Physics and Astronomy Colloquium

semester, 2018


Thursday, January 18th 2018
3:35 pm:
Speaker: Harvey Brown, Philosophy of Physics, University of Oxford
Subject: Quantum Bayesianism (QBism): the way to understand the quantum world

The recent philosophy of Quantum Bayesianism, or QBism, represents an attempt to solve the traditional puzzles in the foundations of quantum theory by denying the objective reality of the quantum state. Einstein had hoped to remove the spectre of nonlocality in the theory by also assigning an epistemic status to the quantum state, but his version of this doctrine was recently proved to be inconsistent with the predictions of quantum mechanics. In this talk, I present plausibility arguments, old and new, for the reality of the quantum state, and expose what I think are weaknesses in QBism as a philosophy of science. (The talk is based on this paper: http://philsci-archive.pitt.edu/12978/


Thursday, January 25th 2018
3:35 pm:
There will be no colloquium this week

Thursday, February 1st 2018
3:35 pm:
Speaker: Mark Bell, University of Minnesota
Subject: Nuclear Weapons and International Politics Today

Nuclear weapons are back in the news. This talk provides an overview of the most important and pressing current issues relating to nuclear weapons and international politics, including ongoing US-North Korea tensions, US nuclear modernization and the US Nuclear Posture Review, the extent of presidential authority over nuclear weapons, the risk of nuclear proliferation by U.S. allies and adversaries, and the recent nuclear ban treaty. The talk places these current issues within a broader historical context and discusses the extent to which today's nuclear concerns represent continuity or change from previous eras.

Faculty Host: Robert Lysak

Thursday, February 8th 2018
3:35 pm:
Speaker: Cristian Batista, University of Tennessee
Subject: Skyrmions and Vortices in Magnetic Systems

The history of magnetism dates back to earlier than 600 b.c., but it is only in the twentieth
century that scientists have begun to understand it, and develop technologies based on this
understanding. The new experimental techniques that were developed over twentieth century
allowed physicists to discover new forms of magnetism that they called “antiferromagnets”.
Unlike ferromagnets, the magnetic moments of antiferromagnets point along different directions
in such a way that the magnetic unit cell has no net magnetic moment. Typical configurations of
antiferromagnets are spiral orderings arising from competing exchange interactions or from the
Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction between magnetic moments embedded
in metallic environments.
The new century started with the observation of a new generation of antiferromagnets
comprising more exotic magnetic textures, such as skyrmion and vortex crystals [1-3]. These
textures were unveiled thanks to the enormous progress made in real and reciprocal space
visualization techniques. We will discuss a few attractive properties of these novel phases and
the simple principles that should guide the experimental search. For instance, we will see that an
external magnetic field can induce a skyrmion crystal phase in hexagonal lattices (lattices with
six equivalent orientations for the spiral ordering) with easy-axis anisotropy [4-10]. Moreover,
we will see that magnetic skyrmions behave as mesoscale particles, which can order in different
three-dimensional structures, such as face centered tetragonal and hexagonal closed packed
crystals [10].
References
[1] U. Rößler, A. Bogdanov, and C. Pfleiderer, Nature 442, 797 (2006).
[2] A. N. Bogdanov and D. A. Yablonskii, Sov. Phys. JETP 68, 101 (1989).
[3] A. Bogdanov and A. Hubert, Journal of Magnetism and Magnetic Materials 138, 255 (1994).
[4] S. Hayami, S.-Z. Lin, and C. D. Batista, Phys. Rev. B 93, 184413 (2016).
[5] A. O. Leonov and M. Mostovoy, Nature Communications 6, 8275 (2015).
[6] Shi- Zeng Lin, Satoru Hayami and C. D. Batista, Phys. Rev. Lett. 116, 187202 (2016).
[7] C. D. Batista, S-Z. Lin, S. Hayami and Y. Kamiya, Reports on Progress in Physics, Volume 79, 8
(2016).
[9] Satoru Hayami, Shi-Zeng Lin, Yoshitomo Kamiya, and Cristian D. Batista, Phys. Rev. B 94, 174420.
[10] Shi-Zeng Lin and C. D. Batista, arXiv:1707.05818v1.

Faculty Host: Natalia Perkins

Thursday, February 15th 2018
3:35 pm:
Speaker: Mark Saffman (University of Wisconsin)
Subject: Quantum computing with simple and complex atoms
Refreshments in atrium after the Colloquium.

Quantum computing is a few decades old and is currently an area where there is great excitement, and rapid developments. A handful of distinct approaches have shown the capability of on demand generation of entanglement and execution of basic quantum algorithms.

One of the daunting challenges in developing a fault tolerant quantum computer is the need for a very large number of qubits. Neutral atoms are one of the most promising approaches for meeting this challenge. I will give a snapshot of the current status of quantum computing in general and atomic quantum computing in particular. The atomic physics underlying our ability to control neutral atom qubits will be described, and I will show how one of the most complicated atoms in the periodic table may lead to some simple solutions to hard problems.

Faculty Host: Paul Crowell

Thursday, February 22nd 2018
3:35 pm:
Speaker: Erez Berg (University of Chicago)
Subject: Critical Metals: Lessons from quantum Monte Carlo studies

Critical phenomena are one of the cornerstones of classical statistical mechanics. Quantum critical points (i.e., continuous phase transitions at zero temperature) in insulating materials are relatively well understood, by analogy with classical critical points in one spatial dimension higher. In contrast, the theory of quantum critical behavior in metals is still, to a large degree, open. Such metallic critical points are believed to play an important role in the physics of several "strongly correlated" materials, such as high temperature superconductors. Fortunately, many classes of metallic quantum critical points can be simulated efficiently using quantum Monte Carlo without the notorious "sign problem", which often hinders numerical simulations of fermionic systems. I will describe some recent progress along these lines, and how it sheds new light on some of the outstanding puzzles in the field.

Faculty Host: Rafael Fernandes

Thursday, March 1st 2018
4:00 pm:
Speaker: Sara Seager, MIT
Subject: Mapping the Nearest Stars for Habitable Worlds
Joint Colloquium with Earth Sciences (Nier Lecture). Note later start time.

Bio:

"Sara Seager is a planetary scientist and astrophysicist at the Massachusetts Institute of Technology where she is a Professor of Planetary Science, Professor of Physics, Professor of Aerospace Engineering, and holds the Class of 1941 Professor Chair. She has pioneered many research areas of characterizing exoplanets with concepts and methods that now form the foundation of the field of exoplanet atmospheres. Her present research focus is on the search for life by way of exoplanet atmospheric “biosignature” gases. Professor Seager works on space missions for exoplanets including as: the PI of the CubeSat ASTERIA; the Deputy Science Director of the MIT-led NASA Explorer-class mission TESS; and as a lead of the Starshade Rendezvous Mission (a space-based direct imaging exoplanet discovery concept under technology development) to find a true Earth analog orbiting a Sun-like star. Among other accolades, Professor Seager was elected to the US National Academy of Sciences in 2015, is a 2013 MacArthur Fellow, is a recipient of the 2012 Sackler Prize in the Physical Sciences, and has Asteroid 9729 named in her honor."

Abstract:

Thousands of exoplanets are known to orbit nearby stars and small rocky planets are established to be common. The ambitious goal of identifying a habitable or inhabited world is within reach. But how likely are we to succeed? We need to first discover a pool of planets in their host star’s “extended” habitable zone and second observe their atmospheres in detail to identify the presence of water vapor, a requirement for all life as we know it. Life must not only exist on one of those planets, but the life must produce “biosignature gases” that are spectroscopically active, and we need to be able to sort through a growing list of false-positive scenarios with what is likely to be limited data. The race to find habitable exoplanets has accelerated with the realization that “big Earths” transiting small stars can be both discovered and characterized with current technology, such that the James Webb Space Telescope has a chance to be the first to provide evidence of biosignature gases. Transiting exoplanets require a fortuitous alignment and the fast-track approach is therefore only the first step in a long journey. The next step is sophisticated starlight suppression techniques for large ground-and space-based based telescopes to observe small exoplanets directly. These ideas will lead us down a path to where future generations will implement very large space-based telescopes to search thousands of all types of stars for hundreds of Earths to find signs of life amidst a yet unknown range of planetary environments. What will it take to identify such habitable worlds with the observations and theoretical tools available to us?

Nier Info:

Professor A.O. Nier

A.O. Nier served as a highly distinguished faculty member of the Physics Department for 42 years starting in 1938. He was actively involved in research up to the time of his death in 1994. A firm believer in “pursuits of knowledge - in areas which cross traditional lines” he had an enormous impact on the geological sciences by his pioneering work on isotope abundances and measurements of many elements which are used in radiometric age determinations of geologic materials. He received many national and international awards in recognition of his discoveries and contributions to Physics, Geological Sciences and many other fields.


Thursday, March 8th 2018
3:35 pm:
Tate Grand Opening

Thursday, March 15th 2018
3:35 pm:
Subject: There will be no colloquium this week due to Spring Break

Thursday, March 22nd 2018
3:35 pm:
Speaker: Pablo Jarillo-Herrero (MIT)
Subject: Magic Angle Graphene: a New Platform for Strongly Correlated Physics

The understanding of strongly-correlated quantum matter has challenged physicists for decades. Such difficulties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum materials. In this talk I will present a new platform to investigate strongly correlated physics, based on graphene moiré superlattices. In particular, I will show that when two graphene sheets are twisted by an angle close to the theoretically predicted ‘magic angle’, the resulting flat band structure near the Dirac point gives rise to a strongly-correlated electronic system. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a Mott-like insulator arising from electrons localized in the moiré superlattice. Moreover, upon doping, we find electrically tunable superconductivity in this system, with many characteristics similar to high-temperature cuprates superconductivity. These unique properties of magic-angle twisted bilayer graphene open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as quantum spin liquids.

Faculty Host: Vlad Pribiag

Thursday, March 29th 2018
3:35 pm:
Speaker: Barry Mauk, APL
Subject: New perspectives on Jupiter’s novel space environment and aurora from NASA’s Juno mission

B. H. Mauk, The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA (Barry.Mauk@jhuapl.edu)

Jupiter’s uniquely powerful auroras are thought to be symptoms of Jupiter’s attempt to spin up its space environment and shed angular moment (albeit minuscule amounts). The processes involved connect together such disparate phenomena as the volcanoes of Jupiter’s moon Io and the Jupiter-unique synchrotron emissions imaged from ground radio telescopes at Earth. While the power sources for auroral processes at Earth and Jupiter are known to be very different, it has been expected that the processes that convert that power to auroral emissions would be very similar. NASA’s Juno mission, now in a polar orbit at Jupiter, is dramatically altering this view about how Jupiter’s space environment operates. Auroral processes are much more energetic than expected, generating beams of electrons with multiple MeV energies and with directional intensities that can be more intense than the electrons within Jupiter’s radiation belts. The most intense auroral emissions appear to be generated by processes that have no precedent within Earth auroral processes. And, the auroral generation processes are poorly correlated, unexpectedly, with any large-scale electric currents thought necessary to regulate the interactions between Jupiter’s spinning atmosphere and space environment. These and other findings are discussed, along with presentation of Juno’s broader mission and discoveries.

Faculty Host: Robert Lysak

Thursday, April 5th 2018
3:35 pm:
Speaker: Alessandra Corsi, Texas Tech
Subject: Multi-messenger time-domain astronomy: GW170817 and the future

On 2017 August 17, the field of gravitational-wave (GW) astronomy made the big leagues with a dazzling discovery. After several GW detections of black hole (BH)-BH mergers with no convincing electromagnetic counterparts, advanced LIGO and Virgo scored their first direct detection of GWs from a binary neutron star (NS) merger, an event dubbed GW170817. Soon after the GW discovery, GW170817 started gifting the astronomical community with an electromagnetic counterpart spanning all bands of the spectrum. In this talk, I will review what we have learned from GW170817, what questions remain open, and what are the prospects for future EM-GW studies of the transient sky.

Faculty Host: Vuk Mandic

Thursday, April 12th 2018
3:35 pm:
Speaker: Doug Glenzinski, Fermilab
Subject: A Rare Opportunity - the Mu2e Experiment at Fermilab

The muon, a heavy cousin of the electron, was discovered in 1936. Since
that time they have only ever been observed to do one of two things: 1)
scatter or 2) decay into final states that include a combination of
charged leptons and neutrinos. A new experiment at Fermilab - the Mu2e
experiment - is going to look for a third thing: a muon trapped in atomic
orbit that interacts with the nucleus to produce an electron and nothing
else. This is a process that's predicted to occur very very rarely, maybe
once every 10^15 decays,(or less!). But this very rare decay will probe
new physics mass scales up to 10,000 TeV/c^2 and may hold the key to
understanding physics at its most fundamental level. The Mu2e experiment
is an ambitious endeavor whose goal is to observe this very rare
interaction for the first time - a discovery that could help reveal a new
paradigm of particle physics.

Faculty Host: Kenneth Heller

Thursday, April 19th 2018
3:35 pm:
Speaker: Victoria Kaspi, McGill University.
Subject: Astronomy's Newest Extragalactic Mystery: Fast Radio Bursts!

Fast Radio Bursts (FRBs) are a newly discovered astrophysical phenomenon consisting of short (few ms) bursts of radio waves.FRBs occur roughly 1000 times per sky per day. From their dispersion measures,these events are clearly extragalactic and possibly generally at cosmological distances. One FRB is known to repeat and indeed has been localized to a dwarf galaxy at redshift 0.2. Nevertheless, the origin of FRBs, whether repeating or not, is presently unknown. In this talk I will review FRB properties as well as highlight efforts to find FRBs, including a new Canadian radio telescope,CHIME, that is predicted to make major progress on the FRB problem.


Thursday, April 26th 2018
3:35 pm:
Speaker: John Bush, MIT
Subject: Hydrodynamic quantum analogs

Droplets walking on a vibrating fluid bath exhibit several features previously thought to be exclusive to the microscopic, quantum realm. These walking droplets propel themselves by virtue of a resonant interaction with their own monochromatic wavefield, and represent the first macroscopic realization of a pilot-wave system of the form proposed for microscopic quantum dynamics by Louis de Broglie in the 1920s. New experimental and theoretical results allow us to rationalize the emergence of quantum-like behavior in this hydrodynamic pilot-wave system in a number of settings, and explore its potential and limitations as a quantum analog.

Faculty Host: J. Woods Halley

Thursday, May 3rd 2018
3:35 pm:
Speaker: Jeffrey Bub, Maryland
Subject: Discussions Over a Beer: Bohr, Einstein, Bell, and all that
Student awards will be distributed at the beginning of the Colloquium.

The Bohr-Einstein debate about the foundations of quantum mechanics is something physicists tend to think of as the sort of thing you might discuss over a beer after you’ve spent the day doing real physics. Following John Bell’s seminal 1964 paper on nonlocality, there’s a new game in town influenced by developments in quantum information. I’ll discuss the significance of this new wave in quantum foundations for the dispute that separated Bohr and Einstein. (Bring your own beer.)

Faculty Host: Michel Janssen

Thursday, May 10th 2018
3:35 pm:
There will be no colloquium this week.

Thursday, September 6th 2018
3:35 pm:
Speaker: Andrew Zangwill, Georgia Institute of Technology
Subject: Walter Kohn and the Creation of Density Functional Theory
Refreshments in atrium after the Colloquium.

Today's most popular method for calculating the electronic structure of atoms, molecules, liquids, solids, and plasmas makes no use of the Schrödinger equation or the many-electron wave function. Instead, it exploits a bold hypothesis: the electron density distribution completely characterizes the ground state of a many-electron system. This hypothesis was advanced in 1964-1965 by the theoretical physicist Walter Kohn and two young postdocs. This talk traces Kohn's educational trajectory (from Nazified Vienna to an internment camp in the Canadian forest to the University of Toronto to Harvard University), his professional trajectory (from applied mathematician to nuclear physicist to solid state physicist to the inventor of density functional theory), and the circumstances which led him to win a share of the 1998 Nobel Prize for Chemistry.

Faculty Host: Michel Janssen

Thursday, September 13th 2018
3:35 pm:
Speaker: Raman Sundrum, University of Maryland College Park
Subject: Fundamental Physics and the Fifth Dimension
Refreshments in atrium after the Colloquium.

The central aspirations, successes and puzzles of Particle Physics will be reviewed against the backdrop of the twin pillars of modern physics: Relativity and Quantum Mechanics. The notion of Naturalness will be introduced as the organizing “gambling” principle behind many searches for new particles and forces beyond the current Standard Model. The interplay between deep theoretical mechanisms of naturalness, such as Extra Spacetime Dimensions and Supersymmetry, and experimental strategies at particle colliders and other facilities will be emphasized.

Faculty Host: Tony Gherghetta

Thursday, September 20th 2018
3:35 pm:
Speaker: Ali Yazdani, Princeton University
Subject: Spotting the elusive Majorana under the microscope
Refreshments in atrium after the Colloquium.

Ettore Majorana famously considered that there may be fermions in nature that are their own antiparticle — and then he mysteriously disappeared just after proposing the idea in 1938. In recent years, we have learned how to engineer materials that harbor quasiparticles that behave similar to fermions Majorana had envisioned. In particular, there has been a focus on one-dimensional topological superconductor that harbor Majorana zero modes (MZM) that can potentially be used to make fault-tolerant topological quantum computation possible. Recently, we have proposed and implemented a platform for realization of topological superconductivity and MZM in chains of magnetic atoms on the surface of a superconductor [1,2]. In this talk, I will describe this platform and the series of experiments we have performed to establish the presence of these exotic quasi-particle using spectroscopic mapping with the STM. [2-4] These include a recent study of the unique spin signature of MZM.[4] Finally, if there is time I will describe some ongoing experiment on realization of MZM in a platform based on chiral quantum spin Hall edge states.

[1] S. Nadj-Perge et al. PRB 88, 020407 (2013).
[2] S. Nadj-Perge et al. Science 346, 6209 (2014).
[3] B. E. Feldman et al. Nature Physics 13, 286 (2016).
[4] S. Jeon et al. Science 358, 772 (2017).


Monday, September 24th 2018
Speaker: Natalia Perkins, University of Minnesota
Subject: The pursuit of Majorana fermions in Kitaev materials
Refreshments in atrium after the Colloquium.

The 1937 theoretical discovery of Majorana fermions has since impacted diverse problems ranging from neutrino physics and dark matter searches to fractional quantum Hall effect, superconductivity and quantum spin liquids (QSLs). In my talk I will focus on the hunting for Majorana fermions in Kitaev materials, which believe to harbor a variety of QSLs. These Kitaev QSLs exhibit two types of fractionalized quasiparticle excitations - itinerant Majorana fermions and gapped Z2 fluxes. In recent years, a remarkable theoretical and experimental progress has been achieved in understanding that these fractionalized quasiparticles and, in particular, Majorana fermions can be effectively probed by conventional spectroscopic techniques such as inelastic neutron scattering, Raman scattering with visible light, and resonant inelastic X-ray scattering.


Thursday, September 27th 2018
3:35 pm:
Physics and Astronomy Colloquium in Physics Tate B50
Speaker: Raymond Jeanloz, UC Berkeley
Subject: Pressure as a Probe for Atoms, Molecules and Planets
Refreshments in atrium after the Colloquium

Experiments using diamond anvils and the largest lasers in the world can exceed the atomic unit of pressure, profoundly changing the nature of atom structure and chemical bonding. Laboratory studies of the metallic hydrogen and helium that dominate large-planet (Jupiter, Saturn) and stellar interiors, and of new forms of ice that comprise icy giant planets (Neptune, Uranus), are complemented by experiments converting simple metals into ionic salts and taking elements to the statistical-atom (Thomas-Fermi-Dirac) regime. Interesting in their own right, and as validation of first-principles quantum simulations, these experiments also inform us about the origins and evolution of planets.

Faculty Host: Cynthia Cattell

Thursday, October 4th 2018
3:35 pm:
Speaker: Professor Nergis Mavalvala, Massachusetts Institute of Technology
Subject: Gravitational wave detectors: past, present and future
Refreshments in atrium after the Colloquium.

The Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves for the first time in 2015, and has continued to make discoveries. I will discuss the instruments that made these discoveries, the science so far, and plans for future improvements and upgrades to LIGO.


Monday, October 8th 2018
Speaker: Fiona Burnell, University of Minnesota
Subject: The emergence of topology in strongly correlated many-body systems
Refreshments in atrium after the Colloquium.

At sufficiently low temperatures, quantum mechanics plays a key role in determining materials’ behaviours. Particularly in systems where interactions are important, this can lead to macroscopic physical properties that are fundamentally different from what we expect both from single-particle quantum mechanics, and from interacting classical systems. Among the possibilities are that two systems may locally look the same, but globally behave very differently. These global differences are described mathematically by a variety of topological quantities, which capture important physical differences such as statistical interactions between particles, and distinctive low-energy physics at the systems' boundaries. I will describe recent developments in our understanding of interacting topological phases in the presence of global symmetries, and how these are connected to the “unusual" (or, in technical terms, anomalous) properties of their low-energy boundary states.


Thursday, October 11th 2018
3:35 pm:
Speaker: Joel Moore, UC Berkeley
Subject: Topology and the electromagnetic responses of quantum materials
Refreshments in atrium after the Colloquium.

This talk starts by reviewing the remarkable theoretical and experimental
progress in topological materials over the past decade. Three-dimensional
topological insulators realize a particular electromagnetic coupling known as
“axion electrodynamics”, and understanding this leads to an improved
understanding of magnetoelectricity in all materials. We then turn to how
topological Weyl and Dirac semimetals can show unique electromagnetic
responses; we argue that in linear response the main observable effect
solves an old problem via the orbital moment of Bloch electrons, and how
in nonlinear optics there should be a new quantized effect, at least approximately.


Thursday, October 18th 2018
3:35 pm:
Speaker: Terry Hwa, University of California, San Diego
Subject: Bacterial growth laws and the origin of dimensional reduction
Refreshments in atrium after the Colloquium.

A cell is the smallest unit of a freely living system. Our understanding of cells is measured by our ability to predict cellular behaviors in response to environmental changes and genetic manipulations. Traditionally, researchers strive to gain insights into cellular behaviors through characterizing the underlying molecular interactions. This ‘bottom-up’ approach however requires a macroscopic number of largely inaccessible parameters in order to be predictive. I will describe a complementary ‘top-down’ approach based on quantitative phenomenology. Extensive quantitative experiments establish that the model bacterium E. coli organizes many of its behavioral responses in very simple manners in accordance to the rate of cell growth. The existence of these simple empirical relations (growth laws) despite myriads of complex molecular interactions is a striking manifestations of a tremendous degree of dimensional reduction occurring in living cells. I will describe how the growth laws can be used to make accurate predictions of cell behaviors without fitting parameters. I will also discuss how the magical dimension reduction can be accomplished by cells through clever strategy of gene regulation.

Faculty Host: Elias Puchner

Thursday, October 25th 2018
3:35 pm:
Speaker: Steven Gubser, Princeton
Subject: Number theory and spacetime
Refreshments in atrium after the Colloquium.

Our description of spacetime relies on the real numbers and hence is wedded to arbitrarily small intervals of length and time. But quantum theory hints at the existence of a smallest possible length, the Planck length. Number theory provides an alternative to the real numbers known as the p-adic numbers. Recent work has argued that quantum field theory defined over the p-adic numbers is holographically dual to a discrete spacetime. Constructions related to p-adic numbers also have a surprisingly prominent role in the early development of the renormalization group. I will explain what the p-adic numbers are and provide some intuition for what they are good for in string theory and beyond. The ultimate aim of using them to understand quantum gravity is ambitious indeed, but I will explain some first steps that give hope for the future.

Bio: Steve Gubser got his PhD from Princeton University in 1998, working with Igor Klebanov on what became the gauge-string duality. He did a post-doc at Harvard, then joined the faculty at Princeton. After a year at Caltech, he returned to Princeton and has been there ever since. He is now the Associate Chair for Undergraduates in Physics, and he is a recent recipient of a Simons Young Investigator award.

Faculty Host: Priscilla Cushman

Thursday, November 1st 2018
3:35 pm:
Speaker: Robert Kleinberg, Columbia University & Boston University
Subject: mK to km: How Millikelvin Physics is Reused to Explore the Earth Kilometers Below the Surface
Refreshments in atrium after the Colloquium.

Investigations of the superfluid phases of liquid helium-3 would seem to have little application to the study of rock formations thousands of meters below the surface of the earth. However, the physicist’s tool box is versatile, and techniques used in one field of study can be reused, with appropriate adaptation, in very different circumstances.

The temperature of liquid helium-3 in the millikelvin range can be measured using an unbalanced-secondary mutual inductance coil set designed to monitor the magnetic susceptibility of a paramagnetic salt. The loss signal is discarded by phase sensitive detection. Now consider the task of measuring the electrical conductivity, at centimeter scale, of the earth surrounding a borehole. Turn the mutual inductance coil set inside out, with secondary coils arranged to be unbalanced with respect to the rock wall. Instead of discarding the loss signal, use it to measure conductivity. A sensor based on this principle has been implemented in a widely deployed borehole geophysical instrument, used to estimate the prevailing direction of the wind millions of years ago, or to decide where to drill the next well in an oilfield.

Nuclear magnetic resonance may seem a very improbable measurement of the rock surrounding a borehole. Conventionally, we place the sample (which might be a human being) inside the NMR apparatus. In borehole deployment, the instrument is placed inside sample, the temperature is as high as 175C, pressure ranges to 140 MPa, and measurements must be made while moving at 10 cm/s. Apparatus with these specifications have been deployed worldwide, and are used to measure a number of rock properties, including the distribution of the sizes of pores in sedimentary rock, and the viscosity of oil found therein. They have also been used for geological and oceanographic studies in northern Alaska, and at the seafloor offshore Monterey, California.

Faculty Host: Shaul Hanany

Thursday, November 8th 2018
3:35 pm:
Speaker: Jason Petta, Princeton University
Subject: Quantum Computing with Electron Spins in Silicon
Refreshments in atrium after the Colloquium.

Electron spins are excellent candidates for solid state quantum computing due to their exceptionally long quantum coherence times, which is a result of weak coupling to environmental degrees of freedom. However, this isolation comes with a cost, as it is difficult to coherently couple two spins in the solid state, especially when they are separated by a large distance. Here we combine a large electric-dipole interaction with spin-orbit coupling to achieve spin-photon coupling [1]. Vacuum Rabi splitting is observed in the cavity transmission as the Zeeman splitting of a single spin is tuned into resonance with the cavity photon. We achieve a spin-photon coupling rate as large as gs/2 = 10 MHz, which exceeds both the cavity decay rate /2 = 1.8 MHz and spin dephasing rate /2= 2.4 MHz, firmly anchoring our system in the strong-coupling regime [2]. Moreover, the spin-photon coupling mechanism can be turned off by localizing the spin in one side of the double quantum dot. These developments in quantum dot cQED, combined with recent demonstrations of high-fidelity two-qubit gates in Si, firmly anchor Si as a leading material system in the worldwide race to develop a scalable quantum computer [3].

1. Mi et al., Science 355, 156 (2017).
2. Mi et al., Nature 555, 599 (2018).
3. Zajac et al., Science 359, 439 (2018).


Thursday, November 15th 2018
3:35 pm:
Speaker: John Marko, Northwestern University
Subject: Physics of chromosome folding and disentanglement
Refreshments in atrium after the Colloquium.

All biological phenomena depend on genetic information which is encoded
into the base-pair sequence along the very long DNA molecules found in all
living cells. The DNAs in chromosomes, in addition to being biologically
important, are amazing physical objects, being 2 nanometers wide and (in
humans) several centimeters in length. I will explain how the cell takes
care of these long, fragile chromosomal DNAs and how it uses DNA itself as
a key mechanical component of the cell nucleus. Then, during and
following DNA replication, our cells face the gigantic challenge of
figuring out how to topologically separate those long polymers from one
another. I will discuss our current understanding of the "lengthwise
compaction" mechanisms underlying this process, focusing on the interplay
between "loop-extruding" SMC complexes (mainly condensin) and
DNA-topology-changing topoisomerase II.

Faculty Host: Elias Puchner

Thursday, November 29th 2018
3:35 pm:
Speaker: Marco Velli, UCLA
Subject: Parker Solar Probe: Understanding Coronal heating and Solar Wind acceleration
Refreshments in atrium after the Colloquium.

The magnetic field is fundamental to solar activity and shapes the inter-planetary environment, as shown by the full three dimensional monitoring of the heliosphere provided by measurements from many past and present interplanetary and remote sensing spacecraft. Magnetic fields are also the source for coronal heating and the very existence of the solar wind; produced by the sun’s dynamo and emerging into the corona, magnetic fields become a conduit for waves, act to store energy, and then propel plasma into the Heliosphere in the form of Coronal Mass Ejections (CMEs). Magnetic fields are also at the heart of the generation and acceleration of Solar Energetic Particle (SEPs) that modify the space weather environment of the Earth and other planets.

Parker Solar Probe (PSP) was launched in August 2018 to carry out the first in situ exploration of the outer solar corona and inner Heliosphere. Direct measurements of the plasma in the closest atmosphere of our star should lead to a new understanding of the questions of coronal heating, solar wind acceleration, and the generation, acceleration and propagation of SEPs.

In this lecture I will start from an introduction to our present knowledge of the magnetized solar corona and wind before describing the PSP scientific objectives, orbit, and instrument suites, and perhapse showing a glimpse from initial data. Emphasis will be on how PSP will confirm or falsify present models as well as the potential new discoveries stemming from the first exploration of the space inside the orbit of Mercury. I will also discuss how synergies with Solar Orbiter might lead us to accurately understand the state of the solar wind all the way from the corona into interplanetary space, a stepping stone
for understanding the dynamics of active magnetized plasmas throughout the universe.

Faculty Host: Robert Lysak

Thursday, December 6th 2018
3:35 pm:
Speaker: Stacy McGaugh, Case Western
Subject: Dynamical Regularities in Galaxies and their Implications for Dark Matter
Refreshments in atrium after the Colloquium.

The flat rotation curves of galaxies were a surprising observation that helped establish the dark matter paradigm. Flat rotation curves are only the first of a series of striking regularities in the dynamics of galaxies. The amplitude of the flat rotation speed is not random; it correlates strongly with the mass observed in stars and gas (the baryonic Tully-Fisher relation). At the centers of galaxies, the dynamical surface density correlates with the observed surface brightness of stars (the central density relation). At all observed radii, the observed centripetal acceleration correlates with the acceleration predicted by the observed distribution of baryons (the radial acceleration relation). These empirical relations inform our thinking about the missing mass problem in ways that were not available when the current paradigm was established.

Faculty Host: J. Woods Halley

Thursday, December 13th 2018
3:35 pm:
There will be no colloquium this week

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