally devise schemes for precisely sculpting a great variety of elementary shapes. Detailed understanding of the underlying reaction-diffusion mechanisms allows us not only to program elementary shapes, but also steer the precipitating reactants into complex flowers, corals, vases, and patterns, with precise control over placement of stems, leaves, etc. via sequential combinatorial assembly of the developing shapes. These findings may hold profound implications for understanding and ultimately expanding upon nature’s morphogenesis strategies, and outline a novel approach to use sequences of dynamic modulations of the environment to steer self-assembly processes as a route to advanced, highly complex microscale materials and devices.

Abstract 18 September 2013 Towards an understanding of growth on the nano-scale: in-situ probes for graphene, nanotube and –wire cvd Stephan Hofmann, Dept. of Engineering, University of Cambridge, Cambridge

The application potential of nano-materials, such as graphene, carbon nanotubes and semiconductor nanowires, hinges entirely on the development of growth and integration techniques that are scalable and allow an adequate level of structural control. Chemical vapor deposition (CVD) now dominates the nanotube/wire market and rapid progress is being made to develop it also for graphene manufacture. Central to most of the current CVD growth processes is thereby the use of a catalyst/seed, in the form of a nanoparticle or poly-crystalline film, respectively. Despite an ever growing body of literature on empirical process calibrations, an understanding of the catalytic CVD growth mechanisms is still largely missing even for common metal catalysts. This leaves key questions unanswered, such as the level of structural growth selectivity/control that can be achieved.

With a focus on diverse applications in the electronics and display industry, we have been trying to address this current lack of understanding by using in-situ metrology, ranging from environmental scanning and transmission electron microscopy to high-pressure X-ray photoelectron spectroscopy, X-ray diffraction and scanning tunnelling microscopy, to directly probe the atomic level mechanisms that govern the growth and device behaviour of these nanomaterials in realistic process environments. This talk will review our current understanding of CNT, graphene [1-6] and Si/Ge nanowire [7-10] CVD based on model catalyst systems, in particular highlighting the importance of kinetic aspects and size- and solubility-dependent mechanisms.

[1] Weatherup et al. Nano Lett. 11 (2011), 4154 [2] Weatherup et al. ACS Nano 6 (2012), 9996 [3] Patera et al. ACS Nano (2013) [4] Wirth et al. Chem. Mat. 24 (2012), 4633

Abstract 10 July 2013 Effects of interactions on tunnelling conductance in topological superconductor/nanowire devices Roman Lutchyn, UCSB

Inspired by a recent experimental observation of zero-bias tunneling conductance in superconductor- semiconductor nanowire devices, I will discuss transport properties of the junctions consisting of a nanowire (Luttinger liquid) coupled to a topological superconductor characterized by the presence of Majorana zero- energy end states. I will analyze the phase diagram for such a system as a function of the interaction strength in the nanowire, and show that its universal low-energy transport is governed by fixed points describing either perfect normal reflection or perfect Andreev reflection. To obtain finite temperature and voltage dependence of the tunneling conductance, we have developed a framework based on real-time Keldysh technique which allows one to understand this transport phenomenon from a very general perspective

Abstract 26 June 2013 Spin currents in ferromagnetic/normal metal hybrids Sebastian Goennenwein, Walther-Messner-Institut, Bayerische Akademie der Wissenschaften, Garching

A pure spin current – i.e., the directed flow of spin angular momentum – is a fascinating manifestation of spin physics in the solid state. In ferromagnet/normal metal hybrid bilayers, pure spin currents can be generated, e.g., by means of spin pumping [1], or via the application of thermal gradients in the so-called spin Seebeck effect [2]. An elegant scheme for detecting spin currents relies on the inverse spin Hall effect: because of spin- orbit coupling, a spin current also induces a charge current, which then can be detected using conventional electronics [1,2].

In the talk, I will introduce and compare our recent experimental investigations of spin current-related phenomena in ferromagnet/normal metal hybrid devices [3-6]. Our results show that spin current generation is possible from both electrically conductive as well as electrically insulating ferromagnets (so-called ferromagnetic insulators). I will address the spatially resolved spin Seebeck effect experiments which we have performed in ferromagnetic insulator/normal metal thin film hybrid structures, and critically discuss a novel magneto-resistance effect arising from the interplay between spin and charge currents in these structures.

References [1] O. Mosendz et al., Phys. Rev. Lett. 104, 046601 (2010). [2] K. Uchida et al., Nature 455, 778 (2008). [3] F. D. Czeschka et al., Phys. Rev. Lett. 107, 046601 (2011). [4] M. Weiler et al., Phys. Rev. Lett. 108, 176601 (2012). [5] M. Weiler et al., Phys. Rev. Lett. 108, 106602 (2012). [6] H. Nakayama et al., Phys. Rev. Lett. 110, 206601 (2013).

Abstract 19 June 2013 Why you should explode your quantum computer: a network of tiny fragments is powerful and practical Simon Benjamin, University of Oxford, UK

A scalable quantum computer could be built by networking together many simple processor cells, thus avoiding the need to create a single complex structure. But quantum links may be very error prone, which seems likely to undermine the practicality of the approach. Remarkably, this need not be so. I will describe an approach to topological-encoded quantum computing using simple, four-qubit cells connected by a very noisy network (e.g. 10% infidelity). Individual NV centres, or small traps, are very relevant as potential cells. We find that intra- cell error rates for initialisation, state manipulation and measurement can simultaneously exceed 0.88% before the protocol fails. Thus the network paradigm matches the robustness of conventional 'monolithic' architectures while offering greater flexibility and scalability. Moreover there is scope for raising the threshold by optimising the theoretical scheme to a given experimental context.

Abstract 29 May 2013 Linking single molecule studies with atomic spectroscopy Ilja Gerhardt, University of Stuttgart

Single organic dye molecules in a cryogenic environment constitute narrow band and high brightness single photon sources. This results mainly from the high collection efficiency inside a solid state system. Unfortunately, single spin manipulations are hard to perform, since the relevant levels are usually singlet states. In this study we combine the spectroscopy on single organic molecules with the spectroscopy on atomic vapor. With a selected molecule, we perfectly match the D-line transitions of atomic . We achieve up to 6x10^5 counts per second (cps) with a width of ~20MHz. Our efforts to select the photons width an atomic line filter (Na-FADOF) are introduced, and the relevant key parameters of such a filter are presented. A novel experiment, introducing slow light on the single photon level will be presented.

Abstract 22 May 2013 Electron tomography for nanostructures: how low can we go Sara Balls, EMAT, University of Antwerp

Nanomaterials play a key role in modern technology, because of their unique physical and chemical characteristics. Such properties are often very sensitive to the three-dimensional (3D) structure of the nanosystems. Electron tomography therefore became a commonly used tool in the 3D investigation of nanomaterials. Most results have been achieved at the nanometer level, but the ultimate goal is to reach 3D electron microscopy with atomic resolution. Although this is not yet a standard technique for all structures, significant progress has recently been achieved using different approaches [1-5].

One of the possibilities to perform electron tomography with atomic resolution is by applying reconstruction algorithms based on compressive sensing. We hereby exploit the fact that nanomaterials at the atomic scale are sparse. The methodology was applied for Au nanorods and the crystal lattice of the nanorods could be reproduced without using prior knowledge on the atomic structure! From these reconstructions, the boundary facets of different rods have been precisely determined and even atomic steps at the surface could be revealed [3]. It is important to note that the atom positions are not assumed to be fixed during the reconstruction process. Therefore, the reconstruction can serve as a starting point to investigate strain in 3D. Further challenges include the visualisation of defects and the characterisation of interfaces in hetero- nanostructures with the same resolution as demonstrated so far.

[1]S. Van Aert, Batenburg KJ, Rossell MD, Erni R, Van Tendeloo G. Nature, 470, 374 (2011) [2]S. Bals, M. Casavola, M. van Huis, S. Van Aert, K.J. Batenburg, G. Van Tendeloo, D. Vanmaekelbergh, Nano Letters, 11, 3420 (2011) [3]B. Goris, S. Bals, W. Van den Broek, E. Carbo-Argibay, S. Gomez-Grana, L.M. Marzan, G. Van Tendeloo, Nature Materials, 11, 930 (2012) [4]M.C. Scott, CC. Chen, M. Mecklenburg, C. Zhu, R. Xu, P. Ercius, et al. Nature, 483, 444 (2012) [5]S. Bals, S. Van Aert, C.P. Romero, K. Lauwaet, M.J. Van Bael, B. Schoeters, B. Partoens, E. Yücelen, P. Lievens, G. Van Tendeloo, Nature Communications, 3, 897 (2012)

Abstract 16 May 2013 Hofstadter’s Butterfly and interaction driven quantum Hall ferromagnetism in graphene Philip Kim, Columbia University, New York

Electrons moving in a periodic electric potential form Bloch energy bands where the mass of electrons are effectively changed. In a strong , the cyclotron orbits of free electrons are quantized and Landau levels forms with a massive degeneracy within. In 1976, Hofstadter showed that for 2-dimensional electronic system, the intriguing interplay between these two quantization effects can lead into a self-similar fractal set of energy spectrum known as “Hofstadter’s Butterfly.” Experimental efforts to demonstrate this fascinating electron energy spectrum have continued ever since. Recent advent of graphene, where its Bloch electrons can be described by Dirac feremions, provides a new opportunity to investigate this half century old problem experimentally. In this presentation, I will discuss the experimental realization Hofstadter’s Butterfly via substrate engineered graphene under extremely high magnetic fields controlling two competing length scales governing Dirac-Bloch states and Landau orbits, respectively. In addition, the strong Coulomb interactions and approximate spin-pseudo spin symmetry are predicted to lead to a variety of integer quantum Hall ferromagnetic and fractional quantum Hall states and the quantum phase transition between them in graphene. I will discuss several recent experimental evidences to demonstrate the role of the electron interaction in single and bilayer graphene.

Abstract 15 May 2013 Quantum Effects in biology Martin Pleno, University of Ulm

In recent years the exploration of the relevance of quantum dynamics for biological functions has experienced a considerable increase in interest thanks to both new theoretical ideas and improved experimental techniques. The surprising longevity of coherent phenomena in warm, wet and noisy biological environments is partially due to an intricate interplay between electronic quantum dynamics and the thermally activated vibrational environment made up of molecules and proteins. On this colloquium I will present background, key questions and an exploration of the fundamentally important interactions of quantum system and environment. I will show how the three key examples of quantum biology - photosynthesis, magnetoreception of birds and olfaction - fit into this framework thus underlining its importance.

Abstract 8 May 2013 Understanding oxide interfaces: from microscopic imaging to electronic phases Shallal Ilani, Dept. of Condensed Matter Physics, Weizmann Institute of Science

In the last decade, the advent of complex oxide interfaces has unleashed a wealth of new possibilities to create materials with unexpected functionalities. A notable example is the two-dimensional electron system formed at the interface between LaAlO3 and SrTiO3 (LAO/STO), which exhibits ferromagnetism, superconductivity, and a wide range of unique magneto-transport properties. A key challenge is to find the microscopic mechanisms that underlie these emergent phenomena.While there is a growing understanding that these phenomena might reflect rich structures at the micro-scale, experimental progress toward microscopic imaging of this system has been so far rather limited due to the buried nature of its interface. In this talk I will discuss our experiments that study this system on microscopic and macroscopic scales. Using a newly-developed nanotube-based scanning electrometer we image on the nanoscale the electrostatics and mechanics of this buried interface. We reveal the dynamics of structural domains in STO, their role in generating the contested anomalous piezoelectricity of this substrate, and their direct effects on the physics of the interface electrons. Using macroscopic magneto-transport experiments we demonstrate that a universal Lifshitz transition between the population of d-orbitals with different symmetries underlies many of the transport phenomena observed to date. We further show that the interactions between the itinerant electrons and localized spins leads to an unusual, gate-tunable magnetic phase diagram. These measurements highlight the unique physical settings that can be realized within this new class of low dimensional systems.

Abstract 24 April 2013 Magnetization dynamics derived from excitations of single magnetic atoms on surfaces Alexander Ako Khajetoorians, Hamburg University, Hamburg

With the development of sub-Kelvin high-magnetic field STM, two complementary methods, namely spin- polarized scanning tunneling spectroscopy (SP-STS) [1] and inelastic STS (ISTS) [2-3], can address single spins at the atomic scale. While SP-STS reads out the projection of the impurity magnetization, ISTS detects the excitations of this magnetization as a function of an external magnetic field. They are thus the analogs of magnetometry and spin measurements pushed to the single atom limit. We have recently demonstrated that it is possible to reliably combine single atom magnetometry with an atom-by-atom bottom- up fabrication to realize complex atomic-scale magnets with tailored properties [4-5]. In this talk, I will address recent developments in probing the spin excitations and magnetization curves of atoms on a multitude of non- magnetic surfaces, and the effects of the electronic structure on the precessional dynamics of the atomic spin. Moreover, I will discuss investigations of the magnetization dynamics [6] of coupled spins as probed with spin- resolved STM techniques and how the relaxation is affected by processes like quantum tunneling and spin- transfer torque. [1] A.A.K., et al. , PRL, 106, 037205 (2011); [2] A. J. Heinrich, et al. , Science, 306, 466 (2004); [3] A.A.K, et al. ,Nature, 467, 1084 (2010); [4] A.A.K., et al., Nature Physics, 8, 497 (2012) [5] A.A.K., et al. , Science, 332, 1062 (2011), [6] A.A.K., et al.,Science, 339, 55 (2013)

Abstract 10 April 2013 Heralded entanglement between widely separated atoms – towards the quantum repeater? Harald Weinfurter, LMU

The quantum repeater, distributing entanglement efficiently between two parties, will be the back bone of future quantum networks. Here we show the functionality of its core element, i.e., the creation and analysis of heralded entanglement between spins of two single Rb-87 atoms trapped independently 20 meters apart. We discuss possible applications of this first element and the next steps towards scalable long distance quantum communication

Abstract 3 April 2013 Controlling the displacement of a single electron spin Tristan Meunier, Institut Néel, CNRS, Grenoble

The ability to displace controllably and on-demand a single electron on a chip is an important prerequisite for the realization of electronic circuits at the single electron level. It indeed opens the route to interconnect nodes of a spin-based quantum nanoprocessor or to perform quantum optics experiments with flying electrons.

I will describe different experimental strategies to displace controllably a single electron initially trapped in a lateral quantum dot and to detect individual flying electrons defined in AlGaAs heterostructures. I will discuss the spin dynamics of the electron while transferring it and show that spin magnetization can indeed be preserved during the electron transfer.

Abstract 27 March 2013 Graphene nano-photonics and carrier dynamics Frank Koppens, ICFO – The Institute of Photonic Sciences, Castelldefels (Barcelona)

Graphene, a two-dimensional sheet of carbon atoms, has recently emerged as a novel material with unique electrical and optical properties, with great potential for novel opto-electronic applications, such as ultrafast photo-detection, optical switches, strong light-matter interactons etc. In this talk I will review the new and strongly emerging field of graphene nano-photonics. In particular, I will show how to exploit graphene as a host for guiding, switching and manipulating light and electrons at the nanoscale [1,2]. This is achieved by exploiting surface plasmons: surface waves coupled to the charge carrier excitations of the conducting sheet. Due to the unique characteristics of graphene, light can be squeezed into extremely small volumes and thus facilitate strongly enhanced light-matter interactions. Additionally, I will discuss novel types of hybrid graphene photodetectors [3] and new excitating results on carrier dynamics and carrier multiplication in graphene. By studying the ultrafast energy relaxation of photo-excited carriers after excitation with light of varying photon energy, we find that electron-electron scattering (and thus carrier multiplication) dominates the energy relaxation cascade rather than electron-phonon interaction [4]. This singles out graphene as a promising material for highly efficient broadband extraction of light energy into electronic degrees of freedom, enabling a new class of high-efficiency optoelectronic and photovoltaic applications.

References [1] J. Chen, M. Badioli, P. Alonso-González, S Thongrattanasiri, F Huth, J Osmond, M. Spasenović, A. Centeno, A. Pesquera, P. Godignon, A. Zurutuza, N. Camara, J. Garcia de Abajo, R. Hillenbrand, F. Koppens, “Optical nano- imaging of gate-tuneable graphene plasmons”, Nature (2012) [2] F. Koppens, D. Chang, J. García de Abajo, “Graphene Plasmonics: A Platform for Strong Light–Matter Interactions”, Nano Letters 11, 3370–3377 (2011). [3] G. Konstantatos, M. Badioli, L. Gaudreau, J. Osmond, M. Bernechea, P. Garcia de Arquer, F. Gatti, F. Koppens, “Hybrid graphene-quantum dot phototransistors with ultrahigh gain”, Nature Nanotechnology (2012) [4] Photo-excitation Cascade and Multiple Carrier Generation in Graphene. K.J. Tielrooij, J.C.W. Song, S.A. Jensen, A. Centeno, A. Pesquera, A. Zurutuza Elorza, M. Bonn, L.S. Levitov, and F.H.L. Koppens. Nature Physics (2012)

Abstract 13 March 2013 Individual molecules investigated by scanning probe microscopy with Atomically Functionalized Tips, Leo Gross, IBM Research, Zurich

We investigated single organic molecules adsorbed on ultrathin insulating films using scanning tunnelling microscopy (STM), noncontact atomic force microscopy (NC-AFM), and Kelvin probe force microscopy (KPFM). With all of these techniques submolecular resolution was obtained due to tip functionalization by atomic manipulation. The techniques yield complementary information regarding the molecular structural and electronic properties.

Using NC-AFM with CO functionalized tips, atomic resolution on molecules was demonstrated [1]. Moreover, different bond orders of individual carbon-carbon bonds in polycyclic aromatic hydrocarbons and fullerenes were distinguished [2]. Using KPFM information about the distribution of charges within molecules was gained by measuring the z-component of the electrostatic field above the molecule, as demonstrated on the tautomerization switch naphthalocyanine [3]. Recently, we investigated other tip functionalizations for AFM that also yielded atomic resolution on molecules, i.e., Cl, Br, Xe, Kr, and NO [4]. The tip dependence will be discussed. The prospects to use high resolution AFM for molecular structure identification [5] and adsorption geometry determination will be discussed.

[1] L. Gross, F. Mohn, N. Moll, P. Liljeroth, G. Meyer, Science 325, 1110 (2009) [2] L. Gross, F. Mohn, N. Moll, B. Schuler, A. Criado, E. Guitian, D. Pena, A. Gourdon, G. Meyer, Science 337, 1326 (2012) [3] F. Mohn, L. Gross, N. Moll, G. Meyer Nature Nanotechnol. 7, 227 (2012) [4] F. Mohn, B. Schuler, L. Gross, G. Meyer, Appl. Phys. Lett. 102, 073109 (2013) [5] L. Gross, F. Mohn, N. Moll, G. Meyer, R. Ebel, W. M. Abdel-Mageed, M.Jaspars, Nature Chem. 2, 821 (2010) Abstract 5 March 2013 Nonlinear optics and single photon detection on a chip Wolfram Pernice, Karlsruhe Institute of Technology

Nanophotonic devices allow for realizing complex optical functionality that is otherwise difficult to achieve with free-space optical setups. While such circuits find a multitude of applications in telecommunication and optical signal processing, their tremendous potential for non-classical optics and quantum computation remains largely unexplored. Here I will present an integrated platform in which key challenges of integrated quantum optics are addressed by combining photonic and superconducting devices. Superconducting nanowires are particularly promising for recording single photon events in a chip-scale framework. Besides offering near- perfect detection efficiency, they also provide a small footprint and scalability as a key step towards integrated quantum optical circuits. Working with a nonlinear optical substrate material furthermore enables active control of photon interaction. Leveraging additional degrees of freedom provided by free-standing mechanical structures thus yields a rich architecture for studying fundamental physics and emerging applications in chip- based metrology.

Abstract 27 February 2013 Semiconductor sources of photon pairs Gregor Weihs, institut für Experimentalphysik, Universität Innsbruck, Austria and Institute for Quantum Computing, University of Waterloo, Canada

For fundamental tests of quantum physics as well as for quantum communications non-classical states of light are an important tool. In our research we focus on developing semiconductor-based and integrated sources of single photons and entangled photon pairs. In my talk I will present two approaches that we have been following towards this goal.

In the first approach we demonstrate efficient photon pair generation in an AlGaAs Bragg-reflection waveguide. Spontaneous parametric down-conversion creates photon pairs at telecommunication . The various phase-matching solutions present in our device can be used to create time-bin or polarization entanglement. This approach can to lead to a fully integrated photon pair source with the pump , active and passive optical devices all on a single semiconductor chip.

In our second approach we use resonant two-photon excitation of a single InAs/GaAs quantum dot to deterministically trigger a biexciton-exciton cascade. We demonstrate Rabi oscillations, Ramsey interference and all-optical coherent control of the quantum dot resulting in single and paired photons with a high degree of indistinguishability. This indistinguishability results in time-bin entanglement, which is a useful variant for long distance communication.

Abstract 20 February 2013 Detection and manipulation of Majorana fermions in condensed matter systems Karsten Flensberg, Niels Bohr Institute, Univ. Of Copenhagen

In recent years a number of proposals for fabrication of topological superconductors in hybrid systems have been put forward, for example by combining conventional superconductors with strong spin-orbit coupling semiconductors. Topological superconductors host Majorana bound states, which are half fermions. They have interesting non-local properties and might be useful for topologically protected quantum computing. The computation is done by performing rotations in the degenerated groundstate manifold, and the talk presents ways of manipulating the state of the Majorana bound states using single particle control.

Abstract 13 February 2013 Experimental Quantum Information Processing with Trapped (Rydberg) Markus Hennrich, University of Innsbruck

Cold trapped ions are well isolated from the environment and can be coherently manipulated by laser light. This makes them a useful system to realize fundamental quantum optics experiments and to build a quantum computer on which one can test quantum algorithms and quantum simulations.

In the first part of my talk, I will introduce trapped ions as a quantum system and discuss their application in quantum information. Then I will present some of our recently realised algorithms, in particular, the generation of GHZ states of up to 14 ion qubits, and the realization of open-system and digital quantum simulations.

In the second part of my talk I will describe a novel approach that will bring together two technologies for quantum systems: trapped ions and Rydberg atoms. This idea promises to combine the advanced quantum control of trapped ions with the strong dipolar interaction between Rydberg atoms. Joining them will form a novel quantum system with advantages from both sides. In particular, it promises to speed up entangling interactions and to make such operations possible in larger ion .

Abstract 6 February 2013 Time-resolved soft X-ray scattering measurements of ordered electronic states in oxides Raanan Tobey, Zernike Institute for Advanced Materials, Univ. of Groningen

Ordered electronic states are ubiquitous in the transition metal oxides. At particular chemical compositions, electronic localization is energetically preferred over itinerancy. In such cases the preferred ground state exhibits ordering in its charges, orbital occupancy, and spin orientation. The lengthscales over which the ordered states extend range from 10s of nanometers to a micron or more. This nanoscale electronic texture is thought to play a fundamental role in emergent behavior such as colossal magneto resistance in manganites and superconductivity in the copper oxides.

In this talk, I will present resonant soft x-ray diffraction (RSXD) as a means for measuring ordered electronic states. By further incorporating short pulses of x-rays we can time resolve the RSXD to measure the evolution of these states after the sample is driven out of equilibrium. As an example, I will demonstrate our capability to visualize the lengthscales over which this magnetic order persists in the manganite La0.5Sr1.5MnO4, and its evolution after perturbation by a laser pulse. Finally, I will present our upcoming measurement to image, in time and space, the evolution of electronic texture.

Abstract 30 January 2013 Vortex electrons probing novel spectroscopic information at the atomic scale Johan Verbeeck, University of Antwerpen

Vortex electron waves are the electron wave counterpart of the better known optical vortices. In optics, they are attracting considerable interest both from a fundamental and application point of view. The essence of wave vortices is the presence of a phase singularity around the propagation axis leading to an extrinsic orbital angular momentum (OAM) in contrast to the intrinsic spin angular momentum (SAM) carried by the polarisation of the wave. A typical way to produce such a vortex wave would be a phase plate with spiralling thickness profile leading to an integer times 2π phase shift when going around the optic axis. This integer is called the topological charge m which links to the orbital angular momentum of mħ per elementary particle in the wave. Applying these ideas to electron waves as available in a transmission electron microscope, one can indeed produce electron vortices making use of a holographic reconstruction technique. Holographic masks containing either a fork or a spiral structure were shown to produce vortex waves with different topological charge separated in space. Making use of the small matter we can even reduce the size of such a vortex wave to the atomic scale as was proven from a direct measurement of an image of the probe as well as from making use of vortex probes in scanning transmission electron microscopy (STEM) imaging while still retaining atomic resolution. An in depth theoretical understanding of electron vortex waves provides a background to guide us in designing new experiments and applications. An example of such experiments is given by the preparation of electrons in specific quantum states like e.g. linear combinations of several vortex modes transforming the electron microscope into an ideal tool for quantum experiments.

In terms of applications for materials science, the use of vortex waves to obtain magnetic information from materials making use of electron energy loss spectroscopy will be discussed. Both experimental and theoretical advances will demonstrate that vortex electrons are an ideal tool to map the magnetization of materials on the atomic scale, forming an alternative to the more commonly used magnetic TEM techniques. The basic principle used here is the breaking of time reversal symmetry that prevents us from observing magnetic information in conventional EELS. Using atomic resolution electron vortices allows obtaining a spectroscopic signal that provides magnetic information on an atom column by atom column basis.