Events List for the Academic Year
Event Time:
Monday, January 27, 2020 | 3:00 pm - 4:00 pm
Event Location:
Hennings 201
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2020-01-27T15:00:00
2020-01-27T16:00:00
Discovering the first cosmic cities: Distant galaxy clusters and the growth of structure in the Universe.
Event Information:
A galaxy cluster can be likened to a city of galaxies. As such we can ask questions such as how does city living affect galaxies? How do they behave differently to galaxies that are located outside of clusters and what are the physical causes of these differences? I will present the search for distant galaxy clusters as one route to answering these questions. By observing the most distant clusters known we may catch galaxies in the act of accreting onto forming clusters and witness the physics of quenching and morphological transformation in action. I will focus on the story on one galaxy cluster, XLSSC 122, which was recently confirmed to lie at a redshift of 2, corresponding to a time 10.4 billion years in the past. XLSSC 122 appears to be a remarkably mature cluster and may well represent one of the first “cosmic cities."
Event Location:
Hennings 201
Event Time:
Thursday, January 23, 2020 | 4:00 pm - 5:00 pm
Event Location:
Hennings 201
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2020-01-23T16:00:00
2020-01-23T17:00:00
Ice sheets: a cartoon
Event Information:
Ice sheet simulations have become much more sophisticated over the last decade in their ability to capture small spatial detail and reproduce actual observed ice sheet behaviour. That does not mean that the underlying models are correct. Here we look "under the hood": the purpose of this talk is to present a survey of the physics in ice sheet models and its implications for the dynamics of ice sheets. I strip away much of the sophistication involved to look at the necessary ingredients in minimal continuum models of ice sheet dynamics that are able to capture land- and ocean-terminating ice sheet dynamics, identifying where the major sources of model uncertainty (as opposed to parameter uncertainty) lie, and how different choices of model closure lead to different qualitative dynamics. I will delve into the importance of iceberg calving for the overall dynamics of ice sheets, demonstrating how the response of an ocean-terminating ice sheet is dependent on the choice of calving parameterization, and outline the major difficulties still faced by thermomechanical ice sheet models in capturing the transition in space from a cold bed, where no sliding can occur, to a temperate one, where sliding often dominates the motion of the ice sheet.
Event Location:
Hennings 201
Event Time:
Thursday, January 23, 2020 | 2:00 pm - 3:00 pm
Event Location:
TRIUMF Auditorium
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2020-01-23T14:00:00
2020-01-23T15:00:00
The human voice, the erhu and the violin
Event Information:
The Western violin and the erhu, the Chinese violin, are at first glance, markedly different instruments, being apparently related only by their bows. The violin soundbox is made of spruce and maple, has a complex shape, and an asymmetrical interior. The erhu soundbox is a simple cylinder closed on one side by a pre-tensioned python skin and open at the other. However, delving into the vibro-acoustics of each structure, deep similarities emerge, both with each other and with that most ancient musical instrument, the human voice. Acoustics is a field that tends to fall between the cracks of physics and engineering, and musical acoustics falls between the cracks of acoustics and music. Thus I will not assume any prior knowledge of the field.
Event Location:
TRIUMF Auditorium
Event Time:
Thursday, January 23, 2020 | 2:00 pm - 3:00 pm
Event Location:
Brimacombe 311
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2020-01-23T14:00:00
2020-01-23T15:00:00
CM Seminar : Quantum Control of Solid-State Quantum Emitters using Optical Pulse Shape Engineering
Event Information:
Abstract: A quantum emitter (QE) is a physical system that can be used to encode a quantum state via some internal degree of freedom (e.g. exciton, electron spin, valley) and is coupled to light via a dipolar transition that enables the conversion of that quantum state into the state of a photon and vice versa. Such QEs can be applied to sources of single and entangled photons for applications in quantum cryptography or quantum imaging [1,2] and a collection of coupled QEs can be used to realize a small quantum simulator or quantum memory node in a distributed quantum network [3]. Our research group has developed optimal quantum control techniques applicable to solid state emitter systems that rely on optical pulse shape engineering to tailor the light-matter interaction [4,5]. In recent years, we have applied such techniques to semiconductor quantum dots, demonstrating the optimization of the speed and fidelity of quantum state manipulation [6] as well as parallel quantum state rotations on inequivalent emitters using a single laser pulse [7,8]. During this seminar, I will highlight some of our recent experiments providing new insight into electron-phonon coupling in quantum dots and enabling the suppression of phonon-mediated decoherence for telecom-compatible emitters.
Biography: Dr. Kimberley Hall is a Professor of Physics at Dalhousie University. She is director of the Ultrafast Quantum Control Group focused on the study and control of materials of interest for solid-state quantum technology and optoelectronic devices. Dr. Hall held the Canada Research Chair in Ultrafast Science from 2007-2017. Dr. Hall received an undergraduate physics degree from the University of Western Ontario, a Ph.D. in physics from the University of Toronto, and carried out postdoctoral work at the University of Iowa prior to joining Dalhousie University in 2004. Dr. Hall is a pioneer in the understanding of spin relaxation in semiconductor nanostructures and spintronic device innovation, has made significant contributions to the understanding of carrier kinetics in diluted magnetic semiconductors and hybrid organic-inorganic perovskites, and has developed optimal quantum control techniques using femtosecond pulse shaping for application to solid-state quantum emitters.
Event Location:
Brimacombe 311
Event Time:
Wednesday, January 22, 2020 | 12:00 pm - 1:00 pm
Event Location:
Henn 318
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2020-01-22T12:00:00
2020-01-22T13:00:00
Entropy Growth and Black Holes
Event Information:
In this talk, I start by reviewing the known ways of increasing the entropy both in classical and in quantum systems, with emphasis given to recent developments (2016-19). I then relate them to black hole physics to show which ones and to which extent they apply to the Bekenstein-Hawking entropy.
Event Location:
Henn 318
Event Time:
Monday, January 20, 2020 | 3:00 pm - 4:00 pm
Event Location:
Hennings 318
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2020-01-20T15:00:00
2020-01-20T16:00:00
Planetary Collisions: From the Moon's Origin to the Dino's Demise
Event Information:
Collisions with asteroids and comets used to be the stuff of science fiction. However, starting with the Apollo missions' revelations about our Moon, it has gradually dawned on the scientific world that collisions between objects from microscopic to planetary scales dominated nearly every aspect of our planetary system's birth and its later evolution. Long after the birth of our planet, a rare asteroid impact initiated the extinction of the dinosaurs. As recently as Feb. 15, 2013 the atmospheric disintegration of a building-sized space rock terrified residents of the city of Chelyabinsk, Siberia. While impact craters are presently rare on the Earth, they dominate the surfaces of many planets and Moons. Impacts on Mars have sent us samples of that planet in the form of meteorites and just might have once transferred living organisms between our two planets. Modern computer models are now revealing just what happens when an irresistible force—a speeding asteroid—meets an immovable object—the Earth, Mars or the Moon.
Event Location:
Hennings 318
Event Time:
Thursday, January 16, 2020 | 4:00 pm - 5:00 pm
Event Location:
Hennings 201
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2020-01-16T16:00:00
2020-01-16T17:00:00
Exploring Spin and Topological Phenomena in Complex Oxide Thin Films
Event Information:
Complex oxide materials exhibit a wide range of electronic and magnetic behavior in bulk and thin films. With advances in oxide thin film deposition techniques, we are now able to realize atomically precise thin films, heterostructures and interfaces of these complex oxide materials that open up a new phase space for materials discovery. The stabilization of unusual ground states in such atomically precise complex oxide materials has led to discoveries of novel spin and topological phenomena. In this talk, I describe two recent examples from our group: spin current transfer in ultra-low loss oxide spinels and topologically protected ground states at an oxide interface. The efficient generation and control of spin currents in these oxide spinels make them promising for future low-power electronics such as spin-wave logic devices and voltage controlled magnetic memory. At the metallic LaTiO3/ SrTiO3 interface, we have uncovered unexpectedly strong spin-orbit coupling in the form of giant Rashba spin splitting and evidence of a Dirac point. Under electrical gating, we have recently found evidence for topologically protected ground states that is consistent with the quantum spin Hall effect. If true, this would be the first experimentally observed oxide topological insulator.
Event Location:
Hennings 201
Event Time:
Thursday, January 16, 2020 | 2:00 pm - 3:00 pm
Event Location:
Brimacombe 311
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2020-01-16T14:00:00
2020-01-16T15:00:00
CM seminar - Probing and Tuning Interlayer Interactions to Control Electronic and Photonic Properties of 2D Heterostructures
Event Information:
Recently, van der Waals heterostructures have emerged as a very powerful platform for designer quantum materials with many fascinating properties that are not possessed by the constituent monolayers achieving novel device functionality. In terms of designer vdW hetero-bilayers, the interlayer interaction is the controlling parameter determining the electronic structures of the heterostructure as a whole.
In this talk I will first discuss directly probing the inter-layer interactions in transition metal dichalcogenide (TMD) heterojunctions through the “lens” of moiré patterns using scanning tunneling microscopy and spectroscopy (STM/S). I will show that the interlayer coupling is strongly dependent on the interlayer atomic alignment of the constituent layers. Moreover, as a consequence of moiré pattern formation, the energy band structure of the hetero-bilayer also shows lateral modulation, forming a 2D electronic superlattice. The moiré pattern “lens” also provides us with a means to measure the 2D strain tensor with high precision and high spatial resolution. In addition, I will show how such strain profile modifies the electronic structures, including converting a type II to a type I band alignment.
As the periodic potential modulation also provides lateral confinement for excitons, an intriguing scenario occurs – the 2D lateral superlattices also form 2D exciton quantum dot arrays which can exhibit interesting optical properties. Recent reports on the observation of “moiré excitons” reveal only the tip of the iceberg of this exciting new frontier. Here I will present another example of how different degrees of freedom including stacking configuration, band alignment, and valley spin, work in concert to compose novel excitonic properties of TMD heterobilayers.
Event Location:
Brimacombe 311
Event Time:
Thursday, January 16, 2020 | 2:00 pm - 3:00 pm
Event Location:
TRIUMF Auditorium
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2020-01-16T14:00:00
2020-01-16T15:00:00
The SNOLAB Science Programme: cutting-edge science from a deep hole in the ground
Event Information:
SNOLAB is a deep underground research facility, hosted 2km beneath the surface of the Earth in a working mine at Creighton, near Sudbury, Ontario. Initially the site of the Sudbury Neutrino Observatory, which unambiguously demonstrated flavour-change in neutrinos created in the fusion processes of the Sun, SNOLAB now hosts a multi-disciplinary programme. Why do we need to go to such great depths to probe the Universe? Deep underground research facilities create ultra-quiet radiation environments through reduced cosmic ray induced backgrounds, and local shielding against ambient natural radioactivity. Such environments are required to address several of the major questions in contemporary astro-particle and sub-atomic physics, such as the search for the Galactic dark matter, and studies of neutrino properties. In addition the capabilities and techniques developed at SNOLAB are finding application in additional research streams such as genetics, nuclear treaty verification and deep subsurface life studies. This talk will provide an overview of the science programme at SNOLAB, will review detector systems used for these studies, and outline future plans for the facility.
Event Location:
TRIUMF Auditorium
Event Time:
Monday, January 13, 2020 | 3:00 pm - 4:00 pm
Event Location:
Hennings 318
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2020-01-13T15:00:00
2020-01-13T16:00:00
CLAUDS: The CFHT Large Area U-band Deep Survey
Event Information:
The CFHT Large Area U-band Deep Survey (CLAUDS) uses ~70 nights of dedicated dark-time imaging (plus significant additional archival data) to map a representative 18.6 square degrees of the Universe to a median depth of U = 27.1 AB (5σ). These are the deepest U-band images ever assembled over this large an area.
These CFHT MegaCam data are combined with the optical (grizy + narrowband) imaging being taken to comparable depths by Subaru’s HSC camera in the same fields. Combining the U and optical data allows the selection of z~2-3 star-forming galaxies and greatly improves photometric redshift performance at z<1, enabling science projects that explore 85% of cosmic time with excellent statistics and that are virtually free of cosmic variance.
With the CLAUDS and Subaru HSC data now in hand and merged, we have entered the scientific exploitation phase, and I will show some example science applications of this unmatched dataset, including galaxy stellar mass functions, galaxy-galaxy mergers, and satellite distributions around massive central galaxies.
Event Location:
Hennings 318
Event Time:
Thursday, January 9, 2020 | 4:00 pm - 5:00 pm
Event Location:
Hennings 201
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2020-01-09T16:00:00
2020-01-09T17:00:00
Pioneer Planet Hunters
Event Information:
The partial award of the 2019 Nobel Prize in Physics for the discovery of exoplanets culminates four remarkable decades of search and development. While astrometry proved fruitless, stellar radial acceleration measurements of high precision revealed reflex motions induced by planetary companions. The UBC group was the first to demonstrate the power of imposed wavelength fiducials in 1979. This encouraged others to join the search – everyone was looking for solar system analogues. The Swiss turned up the first of many, unanticipated `Hot Jupiters’ in few day orbits. I shall review some of the remarkable history, challenges and prior discoveries.
Event Location:
Hennings 201
Event Time:
Thursday, January 9, 2020 | 2:00 pm - 3:00 pm
Event Location:
Brimacombe 311
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2020-01-09T14:00:00
2020-01-09T15:00:00
CM Seminar : 2D moiré superlattices: a new Hubbard model simulator
Event Information:
The Hubbard model, first formulated by physicist John Hubbard in the 1960s, is a simple theoretical model of interacting quantum particles in a lattice. The model is thought to capture the essential physics of high-temperature superconductors, magnetic insulators, and other complex emergent quantum many-body ground states. Although the Hubbard model is greatly simplified as a representation of most real materials, it has nevertheless proved difficult to solve accurately except in the one-dimensional case. Physical realizations of the Hubbard model in two or three dimensions, which can act as quantum simulators, therefore have a vital role to play in solving the strong-correlation puzzle. In this talk, I will discuss a recent experimental realization of the two-dimensional triangular lattice Hubbard model in angle-aligned WSe2/WS2 bilayers, which form moiré superlattices because of the difference in lattice constant between the two 2D materials. We obtain a quantum phase diagram of the two-dimensional triangular lattice Hubbard model near the half filling by probing both the charge and magnetic order of the system. Implications for future studies will also be discussed.
Bio
Kin Fai Mak received his PhD in physics from Columbia University. He is now an associate professor of physics and of applied & engineering physics at Cornell University. His research group explores new physical phenomena in atomically thin materials and their heterostructures. He studies a wide range of materials with very different properties, which include semiconductors, superconductors and magnets etc, and fabricates heteostructures, and devices based on this material platform. To explore new phenomena, he also develops new measurement and imaging techniques suitable for specific problems on hand. Recent research topics in his group include exciton condensation in double layer van der Waals’ heterostructures, strong correlation physics in moire superlattices, 2D magnetism, 2D superconductivity and topological transport phenomena.
Event Location:
Brimacombe 311
Event Time:
Thursday, December 12, 2019 | 5:00 pm - 6:00 pm
Event Location:
Hebb Theatre
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2019-12-12T17:00:00
2019-12-12T18:00:00
Extracting the Universe from the Wave Function
Event Information:
Quantum mechanics is a theory of wave functions in Hilbert space. Many features that we generally take for granted when we use quantum mechanics - classical spacetime, locality, the system/environment split, collapse/branching, preferred observables, the Born rule for probabilities - should in principle be derivable from the basic ingredients of the quantum state and the Hamiltonian. I will discuss recent progress on these problems, including consequences for cosmology and quantum gravity.
Event Location:
Hebb Theatre
Event Time:
Wednesday, December 11, 2019 | 10:30 am - 11:30 am
Event Location:
Hennings 318
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2019-12-11T10:30:00
2019-12-11T11:30:00
Special seminar - Spectroscopy of collective modes in 4d-metal compounds
Event Information:
Abstract
Quantum materials with 4d valence electrons serve as a platform for fundamental concepts such as unconventional superconductivity, Kitaev spin liquids, and solid-state analogues of the Higgs mode in particle physics. However, basic questions about the electronic structure of these materials remain unanswered, because key interaction parameters (including the Hund’s coupling, spin–orbit coupling, and exchange interactions) are comparable in magnitude and their interplay is poorly understood. We will discuss recent inelastic neutron and x-ray scattering experiments on spin waves, spin-orbit excitons, and spin-state transitions, which provide new insight into these interactions and their influence on the phase behavior of model materials including Ca2RuO4, Ca3Ru2O7, SrRu2O6, and RuCl3.
Event Location:
Hennings 318
Event Time:
Tuesday, December 10, 2019 | 2:00 pm - 2:00 pm
Event Location:
TRIUMF Auditorium
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2019-12-10T14:00:00
2019-12-10T14:00:00
First results of the KATRIN experiment on an absolute neutrino mass measurement
Event Information:
Since the Nobel prize-winning discovery of neutrino oscillation, we know that neutrinos have a non-zero mass. However, the absolute mass scale of the most abundant matter particle in the Universe remains unknown. Three fundamentally different approaches aim to determine the neutrino mass: Global fits to cosmological data, neutrinoless double beta decay and kinematic measurements. The latter is the most direct way to determine the mass of the neutrino and is investigated with tritium beta decays in the Karlsruhe Tritium Neutrino (KATRIN) experiment. KATRIN performs spectroscopy of beta-electrons near the tritium endpoint at 18.6 keV by employing a high intensity windowless gaseous tritium source and a high-precision electrostatic spectrometer based on the MAC-E filter principle. The required sensitivity demands novel hardware operating with unprecedented stability and a precise understanding of all systematic effects and their correlations. After a long time of construction, KATRIN performed its first measurement campaign in spring this year and is currently the only experiment capable of improving our knowledge of the neutrino mass in a model-independent way. In September, the first results were released. In this colloquium, I will present the KATRIN experiment and its kinematic measurement approach within the larger picture of neutrino mass measurements. I will then show the first results and the overall future potential.
Event Location:
TRIUMF Auditorium
Event Time:
Tuesday, December 10, 2019 | 12:30 pm - 2:30 pm
Event Location:
Room 309, Hennings Building, 6224 Agricultural Road
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2019-12-10T12:30:00
2019-12-10T14:30:00
“A Monte Carlo inverse treatment planning algorithm for trajectory-based volumetric modulated arc therapy with applications in stereotactic radiosurgery, total body irradiation and patient-specific quality assurance”
Event Information:
Final PhD Oral Examination
Abstract:
The main objective of this thesis is to present a full Monte Carlo (MC)-based inverse treatment planning method for trajectory-based volumetric modulated arc therapy (TVMAT). TVMAT uses continuous and simultaneous gantry and couch rotation to avoid organs at risk (OARs) in the path of beam delivery, and thus it reduces the radiation exposure of normal tissues. However, commercial treatment planning systems do not provide dose calculation of such a beam trajectory. It has been shown that a full MC-based optimization greatly reduces the optimization convergence errors. Previously published approaches to MC-based optimization have not been clinically implemented, and none has been proposed for VMAT or TVMAT so far. In this work, we developed a method that reflects the dynamic multi-leaf collimator (MLC) and gantry-couch trajectory of the actual beam delivery at all stages of the optimization. Dose optimization was performed in a single MC simulation, thereby greatly reducing computation time. We select the initial trajectory (i.e. the range of the gantry, collimator and couch angles) and the initial set of leaf positions, corresponding to a dynamic beam conformal to the target. The MC simulation starts from a phase space scored at the top of the MLC module and uses a beam source that allows simulations of complex continuous beam delivery.
We modified a general-purpose MC code system in order to generate four-dimensional dose files that score individual, time-stamped, energy deposition events in the voxels of the planning target volume and OARs.
Consequently, a relation is established between the space and time coordinates of source particles in the phase space and their contribution to energy deposition. A MC-based direct aperture optimization, with a dose-volume constraint based quadratic objective function, is performed using an in-house code, taking into account the continuous movement of the MLC, gantry and couch between adjacent control points. This method is also applicable to beam delivery with either fixed couch position for VMAT or couch translation for long-field treatments. It is shown that this novel treatment planning algorithm is capable of generating plans that are generally of higher quality than those generated by standard planning systems.
Event Location:
Room 309, Hennings Building, 6224 Agricultural Road
Event Time:
Sunday, December 8, 2019 | 2:00 pm - 3:30 pm
Event Location:
Hebb 100 Theatre (UBC CAMPUS) 2045 East Mall, Vancouver, BC V6T 1Z1
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2019-12-08T14:00:00
2019-12-08T15:30:00
Fix it with Physics! Quirky Solutions to Daily Problems (16th Faraday Show)
Event Information:
The Faraday Show is UBC’s annual science lecture, designed for children, presented by UBC Physics & Astronomy. This year the show will feature creative and quirky ways a physicist might solve everyday problems! Your alarm clock won’t stop? Lightbulbs don’t work? Recycling pop cans is a chore? No problem! Join us to find solutions to these inconveniences and learn the physics and science behind everyday things - through demonstrations and hands-on activities. This show is for children of ALL AGES, and adults who are young at heart!
For more information please contact the outreach program at: outreach@phas.ubc.ca / 604-822-0596, or visit our website at https://outreach.phas.ubc.ca/events/faraday-show/.
Event Location:
Hebb 100 Theatre (UBC CAMPUS) 2045 East Mall, Vancouver, BC V6T 1Z1
Event Time:
Thursday, December 5, 2019 | 4:00 pm - 5:00 pm
Event Location:
Hennings 201
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2019-12-05T16:00:00
2019-12-05T17:00:00
Using new nonlinear methods to improve molecular imaging
Event Information:
Advances in ultrafast lasers, and in technologies to control those lasers, have led to methods which image intrinsic nonlinear optical signatures that were not previously observable in complex materials (such as tissue or Renaissance paintings). Contrast comes from effects such as excited state absorption, ground state depletion, and cross phase modulation - with much less power than a laser pointer. An emerging medical application is in melanoma, which presents serious diagnostic challenges today. More patients die from melanoma after a Stage I diagnosis than after a Stage IV diagnosis, because Stage I cancers (which are treated only by excision) occasionally are actually aggressive tumors - but conventional pathology cannot tell which ones are dangerous. We have shown that femtosecond pump-probe microscopy can distinguish between early-stage tumors which went on to metastatic cancer, and those which did not; and extended studies have unraveled the underlying physical mechanism. I will also present closely related work on nonlinear imaging of perovskites to understand grain boundary effects, and of historical pigments to infer the artist's original colors and intent.
Event Location:
Hennings 201
Event Time:
Thursday, December 5, 2019 | 2:00 pm - 3:00 pm
Event Location:
Brimacombe 311
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2019-12-05T14:00:00
2019-12-05T15:00:00
CM Seminar - Scalable quantum computing with neutral atoms
Event Information:
Abstract:
Quantum computing is a few decades old and is currently an area where there is great excitement, and rapid developments. One of the daunting challenges in developing a practical quantum computer is the need to scale to a very large number of qubits. Neutral atoms are one of the most promising approaches for meeting this challenge.
I will describe the current state of play and survey the optical and atomic physics underlying neutral atom qubits and computation. In the near term hybrid approaches that combine quantum and classical processing have the potential for demonstrating useful quantum speedups. I will give an example of such an approach, and its application to dark matter searches.
Bio sketch:
Mark Saffman is an experimental physicist working in the areas of atomic physics, quantum and nonlinear optics, and quantum information processing. He has made significant contributions to the physics of optical solitons, pattern formation, sources of entangled light, and quantum computing. His current research effort is devoted to the development of neutral atom based quantum computing devices. His research team was the first to demonstrate a quantum CNOT gate between two trapped neutral atoms, and the deterministic entanglement of a pair of neutral atoms. This was done using dipole mediated interactions between highly excited Rydberg atoms. He is currently developing scalable neutral atom platforms using arrays of trapped atoms.
He is a Professor of Physics at the University of Wisconsin-Madison, Director of the Wisconsin Quantum Institute (wqi.wisc.edu), and Chief Scientist for Quantum Information at ColdQuanta, Inc. He is a fellow of the American Physical Society and the Optical Society of America and has been recognized with the Alfred P. Sloan Fellowship and a University of Wisconsin Vilas Associate Award. He also serves as an Associate Editor for Physical Review A.
Event Location:
Brimacombe 311
Event Time:
Wednesday, December 4, 2019 | 12:30 pm - 2:30 pm
Event Location:
Room 200, Graduate Student Centre (6371 Crescent Road)
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2019-12-04T12:30:00
2019-12-04T14:30:00
Final PhD Oral Examination (Thesis Title: “Studies of atmospheric properties for optical ground-based astronomy and methods to enhance laser guide star adaptive optics performance”)
Event Information:
Abstract:
Ground-based astronomy suffers from waveform distortion produced by the turbulent atmosphere, which prevents telescopes from reaching diffraction-limited resolution. Modern large telescopes and next generation extremely-large telescopes use or will use adaptive optics systems with laser guide stars to correct for atmospheric wavefront distortion. The first part of the thesis deals with astronomical site testing and the second part with methods for adaptive optics system improvement.
Meteorological data for 15 observatory sites were studied. Monthly averages of cloud cover, wind speed at 200 hPa, precipitable water vapour, vertical wind velocity and aerosol index were compared for the sites. The long-term evolution over 45 years of the five atmospheric quantities was investigated.
Site testing campaigns to characterize potential telescope sites in terms of optical turbulence. Using scintillometers, ground layer turbulence profiles can be measured. For an assessment of sites long-term statistics are needed. Two campaigns for daytime and nighttime turbulence profiling have been started and preliminary results are described.
Methods for increasing adaptive optics system performance are studied. Polarization modulation of the adaptive optics laser might be an useful method for laser guide star return flux enhancement. An experiment, carried out at the Roque de la Muchachos on La Palma, is described. The method also enables measurements of the Larmor frequency and the magnetic field strength in the mesosphere. The average Larmor frequency is 260.4 kHz, and a corresponding magnetic field of 0.3720 G was determined, which is within 0.05% of the value predicted by the World Magnetic Model. We found a maximum LGS return flux enhancement of 18% for a pulsed amplitude modulation at Larmor frequency compared to amplitude modulation offset from the Larmor frequency. For polarization modulation at the Larmor frequency a 6% increase in LGS return flux was found over polarization modulation offset by 30-kHz from the Larmor frequency.
Adaptive optics system could also benefit from an estimate of the mesospheric sodium density profile. Such profiles can be retrieved by partial amplitude modulation, with pseudo-random binary sequences, of continuous-wave lasers. Results for an experiment at the Large Zenith Telescope in Maple Ridge, and a feasibility study of this method for extremely-large telescopes, are presented. The method could be used on extremely-large telescopes to estimate the sodium density profile with a temporal resolution of a few seconds.
Event Location:
Room 200, Graduate Student Centre (6371 Crescent Road)