March 6, 2019, 3 p.m.
Astrophysical background of gravitational waves: from cosmology to a new era of precision astrophysics
The astrophysical background of gravitational waves (AGWB) is made up by the incoherent superposition of gravitational wave signals emitted by a large number of resolved and unresolved astrophysical sources from the onset of stellar activity until today. I present a theoretical framework to fully characterize the AGWB in terms of energy density anisotropies and polarization and I show the first numerical predictions of the angular power spectra of anisotropies in different frequency bands and of cross-correlations with electromagnetic observables e.g. weak lensing and galaxy number counts. l then illustrate and discuss potential astrophysical implications of this study.
March 7, 2019, 3:30 p.m.
Tails of steady states
I will discuss collective modes which describe the approach to asymptotic equilibrium in steady states of strongly interacting many body systems. These modes can be thought of as time independent analogues of quasinormal modes, which imprint a spatial ringdown pattern on the steady state. I will construct these modes in both hydrodynamics and holography and demonstrate their role in holographic non-equilibrium steady states corresponding to steady flows past obstacles.
March 12, 2019, 1 p.m.
Evolution and signature of cosmological magnetic fields
Magnetic fields are an ubiquitous presence in our Universe but a comprehensive theory describing their generation and evolution is still missing. A broadly accepted paradigm wants these fields to be generated by the amplification of pre-existing weaker fields during the gravitational collapse accompanying structure formation. However the mechanisms behind the generation of these seeds are yet to be understood. An intriguing possibility is that these seed fields are generated in the pre-recombination Universe and then evolve passively until the epoch of structure formation. A clean way to prove the presence of primordial magnetic fields (PMF) is to look at Cosmic Microwave Background (CMB) anisotropies induced by these fields. In this talk, I will discuss the behavior of magnetic fields in the pre-recombination epoch and their imprints on the CMB spectra. I will review the current bounds coming from the combination of various CMB probes such as the Planck satellite and the South Pole Telescope. Furthermore I will also show what we expect to learn in the future from next generation CMB experiments. Finally I will discuss how magnetic fields evolves in the post-recombination regime and their impact on structure formation
March 13, 2019, 7:30 p.m.
Testing General Relativity with Cosmological Observations
General Relativity (GR) is immensely successful. With the late discovery of gravitational waves from black hole and neutron star mergers, it has passed all the tests with flying colors. But so far, all observations have mainly tested the vacuum equations of GR. The most important non-vacuum case, cosmology, is in agreement with GR only after the introduction of two otherwise unknown components, 'Dark Matter' and 'Dark Energy' which amount to about 96% of the total energy budget of the present Universe. This let people in the field question the validity of GR for cosmology. Might it be that GR is flawed on large, cosmological scales? Or in the presence of matter in general? But how can we test Einstein's equation in the presence of matter. Can't we simply move any modification of the Einstein tensor to the right hand side and call it a 'dark matter/energy' component? In my talk I shall discuss possible ways (partially) out of this dilemma. How to test both, the left and the right hand side of Einstein's equations with cosmological observations.
March 21, 2019, 3:30 p.m.
Hydrodynamic transport in the Poiseuille regime
David Rodriguez Fernandez
In the context of describing electrons in solids as a fluid in the hydrodynamic regime, we consider a flow of electrons in a channel of finite width, i.e.~a Poiseuille flow. The electrons are accelerated by a constant electric field. We develop the appropriate relativistic hydrodynamic formalism in 2+1 dimensions and show that the fluid has a finite dc conductivity due to boundary-induced momentum relaxation, even in the absence of impurities. We use methods involving the AdS/CFT correspondence to examine the system in the strong-coupling regime. We calculate and study velocity profiles across the channel, from which we obtain the differential resistance dV/dI. We find that dV/dI decreases with increasing current I as expected for a Poiseuille flow, also at strong coupling and in the relativistic velocity regime. In addition, dV/dI decreases when the coupling is increased. Furthermore, we examine the effect of the Hall-viscosity on the flow of a Fermi liquid. The liquid flows through a lead under the influence of constant electric and magnetic fields. We find that the induced Hall voltage receives an additional contribution due to Hall viscosity. We examine the general behavior of this contribution and show that its dependence on the wire width and density of the liquid can be used to differentiate it from the classical Hall effect. Finally, we present estimates of the Hall signal for GaAs in the hydrodynamic regime, which shows that this effect can be experimentally verified. ------------------------------------------------- Talk based on 1806.10635 and follow-up work involving parity breaking (to appear soon).
March 25, 2019, 1 p.m.
Chiral electrodynamics: from the early Universe to laboratories
March 27, 2019, 11:30 a.m.
Mixed axial-torsional anomaly in Weyl semimetals
March 27, 2019, 3 p.m.
Cosmology with neutral hydrogen intensity mapping
I will introduce the innovative technique of neutral hydrogen (HI) intensity mapping, which allows for precise cosmological measurements in the radio wavelength. After describing the current status of HI intensity mapping measurements, I will focus on forthcoming surveys using instruments like MeerKAT and the SKA to probe dark energy as well as galaxy evolution. My talk will also focus on the challenges that intensity mapping faces, and explain why cross-correlations with optical galaxy surveys will probably dominate the field for the next few years. Finally, I will introduce the GBT-eBOSS cross-correlation project, which hopes to provide the best measurements using HI intensity mapping data to date.
April 4, 2019, 3:30 p.m.
Surprises in holographic quantum field theories with a compact direction
Recently, there has been much interest in connections between energy and entropy in flat space QFTs. These connections are especially relevant in holographic QFTs where they can be proven geometrically in the bulk. However, when the QFT lives on a curved manifold, especially one with a compact direction, do things change? I will discuss two energy conditions which can be proven for holographic QFTS on flat spacetime but which can be violated when the manifold possesses a compact direction: the quantum null energy condition (QNEC) and the achronal averaged null energy condition (ANEC). In particular, I will describe bulk solutions whose dual quantum stress tensor provides explicit counterexamples to these energy conditions.
April 10, 2019, 4 p.m.
A class of minimally modified gravity theories
I will introduce a new class of gravity theories which is as good as GR in the sense that all of constraints are first class and therefore a graviton has only 2 polarisations and the structure of the theory at low energies is thus expected to be stable against quantum corrections. Applying one of examples in this class, i.e. the so-called square root gravity, and tracing the universe back in time, we find a de-sitter attractor with finite Hubble rate. Therefore, this theory is free from cosmological singularity at high energy density limit. A linear perturbation analysis reveals the weak gravitational coupling at such limit, in contrast to Einstein gravity.
April 11, 2019, 3:30 p.m.
The Sachdev-Ye-Kitaev model and large conformal Goldstone fluctuations
The fascinating Sachdev-Ye-Kitaev (SYK) model describing a large number of randomly interacting Majorana fermions represents an ultimate example of the AdS/CFT correspondence in 1+1 space-time dimensions. As pointed out by Kitaev, both the SYK model and its gravity dual possess an emergent conformal symmetry which, however, is spontaneously broken in the infra-red. As such, the soft (or Goldstone) mode in the spectrum of the model emerges which is known to be described by the so-called 'Schwarzian' action. In my talk, after a general exposition to the SYK model, I will concentrate on its quantum deeply infra-red limit where conformal Goldstone fluctuations start to play a paramount role. I will demonstrate how the 'Schwarzian' action can be mapped onto a 'Liouvillian' quantum mechanics and study a long-time limit of 2- and 4-point correlation functions of Majoranas. The range of new results predicted by such mapping encompasses universal power-law decays of correlators as well as an emergent Coulomb blockade physics in the 'complex' version of the SYK model reminiscent to that of conventional mesoscopic quantum dots.
April 16, 2019, 11:30 a.m.
Fidelity and visibility loss in Majorana qubits by entanglement with environmental modes.
n recent years, there has been a large experimental effort towards realizing Majorana qubits for quantum computation. This holds the promise of protection from local noise, since quantum information is stored in non-local degrees of freedom. In practice however, local noise may affect fidelity, and in this talk, I will discuss such an effect in an ideal Majorana qubit system in a topological superconductor, where there is no overlap between Majorana wave functions, by including bosonic modes due to an electromagnetic environment, which couple the Majorana manifold to above-gap continuum quasi-particles. This coupling causes the degenerate ground state of the topological superconductor to be dressed in a polaron-like manner by quasi-particle states and bosons, and the system to become gapless. Topological protection and hence full coherence is only maintained if the qubit is operated and read out within the low-energy spectrum of the dressed states. In particular, I will discuss reduction of fidelity and/or visibility if this condition is violated by a quantum-dot readout that couples to the bare (undressed) Majorana modes.
April 25, 2019, 3:30 p.m.
May 2, 2019, 3:30 p.m.
Hadrons under extreme conditions
Heavy-ion collisions investigate the strong force under the extreme conditions of high temperature and vanishing or small baryon density. While the quark-gluon plasma has been studied theoretically in great detail, the hadronic gas at lower temperatures is usually described using effective low-energy models. Here I'll report on work of the FASTSUM collaboration which aims to give a first-principle description at all temperatures, using lattice QCD simulations both below and above the deconfinement transition. Particular attention is given to baryons and to spectral changes at small but nonzero baryon chemical potential.
May 8, 2019, 3:30 p.m.
Primordial Black Holes during Inflation and the Non-Gaussian Regime
Primordial Black Holes (PBH) could be formed during inflation, possibly giving account for the dark matter in the Universe. The mass and abundances of the PBH depends on the shape of the collapsed regions as well as the statistics of the underlying random density field. These are usually determined assuming gaussian statistics for the inflaton perturbations. In this talk, I will show that this approximation fails for all single field models of inflation producing PBH known so far in the litterature. I will argue that a non-perturbative calculation of the non-gaussianities is necessary and present recent developments in this direction.
May 9, 2019, 3:30 p.m.
Eigenstate thermalization in the Sachdev-Ye-Kitaev model
The eigenstate thermalization hypothesis (ETH) proposes an explanation as to how closed unitary quantum systems can exhibit thermal behavior in pure states. In this work we examine the Sachdev-Ye-Kitaev model as well its IR limit as described by an effective Schwarzian action. We show that, as expected from evidence found using exact diagonalization, the model satisfies ETH. In the Schwarzian limit, we study a specific class of states created by heavy operators and find that they only show a weak form of ETH. Based on arXiv:1707.08013 (with J. Sonner), arXiv:1903.00478 (with P. Nayak and J. Sonner) and work in progress (with P. Nayak and J. Sonner)
May 13, 2019, 11:30 a.m.
Electronic states of pseudospin-1 fermions in lattice ribbons in a magnetic field
May 22, 2019, 11:30 a.m.
Vacancies in Graphene : Dirac Physics and Fractional Vacuum Charges
May 22, 2019, 3:30 p.m.
Probing the Physics of the Early Universe with Gravitational Interferometers
Cosmological inflation predicts the existence of a stochastic background of gravitational waves, whose features depend on the model of inflation under consideration. There exist well motivated frameworks predicting an enhancement of the primordial gravity wave spectrum at frequency scales testable with gravitational wave experiments, with distinctive features as parity violation and non-Gaussianity. I will explain the properties of such scenarios, and their distinctive predictions for what respect the gravity wave power spectrum and bispectrum. I will then discuss how to quantitatively test these predictions with current and future interferometers.
May 22, 2019, 7:30 p.m.
The second kind of impossible
Quasicrystals are exotic forms of matter with symmetries that were once thought to be mathematically impossible. The first known examples were synthesized in the laboratory over 35 years ago, but could Nature have beaten us to the punch? This talk will describe the decades-long search to answer this question, resulting in one of the strangest scientific stories you are ever likely to hear.
May 28, 2019, 1 p.m.
What use are the Baryon Acoustic Oscillations? Why the Linear Point standard ruler?
Baryon Acoustic Oscillations (BAO) are one of the most useful and used cosmological probes to measure cosmological distances independently of the underlying background cosmology. However, in the current measurements, the inference is done using a theoretical clustering correlation function template where the cosmological and the non-linear damping parameters are kept fixed to fiducial LCDM values. How can we then claim that the measured distances are model-independent and so useful to select cosmological models? Motivated by this compelling question we introduce a rigorous tool to measure cosmological distances without assuming a specific background cosmology: the “Purely-Geometric-BAO”. I will explain how to practically implement this tool with clustering data. This allows us to quantify the effects of the standard measurements’ assumptions. I will then focus on a new approach to the problem that leverages a novel BAO cosmological standard ruler: the “Linear Point”. Its standard ruler properties allow us to estimate cosmological distances without the need of modeling the poorly-known late-time nonlinear corrections to the linear correlation function. Last but not least, it also provides smaller statistical uncertainties with respect to the correlation function template fit.
- May 28, 2019, 1 p.m. CS Stefano Anselmi (Observatoire de Paris and IAP) What use are the Baryon Acoustic Oscillations? Why the Linear Point standard ruler?
June 5, 2019, 11:30 a.m.
Coherent electrical control of a single high spin nucleus in silicon
Nuclear electric resonance (NER) enables transitions of a high spin nucleus by modulating its electrical quadrupole interaction with an electric field. In this talk I will show how we found this effect in our single 123-Sb donor device in silicon, with nuclear spin of size 7/2. We demonstrate, for the first time, coherent, purely electrical control of a single high spin nucleus. I will share our theoretical understanding of the microscopic mechanism at play in our device, based on analytical approximations, density functional theory calculations and finite element simulations. Finally, I will discuss future research directions exploiting this versatile system.
- June 5, 2019, 11:30 a.m. Vincent Mourik (Sidney) Coherent electrical control of a single high spin nucleus in silicon
June 5, 2019, 7:30 p.m.
Quantum computing past, present, and future
Quantum computers store and process information at the level of individual atoms, photons, and spins. The strange and counter-intuitive nature of quantum mechanics allows quantum computers to perform computations in ways that classical computers can't. This talk reviews the history of quantum computing, including how they can be constructed, and why quantum algorithms give exponential speed ups over their classical counterparts. The talk presents the current state of the art in experimental realizations of quantum computers and quantum algorithms, and discusses various possible futures for quantum information processing.
- June 5, 2019, 7:30 p.m. CE Seth Lloyd (MIT) Quantum computing past, present, and future
June 6, 2019, 11:30 a.m.
Large contribution of Fermi arcs to the conductivity of Weyl metals
- June 6, 2019, 11:30 a.m. Maxim Breitkreiz (Berlin) Large contribution of Fermi arcs to the conductivity of Weyl metals
June 11, 2019, 2 p.m.
Quantum computing: past, present, and future: Lecture 1
- June 11, 2019, 2 p.m. Seth Lloyd (Lorentz Professor) Quantum computing: past, present, and future: Lecture 1
June 18, 2019, 2 p.m.
Quantum computing: past, present, and future: Lecture 2
- June 18, 2019, 2 p.m. Seth Lloyd (Lorentz Professor) Quantum computing: past, present, and future: Lecture 2
June 25, 2019, 2 p.m.
Quantum computing: past, present, and future: Lecture 3
- June 25, 2019, 2 p.m. Seth Lloyd (Lorentz Professor) Quantum computing: past, present, and future: Lecture 3
Aug. 30, 2019, 4 p.m.
Gratama room, Lorentz Center
Professor Hans van Leeuwen
16:00 Carlo Beenakker: Welcome Wim van Saarloos - Hans van Leeuwen and Statistical Physics, 50+ years of contributions Daniel Bonn - The Physics of Ice Skating Henk Lekkerkerker - CO2 Friend of Foe 17.30 reception
- Aug. 30, 2019, 4 p.m. Professor Hans van Leeuwen (Instituut-Lorentz) Golden Jubilee
Sept. 25, 2019, 7:30 p.m.
- Sept. 25, 2019, 7:30 p.m. CE François Graner (Paris Diderot University) TBA
Oct. 2, 2019, 7:30 p.m.
- Oct. 2, 2019, 7:30 p.m. CE Christopher Jarzynski (University of Maryland) TBA
Nov. 6, 2019, 7:30 p.m.
Jo van den Brand
- Nov. 6, 2019, 7:30 p.m. CE Jo van den Brand (Nikhef/Virgo) TBA
LS = Lorentz Seminar, Casimir room (276), Oort building
CE = Colloquium Ehrenfestii, De Sitter lecture room, Oort building
SBM = Soft & Biological Matter Seminar, Casimir room (276), Oort building
CS = Cosmology Seminar, Casimir room (276), Oort building
ST = String Theory Seminar, Casimir room (276), Oort building