Events: Centre: Physics Centre events at Chalmers University of TechnologyMon, 10 Feb 2020 15:27:45 +0100 Lassila, Physics and Astronomy<p>N6115, seminar room, Kemigården 1, Fysik Origo</p><p>​Title on Master thesis: Holographic Duality and Strongly Interacting Quantum Matter</p><h2 class="chalmersElement-H2">​Abstract: </h2> <div><span style="background-color:initial">This thesis is devoted to the applications of holographic duality to condensed matter physics. It</span></div> <div>is centered around a ’bottom-up’ approach where the starting point is the postulation of a reasonable</div> <div> </div> <div>gravitational bulk theory action, as opposed to the ’top-down’ models where a specific duality is derived</div> <div> </div> <div>from a string theory setting. The main advantage with the holographic approach to condensed matter</div> <div> </div> <div>physics is the potential ability to perform computations for strongly interacting many-body systems</div> <div> </div> <div>which does not have a quasiparticle description. The duality maps a strongly coupled quantum field</div> <div> </div> <div>theory to a weakly interacting gravitational theory which in principle can be solved perturabtively using</div> <div> </div> <div>ordinary general relativity. An introduction to some of the main topics of ’bottom-up’ holography is</div> <div> </div> <div>covered. This includes a brief introduction to large N field theories, the AdS/CFT correspondance, the</div> <div> </div> <div>holographic dictionary, the holographic renormalization group, holographic thermodynamics, and the</div> <div> </div> <div>Hawking-page transition and its interpretation in the light of AdS/CFT. Finally, a minimal bottom-up</div> <div> </div> <div>model for holographic superconductivity is studied. By imposing a mixed boundary condition at the</div> <div> </div> <div>boundary of AdS space, a dynamical photon is incorporated in the strongly coupled superconductor.</div> <div> </div> <div>This allows charged collective excitations, e.g. plasmons, to be studied. A linear response analysis of</div> <div> </div> <div>the minimal holographic superconductor is performed numerically in an attempt to compute plasmon</div> <div> </div> <div>dispersion relations.​</div> <div> </div> Norling, Bioscience<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>Title of doctoral thesis: &quot;Liposomes for mucosal vaccine delivery: physicochemical characterization and biological application</p><h2 class="chalmersElement-H2">Abstract:</h2> <div><span style="background-color:initial">Liposomes are attractive vaccine carriers due to their potential to act as adjuvants, and to the fact that their composition and characteristics are virtually endlessly customizable. However, the precise physicochemical profile of an ideal carrier liposome for mucosal vaccines is still widely unknown, and how different properties affect key steps in the acquisition of protective immunity remains to be elucidated. Additionally, there is no consensus in the field regarding characterization of vaccine formulations, often with incomplete reporting of properties as a result. The focus of this work is therefore twofold: i) to contribute to a better understanding of how the physicochemical profile of vaccine carrier liposomes impacts the development of protective immunity using models at different levels of complexity, and ii) to improve and simplify the physicochemical characterization of liposomes through development and use of new analytical methods. </span></div> <span style="background-color:initial"><div> </div></span><div><span style="background-color:initial">The work in the first area consists of, firstly, an in vivo characterization of the biological response to vaccine liposomes carrying a vaccine protein and characterized by varying surface hydrophilicity (PEGylation). This study showed that non-PEGylated vaccine liposomes more efficiently induced local cell- and antibody-mediated immune responses, as well as better protection against a lethal virus challenge than both PEGylated liposomes and free vaccine protein. Secondly, in vitro studies focused on how liposome stiffness influences dendritic cells, investigating effects on uptake, antigen presentation and cellular activation. These investigations demonstrated that stiff, gel phase liposomes were able to more efficiently activate dendritic cells and induce significantly higher levels of antigen presentation and co-stimulatory signaling compared to both soft, fluid phase liposomes, and free vaccine protein. <br /><br />The work in the second part comprises two studies: a surface plasmon resonance-based method to characterize the influence on liposome deformation from specific multivalent interactions with supported cell membrane mimics, and a waveguide microscopy technique for characterization of optical properties of individual liposomes. While the latter method might become valuable in the context of quantifying the efficiency of dye labelling of liposomes, the surface plasmon resonance study offered information on how liposome deformation depends on membrane stiffness and ligand-receptor pair density. Taken together, the work presented in this thesis demonstrate the value of multidisciplinary approaches to complex biological and medical challenges.</span></div> <div> </div> Properties of Semiconductor Moire Crystals<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​​Welcome to a Graphene Centre Seminar with Xiaoqin (Elaine) Li, Austin, Texas USA</p><strong>Abstra​ct:</strong><div><span style="font-size:14px"><span></span>A new type of superlattice, known as the moiré superlattice, form when two monolayers of van der Waals materials are stacked to form a heterostructure. A periodic energy modulation, as well as distinct optical selection rules, give rise to rich optical properties that remain largely unexplored in semiconductor moiré crystals. I will discuss how the moiré potential in twisted transition metal dichalcogenide bilayers changes the exciton resonances and diffusion in a manner controllable by the twist angle. Additional insights are obtained by comparing heterostructures prepared with either chemical vapor deposition or mechanical stacking. There are many exciting opportunities for exploring fundamental condensed matter physics and novel optoelectronic devices such as an array of single-photon emitters in these moiré crystals.</span><br /></div> Yi, Nuclear Engineering<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>​ Title of thesis: A neutron noise solver based on a discrete ordinates method.</p><h2 class="chalmersElement-H2">Abstract:</h2> <div><span style="background-color:initial">A</span><span style="background-color:initial"> neutron noise transport modelling tool is presented in this thesis. The simulator allows to determine the static solution of a critical system and the neutron noise induced by a prescribed perturbation of the critical system. The simulator is based on the neutron balance equations in the frequency domain and for two-dimensional systems. The discrete ordinates method is used for the angular discretization and the diamond finite difference method for the treatment of the spatial variable. The energy dependence is modelled with two neutron energy groups. The conventional inner-outer iterative scheme is employed for solving the discretized neutron transport equations. For the acceleration of the iterative scheme, the diffusion synthetic acceleration is implemented.</span></div> <div><br /></div> <div> </div> <div>The convergence rate of the accelerated and unaccelerated versions of the simulator is studied for the case of a perturbed infinite homogeneous system. The theoretical behavior predicted by the Fourier convergence analysis agrees well with the numerical performance of the simulator. The diffusion synthetic acceleration decreases significantly the number of numerical iterations, but its convergence rate is still slow, especially for perturbations at low frequencies.<br /><br /></div> <div> </div> <div>The simulator is further tested on neutron noise problems in more realistic, heterogeneous systems and compared with the diffusion-based solver. The diffusion synthetic acceleration leads to a reduction of the computational burden by a factor of 20. In addition, the simulator shows results that are consistent with the diffusion-based approximation. However, discrepancies are found because of the local effects of the neutron noise source and the strong variations of material properties in the system, which are expected to be better reproduced by a higher-order transport method such as the one used in the new solver. </div> <div> </div> <div><br /></div> <div> </div> <div>Keywords: Neutron noise, nuclear reactor modelling, deterministic neutron transport methods, discrete ordinates, diffusion synthetic acceleration, convergence analysis</div> <div> </div> <div><br /></div> <div> </div> <div>​<br /></div> feedback of a mechanical oscillator<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​Welcome to a Linnaeus Colloquium by Tobias Kippenberg, Institute of Physics Swiss Federal Institute of Technology Lausanne, EPFL, Switzerland</p><strong>Abstract:</strong><div> <div>In real-time quantum feedback protocols (1), the record of a continuous measurement is used to stabilize a desired quantum state. Recent years have seen spectacular advances in a variety of well-isolated micro-systems, including microwave photons(2) and superconducting qubits(3). By contrast, the ability to stabilize the quantum state of a tangibly massive object, such as a nano-mechanical oscillator, remains a difficult challenge. The main obstacle is environmental decoherence, which places stringent requirements on the timescale in which the state must be  measured. Using cavity optomechanical coupling(4, 5) we report on a position sensor that is capable of resolving the zero-point motion of a solid-state, 4.3 MHz frequency nanomechanical oscillator in the timescale of its thermal decoherence(6), a basic requirement for preparing its ground-state using feedback as well as (Markovian) quantum feedback. The sensor is based on evanescent coupling to a high-Q optical microcavity(7), and achieves an imprecision 40 dB below that at the standard quantum limit for a weak continuous position measurement(8), while maintaining an imprecision-back-action product within a factor of 5 of the Heisenberg uncertainty limit. As a demonstration of its utility, we use the measurement as an error signal with which to feedback cool the oscillator. Using radiation pressure as an actuator, the oscillator is cold-damped(9) with unprecedented efficiency: from a cryogenic bath temperature of 4.4 K to an effective value of 1.1 mK, corresponding to a mean phonon number of 5 (i.e., a ground state probability of 16%). The measurement reveals strong backaction-imprecision correlations, which we observe as quantum mechanical sideband asymmetries, as well as pondermotive squeezing of the light field(10). Our results set a new benchmark for the performance of a linear position sensor, and signal the emergence of mechanical oscillators as practical subjects for measurement-based quantum control.  We moreover demonstrate the existence of such quantum correlations due to the optomechanical interaction at room temperature (11) and demonstrate that the correlations enable quantum enhanced force sensing (termed “variational measurements”). This scheme, rather than utilizing squeezed vacuum, uses the quantum correlations produced in the interferometer for enhanced force sensing. Closing, we will describe recent progress which uses soft clamping and strain engineering, which has enabled to attain mechanical quality factor exceeding 800 million at room temperature, implying a mechanical oscillator undergoing more than hundreds of oscillations during the thermal decoherence time. These results signal the emergence of room temperature quantum feedback, and room temperature quantum control of mechanical oscillators. Time permitting, I will also show emerging hybrid optomechanical technologies, ranging from non-reciprocal photonic circuits using optomechanics, to quantum limited amplifiers based on ground state cooled mechanical oscillators.</div> <div><strong>References:</strong></div> <div>1.<span style="white-space:pre"> </span>H. Wiseman, Quantum theory of continuous feedback. Physical Review A 49, 2133 (1994).</div> <div>2.<span style="white-space:pre"> </span>C. Sayrin et al., Real-time quantum feedback prepares and stabilizes photon number states. Nature 477, 73 (Sep 1, 2011).</div> <div>3.<span style="white-space:pre"> </span>R. Vijay et al., Stabilizing Rabi oscillations in a superconducting qubit using quantum feedback. Nature 490, 77 (Oct 4, 2012).</div> <div>4.<span style="white-space:pre"> </span>T. Kippenberg, H. Rokhsari, T. Carmon, A. Scherer, K. Vahala, Analysis of Radiation-Pressure Induced Mechanical Oscillation of an Optical Microcavity. Physical Review Letters 95,  (2005).</div> <div>5.<span style="white-space:pre"> </span>M. Aspelmeyer, T. J. Kippenberg, F. Marquardt, Cavity optomechanics. Reviews of Modern Physics 86, 1391 (December 30, 2014, 2014).</div> <div>6.<span style="white-space:pre"> </span>D. J. Wilson et al., Measurement and control of a mechanical oscillator at its thermal decoherence rate. Nature doi:10.1038/nature14672,  (2015, 2014).</div> <div>7.<span style="white-space:pre"> </span>E. Gavartin, P. Verlot, T. J. Kippenberg, A hybrid on-chip optomechanical transducer for ultrasensitive force measurements. Nature nanotechnology 7, 509 (Aug, 2012).</div> <div>8.<span style="white-space:pre"> </span>A. A. Clerk, M. H. Devoret, S. M. Girvin, F. Marquardt, R. J. Schoelkopf, Introduction to quantum noise, measurement, and amplification. Reviews of Modern Physics 82, 1155 (2010).</div> <div>9.<span style="white-space:pre"> </span>M. Pinard, P. Cohadon, T. Briant, A. Heidmann, Full mechanical characterization of a cold damped mirror. Physical Review A 63,  (2000).</div> <div>10.<span style="white-space:pre"> </span>V. Sudhir, D. Wilson, A. Ghadimi, T. J. Kippenberg, Appearance and disappearance of quantum correlations in measurement-based feedback control of a mechanical oscillator. Phys. Rev.  X  (2017).</div> <div>11.<span style="white-space:pre"> </span>V. Sudhir, D. Wilson,  T. J. Kippenberg, Room temperature quantum correlations of a mechanical oscillator. Phys. Rev.  X  (2017).</div> <div>12.<span style="white-space:pre"> </span>A. Ghadimi et al., Elastic strain engineering for ultralow mechanical dissipation, Science (2018)</div> <div><br /></div></div> Åvall, Physics<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>​ Title of doctoral thesis: &quot;Structure and dynamics in liquid battery electrolytes.</p>​<br /><strong>Abstract</strong>: To be announced.<br /> of changing glaciers - processes and mechanisms of ice sheets<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>​Welcome to a colloquim by Angelika Humbert, Alfred Wegener Institute.</p><font color="#212121"><span style="font-size:20px">Abstract:</span></font><img src="/SiteCollectionImages/Centrum/Fysikcentrum/Blandade%20bilddimensioner/Angelika.JPG" class="chalmersPosition-FloatRight" alt="" style="margin:5px;height:215px;width:165px" /><div><div><span style="background-color:initial">Ice sheets are complex systems with processes from the micro-scale to </span><span style="background-color:initial">the kilometer scale are interacting with many of those interactions </span><span style="background-color:initial">being non-linear. Two processes with major influence on the dynamics of </span><span style="background-color:initial">the entire system are still lacking physically based process </span><span style="background-color:initial">description: calving of icebergs and sliding of glaciers across the </span><span style="background-color:initial">bedrock. Both mechanisms are of crucial for simulating future change of </span><span style="background-color:initial">ice sheets in Greenland and Antarctica. After a brief introduction into </span><span style="background-color:initial">the continuum mechanics of ice sheets, we will be discussing climatic </span><span style="background-color:initial">forcing of ice sheets, the role of calving, sliding and ice sheet </span><span style="background-color:initial">simulations for projecting future sea level change.</span></div></div> Ångqvist, Physics<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>Title of thesis: &quot;Atomic scale modeling of ordering phenomena</p>​<br />Abstract: To be announced pathways of hot electrons in graphene<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​Welcome to a Graphene Centre Seminar with Bernard Placais, CNRS, Paris, France.</p> QED with atomic hydrogen<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>Welcome to a colloquium by Professor​ Thomas Udem, Max-Planck Institut of Quantum Optics.</p><h2 class="chalmersElement-H2">​Abstract:</h2> <div><span style="background-color:initial">Precise determination of transition frequencies of simple atomic systems are required for a</span></div> <div> </div> <div>number of fundamental applications such as tests of quantum electrodynamics (QED), the</div> <div> </div> <div>determination of fundamental constants and nuclear charge radii. The sharpest transition in</div> <div> </div> <div>atomic hydrogen occurs between the metastable 2S state and the 1S ground state with a</div> <div> </div> <div>natural line width of only 1.3 Hz. Its transition frequency has been measured with almost 15</div> <div> </div> <div>digits accuracy using an optical frequency comb and a cesium atomic clock as a reference [1].</div> <div> </div> <div>A measurement of the Lamb shift in muonic hydrogen is in significant contradiction to the</div> <div> </div> <div>hydrogen data if QED calculations are assumed to be correct [2]. In order to shed light on</div> <div> </div> <div>this discrepancy the transition frequency of one of the broader lines in atomic hydrogen has</div> <div> </div> <div>to be measured with very good accuracy [3].</div> <div> </div> <div><br /></div> <div> </div> <div>[1] C. G. Parthey et al., Phys. Rev. Lett. 107, 203001 (2011).</div> <div> </div> <div>[2] A. Antognini et al., Science 339, 417, (2013).</div> <div> </div> <div>[3] A. Beyer et al., Science 358, 79 (2017).</div> <div> </div> <div>​</div> devices in bilayer graphene<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​​​Welcome to a Graphene Centre Seminar with​ Klaus Ensslin, ETH, Zurich , Switzerland</p> optics, extraordinary momentum and spin in evanescent waves, optical analog of topological insulators, and the quantum spin Hall effect of light<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​Welcome to a Linnaeus Colloquium with Franco Nori, RIKEN, Japan</p><strong>Abstract:</strong><br /><div><div>This talk provides a brief overview to some aspects of parity-time symmetric optics, extraordinary momentum and spin in evanescent waves, optical analog of topological insulators, and the quantum spin Hall effect of light.  </div> <div> </div> <div>1.<span style="white-space:pre"> </span>Parity-Time-Symmetric Optics </div> <div>Optical systems combining balanced loss and gain provide a unique platform to implement classical analogues of quantum systems described by non-Hermitian parity–time (PT)-symmetric Hamiltonians [1]. Such systems can be used to create synthetic materials with properties that cannot be attained in materials having only loss or only gain. We report PT-symmetry breaking in coupled optical resonators. We observed non-reciprocity in the PT-symmetry-breaking phase due to strong field localization, which significantly enhances nonlinearity. In the linear regime, light transmission is reciprocal regardless of whether the symmetry is broken or unbroken. We show that in one direction there is a complete absence of resonance peaks whereas in the other direction the transmission is resonantly enhanced, which is associated with the use of resonant structures. Our results could lead to a new generation of synthetic optical systems enabling onchip manipulation and control of light propagation. </div> <div> </div> <div>2.<span style="white-space:pre"> </span>The quantum spin Hall effect of light: photonic analog of 3D topological insulators. </div> <div>Maxwell’s equations, formulated 150 years ago, ultimately describe properties of light, from classical electromagnetism to quantum and relativistic aspects. The latter ones result in remarkable geometric and topological phenomena related to the spin-1 massless nature of photons. By analyzing fundamental spin properties of Maxwell waves, we show [2] that free-space light exhibits an intrinsic quantum spin Hall effect —surface modes with strong spin-momentum locking. These modes are evanescent waves that form, for example, surface plasmon-polaritons at vacuum-metal interfaces. Our findings illuminate the unusual transverse spin in evanescent waves and explain recent experiments that have demonstrated the transverse spin-direction locking in the excitation of surface optical modes. This deepens our understanding of Maxwell’s theory, reveals analogies with topological insulators for electrons, and offers applications for robust spindirectional optical interfaces.  Related work can be found in [3]. </div></div> <div><br /></div> <div><a href="/en/departments/mc2/calendar/Documents/franco_nori.pdf"><img class="ms-asset-icon ms-rtePosition-4" src="/en/departments/mc2/calendar/_layouts/images/icpdf.png" alt="" />Abstract and references (pdf</a>)</div> <div><br /></div> ​ by Patrice Simon<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>​ Welcome to a GPC Colloquium by Dr, Distinguished Professor​ Patrice Simon, University Paul Sabatier Toulouse, France Title: Electrochemistry at nanoporous electrodes: 2- and 3-D electrodes for Electrochemical Capacitor applications</p><h2 class="chalmersElement-H2">​Abstract:</h2> <div>This presentation will give an overview of the research work we achieved on capacitive (porous carbon) and pseudocapacitive materials and the challenges/limitations associated with the development of these materials. Starting with porous carbons, we will present the state-of-the art of the fundamental of ion adsorption mechanism in porous carbons and its practical applications. Moving from double layer to pseudocapacitive materials, we will show how the control of the electrodes structure can help in preparing high energy electrodes using 2-Dimensional MXene materials.​</div> Bands in Flatlands<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​Welcome to a Graphene Centre Seminar with ChunNing (Jeanie) Lau, Ohio, USA</p> and reversing a quantum jump mid-flight<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>Joint Linnaeus and GPC Colloquium with Michel Devoret, Yale University, USA​</p><div><span style="font-weight:700"><img src="/SiteCollectionImages/Institutioner/MC2/Föreläsningar/M%20Devoret.jpg" class="chalmersPosition-FloatRight" alt="" style="margin:5px;height:235px;width:200px" />Abstract:</span></div> <div>Measurements in quantum physics, unlike their classical physics counterparts, can fundamentally yield discrete and random results. Historically, Niels Bohr was the first to hypothesize that quantum jumps occurred between two discrete energy levels of an atom. Experimentally, quantum jumps were only directly observed many decades later, in an atomic ion driven by a weak deterministic force under strong continuous energy measurement. The times at which the discontinuous jump transitions occur are reputed to be fundamentally unpredictable. Despite the non-deterministic character of quantum physics, is it possible to know if a quantum jump is about to occur? </div> <div>Our work<span style="font-size:10.5px;line-height:0;vertical-align:baseline;top:-0.5em">1</span> provides a positive answer to this question: we experimentally show that the jump from the ground state to an excited state of a superconducting artificial three-level atom can be tracked as it follows a predictable “flight” by monitoring the population of an auxiliary energy level coupled to the ground state. The experimental results demonstrate that the evolution of the jump — once completed — is continuous, coherent, and deterministic. Based on these insights and aided by real-time monitoring and feedback, we then pinpoint and reverse one such quantum jump “mid-flight”, thus deterministically preventing its completion. Our findings, which agree with theoretical predictions essentially without adjustable parameters, lend support to the modern formulation of quantum trajectory theory; most importantly, they may provide new ground for the exploration of real-time intervention techniques in the control of quantum systems, such as the early detection of error syndromes.</div> <div><br /></div> <div>1.<span style="white-space:pre"> </span>Z. Minev et al., Nature 570, 200–204 (2019)</div> <div><br /></div> imaging of exoplanets<p>PJ, lecture hall, Fysikgården 2B, Fysik Origo</p><p>Welcome to a colloquium by Markus Janson, Stockholm University.</p><h2 class="chalmersElement-H2">​Abstract: </h2> <div>Since the first discoveries of plants in orbit around other stars in the early/mid-1990s, the field of exoplanet research has rapidly expanded, and now encompasses thousands of exoplanet detections, as well characterization of the detected systems to various degrees of depth. In this talk, I will discuss the different techniques used to detect and study exoplanets, with a particular emphasis on high-contrast imaging, which allows to probe both orbital, physical and atmospheric properties of the planet. High-contrast imaging also allows to study planets during their actual phase of formation, and their interactions with the surrounding protoplanetary disk in which they form. While the sensitivities with currently existing telescopes generally only allow for the detection of classes of planets quite unlike the Earth, improvements in technology are continuously enhancing these sensitivities. I will discuss the pathways toward the ability to detect and characterize true Earth analogs, which will (most likely) be required to assess the frequency and distribution of habitability and life in the universe.​</div>