Center for Theoretical Physics

Warsaw, Poland

Center for Theoretical Physics

Warsaw, Poland
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News Article | April 17, 2017

Parameter regions with different behavior of the classical bound for an XXZ-like Hamiltonian with two parameters. Credit: ICFO Classical correlations are part of our everyday life. For instance, if one always puts on a pair of socks of the same color and shape, looking at the color or shape of one sock determines the color or shape of its pair. Even more, by observing the color and shape of one sock and we can simultaneously know the color and shape of the other one. In the quantum realm, Heisenberg's uncertainty principle states that accurately measuring a pair of properties of an atom puts a limit to the precision of measurement you can obtain on the same properties of another atom. Therefore, if the socks are said to be entangled, observing the color of one sock would allow us to predict the color of the other. However, if we also observe the shape of the sock, this would "disturb" the color, making it completely unpredictable to a certain extent. This weird "synchronization" between particles is defined as quantum entanglement, and is one of the intrinsic features of the quantum world. In nature, there exists a much stranger form of so-called nonlocal correlations, which are manifested by some entangled states between atomic particles. By making the minimal assumptions that properties of objects (shape/color) exist regardless of our knowledge of them, and that information cannot propagate instantaneously, one finds that quantum physics can generate correlations that are incompatible with these two apparently reasonable principles. Although extremely fascinating to study, these nonlocal correlations are very hard to characterize in systems composed of many particles for three reasons. First, classical correlations are mathematically very complex to study; second, quantum many-body states are very complex to describe due to the exponential growth of their described states; and, third, currently available experimental techniques are rather limited, constraining the measurements that can be performed in the laboratory. In order to explore the role of nonlocal correlations in many-body quantum systems, one thus has to address these three problems at the same time. In a recent paper published in Physical Review X, a team of scientists from MPQ in Munich, ICFO in Barcelona, University of Innsbruck and the Center for Theoretical Physics of the Polish Academy of Sciences have proposed a simple test to study nonlocal correlations in quantum many-body systems. They have studied whether nonlocal correlations appear in natural systems as ground states of some spin Hamiltonians, such as electrons (described by their spin degree of freedom) in a system of one spatial dimension. By combining numerical and analytical results, they have shown that some Hamiltonians that have been studied by physicists for some decades have a state of minimal energy that can display nonlocal correlations. As the first author, Jordi Tura, has commented, "We provide a set of tools to study a problem that has always been complicated on its own. The techniques we developed are much simpler than previous ones. If you wanted to implement them in the lab, you would just need to ensure that the system is prepared in a state of sufficiently low energy." The results sheds some light onto this fascinating problem, hopefully sparking further progress in our understanding of nonlocality in quantum many-body systems. More information: J. Tura, G. De las Cuevas, R. Augusiak, M. Lewenstein, A. Acín, J. I. Cirac, Energy as a detector of nonlocality of many-body spin systems, Phys. Rev. X, 7, 021005 (2017)

Awad A.,Center for Theoretical Physics | Awad A.,Ain Shams University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We use a phase space approach to study possible consequences of fixed points in a single fluid flat Friedmann-Lemaître-Robertson-Walker (FLRW) models with pressure p(H), where H is the Hubble parameter. One of these consequences is that a fluid with a differentiable pressure, i.e., a finite adiabatic speed of sound, reaches a fixed point in an infinite time and has no finite-time singularities of types I, II, and III described by Nojiri, Odintsov, and Tsujikawa. It is impossible for such a fluid to cross the phantom divide in a finite time. We show that a divergent dp/dH, or the speed of sound, is a necessary but not sufficient condition for phantom crossing. We use pressure properties, such as asymptotic behavior and fixed points, to qualitatively describe the entire behavior of a solution in flat FLRW models. We discuss FLRW models with bulk viscosity η∼ρr, in particular, solutions for r=1 and r=1/4 cases, which can be expressed in terms of the Lambert-W function. The last solution behaves as either a nonsingular phantom fluid or a unified dark fluid. Using causality and stability constraints, we show that the universe must end as a de Sitter space. Relaxing the stability constraint leads to a de Sitter universe, an empty universe, or a turnaround solution that reaches a maximum size and then recollapses. © 2013 American Physical Society.

Ferrari R.,National Institute of Nuclear Physics, Italy | Ferrari R.,Center for Theoretical Physics
Acta Physica Polonica B | Year: 2013

On the basis of extended simulations, we provide some results concerning the spectrum of Massive SU(2) Yang-Mills on the lattice. We study the "time" correlator of local gauge invariant operators integrated over the remaining three dimensions. The energy gaps are measured in the isospin I = 0,1 and internal spin J = 0,1 channels. No correlation is found in the I = 1, J = 0 channel. In the I = 1, J = 1 channel and far from the critical mass value mc, the energy gap roughly follows the bare valuem (vector mesons). In approaching the critical value mc at β fixed, there is a bifurcation of the energy gap: one branch follows the value m, while the new is much larger and it shows a more and more dominant weight. This phenomenon might be the sign of two important features: the long range correlation near the fixed point at β → ∞ implied by the low energy gap and the screening (or confining) mechanisms across the m = mc associated to the larger gap. The I = 0, J = 0,1 gaps are of the same order of magnitude, typically larger than the I = 1, J = 1 gap (for m > mc). For m ∼ m c, both I = 0 gaps have a dramatic drop with minima near the value m. This behavior might correspond to the formation of I = 0 bound states both in the J = 0 and J = 1 channels.

Sanders J.L.,Center for Theoretical Physics | Sanders J.L.,Institute of Astronomy | Binney J.,Center for Theoretical Physics
Monthly Notices of the Royal Astronomical Society | Year: 2015

We extend models of our Galaxy based on distribution functions that are analytic functions of the action integrals to extended distribution functions (EDFs), which have an analytic dependence on metallicity as well. We use a simple, but physically motivated, functional forms for the metallicity of the interstellar medium as a function of radius and time and for the star formation rate, and a model for the diffusion of stars through phase space to suggest the required functional form of an EDF.We introduce a simple prescription for radial migration that preserves the overall profile of the disc while allowing individual stars to migrate throughout the disc. Our models explicitly consider the thin and thick discs as two distinct components separated in age.We show how an EDF can be used to incorporate realistic selection functions in models, and to construct mock catalogues of observed samples. We show that the selection function of the Geneva-Copenhagen Survey (GCS) biases in favour of young stars, which have atypically small random velocities. With the selection function taken into account our models produce good fits of the GCS data in chemo-dynamical space and the Gilmore & Reid (1983) density data. From our EDF, we predict the structure of the Sloan Extension for Galactic Understanding and Exploration G-dwarf sample. The kinematics are successfully predicted. The predicted metallicity distribution has too few stars with [Fe/H]≃-0.5 dex and too many metal-rich stars. A significant problem may be the lack of any chemical-kinematic correlations in our thick disc. We argue that EDFs will prove essential tools for the analysis of both observational data and sophisticated models of Galaxy formation and evolution. © 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Alicea J.,California Institute of Technology | Fendley P.,Center for Theoretical Physics
Annual Review of Condensed Matter Physics | Year: 2016

We concisely review the recent evolution in the study of parafermions-exotic emergent excitations that generalize Majorana fermions and similarly underpin a host of novel phenomena. First we generalize the intimate connection between the -symmetric Ising quantum spin chain and Majorana fermions to -symmetric chains and parafermions. In particular, we highlight how parafermion chains host a topological phase featuring protected edge zero modes. We then tour several blueprints for the laboratory realization of parafermion zero modes-focusing on quantum Hall superconductor hybrids, quantum Hall bilayers, and two-dimensional topological insulators-and describe striking experimental fingerprints that they provide. Finally, we discuss how coupled parafermion arrays in quantum Hall architectures yield topological phases that potentially furnish hardware for a universal, intrinsically decoherence-free quantum computer. © Copyright 2016 by Annual Reviews. All rights reserved.

News Article | February 27, 2017

Zachary Hulcher was once set on becoming a lawyer. In high school, he took part in mock trials and competed in youth judicial, playing the role of legal counsel and presenting cases in front of a student jury. He says his inspiration came partly from the television show Law and Order: “There’s drama, there’s action, you send people to jail, and you get to argue with people — and I loved arguing with people.” But all that changed one day, sometime during his junior year, when he happened to flip through his physics textbook. In an idle moment at school, he turned to the very back of the book and started to read the chapter about special relativity. Physics, he discovered, put mathematics and science into an almost fantastical perspective. “Ideas that come out of that one chapter are time travel, atomic bombs, things warping when they go really fast, and all these things that shouldn’t be real, but are,” Hulcher says. Hulcher is currently a senior at MIT, majoring in physics as well as computer science and electrical engineering, with a minor in math. “I love the creative process and figuring out how elegant solutions to real problems arise out of seeming chaos,” he says. He is a recipient of the 2017 Marshall Scholarship, awarded each year to up to 40 U.S. students who will pursue graduate degrees at universities in the United Kingdom. Next year, Hulcher will be working toward a PhD in high energy physics at Cambridge University, where he hopes to work on both experimental and theoretical problems of the Standard Model of particle physics, which governs every aspect of the known universe except for gravity. Hulcher was born and raised in Montgomery, Alabama. His mother and father are managers for Alabama’s environmental management agency. Hulcher grew up playing basketball with his younger brother in the family’s backyard. The brothers, who towered over their classmates — Hulcher is 6 feet 4 inches tall and his “little” brother, Jacob, is 6 feet 8 inches — joined their church league, and eventually played for their middle and high school teams. Along with basketball, Hulcher played football and was on the track and field team, balancing an unrelenting schedule of games and practices with an increasingly challenging course load. Hulcher attended the Montgomery Catholic Preparatory School System from kindergarten through high school in Montgomery, where he was valedictorian and a National Merit Scholar. In his freshman year he began taking math and physics classes with Joe Profio, a teacher who, recognizing that Hulcher was one of the top students in his class, urged him to join the school’s math teams. Hulcher soon found himself taking long drives to math competitions across the state with Profio and his classmates. During those drives, Profio would talk about math at a deeper level than he could present in class, and Hulcher credits his passion for physics and math to these inspiring talks. “Our conversations obliterated the idea that the only beauty in the world is found in an imaginary place in a book — beauty was all around me, if I would only look through the right lens,” Hulcher says. It was around that time that Hulcher says “the wheels started cranking to do science.” The answer to how and where to direct this newfound momentum came from an unlikely source, another TV show. “I was watching NCIS one day, and one of the characters is from MIT, and I thought, ‘I’m starting to like more science. I should apply there,’ and I did,” Hulcher recalls. When Hulcher set foot on the campus for the first time — also the first time he had been anywhere north of Washington, D.C. — he was immediately drawn to the physics seminars held during Campus Preview Weekend. “I remember an event called something like ‘physics til you drop,’ and two students were standing at a blackboard, doing physics until 5 or 6 am, long past when I could stay awake,” Hulcher says. “People would ask them questions about quantum mechanics, string theory, general relativity, anything, and they would try to answer them on the board. I was pretty hooked.” He quickly landed on physics as a major but also chose computer science and electrical engineering, a decision based largely on conversations with his roommate, who was also majoring in the subject. When Hulcher took classes that explored quantum computing — the idea that quantum elements such as elementary particles can perform certain calculations vastly more efficiently than classical computers — he realized “all of computing is not just a computer science problem. It’s a physics problem. That’s just cool.” In the summer following his sophomore year, Hulcher traveled to Geneva, Switzerland, to work at the Compact Muon Solenoid experiment (CMS) at CERN’s Large Hadron Collider, the world’s largest and most powerful particle accelerator. There, he helped to implement an alarm system that monitors the accelerator’s major systems and distributes information to key people in the event of a failure. He returned again the following summer, this time as a theorist. The LHC uses giant magnets to steer beams of atoms, such as lead ions, toward each other at close to the speed of light. Hulcher, working as a research assistant with Krishna Rajagopal of MIT's Department of Physics and the Center for Theoretical Physics, was interested in the hot plasma of quarks and gluons produced when two lead ions collide. “The plasma doesn’t last very long before it returns to some other state of matter,” Hulcher says. “You don’t even have time to blast it with light to see it; it would just disappear before the light got there. So you need to use events inside it to study it.” Those events involve jets of particles that spew out from the plasma following a collision between two lead ions. Hulcher worked with Rajagopal and Daniel Pablos, a University of Barcelona graduate student, to help implement a model for how these jets of particles propagate through the resulting plasma. Hulcher recently helped to present the team’s results at a workshop in Paris and is finishing up a paper to submit to a journal — his first publication. In addition to his research work, Hulcher has racked up a good amount of teaching experience. As a teaching assistant for MIT’s Department of Physics, he has graded weekly problem sets for classes in classical mechanics and electricity and magnetism. He tutors fellow students in electrical engineering and computer science subjects, and he has spent the last year as eligibles chair of the MIT chapter of the engineering honor society Tau Beta Pi. Through the MIT International Science and Technology Initiatives (MISTI), Hulcher has traveled around the world, to Italy, Mexico, and most recently, Israel, teaching students subjects including physics, electrical engineering, and entrepreneurship. Of all the relationships he’s developed through his time at MIT, he counts those with most of his teammates as some of the strongest. Hulcher joined MIT’s football team as a freshman offensive lineman; he says he will remember hanging out on long nights, p-setting with his friends from the football team. He will also remember MIT as a really long rollercoaster, he says. As for what’s next, Hulcher says the plan for now is “to keep liking physics.” If that happens, he hopes to become a researcher and professor, to help students see the world through physics. “I fell in love with physics,” Hulcher says. “I appreciate light bouncing off a mirror, and smoke billowing up, and light moving through it in a different way. I appreciate looking up at the stars and thinking about what’s out there. The small things I took for granted when I didn’t know much about them, I appreciate now. Everything is just a little prettier.”

Ferrari R.,Center for Theoretical Physics | Ferrari R.,University of Milan
Acta Physica Polonica B | Year: 2012

In the present paper, we study the limit of zero mass in non-Abelian gauge theories both with Higgs mechanism and in the nonlinear realization of the gauge group (Stückelberg mass). We argue that in the first case the longitudinal modes undergo a metamorphosis process to the Goldstone scalar modes, while in the second, we guess, a decoupling process associated to a phase transformation. The two scenarios yield strikingly different behaviors at high energy, mainly ascribed to the presence of a massless Higgs doublet among the physical modes in the case of Higgs mechanism (i.e. not only the Higgs boson). The aim of this work is to show that the problem of unitarity at high energy in non-Abelian gauge theory with no Higgs boson can open new perspectives in quantum field theory.

Krefl D.,Center for Theoretical Physics
Journal of High Energy Physics | Year: 2014

The β-ensemble with cubic potential can be used to study a quantum particle in a double-well potential with symmetry breaking term. The quantum mechanical perturbative energy arises from the ensemble free energy in a novel large N limit. A relation between the generating functions of the exact non-perturbative energy, similar in spirit to the one of Dunne-Ünsal, is found. The exact quantization condition of Zinn-Justin and Jentschura is equivalent to the Nekrasov-Shatashvili quantization condition on the level of the ensemble. Refined topological string theory in the Nekrasov-Shatashvili limit arises as a large N limit of quantum mechanics. © 2014 The Authors.

Janiuk A.,Center for Theoretical Physics | Czerny B.,pernicus Astronomical Center
Monthly Notices of the Royal Astronomical Society | Year: 2011

We discuss two important instability mechanisms that may lead to the limit-cycle oscillations of the luminosity of the accretion discs around compact objects: ionization instability and radiation pressure instability. Ionization instability is well established as a mechanism of X-ray novae eruptions in black hole binary systems, but its applicability to active galactic nuclei (AGN) is still problematic. Radiation pressure theory has still a very weak observational background in any of these sources. In this paper, we attempt to confront the parameter space of these instabilities with the observational data. At the basis of this simple survey of sources properties, we argue that the radiation pressure instability is likely to be present in several Galactic sources with the Eddington ratios being above 0.15 and in AGN with the Eddington ratio above 0.025. Our results favour the parametrization of the viscosity through the geometrical mean of the radiation and gas pressure in both Galactic sources and AGN. More examples of the quasi-regular outbursts in the time-scales of 100 s in Galactic sources and hundreds of years in AGN are needed to formulate firm conclusions. We also show that the disc sizes in the X-ray novae are consistent with the ionization instability. This instability may also considerably influence the lifetime cycle and overall complexity in the supermassive black hole environment. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Krefl D.,Center for Theoretical Physics
Letters in Mathematical Physics | Year: 2016

We invoke integrals of Mellin–Barnes type to analytically continue the Gopakumar–Vafa resummation of the topological string free energy in the string coupling constant, leading to additional non-perturbative terms. We also discuss in a similar manner the refined and Nekrasov–Shatashvili limit version thereof. The derivation is straight-forward and essentially boils down to taking residue. This allows us to confirm some related conjectures in the literature at tree-level. © 2016 Springer Science+Business Media Dordrecht

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