Events List for the Academic Year
Event Time:
Friday, August 2, 2024 | 8:00 am - 10:00 am
Event Location:
Zoom - https://ubc.zoom.us/j/68355025780?pwd=QCKYuMaLKywTlUaZiwJu6H4obKhniI.1 Passcode: 8223191
Add to Calendar
2024-08-02T08:00:00
2024-08-02T10:00:00
Novel reconstruction techniques for detecting low mass dark matter in the SuperCDMS experiment and characterization of SuperCDMS SNOLAB detectors
Event Information:
Abstract:
The existence of dark matter has been inferred through many astrophysical evidences. However, much about its nature is unknown to this day. The several decades-long search for dark matter has given rise to many experiments and even more dark matter candidates. SuperCDMS is a direct detection experiment which uses cryogenic detectors to probe interactions of dark matter particles with Standard Model particles.
Work done towards advancing two frontiers of the SuperCDMS SNOLAB experiment will be the main points of discussion in this thesis – (a) the characterization of the new SuperCDMS SNOLAB detectors, and, (b) the development of novel event reconstruction techniques for the experiment.
After introducing the motivation for dark matter and the current experimental techniques used to detect it, the thesis will introduce SuperCDMS detection principles and pre-requisite knowledge to motivate and understand most of the work described within. First, preliminary results from testing and characterizing SuperCDMS detector towers at the Cryogenic Underground TEst facility, a low background test facility at SNOLAB in Sudbury, Canada will be presented. The remainder of this thesis will focus on novel reconstruction techniques critical to achieving the experiment’s projected sensitivity post commissioning.
This discussion is sub-divided into two. The first aspect will present a detailed discussion of advanced reconstruction algorithms to fit data sampled at non-uniform speeds to keep within the bandwidths of the readout electronics and maintain low trigger thresholds at SuperCDMS SNOLAB. The second major development discussed will be a novel reconstruction technique called the N×M filter, which fits N channels with M shapes/templates simultaneously and develops a pipeline which integrates machine learning to achieve excellent resolution improvement.
The key outcomes of this thesis are (a) capability demonstration of the SuperCDMS SNOLAB detectors, (b) development of a less memory intensive algorithm to process non-uniformly sampled data, and, (c) demonstration of a two-fold improvement and nearly a four-fold improvement in energy resolution in old and new data sets using the N×M filter, respectively.
Event Location:
Zoom - https://ubc.zoom.us/j/68355025780?pwd=QCKYuMaLKywTlUaZiwJu6H4obKhniI.1 Passcode: 8223191
Event Time:
Wednesday, July 31, 2024 | 10:00 am - 11:00 am
Event Location:
BRIM 311
Add to Calendar
2024-07-31T10:00:00
2024-07-31T11:00:00
Inside Nature Physics
Event Information:
Abstract: What happens to your paper after you submit it to a journal is not always clear from the outside, but it’s helpful for authors to understand the editorial process so that they can navigate it smoothly. In this talk, we will unpack this process and explain how editors make their decisions. We will introduce Nature Physics (and other journals in the Nature Portfolio) and describe how we see our role in the scientific community and what we are trying to achieve. Building on that, we will discuss how we select papers for peer review based on the likely breadth of interest in the results that they contain, and how we interpret the information in peer review reports in terms of technical and editorial concerns. We’ll also discuss some techniques for writing papers and leave plenty of time for questions and discussion.
Bio: Before joining Nature Physics in 2017, David carried out theoretical research on graphene and other two-dimensional materials, and topological materials. He completed a Ph.D at Lancaster University in 2007, and then did post-doctoral work at the University of Manitoba (Canada) and the University of Maryland (USA) before undertaking an Assistant Professorship at Nordita, the Nordic Institute for Theoretical Physics in Stockholm, Sweden. David is based in the Nature Physics Berlin office.
Event Location:
BRIM 311
Event Time:
Wednesday, July 17, 2024 | 9:00 am - 10:00 am
Event Location:
Zoom
Add to Calendar
2024-07-17T09:00:00
2024-07-17T10:00:00
Special Seminar: Wolf Widdra - Institute of Physics, Martin-Luther-Universität Halle-Wittenberg: Laser-based double photoemission spectroscopy at surfaces
Event Information:
Abstract:With the recent progress in high-order harmonic generation (HHG) using femtosecond lasers, laboratory photoelectron spectroscopy with an ultrafast, widely tunable vacuum-ultraviolet light source has become available. Whereas HHG-based photoemission experiments at kilohertz repetition rates have been severely limited by the space-charge effects in the past, the new development of compact HHG light sources with megahertz repetition rates allows for efficient photoemission and double photoemission experiments as is demonstrated here [1-7].I will present momentum-resolved photoemission experiments with photon energies between 14 and 40 eV that demonstrate the high performance of the setup [3,4,6]. In addition, the combination of two time-of-flight spectrometers with coincidence detection electronics opens the way for efficient and long-term stable double photoemission experiments at variable photon energies [1,6-8]. For the noble metal (001) surface of Ag, we present a detailed analysis of double photoemission data and will compare them with similar data for the NiO(001) surface. The electron-electron pair distribution shows a sharp sum-energy onset, which corresponds to one hole in the Ag 3d band (4.5 eV below the Fermi level) and a second excitation from the Ag sp band. Simultaneously, an intense energy sharing between the electrons in the pair is visible indicating strong electron-electron correlations [5]. For thin films of C60, a molecular-orbital resolved correlation energy is determined based on double photoemission data at various photon energies.
References:[1] M. Huth, C.-T. Chiang, A. Trützschler, F. O. Schumann, J. Kirschner, and W. Widdra; Applied Physics Letters 104, 061602 (2014).[2] A. Blättermann, C.-T. Chiang, and W. Widdra; Physical Review A 89, 043404 (2014).[3] C.-T. Chiang, M. Huth, A. Trützschler, M. Kiel, F. O. Schumann, J. Kirschner, and W. Widdra, New Journal of Physics 17, 013035 (2015).[4] C.-T. Chiang, M. Huth, A. Trützschler, F. O. Schumann, J. Kirschner, and W. Widdra, Electron Spectroscopy and Related Phenomena 200, 15-21(2015).[5] A. Trützschler, M. Huth, C.-T. Chiang, R. Kamrla, F. O. Schumann, J. Kirschner, and W. Widdra, Phys. Rev. Lett. 118, 136401(2017).[6] M. Huth, A. Trützschler, C.-T. Chiang, R. Kamrla, F. O. Schumann, and W. Widdra, J. Appl. Phys. 124, 164504 (2018).[7] C.-T. Chiang, A. Trützschler, M. Huth, R. Kamrla, F. O. Schumann, and W. Widdra, Prog. Surf. Sci. 95, 100572 (2020).
Speaker Bio: Prof. Wolf Widdra is a Professor at the the Institute of Physics, Martin-Luther-Universität Halle-Wittenberg, Halle, Germany.
Event Location:
Zoom
Event Time:
Thursday, July 11, 2024 | 4:00 pm - 5:00 pm
Event Location:
Hennings 201
Add to Calendar
2024-07-11T16:00:00
2024-07-11T17:00:00
Physics of Life
Event Information:
~this talk is hosted by Sabrina Leslie and Steve Plotkin as an event connected to the Frontiers in Biophysics Conference on July 12 2024 at the downtown SFU campus - all are welcome to attend!~
Wed 10 July: Simon Fraser Physics Colloquium
Thu 11 July: UBC Physics Colloquium
Physics of Life
Physicists have been interested in the phenomena of life for centuries. During the twentieth century there were many dramatic successes at the interface of physics and biology, often changing the course of biology but leaving physics largely untouched. Something changed circa 2000, and biological physics emerged fully as a part of physics itself.
In the first part of this talk I will review some of this history, drawing on the recent Decadal Survey of the field organized by the US National Academy of Sciences. This survey led to the identification of four big questions that organize the physicists’ exploration of the living world. In the second part, I’ll describe work my colleagues and I have done addressing two of these questions. (1) How do macroscopic functions of life emerge from interactions among microscopic constituents? Here we use ideas from statistical physics to uncover surprisingly precise scaling behaviors in the dynamics of neurons and behavior. (2) How do living systems represent and process information? Here we use the early events in a developing fly embryo to show how maximizing information subject to physical limits generates successful predictions for system behavior, with no free parameters. These ideas have a chance of being more general than the examples we have chosen.
Fri 12 July: Frontiers in Biophysics
Ambitions for theory in the physics of life
Can we have a theoretical physicist’s understanding of life that has the generality we expect in physics yet engages with the myriad details that are characteristic of biology? The improved quality of data on these complex systems demands more ambitious theorizing. I will provide a brief survey of different approaches, and then focus on the idea that living systems are characterized by the optimization of information flow subject to physical constraints. This has a long history in the context of neural coding, and more recently my colleagues and I have explored the same idea in genetic networks, using the early events in the fly embryo as an example. We have uncovered a surprising precision in this system, and shown how optimization principles provide parameter-free predictions for the architecture and dynamics of the relevant genetic networks. These ideas could be more general, and I’ll speculate on unifying principles.
Bio:
I am interested in the interface between physics and biology, broadly interpreted. A central theme in my research is an appreciation for how well things work in biological systems. It is, after all, some notion of functional behavior that distinguishes life from inanimate matter, and it is a challenge to quantify this functionality in a language that parallels our characterization of other physical systems. Strikingly, when we do this (and there are not so many cases where it has been done!), the performance of biological systems often approaches some limits set by basic physical principles. While it is popular to view biological mechanisms as an historical record of evolutionary and developmental compromises, these observations on functional performance point toward a very different view of life as having selected a set of near optimal mechanisms for its most crucial tasks. Even if this view is wrong, it suggests a theoretical physicist's idealization; the construction of this idealization and the attempt to calibrate the performance of real biological systems against this ideal provides a productive route for the interaction of theory and experiment, and in several cases this effort has led to the discovery of new phenomena. The idea of performance near the physical limits crosses many levels of biological organization, from single molecules to cells to perception and learning in the brain, and I have tried to contribute to this whole range of problems.
Learn More:
Read his faculty webpage: http://www.princeton.edu/~wbialek/wbialek.html
Find out more about the Frontiers in Biophysics Conference at SFU this year: https://www.frontiers-biophysics.ca/
Event Location:
Hennings 201
Event Time:
Thursday, July 4, 2024 | 10:00 am - 12:00 pm
Event Location:
Henn 318
Add to Calendar
2024-07-04T10:00:00
2024-07-04T12:00:00
Scaling theories and simulation of ductile yielding in amorphous solids
Event Information:
Abstract :
Amorphous solids are a diverse class of materials that have significant interest owing to their ubiquity in industry, yet a unifying theory to describe their mechanical response to load under temperature is lacking. Using a combination of highly parallelized numerical routines to simulate an elastoplastic model (EPM) of amorphous solids, as well the corresponding mean-field theory, I develop a scaling theory for the yielding of amorphous solids for non-zero temperature and driving rates. First, I simulate very large systems at zero temperature. Here, ductile yielding proceeds through localized rearrangements that, under sufficient load, self-organize into extended avalanches. I study the appearance of the recently described stability plateau, which violates the existing athermal scaling theories for amorphous yielding. Using finite-size scaling, I show that this deviation originates in the spatial extent of the largest avalanches, and that this plateau in turn affects the energy available to avalanches. Consequently, this changes the scaling description for amorphous yielding at zero temperature.Second, I introduce a temperature-dependent failure to the EPM. With extensive numerical simulations, as well as scaling arguments from mean-field theory, I map out a phase-diagram for different regimes of behaviour, depending on both temperature and the rate at which energy is loaded into the system. I verify the boundaries of the phase-diagram by showing changes in behaviour across each of the phase lines, test predictions for avalanche size and flow stress in each flow regime. Contrary to recently proposed theories based on mean-field modelling alone, I show that the competition between driving rate and temperature differs between the continuously flow regime (in which avalanches merge) and the intermittent flow regime. Finally, motivated by experimental data on the creep of disordered mylar sheets, I study creep-flow in amorphous solids by considering an EPM at fixed stress and non-zero temperature. Here, we show that creep proceeds through cascades of correlated activity that occur over extremely long timescales. These ``thermal avalanches'' have long periods of quiescence, yet I argue they obey the same scaling laws and underlying physics as the mechanical avalanches of the ductile yielding transition.
Event Location:
Henn 318
Event Time:
Wednesday, July 3, 2024 | 10:00 am - 11:00 am
Event Location:
BRIM 311
Add to Calendar
2024-07-03T10:00:00
2024-07-03T11:00:00
Solids with random stacking: the curious case of lithium
Event Information:
Abstract: Close-packing of spheres is a problem with a long, beautiful history that spans centuries. To achieve maximal density, we must first arrange spheres into triangular layers and stack them. Each layer can sit in two possible positions. I will describe the solid that arises from random stacking, i.e., when each twofold choice is made at random. I will introduce a related problem — that of stacking spheres with the least possible density, while retaining stability. The conjectured solution involves stacking honeycomb layers, with a six-fold choice at each layer. I will describe random stacking in this case as well.
I will then present a decades-long puzzle: the low-temperature structure of lithium and sodium. Both have irregular structures with many conflicting claims. We propose an explanation that borrows ideas from the field of frustrated magnetism. Lithium and sodium form close-packed structures, with stacked triangular layers. Due to a hidden gauge symmetry, all stacking configurations have the same electronic energy. We have a ‘frustrated’ solid where an infinite family of crystal structures compete.
References: Phys. Rev. E 108, 035003 (2023),arXiv:2405.15865
Event Location:
BRIM 311
Event Time:
Wednesday, June 26, 2024 | 11:00 am - 12:00 pm
Event Location:
Hennings Room 309
Add to Calendar
2024-06-26T11:00:00
2024-06-26T12:00:00
Decoherence by Warm Horizons
Event Information:
Recently Danielson, Satishchandran, and Wald (DSW) have shown that quantum superpositions held outside of Killing horizons will decohere at a steady rate. This occurs because of the inevitable radiation of softphotons (gravitons), which imprint a electromagnetic (gravitational) ``which-path'' memory onto the horizon. Rather than appealing to this global description, an experimenter ought to also have a local description for the cause of decoherence. One might intuitively guess that this is just the bombardment of Hawking/Unruh radiation on the system, however simple calculations challenge this idea -- the same superposition held in a finite temperature inertial laboratory does not decohere at the DSW rate. In this work we provide a local description of the decoherence by mapping the DSW set-up onto a worldline-localized model resembling an Unruh-DeWitt particle detector. We present an interpretation in terms of random local forces which do not sufficiently self-average over long times. Using the Rindler horizon as a concrete example we clarify the crucial role of temperature, and show that the Unruh effect is the only quantum mechanical effect underlying these random forces. A general lesson is that for an environment which induces Ohmic friction on the central system (as one gets from the classical Abraham-Lorentz-Dirac force, in an accelerating frame) the fluctuation-dissipation theorem implies that when this environment is at finite temperature it will cause steady decoherence on the central system. Our results agree with DSW and provide the complementary local perspective.
Event Location:
Hennings Room 309
Event Time:
Tuesday, June 25, 2024 | 10:00 am - 12:00 pm
Event Location:
Zoom : https://ubc.zoom.us/j/2140943545?pwd=RGdIb0swbmRxM0QrWEtWejY2VGpVUT09, Meeting ID: 214 094 3545 , Passcode: 876743
Add to Calendar
2024-06-25T10:00:00
2024-06-25T12:00:00
The Pendulum Lab: Understanding Common Experiences and Pitfalls in a Lab With an Intentional Model Failure
Event Information:
Abstract:
In this thesis we investigate how students experienced the pendulum lab, a one-week lab in the first year lab curricula. This lab course is designed to teach scientific critical thinking through an iterative process of collecting data, making a quantitative comparison and reflecting on that comparison. In the pendulum lab, students are asked to compare the periods of a pen[1]dulum at 10 and 20 degrees. The quantitative comparison is performed using a modified statistical test that we refer to as the t-score. The lab is designed such that a reasonable measurement of the periods shows that they are different. But this result contradicts the equation for the period of a pendulum that students learn in first year physics courses because of an approximation. Previous work has shown that students who see the lab as a model confirmation activity struggle with labs like this.
Our study set out to see if asking students to hypothesize would hinder their ability to interpret their data correctly because of confirmation bias. But instead, we found that students’ main struggles were because of issues with inconclusive results. Confirmation bias did play a role in some of our data, but the impact was limited. We also found that only half of the students were surprised by their results in this lab. Most students did not know about the equation for the period of a pendulum if they took this lab in the Fall semester. However, students who took the lab in the Spring semester did know about the pendulum equation and showed indications of confirmation bias. We also found that only half of the students in the lab performed a high enough quality measurement to uncover the difference, which was far below our expectations.
We proposed and implemented an intervention to deal with inconclusive results by redesigning how students interpret their t-scores. We also added more time to the lab so more students could get high quality results. These changes were not extremely successful but provided useful information for future improvements to the lab.
Event Location:
Zoom : https://ubc.zoom.us/j/2140943545?pwd=RGdIb0swbmRxM0QrWEtWejY2VGpVUT09, Meeting ID: 214 094 3545 , Passcode: 876743
Event Time:
Friday, June 7, 2024 | 11:00 am - 12:00 pm
Event Location:
BRIM 311
Add to Calendar
2024-06-07T11:00:00
2024-06-07T12:00:00
Many-body localization in the disordered Fermi-Hubbard model
Event Information:
Abstract: How isolated quantum systems reach thermal equilibrium is a long-standing question of continuing interest. The absence of equilibration in some systems is also well known, notably Anderson localization in noninteracting systems with quenched disorder. However, it has only relatively recently been understood that the absence of equilibration can persist in the presence of interactions, dubbed many-body localization. While most of the theoretical work in this area has focused on spin systems, in which there is just one local degree of freedom, systems with multiple coupled degrees of freedom are of interest, not least because most experimental studies of many-body localization use cold atoms described by the Hubbard model. This talk will review this context and explore the specific case of the disordered Fermi-Hubbard model. With two coupled local degrees of freedom, charge and spin, how does disorder in one of these influence localization in the other? Writing the Hamiltonian in terms of charge- and spin-specific integrals of motion, we extract time scales associated with charge-charge, spin-spin, and charge-spin coupling and connect these with the growth of entanglement.
Event Location:
BRIM 311
Event Time:
Monday, June 3, 2024 | 12:00 pm - 1:00 pm
Event Location:
HENN 318
Add to Calendar
2024-06-03T12:00:00
2024-06-03T13:00:00
Chaotic Instability in the BFSS matrix model
Event Information:
UBC Theoretical High Energy Physics Seminar
Abstract:
Recently, chaotic scattering has been studied in the context of String Theory. Chaotic scattering occurs when the particle motion in ascattering region cannot be exactly solved. This is a more general situation than the familiar solvable scattering problems. In this talk,as a simple example, we first introduce chaotic scattering in a four-hill potential model and then present self-similar structures(fractals) appearing in the initial value space and the Cantor set in the time delay function. A method for calculating the fractal dimension is also explained. Then we discuss chaotic scattering in the Banks-Fischler-Shenker-Susskind (BFSS) matrix model, which is a non-perturbative formulation of a superstring theory. This model can also be interpreted as a matrix regularization of a supermembrane theory, in which the basic degrees of freedom are membranes. The potential of this model has flat directions, which are related to the instability of the supermembrane theory. We investigate classical motions of a spherical membrane and show that this instability can be regarded as chaotic scattering.
Event Location:
HENN 318
Event Time:
Friday, May 31, 2024 | 10:00 am - 11:00 am
Event Location:
McLeod 3038
Add to Calendar
2024-05-31T10:00:00
2024-05-31T11:00:00
Inter-valley coherence, intrinsic and extrinsic spin-orbit coupling in rhombohedral graphene
Event Information:
Abstract: Rhombohedral graphene multilayers provide a clean and highly reproducible platform to explore the emergence of superconductivity and magnetism in a strongly interacting electron system. The high density of states near the van Hove singularities lead to a variety of broken symmetry phases – including exotic forms of spin and valley ferromagnetism [2, 3]. Because of their combined spin and valley ‘isospin’ degrees of freedom, these Stoner magnets exhibit an approximate SU(4) symmetry with a near degeneracy in the many body phase diagram. In reality, this SU(4) symmetry is only approximate, weak symmetry breaking arises even at the single particle level – in the form of intrinsic spin-orbit coupling. Here, we use high resolution thermodynamic compressibility measurements and nanoSQUID on tip (nSOT) magnetometry to study the effects that intrinsic spin-orbit coupling has on the magnetic phase diagram in rhombohedral graphene. By supporting rhombohedral graphene on a WSe2 substrate, ‘extrinsic’ strong spin-orbit coupling can be proximitized, significantly altering the magnetic phase diagram. We demonstrate the presence of an inter-valley coherent quarter metal which becomes strongly spin-valley locked when supported by WSe2. Our results shed light on the role proximity induced Ising spin-orbit coupling and intrinsic spin-orbit coupling plays in selecting the ground state in correlated graphene systems.
References:
[1] Zhou, H., Xie, T., Taniguchi, T. et al. Nature 598, 434–438 (2021).
[2] Zhou, H., Xie, T., Ghazaryan, A., et al. Nature 598, 429–433 (2021).
[3] Arp, T., et. al. arXiv:2310.03781 (2023).
Event Location:
McLeod 3038
Event Time:
Thursday, May 30, 2024 | 2:00 pm - 3:00 pm
Event Location:
MacLeod 3038 (https://maps.ubc.ca/?code=MCLD)
Add to Calendar
2024-05-30T14:00:00
2024-05-30T15:00:00
Next-Generation Microcombs for Compact Optical Frequency Division Systems
Event Information:
Dear colleagues,We invite you to the next SSCS Vancouver Seminar on Thursday, May 30th, at 2 pm by Prof. Kerry Vahala from Caltech. Kerry is a world authority on frequency combs. Please remember to mark your calendar!
Abstract: Optical frequency division (OFD) enables transfer of stability from references such as atomic transitions and optical cavities to microwave and radio-frequency signals. Enabled by self-referenced frequency combs, the most accurate clocks (optical clocks) and lowest phase-noise microwave signal sources are based upon this method. In recent years, a miniature chip-based comb (microcomb) is being studied for creation of compact OFD systems. I will review the physical principles of microcomb operation along with recently demonstrated microcomb devices that mode lock by formation of femtosecond pulse pairs. Finally, a high-performance microwave signal source is described wherein microcombs implement the method of 2-point OFD using a compact cavity reference.Biography:
Kerry Vahala is Professor of Applied Physics at Caltech and holds the Jenkins Chair in Information Science and Technology. His research on chip-based high-Q optical resonators and related nonlinear optical devices has advanced miniature frequency and time systems, microwave sources, parametric oscillators, astrocombs and gyroscopes. Vahala also made early contributions to the subject of cavity optomechanics and demonstrations of chip-based devices to cavity QED phenomena. A member of the National Academy of Engineering and Fellow of the IEEE and Optica, he received the IEEE Sarnoff Medal for research on quantum-well laser dynamics, the Alexander von Humboldt award and MPQ Distinguished Scholar Award for work on ultra-high-Q optical microcavities, a NASA achievement award for application of microcombs to exoplanet detection, and the Optica Paul F. Forman Team Engineering Excellence Award for a 2-photon optical clock. Vahala is the Executive Officer of the Department of Applied Physics and Materials Science at Caltech.
Event Location:
MacLeod 3038 (https://maps.ubc.ca/?code=MCLD)
Event Time:
Thursday, May 30, 2024 | 11:00 am - 12:30 pm
Event Location:
HENN 318
Add to Calendar
2024-05-30T11:00:00
2024-05-30T12:30:00
Efficient field theories for big data experiments
Event Information:
*This talk is presented live in HENN 318 and via Zoom:
Meeting URL: https://ubc.zoom.us/j/63645767535?pwd=ocFqc2BzhwbnSNoGY7iabbjlvxVXTM.1
Meeting ID: 636 4576 7535
Passcode: 147999
Bio:Dave Sutherland is a theoretical physicist. After completing his PhD in 2016 at the Cavendish laboratory in Cambridge, he worked as a postdoc in UC Santa Barbara, and a Marie Skłodowska-Curie COFUND fellow at INFN Trieste, before moving to Glasgow as a lecturer in 2022. He has worked on various aspects of model building, effective field theory, amplitudes, and their applications to phenomenology.
Abstract:In particle physics, we have lots of data, but we are unsure exactly what to look for within it. I will show how simple principles from field and scattering theory can help guide this search, maximize our chance of a fundamental discovery, and guarantee that we understand the nature of electroweak symmetry breaking in the coming decades.
Links:
See his University of Glasgow faculty webpage here: University of Glasgow - Schools - School of Physics & Astronomy - Our staff - David Sutherland
Event Location:
HENN 318
Event Time:
Friday, May 24, 2024 | 10:00 am - 12:00 pm
Event Location:
Henn 318
Add to Calendar
2024-05-24T10:00:00
2024-05-24T12:00:00
Negative Lambda Quantum Cosmology
Event Information:
Abstract: We present a model of quantum cosmology based on anti-de Sitter/conformal field theory (AdS/CFT) holography. The spacetimes in our construction are time-symmetric, big-bang/big-crunch cosmologies with a negative cosmological constant $\Lambda$. In the simplest version of our model the cosmology lives inside a spatially finite bubble within an otherwise empty AdS spacetime. By studying the thermodynamic and geometric properties of this spacetime, we provide evidence that the ``bubble of cosmology'' spacetime has a well-defined dual CFT description.
It is also desirable to have a cosmology which is globally homogeneous and isotropic. We present an upgraded model in which this is the case.
Although a homogeneous cosmology is not asymptotically AdS and hence cannot be described directly by AdS/CFT, the time-reflection symmetry of the spacetime allows us to perform an analytic continuation, following which the spacetime is an asymptotically AdS Euclidean wormhole. If we assume that the cosmology is spatially flat a second analytic continuation obtains a Lorentzian traversable AdS wormhole. The AdS wormhole spacetimes can be described using holography: they are dual to a pair of three-dimensional CFTs coupled via a four-dimensional theory.
We explain how an anomalously large amount of negative energy can support the traversable wormhole, and we begin to populate the holographic dictionary relating observables in the wormhole/cosmology to observables in the microscopic theory. Finally we show that time-dependent scalar fields naturally enable these cosmologies to contain a period of accelerated expansion, suggesting that our $\Lambda<0$ models could ultimately provide the framework for a fully microscopic description of our universe.
Event Location:
Henn 318
Event Time:
Thursday, May 23, 2024 | 2:00 pm - 4:00 pm
Event Location:
Henn 309
Add to Calendar
2024-05-23T14:00:00
2024-05-23T16:00:00
A study of sporadic pulsars and radio transients with the CHIME telescope
Event Information:
Lay abstract:
Pulsars are the remnants of massive stars. Their rapid and precise rotation allows us to use them as tools to test many theories of physics. Pulsars come in many flavours; some are sporadic and only emit radio waves occasionally. This thesis uses the Canadian Hydrogen Mapping Experiment (CHIME) to study pulsars of all flavours. First, I developed a new processing pipeline so that we can fully utilise the capabilities of CHIME/Pulsar. Then, I create a technique, LuNfit, to characterise the single pulses of pulsars. I then apply LuNfit to 35 pulsars with a 477-hour CHIME/Pulsar observation campaign. Finally, I discovered a new long-period transient, CHIME J0630+25, with a period of 421 seconds.
This marks only the fourth object in this new class. Their nature remains unknown, but we speculate that CHIME J0630+25 is likely a white dwarf or a neutron star.
Event Location:
Henn 309
Event Time:
Tuesday, May 14, 2024 | 1:00 pm - 2:00 pm
Event Location:
BRIM 311
Add to Calendar
2024-05-14T13:00:00
2024-05-14T14:00:00
Graphene multilayers: from unconventional superconductors to quantum devices
Event Information:
Abstract: Crystalline graphene multilayers present a rich playground to explore correlated electronic phenomena in a tunable and ultra-clean setting. For instance, Bernal bilayer graphene and rhombohedral trilayer graphene host multiple symmetry-broken metallic phases at low temperature, as well as unconventional superconductors with different pairing symmetries. The rich phase diagram of these systems can further be tuned through proximity to WSe2, which induces spin-orbit coupling in the graphene layers and leads to a dramatic enhancement of superconductivity that remains poorly understood. I will first discuss the lessons learned from our theoretical exploration of graphene multilayers with induced spin-orbit coupling, focusing on various types of magnetic and inter-valley coherent ground states and their possible connections to superconductivity. I will then outline a recipe to engineer topological superconductivity in graphene multilayers using gate-defined Josephson junctions. Such a platform provides a promising alternative to traditional architectures for Majorana zero-modes due to its purity, gate tunability and atomically thin nature.
Speaker Bio: Étienne Lantagne-Hurtubise completed his PhD at UBC, and worked at QMI with Professor Marcel Franz. He is currently a postdoctoral fellow at Caltech.
Event Location:
BRIM 311
Event Time:
Monday, May 13, 2024 | 12:30 pm - 2:30 pm
Event Location:
TRIUMF Theory Room, 4004 Wesbrook Mall and zoom; https://ubc.zoom.us/j/68938408525?pwd=MVBBK05ZQWdCK2tJKzNGUXZaazJhdz09 Passcode: 959424
Add to Calendar
2024-05-13T12:30:00
2024-05-13T14:30:00
Probing Beyond Standard Model Physics Through Ab Initio Calculations of Exotic Weak Processes in Atomic Nuclei
Event Information:
"Exotic weak decays offer a unique way to probe physics beyond the Standard Model in a low-energy regime using the atomic nucleus as a window to complement the high-energy searches done at particle accelerator facilities. However, in order to extract the relevant physics parameters from experimental observations, inputs from nuclear theory are required.
The hypothetical neutrinoless double beta decay has gathered a lot of interest, as its observation would answer many standing questions in particle physics. First, it would unveil fundamental properties of the most abundant yet most elusive massive particle: the neutrino. A simple observation of this decay would imply the neutrino to be Majorana, meaning that it is its own antiparticle, as well as give insight into its absolute mass. Furthermore, the existence of this decay would explain the matter/antimatter asymmetry of the universe.
In order to extract the neutrino mass and potential couplings to more exotic mechanisms, as well as compare sensitivities of experiments using different isotopes, the nuclear matrix element must be obtained from nuclear theory. Unfortunately, the different models that have historically been used to compute this quantity have shown a large spread with no means of quantifying their respective uncertainties, greatly hindering the experimental precision.
In this thesis, we use recent advances in ab initio methods, which profit from the rapid increase in computational power to calculate nuclear observables directly from the interaction between the nucleons. In particular, we use the ab initio valence-space in medium similarity renormalization group method to compute the matrix element of all relevant candidate isotopes for experimental searches. We further develop a new machine learning emulator that greatly increases the speed of calculations. Using this emulator, we probe the full input parameter space of the calculation to give the first statistical uncertainty on the matrix element.
Our results show smaller values than previous models and are consistent with other ab initio methods. This provides a much tighter constraint than the spread coming from previous models, greatly clarifying the picture for both current and future experimental searches of the decay. “
Event Location:
TRIUMF Theory Room, 4004 Wesbrook Mall and zoom; https://ubc.zoom.us/j/68938408525?pwd=MVBBK05ZQWdCK2tJKzNGUXZaazJhdz09 Passcode: 959424
Event Time:
Friday, May 10, 2024 | 9:00 am - 11:00 am
Event Location:
Henn 309 and Zoom, https://ubc.zoom.us/j/69190854282?pwd=amdGR3ovSnhDc0lSaXR6bzNuTkZYQT09
Add to Calendar
2024-05-10T09:00:00
2024-05-10T11:00:00
Classical descriptions of quantum computations: Foundations of quantum computation via hidden variable models, quasiprobability representations, and classical simulation algorithms
Event Information:
[abstract] Quasiprobability representations serve as a bridge between classical and quantum descriptions of physical systems. In these representations, nonnegativity allows for a probabilistic interpretation, aligning the description with classical physics.
However, the capacity to model quantum systems hinges on the use of negative quasiprobabilities. Accordingly, negativity is considered a hallmark of genuinely quantum behaviour. This principle has been applied to quantum information processing where negativity in the Wigner function is necessary for a quantum computational advantage. This is demonstrated by an efficient classical simulation algorithm for quantum computations in which all components of the computation remain nonnegative.
However, when constructing quasiprobability representations for quantum computation to which this statement applies, a marked difference arises between the cases of even and odd Hilbert space dimension. We find that Wigner functions with the properties required to describe quantum computation do not exist in any even dimension. We establish that the obstructions to the existence of such Wigner functions are cohomological.
In order to recover the properties required for classical simulation of quantum computation in any dimension, some constraints that traditionally define a Wigner function must be relaxed. We consider several examples of these generalized quasiprobability representations, and we find that when sufficiently general representations are admitted, no negativity is required to represent universal quantum computation.
The result is a hidden variable model that represents all elements of universal quantum computation probabilistically. Since this model can simulate any quantum computation, the simulation must be inefficient in general. However, in certain restricted settings, the simulation is efficient, allowing for a broader class of magic state quantum circuits to be efficiently classically simulated than those covered by the stabilizer formalism and Wigner function methods.
With this hidden variable model, we present a formulation of quantum mechanics that replaces the central notion of state, as a complex vector or density operator, with a bit string. This formulation applies to universal quantum computation, and hence all finite-dimensional quantum mechanics. Thus, we present a surprising response to Wheeler’s "It from Bit" challenge. Alongside coherence, entanglement, and contextuality, this provides a new approach to characterizing quantum advantage.
Event Location:
Henn 309 and Zoom, https://ubc.zoom.us/j/69190854282?pwd=amdGR3ovSnhDc0lSaXR6bzNuTkZYQT09
Event Time:
Wednesday, May 1, 2024 | 12:00 pm - 2:00 pm
Event Location:
QMI 188 (2355 East Mall)
Add to Calendar
2024-05-01T12:00:00
2024-05-01T14:00:00
Emergent optical and electronic properties in atomically thin rhombohedral-stacked transition metal dichalcogenides
Event Information:
Abstract:
Rhombohedral(R)-stacked TMD means the neighbouring layers are oriented in the same direction, which can be obtained through either chemical synthesis or artificial stack with a small twist. The investigation into how the stacking order determines the properties of TMD homobilayers is crucial for understanding the exotic physics observed in two-dimensional semiconductors.
Here we use various optical spectroscopy techniques to explore the emergent excitonic and correlated phenomena in both homogeneous and twisted TMD homobilayers of rhombohedral stacking. Specifically, we observe a spontaneous electrical polarization arising from the asymmetric interlayer-coupling-induced Berry phase in R-stacked MoS2 bilayer. Utilizing this polarization, we achieve an efficient and scalable photovoltaic effect in a Gr/R-MoS2/Gr heterostructure. By employing non-degenerate pump-probe photocurrent spectroscopy, we disentangle the competition between thermal and electronic effects, extracting a 2ps intrinsic photocurrent speed.
More importantly, the out-of-plane electrical polarization in R-stacked MoS2 can be switched through in-plane sliding motion, which is referred to as sliding ferroelectricity. By harnessing the coupling between electronic polarization and excitonic effects, we demonstrate an optical method to probe the domain wall motion in both R-stacked MoS2 homo-bilayer and tri-layer.
Finally, we report the discovery of a series of correlated insulating states at both integer and fractional fillings, arising from Γ-valley flat bands, in a small-angle twisted MoSe2 homo-bilayer. We observe a Mott-insulator state instead of a semi-metal on the half-filled honeycomb lattice, in contrast to the theoretical prediction based on continuum model. The observed phenomenon is consistent with the picture of semi-metal to insulator transition at large U/t limit. Our exploration on the moire homo-bilayer in rhombohedral stacking offers a new opportunity to simulate the Mott-Hubbard physics with spin SU (2) symmetry.
Event Location:
QMI 188 (2355 East Mall)
Event Time:
Monday, April 29, 2024 | 10:00 am - 12:00 pm
Event Location:
14th floor meeting room, BC Cancer Research Institute
Add to Calendar
2024-04-29T10:00:00
2024-04-29T12:00:00
An analysis of imaging and biological effects impacting theranostic dosimetry using radiopharmaceutical pairs
Event Information:
Abstract:
Radiopharmaceutical therapy (RPT) is a safe and effective cancer treatment using alpha or beta emitting radiopharmaceuticals that specifically target cancer cells to selectively destroy cancer tissue while sparing healthy cells. Treatment can be personalized on a patient-by-patient basis using dosimetry to determine suitable administered activities for subsequent treatment cycles. Dosimetry requires obtaining quantitative single photon emission computed tomography (SPECT) images which can only be done using gamma emitting radioisotopes.
Not all therapeutic radioisotopes are suitable for SPECT imaging. In such cases, it may be necessary to use an imaging surrogate to predict the radiation dose from the therapeutic isotope, either pre-therapy or during combination RPT. However, these methods may introduce inaccuracies into the dosimetry estimate. This dissertation aims to investigate some “theranostic pair” radiopharmaceuticals and determine if these pairs may be suitable for theranostic dosimetry.
In addition to a comprehensive literature review of theranostic dosimetry and the validity of multiple theranostic pairs used clinically and pre-clinically, three Monte Carlo based simulation studies are
performed:
- First, an investigation into the theranostic pair 177Lu (a beta/gamma
emitter) and 90Y (a beta emitter) to determine if Bremstrahhlung photons emitted by 90Y reduce the accuracy of quantitative SPECT imaging of 177Lu
- Then, simulations of 225Ac (an alpha emitter) and 177Lu within prostate cancer cells were performed and used to create nucleus absorbed dose kernels which were convolved with multicellular tumour maps of varying morphologies (i.e. hypoxic, necrotic, and normoxic tumour
phenotypes) to assess the absorbed dose distribution differences between particulate radiation from 225Ac and 177Lu on a microscopic scale
- Finally, the proposition of a novel method using 99mTc (a gamma
emitter) to improve bone marrow dosimetry is discussed and tested. Bone marrow dosimetry during RPT for prostate cancer with 177Lu labelled pharmaceuticals is extremely challenging, and we propose using 99mTc-sulfur colloids to assist in the determination of bone marrow location during imaging and subsequently use 177Lu for bone marrow dosimetry, which requires simultaneous SPECT imaging of 177Lu and 99mTc.
We test the feasibility of this and suggest additions to clinical scatter correction methods to reduce the impact of photon contamination from 177Lu on 99mTc images.
Event Location:
14th floor meeting room, BC Cancer Research Institute