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# The David and Edith Harris Physics Colloquium Series

## SPRING 2015 Schedule

**
****Thursdays - Socials:
3:30pm in 4-349 (The Pappalardo Room) // Talk: 4:00pm in 10-250 (unless otherwise noted)**

**Other Colloquia:**

Center for Theoretical Physics Colloquia and Seminars

Center for Ultracold Atoms Seminars (CUA)

Chez Pierre Seminars in Condensed Matter Physics at MIT

LNS Nuclear and Particle Physics Colloquia

Modern Optics and Spectroscopy Seminars

**FEBRUARY 5, 2015**

**MARKUS OBERTHALER**

University of Heidelberg

Host: Vladan Vuletic

"Quantum metrology with Bose Einstein Condensates"

One aspect of metrology, the science of measurement, is the exploration of the ultimate precision limit. It is known for quite some time that the new possibilities in quantum mechanics allow the surpassing of the ultimate classical precision limit given by counting statistics. Quantum metrology is about the exploration of these new limits. The goal is the generation and characterization of useful quantum mechanical resources for going beyond the classical precision limits. Since the gain in precision is intimately connected to quantum entanglement in many particle systems these investigations are also interesting from the fundamental point of view.

In this colloquium I will discuss in detail how Bose Einstein condensates can be used to generate entangled many particle states which push atom interferometry beyond the classical limits. I will use the system of two component atomic condensates as a model system for explaining how quantum correlations arise and how they can be used for improved estimation of a phase shift in an atom interferometer. The simplest form of useful many particle quantum states are spin squeezed states which can be classified as Gaussian states. I will also report on the latest results revealing that Bose Einstein condensates make it possible to generate deterministically non-gaussian states. The experimental extraction of a bound of the quantum Fisher information implies that these states also surpass the classical limits of the phase estimation precision.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

**FEBRUARY 12, 2015**

**ANNA WATTS**

University of Amsterdam

Host: Deepto Chakrabarty

"The Physics and Forensics of Neutron Star Explosions"

Neutron stars are the densest objects in the Universe, attaining physical conditions that cannot be replicated on Earth. The nature of matter at such extreme densities is one of the great unsolved problems in modern science and this makes neutron stars unparalleled laboratories for nuclear physics and QCD. Astronomers have several techniques at their disposal to study neutron star composition, but some of the most promising involve using the magnetic and thermonuclear explosions that periodically rock many neutron stars. I will introduce these explosive phenomena and explain how we can sift through the photonic debris to find clues to the underpinning dense matter physics.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

**FEBRUARY 19, 2015**

**JEAN DALIBARD**

Collège de France

Hosted by Wolfgang Ketterle

"Quantum Gases in Low Dimension: from Atom Circuits to Topological States of Matter"

In his famous novel "Flatland" published in 1884, the English writer Edwin Abbott imagined a social life in a two-dimensional world. With a very original use of geometrical notions, E. Abbott produced a unique satire of his own society. Long after Abbott's visionary allegory, microscopic physics has provided a practical path for the exploration of low-dimensional worlds. With the realization of quantum wells for example, it has been possible to produce two-dimensional gases of electrons. The properties of these fluids dramatically differ from the standard three-dimensional case, and some of them are still lacking a full understanding.

During the last decade, a novel environment has been developed for the study of low-dimensional phenomena. It consists of cold atomic gases confined in laser traps, which can be shaped in arbitrary forms such as disks or annuli, in which permanent currents can be established. The talk will discuss some aspects of this research and explain how artificial magnetism can be implemented on these gases, raising the possibility to generate topological states of matter analogous to those appearing in Quantum Hall physics.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

**FEBRUARY 26, 2015**

**ANDREI KOUNINE**

MIT

Hosted by Peter Fisher

"Results from the AMS Experiment on the International Space Station"

Alpha Magnetic Spectrometer (AMS-02) is a general purpose high energy particle detector deployed on the International Space Station in 2011. It conducts a unique long duration mission of fundamental physics research in space. To date the detector collected over 60 billion cosmic ray events. In this presentation, I will discuss the AMS detector and its physics objectives: a search for understanding of dark matter, antimatter, the origin of cosmic rays, and the exploration of new physics phenomena.

The presentation will then focus on recent AMS results on the positron fraction, the individual fluxes, and the combined electron and positron flux. These measurements are complementary one to another. Together they provide a deeper understanding of the origin of high energy cosmic rays and shed more light on the existence of new phenomena in cosmic rays.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

**MARCH 5, 2015**

FRANCIS GAVIN

MIT

Hosted by Peter Fisher

*PHYSICS IN THE INTEREST OF SOCIETY COLLOQUIUM*

"Nuclear Statecraft – History and Strategy in America’s Atomic Age"

There is a widely held belief that we are at a profound and critical juncture in world politics. Nuclear proliferation, strategy and policy have risen to the top of the global policy agenda, and issues ranging from a nuclear Iran to the global zero movement are generating sharp discussion and debate. What is the best way to understand these problems and recommend the best policies to deal with these questions?

The key is to examine the historical origins of our contemporary nuclear world. Surprisingly, this is rarely done. Nuclear Statecraft challenges key elements of the widely accepted, stylized narrative about the history of the atomic age and the consequences of the nuclear revolution. On a wide range of issues -- including the strategy of flexible response, the influence of nuclear weapons during the Berlin Crisis, Nixon’s nuclear strategies, the origins and motivations for U.S. nuclear nonproliferation policy, and how to assess the nuclear dangers we face today, to name a few -- we have gotten our history wrong. In this talk, I will focus on the little known history of the origins of the United States decision to vigorous pursue nuclear nonproliferation, and in particular, the Nuclear Nonproliferation Treaty.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 8-329 *NOTE ROOM CHANGE

**MARCH 12, 2015**

**SURYA GANGULI**

Stanford University

Hosted by Nikta Fakhri

"The Statistical Physics of Deep Learning: on the Beneficial Roles of Dynamic Criticality, Random Landscapes, and the Reversal of Time"

Neuronal networks have enjoyed a resurgence both in the worlds of neuroscience, where they yield mathematical frameworks for thinking about complex neural datasets, and in machine learning, where they achieve state of the art results on a variety of tasks, including machine vision, speech recognition, and language translation. Despite their empirical success, a mathematical theory of how deep neural circuits, with many layers of cascaded nonlinearities, learn and compute remains elusive. We will discuss three recent vignettes in which ideas from statistical physics can shed light on this issue. In particular, we show how dynamical criticality can help in neural learning, how the non-intuitive geometry of high dimensional error landscapes can be exploited to speed up learning, and how modern ideas from non-equilibrium statistical physics, like the Jarzynski inequality, can be extended to yield powerful algorithms for modeling complex probability distributions. Time permitting, we will also discuss the relationship between neural network learning dynamics and the developmental time course of semantic concepts in infants.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 8-329 (NOTE ROOM CHANGE)

**MARCH 19, 2015**

**CHRISTOPHER FRYER**

Los Alamos National Laboratory

Hosted by Deepto Chakrabarty

"Diagnosing Supernovae: Nature’s High-Energy Density Laboratory"

The engine behind core-collapse supernovae reaches temperatures above 10MeV and densities above nuclear densities, an ideal setting to study the behavior of matter in these extreme conditions. But, to do so, we must understand the observations of supernovae sufficiently well to use this data to probe the central engine. We will review the growing suite of supernova observations and what we now understand about supernovae. We will also discuss the current issues in using these diagnostics and what the future holds to improve our understanding of both the observations and their constraints on the central supernova engine.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

MARCH 26, 2015 - NO COLLOQUIUM DUE TO SPRING BREAK

**APRIL 2, 2015**

**JACQUELINE HEWITT**

MIT Kavli Institute for Astrophysics and Space Research

Host: Deepto Chakrabarty

"Probing the Cosmic Dawn and Cosmology with the 21cm Hydrogen Line"

Measurements of the cosmic microwave background at redshift z ~ 1100 give us information about the initial density fluctuations that seeded subsequent gravitational collapse and structure formation. Observations of galaxies and clusters at z <~ 8 give us information about the outcome of this structure formation. Between those redshifts lies a modern frontier of cosmology - the cosmic dawn that marked the formation of the first stars and galaxies and the reionization of the intergalactic medium. Direct observations of this phase of the universe’s history are currently lacking. A particularly promising technique is that of mapping hydrogen structures using the redshifted 21cm radio line. Several recently-completed low frequency radio arrays are now operating and providing us with an early glimpse into the process of structure formation. Future larger arrays may further elucidate this history and possibly even probe the initial conditions believed to result from inflation.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

**APRIL 9, 2015**

**CRISTIAN URBINA**

CEA-Saclay

Hosted by Pablo Jarillo-Herrero

"Manipulating the Quantum State of a Single Cooper Pair in a One-Atom Contact"

The Josephson effect describes the flow of supercurrent through a weak link such as a tunnel junction, nanowire, or molecule between two superconductors. It is the basis for a great variety of circuits and devices like magnetometers, quantum amplifiers, and qubits — having a tremendous impact on both fundamental and applied science. The most general description of the Josephson effect, valid for all types of weak links, is based on the key concept of Andreev bound states. These are doublets of localized states with energies smaller than the superconducting gap, which can be viewed as the two possible states of a Cooper pair localized at the weak-link.

I will illustrate this physics with experiments on the simplest Josephson weak-link: a one-atom contact between two superconductors. In particular, I will describe the time-domain manipulation of quantum superpositions of Andreev bound states.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

**APRIL 16, 2015**

**NIMA ARKANI-HAMED**

Institute for Advanced Study

Hosted by The MIT Society of Physics Students

"Space-Time, Quantum Mechanics and Scattering Amplitudes"

Scattering amplitudes for gluons and gravitons have extraordinary properties that are completely invisible in the textbook formulation of quantum field theory using Feynman diagrams. Apart from the simplest processes, Feynman diagram calculations are extremely complicated, while the final results turn out to be amazingly simple, exhibiting hidden infinite-dimensional symmetries. This strongly suggests the existence of a new formulation of quantum field theory where locality and unitarity are derived concepts, while other principles are made more manifest.

The past few years have seen significant advances towards uncovering this new picture, exposing a surprising link between this very basic physics and new mathematical structures in algebraic geometry. In this talk I will review these ideas, and discuss a number of directions in which this field is likely to develop in the coming years.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

**APRIL 23, 2015**

**ALEXANDER POLYAKOV**

Princeton University

Hosted by The MIT Physics Graduate Student Council

"The Praise for Quantum Field Theory"

I will try to demonstrate this by briefly discussing some fundamental problems, solved and unsolved.

The problems will include critical phenomena and conformal bootstrap, quark confinement and gauge/string duality, cosmological constant and turbulence and may be more.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

**APRIL 30, 2015**

**ARUP CHAKRABORTY
**MIT

Hosted by Mehran Kardar

"Bringing Together Physics, Biology, and Medicine to Hit HIV Where it Hurts"

HIV is a highly mutable virus, which evades natural and vaccine-induced immune responses and is the causative agent for the AIDS epidemic. I will describe methods, rooted in statistical physics, which aim to determine the fitness landscape of HIV – i.e., a definition of the collective sets of mutations that allow the virus to maintain fitness and evade immunity, and those combinations of mutations that cripple it. The “Hamiltonian” that describes this fitness landscape is analogous to the Hopfield Hamiltonian for associative memory in neural networks. I will show how this Hamiltonian reveals encoded “memories” in the HIV population of host-pathogen riposte won by the virus, and scaling laws that describe this phenomenon. I will also present how evolutionary dynamics with our inferred fitness landscape can predict HIV evolution in individual patients, and how we have harnessed this knowledge, along with other experimental tests, to design a therapeutic vaccine against HIV which is being advanced to pre-clinical trials.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

**MAY 7, 2015**

**MICHAEL BRENNER
**Harvard University

Hosted by Jeremy England

"Towards Artificial Living Materials"

Biological systems provide an inspiration for creating a new paradigm for materials synthesis. Imagine it were possible to create an inanimate material that could both perform some function, e.g. catalyze a set of reactions, and also self replicate. Changing the parameters governing such a system would allow the possibility of evolving materials with interesting properties by carrying out "mutation-selection" cycles on the functional outcomes. Although we are quite far from realizing such a vision in the laboratory, recent experimental advances in coating colloidal scale objects with specific glues (e.g. using complementary DNA strands) have suggested a set of theoretical models in which the possibilities of realizing these ideas can be explored in a controlled way. This talk will describe our ongoing efforts to explore these ideas using theory and simulation, and also small scale experiments.

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

**MAY 14, 2015**

**MICHEL DEVORET
**Yale University

Hosted by Isaac Chuang

"Implementing Cat-Codes in Josephson Quantum Circuits"

Physical systems usually exhibit quantum phenomena, such as state superposition and entanglement, only when they are sufficiently decoupled from a lossy environment. Paradoxically, a specially engineered interaction with the environment can become a resource for the generation and protection of quantum states. Moreover, this notion can be generalized to the confinement of a system into a manifold of quantum states that consists of all coherent superpositions of multiple stable steady states. It has now become practically feasible to confine the state of a harmonic oscillator to the quantum manifold spanned by two coherent states of opposite phases. In a recent experiment [1], we have observed a superposition of two coherent states, also known as a Schrodinger cat state, spontaneously squeeze out of vacuum, before decaying into a classical mixture. The dynamical protection of logical qubits built from Schrodinger cat states is based on an engineered driven-dissipative process in which photon pairs are exchanged rather than single photons. The recent class of experiments in which qubits are encoded using cat states opens a new avenue in quantum information processing with superconducting circuits.

*[1] Leghtas et al., Science 347, 853 (2015)*

Time: 4:00 pm

Place: Room 10-250

Refreshments @ 3:30 pm in 4-349 (Pappalardo Community Room)

*Last updated on April 27, 2015 1:30 PM*