Past Colloquia

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Prof. Shunsaku Horiuchi (Physics, Virginia Tech)

"The Hunt for Dark Matter"

Four fifths of the matter in the universe consists of something completely different from the "ordinary matter" we know and love. I will explain why this "dark matter" is an unavoidable ingredient to understand the universe as we observe it, and I will introduce the strategies to search for dark matter as a particle. I will then focus on an example of a possible hint of discovery, and describe the importance of astrophysics to elucidate its validity. I will finish by outlining ways forward in this exciting hunt.

Host: Mark Pitt

Dr. Mark Stiles (NIST, Gaithersburg, MD)

"Energy-Efficient Neuromorphic Computing with Magnetic Tunnel Junctions"

Human brains can solve many problems with orders of magnitude more energy efficiency than traditional computers. As the importance of such problems, like image, voice, and video recognition increases, so does the drive to develop computers that approach the energy efficiency of the brain. Magnetic devices, especially tunnel junctions, have several properties that make them attractive for such applications. Their conductance depends on the state of the ferromagnets making it easy to read information that is stored in their magnetic state. In addition, the spin current can manipulate the magnetic state. Based on this electrical control of the magnetic state, magnetic tunnel junctions are actively being developed for integration into CMOS integrated circuits to provide non-volatile memory. This development makes it feasible to consider other geometries that have different properties. I describe computing primitives that have been constructed based on the different functionalities of magnetic tunnel junctions. The first group of these uses tunnel junctions in their superparamagnetic state for a population coding scheme or for stochastic computing. The second uses them as non-linear oscillators in the first nanoscale “reservoir” for reservoir computing.

Host: Satoru Emori

Prof. Ewelina Hankiewicz (Theoretical Physics, University of Wurzgburg)

"Unconventional Superconductivity in Topological Insulators and Rashba 2DEGS"

A topological insulator in the proximity to an s-wave superconductor is the prefect material to detect signatures of Majorana fermions. S-wave superconductor on the top of the surface states of 3D TI generates s-wave and p-wave pairing mixture in the surface state due to the spin-momentum locking [1,2]. We predict that in the Josephson junction setup, namely superconductor (S) /surface state of topological insulator/superconductor (S), existence of this p-wave component leads to novel features in transport like superconducting Klein tunneling i.e. the perfect transmission of hybridized Majorana states for normal incidence, the non-sinusoidal current phase relation [2] and unusual phase-dependent thermal conductance [3]. Further, we propose the experimental setups to observe signatures of Majorana fermions in the ac Josephson effect on TI hybrid structures [4] and in phase controlled Josephson junctions based on Rashba 2DEGs [5,6]. [1] L. Fu and C. L. Kane, Phys. Rev. Lett. 100, 096407 (2008). [2] G. Tkachov and E. M. Hankiewicz, Phys. Rev. B 88, 075401 (2013). [3] B. Sothmann and E. M. Hankiewicz Phys. Rev. B 94, 081407(R) (2016); B. Sothmann, F. Giazotto, and E. M. Hankiewicz New J. Phys. 19, 023056 (2017). [4] F. Dominguez, O. Kashuba, E. Bocquillon, J. Wiedenmann, R. S. Deacon, T. M. Klapwijk, G. Platero, L. W. Molenkamp, B. Trauzettel, and E. M. Hankiewicz Phys. Rev. B 95, 195430 (2017). [5] H. Ren, F. Pientka, S. Hart, A. T. Pierce, M. Kosowsky, L. Lunczer, R. Schlereth, B. Scharf, E. M. Hankiewicz, L. W. Molenkamp, B. I. Halperin and A. Yacoby, Nature 569, 93(2019). [6] B. Scharf, F. Pientka, H. Ren, A. Yacoby, and E. M. Hankiewicz Phys. Rev. B 99, 214503 (2019).

Host: Giti Khodaparast

Prof. Rodney D. Van Meter Keio University

"The quantum Internet"

The Quantum Internet will transform how we communicate, compute and measure our universe. Using long-distance entanglement, we can execute cryptographic functions that strengthen the security of classical communications, measure physical phenomena more precisely than purely classical systems, and ultimately couple quantum computers together to extend the size of problems they can solve. But how can we build such a network? Experimental progress toward quantum repeaters -- the quantum equivalent of the Internet's switches and routers -- is moving at a dizzying rate, and theorists have proposed half a dozen approaches to managing errors to create high-fidelity entanglement along a chain of repeaters. The next frontier is extending from one-dimensional chains to complex topologies, and on to a network of networks -- a true Quantum Internet. In this talk, I will bridge the physics and the engineering to give a picture of the current status and open research problems.

Host: Sophia Economou

Prof. Bhuvana Srinivasan (Aerospace & Ocean Engineering, Virginia Tech)

“Numerical Studies of High-energy Density Fusion and Astrophysical Plasma's”

Experimental efforts to study nuclear fusion and astrophysical plasmas have produced significant scientific advances but remain challenged by diagnostic access and limitations. Hence, there is a need for high-fidelity computational models in plasma physics to support experiments. Plasma physics will be introduced in this talk along with a brief description of plasma models and their limitations. Recent advances in plasma fluid and kinetic modeling have supported development of sophisticated simulation tools. A hierarchy of models, ranging from magnetohydrodynamic (MHD) to fully kinetic, are developed and applied across a wide range of parameter regimes in the Plasma Dynamics Computational Laboratory at Virginia Tech. Some representative applications will be presented with a focus on high-energy-density plasmas for fusion and astrophysical studies. Other research topics being pursued in the computational laboratory will be discussed briefly.

Host: Nahum Arav

Exam Day (No Classes)

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University and Graduate Ceremonies (No Classes)

Holiday (No Classes)

Holiday (No Classes)

No Colloquium (Physics Faculty Meeting)

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Special Colloquium

Prof. Alexandru Petrescu (University of Sherbrooke)

Quantum information processing with superconducting circuits: Purcell effect and the measurement problem

With recent advances in state preparation, gate, and measurement operations, superconducting circuit architectures are now leading candidates for quantum information processing. As micro-fabricated circuits are scaled up towards a practical quantum processor, strict requirements on the fidelity of operations required for quantum computation are imposed. For theorists, this mandates the development of accurate models describing the dynamics of complex superconducting circuits subject to strong drives. This talk will begin with an elementary introduction to such systems and their description in terms of quantum electrodynamics, the fundamental theory of light-matter interactions. We will then address the problem of the Purcell effect, which is the enhancement of the decay rate of a single qubit due to a linear electromagnetic environment, and show how convergent results can be obtained without any artificial high-frequency cutoffs. We will also explain how the Purcell rate is further enhanced in the presence of the drive fields typically used to measure qubits, which is a ubiquitous problem encountered in present-day experiments.

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Special Colloquium

Prof.l Giacomo Torlai (Flatiron Institute)

"Description:A Data-Driven Approach to Quantum Many-Body Physics "

Quantum many-body physics studies the collective properties of complex systems made of a large number of interacting particles. These may be real materials, quantum matter synthetically engineered in laboratories, as well as quantum computers and simulators. The challenge in uncovering their fundamental properties stems from the intrinsic complexity underlying quantum many-body states. This complexity is somewhat reminiscent of the so-called “curse of dimensionality” encountered in data science applications and routinely solved by machine learning algorithms, a disruptive technology which is now leading a profound revolution in the world of industry and information. In this talk, I will embrace a data-driven perspective on the ultimate “big data” problem in physics: the quantum wavefunction. Motivated by the remarkable experimental advances in producing high-quality quantum data, I will show how to repurpose generative neural networks to learn wavefunctions from measurements, and how these algorithms can be integrated in the experimental stack of quantum simulation platforms to enhance their capabilities. As a demonstration, I will present the reconstruction of a cold-atom wavefunction from experimental data produced by a programmable quantum simulator, including the extraction of its entanglement entropy. I will conclude by discussing what lies ahead in this new multidisciplinary field interfacing machine learning, computational physics and quantum information.

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Special Colloquium

Dr. Marie Boer (University of New Hampshire)

“Tomographic Views of Nucleon using Relativistic Light Particles”

The concept of atoms as elementary blocks of matter was born during antiquity. Last century, nuclear physics experiments discovered that atoms contain a nucleus made of positive and neutral particles, respectively called protons and neutrons (nucleons), themselves made of elementary particles, called quarks and gluons (partons). Quantum Chromodynamics is the theory describing parton interactions, where gluons are vector of the strong force between quarks, ensuring their confinement. The development of particle accelerators allowed for many experimental discoveries about this fundamental structure and led to models describing the distribution and dynamics of the partons inside the nucleon. Recent progress has allowed multi-dimensional representations, accounting for correlations between partons' spatial distributions and their momenta. In particular, Generalized Parton Distributions (GPDs) are probabilistic functions accessible from exclusive reactions, i. e. reactions where all products are known, in a kinematic regime where the partons can be probed individually. Interpretations of GPDs leads for instance to tomographic descriptions of the nucleon. In this presentation, I will review our current understanding of the nucleon structure and motivate more complex representations. As an example, I will discuss the potential of Compton-like scattering processes off quarks ("light scattering") for 3-dimensional tomographic views of the nucleon and for testing the fundamental properties of the GPDs. I will talk about several current and future experiments, in particular at the Jefferson Laboratory (Newport News, VA), and discuss their potential in challenging new measurements accessing GPDs

Host: Camillo Mariani

No Colloquium (Tenure Track Physics Faculty Meeting)

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No Colloquium (Due to COVID-19)

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No Colloquium (Due to COVID-19)

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No Colloquium (Due to COVID-19)

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"Exam Day" (No Colloquium)

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"University Commencement "(No Colloquium)

Prof. Camillo Mariani (Physics, Virginia Tech)

"Neutrinos-an Interesting Journey"

Neutrino physics has made tremendous strides in the past decade; the next challenge is to determine if neutrinos violate charge-parity (CP) symmetry, and to identify the neutrino mass hierarchy. The focus of my research group is to understand neutrino interactions in matter and their role in the discovery of the CP violating phase in the neutrino sector. Neutrino can also be used for other applications like nuclear reactor monitoring. The latter presents multiple experimental challenges, but there has been huge technology advancement in the last few years, one lead by a group at VT. More specifically, I will discuss our VT group research efforts preparing the next generation neutrino detectors and experiments. Throughout the talk we will rediscover the importance of nuclear physics and its influence in describing the neutrino interactions with matter.

Host: Mark Pitt

Prof. Vito Scarola (Physics, Virginia Tech)

Quantum Analog Simulation with Ultracold Atoms inOptical Lattices: Opportunities and Challenges

Quantum analog simulationoffers promise in effectively solving intractable quantum many-bodyproblems. One class of problems in particular, disordered Hubbard models,provide simple reduced models of strongly correlated materials, such as copperoxide-based compounds or disordered superconductors. Yet unbiasednumerical studies have not settled debates regarding the essential physicscaptured by Hubbard models. Progress in another seemingly unrelated areacan help with this mathematical problem. Cooling neutral atoms toquantum degeneracy has enabled the precise construction and manipulation oflarge multi-particle quantum states. Lasers defining opticallattices constrain the atoms so that their motion is very accurately capturedby Hubbard models. As a result, these experiments are being used toeffectively perform quantum analog simulation of Hubbard models. Work inmy theory group seeks to guide experimental setups in these simulations. I willreview experimental setups and discuss recent progress in using opticallattices as quantum analog simulators of Hubbard models.

Host: Mark Pitt

Prof. Tatsu Takeuchi (Physics, Virginia Tech)

"Successes and Failures in Theoretical Particle Physics"

I will review the history of the development of the Standard Model of Particle Physics during the past half-century or so, and the various theoretical ideas (the good,the bad, and the ugly) that were proposed in the process: some to be firmly established, while others, though quite popular in their heyday, have been proven wrong by experiment.

Host: Mark Pitt

Prof. Arun Bansil (Northeastern University)

"Topological Quantum Phases, Novel Superconductors, and Ultra-Thin Films Beyond Graphene"

I will discuss some of our recent work aimed at understanding the electronic structure and spectroscopy of novel superconductors, topological materials, and atomically thin 2D films beyond graphene. [1-5] Illustrative examples will include: (i) How by exploiting electronic structure techniques we have been able to successfully predict and understand the characteristics of many new classes of topologically interesting materials, including magnetic topological materials; (ii) How atomically thin ‘beyond graphene’ 2D and layered materials offer exciting new possibilities for manipulating electronic structures and provide novel platforms for fundamental science and applications; And, (iii) with regard to the high-Tc’s, I will discuss recent breakthroughs in modeling the insulating pristine compounds and their transition from the insulating to the metallic state with doping without invoking any free parameters such as the Hubbard effective U parameter. A first-principles description of the competing stripe and magnetic phases in the cuprates also then becomes possible, providing a new pathway for modeling correlated materials more generally. [1] Y. Zhang et al., Proceedings of the National Academy of Sciences 117, 68 (2020). [2] A. Bansil, H. Lin and T. Das, Reviews of Modern Physics 88,021004 (2016). [3] C. Hu et al., Science Advances 6, eaba4275 (2020). [4] I. Belopolski etal., Science 365, 1278 (2019). [5] Z. Hennighausen etal., Nanoscale 11, 15929 (2019). Bio sketch of Professor Arun Bansil, Northeastern University Bansil is a University Distinguished Professor of physics at Northeastern University(NU). He served for over two years at the US Department of Energy managing the flagship Theoretical Condensed Matter Physics program (2008-10). He is an academic editor of the international Journal of Physics and Chemistry of Solids (1994-), the founding director of NU’s Advanced Scientific Computation Center (1999-) and serves on various international editorial boards and commissions. He has authored/co-authored over 400 technical articles and edited 17 volumes of conference proceedings covering a wide range of topics in condensed matter and materials physics, and a major book on X-Ray Compton Scattering (Oxford University Press, Oxford, 2004). Bansil is a Highly Cited Researcher (ISI Web of Science/Clarivate Analytics, 2017-2020).

Host: Kyungwha Park

No Colloquium

Host: Vito Scarola

Prof. Bruce Vogelaar (Physics, Virginia Tech)

The CNO energy-production mechanism in the universe is detected by Borexino

An international team of about 100 scientists of the Borexino Collaboration reported in Nature this week a spectral measurement of neutrinos from the sun, directly revealing for the first time operation of the carbon-nitrogen-oxygen (CNO) fusion-cycle which is expected to be the dominant energy source powering stars heavier than the Sun. How this was accomplished will be explained, along the implications, and likelihood of even improved results. Some aspects of Virginia Tech's role just might be highlighted.

Host: Mark Pitt

Fall Semester Exams (No Colloquium)

Host: Vito Scarola/i>

University & Graduate Commencement Ceremonies
No Colloquium

Holiday (No Classes)