Scientific Series

The precise determination of the entanglement of an interacting quantum many-body systems is now appreciated as an indispensable tool to identify the fundamental character of the ground state of such systems. This is particularly true for unconventional ground states harbouring non-local topological order or so-called quantum spin liquids that evade a standard description in terms of correlation functions. With the entanglement entropy emerging as one of the central measures of entanglement, recent progress has focused on a precise characterization of its scaling behaviour, in particular in the determination of (subleading) corrections to the prevalent boundary-law. 

In the past years, much progress has been made for certain spin, bosonic, and even fermionic quantum many-body systems. However, a large class of interacting models is thought to be exempt from numerical studies due to the fermion sign problem. At its heart, it occurs when the statistical weights in the simulation are positive and negative resulting in an exponential scaling of the algorithm instead of a polynomial one. In this work, we study the connection of the sign problem and the entanglement entropy using Determinantal Quantum Monte Carlo, the method of choice for unbiased, large-scale simulations of fermionic systems. We show that there is a strong correlation between the behavior of the entanglement entropy and the sign problem and that the particular structure of the ‘observable’ entanglement entropy to some extent allows to handle the sign problem much better than for usual correlation functions.

Bell inequalities bound the strength of classical correlations arising between outcomes of measurements performed on subsystems of a shared physical system. The ability of quantum theory to violate Bell inequalities has been intensively studied for several decades. Recently, there has been an increased interest in studying physical correlations beyond the scenario of Bell inequalities, to more general network structures involving many sources of physical states and observers that may be measuring on subsystems of independent states. Much less is known about the nature of physical correlations in networks as compared to standard Bell inequalities. In this talk we will discuss the motivation and interest for studying such network correlations, review the recent progress in understanding such networks, and discuss the many open questions and new possible directions for research on this topic.

I will discuss ongoing work developing Hamiltonian truncation methods for studying strongly-coupled IR physics originating from a perturbed UV conformal field theory. This method uses a UV basis of conformal Casimir eigenstates, which is truncated at some maximum Casimir eigenvalue, to approximate the low energy spectrum of the IR theory. So far, such methods have been limited to theories in 2D, and I will present a new framework for generalizing this approach to higher dimensions. Focusing specifically on the case of scalar fields in 3D, I will then show tests of this framework by comparing with known analytic results at weak coupling and in the O(N) model at large-N, before discussing the possible application to strongly-coupled systems like the 3D Ising model.

The Advanced LIGO observatories have successfully completed their first science run. Data were collected from September 2015 to January 2016, with a sensitivity a few times better than initial instruments in the hundreds of Hertz band. In this talk I will describe the searches for gravitational-wave transients performed during the first few weeks of the science run. Furthermore, I will describe the methods devised to characterize transient gravitational-wave sources and their applications in the advanced gravitational-wave detector era.

Precision atom interferometry is poised to become a powerful tool for discovery in fundamental physics.  Towards this end, I will describe recent, record-breaking atom interferometry experiments performed in a 10-meter drop tower that demonstrate long-lived quantum superposition states with macroscopic spatial separations.  The potential of this type of sensor is only beginning to be realized, and the ongoing march toward higher sensitivity can enable a diverse science impact, including laboratory tests of general relativity and the equivalence principle, searches for dark matter, probes of quantum mechanics, and detection of gravitational waves.  With the recent LIGO results, gravitational wave astronomy is particularly compelling since it provides a new window into the universe, collecting information about astrophysical systems and cosmology that is difficult or impossible to acquire by other methods.  Atom interferometric gravitational wave detection offers a number of advantages over traditional approaches, including simplified detector geometries, access to low frequencies that are complementary to LIGO, and substantially reduced antenna baselines.

Quantum theory can be understood as a theory of information processing in the circuit framework for operational probabilistic theories. This approach presupposes a definite casual structure as well as a preferred time direction. But in general relativity, the causal structure of space-time is dynamical and not predefined, which indicates that a quantum theory that could incorporate gravity requires a more general operational paradigm. In this talk, I will describe recent progress in this direction. First, I will show how relaxing the assumption that local operations take place in a global causal structure leads to a generalized framework that unifies all signaling and non-signaling quantum correlations in space-time via an extension of the density matrix called the process matrix. This framework also contains a new kind of correlations incompatible with any definite causal structure, which violate causal inequalities, the general theory of which I am going to present. I will then present an extension of the process matrix framework, in which no predefined causal structure is assumed even locally. This is based on a more general, time-neutral notion of operation, which leads to new insights into the problem of time-reversal symmetry in quantum mechanics, the meaning of causality, and the fact that we remember the past but not the future. In the resultant generalized formulation of quantum theory, operations are associated with regions that can be connected in networks with no directionality assumed for the connections. The theory is compatible with timelike loops and other acausal structures.

I will discuss a class of non-compact solutions to the Strominger-Hull system, the first order system of equations for preserving N=1 supersymmetry in heterotic compactifications to four dimensions. The solutions consists of the conifold and its Z2 orbifold with Abelian gauge fields and non-zero three-form flux. The heterotic Bianchi Identity is solved in a large charge limit of the gauge fields, where it is shown that the topological term p1(TX) can be consistently neglected. At large distances, these solutions are locally Ricci-flat. For a given flux, the family of solutions has three real parameters, the size of the pair of two spheres in the IR and the dilaton zero mode. There exists an explicit analytic solution for the decoupled near horizon region where for a given flux, the size of the cycles is frozen and the only parameter is the dilaton zero mode. This near horizon region also has an exactly solvable worldsheet CFT. When one of the two cycles has vanishing size the near horizon region disappears, but a solution on the unorbifolded resolved conifold still exists.

Asymptotically AdS spacetimes with reflecting boundary conditions represent a natural setting for studying superradiant instabilities of rotating or charged black holes. In the first part of this talk, I prove that all asymptotically AdS black holes with ergoregions in dimension d ≥ 4 are linearly unstable to gravitational perturbations. This proof uses the canonical energy method of Hollands and Wald in a WKB limit. In the second part of the talk, I consider a charged Reissner-Nordstrom-AdS black hole---which is superradiantly unstable to charged scalar field perturbations at the linear level---and study the full *nonlinear* evolution of the instability. In this special case, the instability occurs even for spherically symmetric perturbations, which simplifies the analysis and allows for the use of numerical general relativity simulations. Our results show that nonlinear backreaction causes the black hole to lose charge and mass to the scalar field as the instability proceeds. Eventually, higher scalar field harmonics become nonsuperradiant, and they are reabsorbed into the black hole. The final state is described by a “hairy” black hole, surrounded by a scalar condensate in the fundamental (lowest) mode. I discuss implications of this work on the original problem of the rotating black hole superradiant instability.

The kinetic Sunyaev-Zel'dovich effect is a direct probe of the distribution and velocity of electrons on cosmological scales.  Recent progress in Cosmic Microwave Background observations allow statistical detections of this subtle effect originating from a number of different tracers populations.  In my talk, I will review the observational status, highlight the consequences for astrophysics and cosmology and discuss future directions.