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The David and Edith Harris Physics Colloquium Series
FALL 2016 Schedule
OCTOBER 27, 2016
Host: Isaac Chuang
"What Should We Do with a Small Quantum Computer?"
A large-scale quantum computer would be able to solve problems that existing classical computers would take much longer than the age of the universe to solve. This would have dramatic implications for cryptography, chemistry, material science, nuclear physics and probably other areas that are still unknown. But what about quantum computers that will be available in the next few years? Experimentalists working with ion traps and superconducting qubits have plans to build quantum computers with 50-100 qubits capable of performing some thousands of quantum gates. The company D-Wave is already selling devices with over 1000 qubits, although they can only run a single algorithm (the adiabatic algorithm) and they suffer high rates of noise. An important milestone for these early quantum computers would be to demonstrate "quantum supremacy"; that is, solving a computational problem that could not be solved using classical computers without an astronomical amount of time.
In this talk, I will analyze two algorithms that can be run on current and near-term quantum computers. First I will look at the adiabatic algorithm, which has shown promise in its ability to use quantum tunneling to solve optimization problems more efficiently than classical local search. Here I will show that a different classical algorithm (using ideas from condensed-matter theory) can simulate the adiabatic algorithm in these cases. This fast simulation suggests that adiabatic tunneling does not outperform classical computing and thus is not a promising approach to quantum supremacy. Second, I will discuss simple variational quantum algorithms that try to approximately minimize some objective function. I will describe methods of running these algorithms efficiently and will show that conjectures from complexity theory imply that these algorithms cannot in general be simulated by classical computers.
NOVEMBER 3, 2016
Host: MIT Graduate Women in Physics (Jasmine Brewer)
"Dark Matters of Graphene"
Dark matter remains one of the principal motivators for new physics beyond the Standard Model. Although it comprises the vast majority of the matter in the Universe, its properties continue to elude us. For decades, Weakly Interacting Massive Particles (WIMPs) have served as the primary theoretical paradigm for dark matter. However, as a wide variety of experiments put such models to the test with no definitive detections, we are challenged to reevaluate this canonical scenario. I will discuss the theoretical motivations and experimental prospects for moving beyond the WIMP paradigm. The focus will be on direct detection experiments, which aim to discover dark matter via its scattering off targets located deep underground. I will present a new proposal to use two-dimensional materials, such as graphene, as targets for dark matter that is lighter than a WIMP. This proposal provides the first opportunity for directional detection down to MeV masses, and can be implemented by the PTOLEMY experiment.
NOVEMBER 10, 2016
M. CRISTINA MARCHETTI
Host: Mehran Kardar
"Active Matter: From Colloids to Living Cells"
Collections of self-propelled entities, from living cells to engineered microswimmers, organize in a rich variety of active fluid and solid states, with novel rheological and mechanical properties. In this talk I will describe the behavior of such “active materials”, focusing on two examples of liquid-solid transitions driven by active processes. The first is the spontaneous assembly of active colloids in coherent mesoscale structures with life-like properties. I will describe a minimal model of purely repulsive active particles that exhibits motility-induced phase separation into gas and dense liquid phases and jams at high density when crowding overcomes activity. This athermal phase separation has all the characteristic of a liquid-gas spinodal and captures aggregation of the surface-dwelling bacterium Myxococcus xanthus. The second example is motivated by the experimental observation of glassy dynamics in confluent epithelial cell monolayers, where there are no gaps between cells and the packing fraction is always unity. I will present a new theoretical model that captures this behavior by predicting a liquid-solid transition that occurs at constant density as a function of cell properties, such as motility, contractility, and cell-cell adhesion.
NOVEMBER 17, 2016
MORDECHAI (MOTI) SEGEV
Israel Institute of Technology
Host: Marin Soljačić
"Photonic Topological Insulators"
The recent breakthroughs on photonic topological insulators will be discussed, with an emphasis on fundamental aspects that are universal to many waves systems in nature, as well as on new ideas ranging from topological lasers to disorder-induced photonic topological phenomena.
DECEMBER 1, 2016
Host: Janet Conrad
"The High-energy X-ray View of the Galactic Center"
The inner parsecs of the Galaxy contain one of the highest concentration of high-energy sources in the Milky Way. The supermassive black hole (Sagittarius A*), pulsar wind nebulae, supernova remnants, X-ray binaries, and hot interstellar gas are copious emitters of X-rays and gamma-rays. In addition, this region contains a large density of dark matter, making it an important source of both dark matter interaction signatures and backgrounds to dark matter searches. NuSTAR provides a view of the hard X-ray (3-79 keV) band, a critical bridge between the soft X-ray and gamma-ray emission, with unprecedented angular resolution. I will present the first sub-parsec scale images of the Galactic Center above 20 keV, obtained with NuSTAR. This view provides new insight into the distribution of stellar remnant and cosmic ray populations near Sgr A*. I will also detail a novel use of the NuSTAR instrument’s view of the Galactic Center, which allows it to deliver leading constraints on the radiative decay of dark matter, in particular from sterile neutrinos.
DECEMBER 8, 2016
Host: MIT Society of Physics Students (Caitlin Fischer)
"Extracting the Universe from the Wave Function"
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.
PAST FALL 2016 COLLOQUIA
SEPTEMBER 8, 2016
UC Santa Barbara
Host: MIT Physics Graduate Student Council (Tzer Han Tan)
"Quantum Crystals, Quantum Computing and Quantum Cognition"
Quantum mechanics is down to earth - quite literally - since the electrons within the tiny crystals found in a handful of dirt manifest a dizzying world of quantum motion. Each crystal has its own unique choreography, with the electrons entangled in a myriad of quantum dances. Quantum entanglement also holds the promise of futuristic Quantum Computers - which might be comprised of electron and nuclear spins inside diamond, or of atoms confined in traps, or of small superconducting grains, among a plethora of suggested platforms. In this talk I will describe ongoing efforts to elucidate the mysteries of Quantum Crystals, to design and assemble Quantum Computers, before ruminating about “Quantum Cognition” - the proposal that our brains are capable of quantum processing.
SEPTEMBER 15, 2016
Host: Mehran Kardar
"Building Microbial Communities from the Bottom Up"
Microbes exist in complex, multi-species communities with diverse interactions that play an essential role in both human health as well as the health of the planet. Over the last decade tremendous progress has been made in characterizing these communities, but the lack of experimentally tractable model systems has made it difficult to discern the rules governing microbial community assembly and function. In this talk I will describe our recent experimental efforts to develop a bottom-up approach to understanding the dynamics of these communities. We have begun by quantifying the network of pairwise competitive outcomes among species within a model microbial community. We find that simple assembly rules incorporating just these pairwise competitive outcomes are surprisingly successful in predicting the outcome of multi-species competition, indicating that higher-order interactions among species can often be neglected. While deterministic dynamics are sufficient to explain these experiments, we find that stochastic effects can dominate during colonization of a new environment such as the gut of the worm C. elegans, leading to strong heterogeneity between the microbial communities in individual animals. These results illustrate how a bottom-up approach of characterizing individual interactions can explain the emergent behavior within complex multi-species communities.
SEPTEMBER 22, 2016
Host: Isaac Chuang
"Towards Quantum Computing with Superconducting Circuits: Extending the Lifetime of Information Through Quantum Error Correction"
Dramatic progress has been made in the last decade and a half towards realizing solid-state systems for quantum information processing with superconducting quantum circuits. Artificial atoms (or qubits) based on Josephson junctions have improved their coherence times more than a million-fold, have been entangled, and used to perform simple quantum algorithms. The next challenge for the field is demonstrating quantum error correction that actually improves the lifetimes, a necessary step for building more complex systems. Here, we demonstrate a fully operational quantum error correction system, based on a logical encoding comprised of superpositions of cat states in a superconducting cavity. This system uses real-time classical feedback to encode, track the naturally occurring errors, decode, and correct, all without the need for post-selection. Using this approach, we reach for the first time, the break-even point for QEC and preserve quantum information through active means. Moreover, the performance of the system matches with predictions, and can be dramatically improved by making the protocol more fault tolerant. Mastering the practice of error correction, and understanding the overhead and complexity required, are the main scientific challenges remaining for reaching scalable quantum computation with this technology.
SEPTEMBER 29, 2016
Host: Deepto Chakrabarty
"Observing the Signature of a Single Prolific R-Process Event in an Ultra-Faint Dwarf Galaxy"
The heaviest chemical elements in the periodic table are synthesized through the rapid neutron-capture (r-) process but the astrophysical site where r-process nucleosynthesis occurs is still unknown. The best candidate sites are ordinary core-collapse supernovae and mergers of binary neutron stars. Through their stars, 13 billion year old ultra-faint dwarf galaxies preserve a "fossil" record of early chemical enrichment that provides the means to isolate and study clean signatures of individual nucleosynthesis events. Until now, ultra-faint dwarf galaxy stars displayed extremely low abundances of heavy elements (e.g. Sr, Ba). This supported supernovae as the main r-process site. But based on new spectroscopic data from the Magellan Telescope, we have found seven stars in the recently discovered ultra-faint dwarf Reticulum II that show extreme r process overabundances, comparable only to the most extreme ancient r-process enhanced stars of the Milky Way's halo. This r-process enhancement implies that the r-process material in Reticulum II was synthesized in a single prolific event. Our results are clearly incompatible with r-process yields from an ordinary core-collapse supernova but instead consistent with that of a neutron star merger. This first signature of a neutron star merger in the early universe holds the key to finally, after 60 years, identifying the cosmic r-process production site, in addition to being a uniquely stringent constraint on the metal mixing and star formation history of this galaxy from the early universe.
OCTOBER 6, 2016
Tel Aviv University
Host: Or Hen
"The Writing Army of Judah"
An ostracon is a piece of pottery or stone that contains writing. We have been studying ostraca written in the ancient Hebrew alphabet from the first Temple era (Iron Age II).
We performed an algorithmic handwriting analysis of 16 ostraca unearthed at an excavation in the far-flung desert fort of Arad, and deduced that the texts had been written by at least six authors. The content of the inscriptions disclosed that reading and writing abilities had existed throughout the Judah military chain of command, from the highest echelon all the way down to the deputy quartermaster of the fort.
Considering the remoteness of Arad, the small garrison stationed there, and the narrowly constrained period of the inscriptions, this finding indicates a high literacy rate within Judah’s administrative apparatus – and provides a suitable background for the composition of a critical mass of biblical texts.
OCTOBER 13, 2016
Host: Krishna Rajagopal
"New Physics Gets a Boost: Jet Substructure at the Large Hadron Collider"
Collisions at the Large Hadron Collider (LHC) are dominated by jets, collimated sprays of particles that arise from quantum chromodynamics (QCD) at high energies. With the remarkable performance of the ATLAS and CMS detectors, jets can now be characterized not just by their overall direction and energy but also by their substructure. In this talk, I highlight the increasingly important role that jet substructure is playing in searches for dark matter and other new physics at the LHC, especially when exploring extreme kinematic regimes involving large Lorentz boosts. I also explain how innovative theoretical studies of jet substructure have taught us surprising lessons about QCD, revealing new probes of hot dense matter and universal features of gauge theories.
OCTOBER 20, 2016
Host: Nergis Mavalvala
THE PAPPALARDO DISTINGUISHED LECTURE IN PHYSICS
"Probing the Dark Universe with Galaxy Surveys"
The expansion of the universe and the growth of structure in it are dominated by two constituents that make up the 95 percent of the universe that is unexplained by the standard model, known as dark matter and dark energy. An understanding of these unseen components is critical to answering the most fundamental questions about our universe: how the it began, why it is accelerating, and what is the nature of most of its mass. A new generation of sky surveys are beginning to map the universe’s expansion history and evolution of structure over the last ~ 12 billion years, using statistical constraints from hundreds of millions of galaxies. I will outline the landscape of current and near future cosmological sky surveys, including early results from the Dark Energy Survey, and expected measurements from the upcoming Dark Energy Spectroscopic Instrument and the Large Synoptic Survey Telescope. Making use of these data to understand the nature of dark energy and dark matter also requires large numerical simulations of the evolution of the matter distribution and a modeling approach for connecting these simulations to observations of the galaxy distribution. I will present recent developments in how we are using simulations, modeling of the galaxy-halo connection, and large galaxy surveys together to probe the physics of the dark universe as well as the physics of galaxy formation.
Last updated on October 24, 2016 10:56 AM