The CTP Lunch Club meets every Friday at noon in the Cosman seminar room, 6C-442 (provided that there are sufficient speakers). Pizza will be provided begining at 11:40am.
The seminars are designed for graduate students and should be accessible to all students. First year students are particularly encouraged to attend so that they may learn about research being performed in the CTP. The goal is learning, and to encourage participation, the seminars will be for students only.
Email notification of the club will be sent to the ctp-all, ctp-postdocs and ctp-students email lists as appropriate. If you wish to speak, or have suggestions about speakers and/or possible workshop topics, please contact the organizers, Yonatan Kahn at ykahn[at]mit[dot]edu, Daniel Kolodrubetz at dkolodru[at]mit[dot]edu, and Daniel Roberts at drob[at]mit[dot]edu.
Dynamics and universality in quantum quenches with ultracold atom
Which tools can we count-on to study the dynamics of interacting quantum systems following a quantum quench? I will open my talk with a (biased) review on recent experiments with ultracold atoms. I will than move to the theoretical efforts to explain them, focusing on the concepts of quantum criticality and universality. Finally, I will present our recent predictions for a quantum quench in which two independent condensates are suddenly coupled. Combining numerics, semi-classics, and field-theories, we find that appropriately rescaled physical observables follow a universal time-evolution. Using this method, we can hope to describe the dynamics of many-body quantum systems without knowing their microscopic details.
Faculty and post-docs are welcome.
Dynamics of broken symmetry
I will explore the far from equilibrium response of a superfluid using the gauge-gravity duality (also known as `AdS/CFT'). This duality, mapping a quantum critical theory to one involving gravity, has emerged in recent years as a powerful tool in the study of strongly coupled systems. Using this approach I will establish the dynamical phase diagram corresponding to quantum quenches of the order parameter source field. The resulting setup can be solved in great detail even far from equilibrium and gives rise to three distinct dynamical regimes (phases). I will end with some comments about the genericity of this behavior found for time-reversal invariant systems with broken symmetry.
Faculty and post-docs are welcome.
Wigner solves M-theory
Despite the technical sounding abstract, apart from a brief M-theory motivation, the talk will be accessible to all graduate students. We will be exploring ordinary N-dimensional integrals.
The low energy field theory on N coincident M2-branes is ABJM theory, which by AdS/CFT is dual to M-theory on AdS_{4} x S^{7}. Supersymmetric localization reduces the partition function of ABJM theory on S^{3} to a matrix model problem. (By AdS/CFT this is equal to the partition function of M-theory.) In the large-N limit this matrix model can be solved by two very different methods introduced by Wigner:
1.) Taking the continuum limit of the saddle point equations reminiscent of the derivation of Wigner's Semicircle Law,
2.) Determining the free energy of an equivalent non-interacting fermion system in the semiclassical expansion developed by Wigner.
The results provide some of the most detailed information about M-theory we have currently.
Faculty and post-docs are welcome.
Non-linear tidal interactions in close binary systems
Tidal forces in compact binaries can alter the orbital evolution of the system. Typically, these interactions are described by a network of standing waves within a star linearly driven by the external tidal potential. However, non-linear fluid interactions within stars can introduce instabilities that will excite standing waves away from their linear saturation values. These interactions come from over-lap integrals between the linear modes and cause instabilities including parametric (3-mode) instabilities as well as more general, off-resonance non-linear driving. These large non-linear modal networks may be important for stellar binaries, compact-object coalescence signals in gravitational-wave detectors, and exo-planet populations. We study the non-linear coupling between networks of g-modes in sun-like stars in order to estimate the saturation of these non-linear modal networks and estimate the effect on orbital dynamic.
Faculty and post-docs are welcome.
Psi-Epistemic Theories: The Role of Symmetry
Tidal forces in compact binaries can alter the orbital
Formalizing an old desire of Einstein, "psi-epistemic theories" try to reproduce the predictions of quantum mechanics, while viewing quantum states as ordinary probability distributions over underlying objects called "ontic states." Regardless of one's philosophical views about such theories, the question arises of whether one can cleanly rule them out, by proving no-go theorems analogous to the Bell Inequality. In the 1960s, Kochen and Specker (who first studied these theories) constructed an elegant psi-epistemic theory for Hilbert space dimension d=2, but also showed that any deterministic psi-epistemic theory must be "measurement contextual" in dimensions 3 and higher. Last year, the topic attracted renewed attention, when Pusey, Barrett, and Rudolph (PBR) showed that any psi-epistemic theory must "behave badly under tensor product." In this paper, we prove that even without the Kochen-Specker or PBR assumptions, there are no psi-epistemic theories in dimensions d>=3 that satisfy two reasonable conditions: (1) symmetry under unitary transformations, and (2) "maximum nontriviality" (meaning that the probability distributions corresponding to any two non-orthogonal states overlap). On the other hand, we also show the surprising result that without the symmetry restriction, one can construct maximally-nontrivial psi-epistemic theories in every finite dimension d.
Faculty and post-docs are welcome.
Entanglement and Quantum Computation in the 1D Quantum Ising Model
The 1D Ising model, and its extension known as the XY chain, are characterized by free fermionic excitations. Despite this apparent simplicity, they present a rich phase diagram and constitute a wonderful toy model that has intrigued theoretical physicists for decades. This interest, of course, extended to the study of its entanglement entropy. I will discuss the analytical results obtained for the Renyi entropies through exact solutions in the thermodynamic limit, showing how the Conformal Field Theory prediction is recovered in the scaling limit close to the Quantum Phase Transitions. I will also show the peculiar behavior of the entropies close to a bi-critical point, where the CFT is not applicable, and where one observes an essential singularity, for which infinitesimal changes in the parameters of the model can trigger macroscopic differences in the entanglement. Finally, I will discuss how the entanglement entropy can be used to characterize the different computational power (under the adiabatic quantum algorithm) of the different phases of the model and I will argue that this difference can be traced in the existence of (Majorana) edge states, as shown by Kitaev.
Faculty and post-docs are welcome.
Critical Phenomena in Supercooled Liquids
I will first introduce salient features of supercooled liquids, calling for theoretical explanation. In particular there seem to exist novel critical phenomena hidden in the physics of supercooled liquids. To this end, I will present an effective field theory approach to quantitatively capture universal aspects of such phenomena.
Faculty and post-docs are welcome.
Classical Space-times from the S-Matrix
We show that classical space-times can be derived directly from the S-matrix for a theory of massive particles coupled to a massless spin two particle. As an explicit example we derive the Schwarzchild space-time as a series in ${G}_{N}$. At no point of the derivation is any use made of the Einstein-Hilbert action or the Einstein equations. The intermediate steps involve only on-shell S-matrix elements which are generated via BCFW recursion relations and unitarity sewing techniques. The notion of a space-time metric is only introduced at the end of the calculation where it is extracted by matching the potential determined by the S-matrix to the geodesic motion of a test particle. Other static space-times such as Kerr follow in a similar manner. From a technical standpoint our methodology can also be utilized to calculate quantities relevant for the binary inspiral problem more efficiently then the more traditional Feynman diagram methodology.
Faculty and post-docs are welcome.
Critical Phenomena in Supercooled Liquids
I will first introduce salient features of supercooled liquids,
calling for theoretical explanation. In particular there seem to exist
novel critical phenomena hidden in the physics of supercooled liquids.
To this end, I will present an effective field theory approach to
quantitatively capture universal aspects of such phenomena.
Technical References:
(1) S. Yaida, "Effective Field Theory for Supercooled Liquids," arXiv:1212.0857 [cond-mat.dis-nn].
(2) E. Dyer, J. Lee, and S. Yaida, "Critical Exponents for Supercooled Liquids," arXiv:1302.2917 [cond-mat.dis-nn].
Nontechnical Reference:
(a) http://www.youtube.com/watch?v=Rsu30g6Rmp4
Faculty and post-docs are welcome.
M-theory and Quantum Mechanics
We will begin with an introduction to the conjecture that M-theory is holographically dual to a supersymmetric quantum mechanics. Although numerous checks have been done on this duality in the past 15 years, most of them crucially depend on the power of supersymmetry. The thermal free energy, on the other hand, is a non-supersymmetric quantity that should match if the duality is true. We will describe the strategy to compute the free energy for various supersymmetric quantum mechanics by solving the truncated Schwinger-Dyson equations.
Faculty and post-docs are welcome.
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