Events: Mikroteknologi och nanovetenskap events at Chalmers University of TechnologyWed, 22 Jan 2020 15:27:52 +0100 transport and symmetry breaking in microscopic power grids<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​Linneaus Colloquium with Peter Rabl, TU Vienna, Austria​</p><strong>​Abstract:</strong><div>I this talk I will address the general problem of energy transport in a microscopic network of coupled harmonic oscillators, where it is the goal, for example, to deliver energy from a quantum generator to a quantum engine. I will show that transport in such systems differs strongly from the usual Ohm's or Fourier's law and exhibits various anomalies,  which arise from the competition between coherent and incoherent processes in combination with nonlinear saturation effects. Specifically, one finds that such networks exhibit a non-equilibrium phase transition between a noise-dominated and a coherent transport regime. This transition is closely related to the phenomenon of PT-symmetry breaking in classical gain-loss systems, but is identified here as a very generic mechanism in active quantum networks. As an outlook I will show that in dissipative spin systems the same PT-symmetry breaking mechanism results in novel types of phase transitions, which do not exhibit the usual features of phase co-existence or symmetry-breaking. <br /></div> physics of graphene, possibly the most nonlinear material we know<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​Graphene Centre Seminar with Dmitry Turchinovich, Bielefeld University, Germany</p>​​<strong>Abstract</strong>:<div><p>The interaction of graphene with THz electromagnetic waves is dominated by the collective, thermodynamic response of conduction electrons in graphene to the THz excitation. Due to very efficiently energy transfer from the incident electromagnetic field to electronic system of graphene, facilitated by the excellent transport properties of Dirac electrons, the electronic temperature in graphene becomes strongly modulated by the THz excitation. As a result, the THz conductivity of graphene becomes highly nonlinear, leading to very strong THz saturable absorption and extremely efficient THz high harmonics generation in graphene. The effective nonlinear coefficients of graphene in the THz frequency range exceed that of any known material by many orders of magnitude. This possibly makes graphene the most nonlinear material we know.</p> </div> ceremony: GCC PhD award 2019, Marlene Bonmann<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p></p>​<img src="/SiteCollectionImages/Centrum/Grafencentrum/M%20Bonmann.jpg" class="chalmersPosition-FloatLeft" alt="" style="margin:5px;width:150px;height:163px" />In conjunction with the first GCC seminar of the year we welcome you to listen to Marlene Bonmann, PhD student at Terahertz and Millimetrewave laboratory, Microtechnology and Nanoscience and winner of the GCC PhD award 2019.<div><br /></div> <div>Marlene will present her PhD thesis with the title &quot;<a href="" target="_blank">Graphene field-effect transistors and devices for advanced highfrequency applications​</a>&quot;.</div> Scigliuzzo, Microtechnology and Nanoscience<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​Mid term seminar with the title: Phononic and photonic interaction with superconducting circuits</p>​Abstract: the first part of the talk will summarize our work on understanding the spurious conversion from photon to phonon in a superconducting resonators realized on a piezoelectric substrate. In the second part we will show how to use a qubit as primary thermometry of propagating microwave modes and measure the photon occupation number in the transmission lines at the base plate of our dilution refrigerator. superconductors and Majorana fermions<p></p><p>​ We we​lcome you to attend this year first GPC Physics Colloquium by  Annica Black-Schaffer, Uppsala University.</p><h2 class="chalmersElement-H2">​Abstract:</h2> <div>Topological superconductors with Majorana fermion quasiparticles form a newly discovered class of matter. The Majorana fermion can be seen as half an electron, or more accurately, the electron wave function has split up into two completely separate parts. I will explain the basic physics behind topological superconductors and why Majorana fermions appear, briefly review the current experimental status, and report a few of our recent findings.​</div> Marc Seja, Microtechnology and Nanoscience<p>C511, seminar room, Kemivägen 9, MC2-huset</p><p>​Licentiate seminar with the title Transport in mesoscopic superconducting devices Kevin Marc Seja is a PhD student at the Applied Quantum Physics Laboratory. Main supervisor: Tomas Löfwander. Examiner: Göran Johansson Opponent at the seminar: Thilo Bauch</p>​<span style="font-size:14px"><span style="background-color:initial">A field of growing interest within the last few decades is the study of superconductivity in mesoscopic-scale heterostructures. Mesoscopic refers to sizes between the atomic and macroscopic scales. Here, the size of heterostructures can be comparable to the inherent scale of superconductivity, the superconducting coherence length, and give rise to new physical phenomena.</span></span><div><br /><span style="font-size:14px"><span style="background-color:initial"></span></span><div><span style="font-size:14px">The focus of this work is on mesoscopic hybrid structures consisting of superconducting, normal-metal, and magnetic regions. The combination of these different types of materials and the competition between interactions such as magnetism and superconductivity can then be used to design structures with novel effects. This is not only interesting from a fundamental point of view but equally relevant for technological applications. The magnet-superconductor hybrid structures examined in this work, for example, give rise spin-polarized Andreev bound states, a promising ingredient to superconducting spintronics.</span></div> <div><span style="font-size:14px"><br /></span></div> <div><span style="font-size:14px">We study transport in such hybrid systems under current bias to investigate the effects of such Andreev bound states on nonequilibrium properties. As part of this work, we develop a general calculation scheme for current-bias nonequilibrium within the quasiclassical theory of superconductivity. We use this scheme to study charge and spin imbalance in a normal-metal/superconductor structure with a spin-active interface. Our results show that transport in systems with spatially extended tunnel barriers is more accurately described by this current-bias picture compared to a voltage-bias description traditionally used in the theoretical literature for narrow constrictions. We find that the presence of Andreev bound states at a spin-active interface between normal-metal and superconducting regions strongly influence the charge as well as spin transport in such structures.</span></div> </div>'s Thesis presentation with Gagandeep Bhatia, MPCAS<p>Fasrummet, meeting room, Kemivägen 9, MC2-huset</p><p>​Title: FOPPL-prime: A First Order Probabilistic Programming Language (Implementation and Applications)</p> 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> 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>,-MC2.aspx,-MC2.aspxEwa Simpanen, Microtechnology and Nanoscience<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​Title: Longer Wavelength GaAs-Based VCSELs for Extended-Reach Optical Interconnects</p><span><span class="text-normal page-content"><div>Ewa is a PhD student at the Photonics Laboratory</div></span></span><span><span class="text-normal page-content"> <div>Examiner: Prof. Anders Larsson<br /></div> <div>Main supervisor: <span style="display:inline-block"><span style="display:inline-block">Associate Professor Johan Gustavsson</span></span></div> </span></span>,-Microtechnology-and-Nanoscience.aspx,-Microtechnology-and-Nanoscience.aspxEdoardo Trabaldo, Microtechnology and Nanoscience<p>Kollektorn, lecture room, Kemivägen 9, MC2-huset</p><p>​Title: Noise and electrical properties of YBCO nanostructures</p>​<br />Edoardo is a PhD student at the Quantum Device Physics Laboratory<br />Examiner: Prof. Floriana Lombardi<br />Main supervisor: Associate Professor Thilo Bauch<span><span class="text-normal page-content"><div><br /><span style="display:inline-block"><span style="display:inline-block"></span></span></div></span></span><span><span class="text-normal page-content"> </span></span> 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> 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> ​ 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>