Physics

Many non-trivial ideas have been proposed to resolve singularities in quantum gravity. In this talk I argue that singularity resolution can be trivial in gravitational path integrals, because geodesically incomplete singular spacetimes are usually not included in the sum. For theories where this holds, there is no need to develop non-trivial ideas on singularity resolution. Instead, efforts should better be directed to understand tunneling processes and complex-valued spacetimes.

The semi-classical limit of spin foams leads to the Area Regge action. It was long thought that this action leads to flatness and does, in particular, not allow for propagating gravitons. I will present the first systematic studies of the continuum limit of the Area Regge action, using different versions of regular hypercubic lattices. These studies have shown that the Area Regge action does in its continuum limit, lead to leading order to general relativity, and thus to propagating gravitons. The higher order corrections depend on the choice of triangulation for the hypercubic lattice. However, there seems to be a preferred choice, for which the Area Regge action is not singular. In this case the correction term approximates the square of the Weyl curvature tensor, and can be interpreted to arise from an area metric dynamics. We therefore conjecture that the continuum limit of spin foams is described by an area metric theory.

The interplay between strong electronic interaction and non-trivial topology in magic-angle twisted bilayer graphene (tBLG) yields many intriguing phenomena that range from superconductivity to a spontaneous quantum anomalous Hall state with Chern number ±1. WIth the equilibrium phase diagram under much scrutiny, a better theoretical understanding of the excitation spectrum of tBLG is crucial to reveal experimental signatures of competing phases and discern possible pathways of controlling their behavior out of equilibrium. To this end, we study an effective Wannier-orbital model from Kang and Vafek (2019) in the strong coupling limit, which captures the Chern state at three quarters filling as well as its competition with proximal stripe charge order. We compute the density and chiral excitation spectra as well as the charge gap as a function of the overlap of neighboring Wannier orbitals. Our results provide an experimental signature to detect the transition from the stripe phase to the Chern phase and offer insight into steering of the quantum anomalous Hall state via low-frequency driving.

Monitored quantum circuits, composed of local unitary operators and projective measurements, have recently emerged as a rich setting for studying non-equilibrium quantum dynamics. In such systems, sufficient densities of measurements can protect a highly-monitored steady state phase with area law entanglement. Furthermore, it has been shown that such area law phases can host a measurement-protected Ising ferromagnetic order. However, it is not yet known whether such measurement-protected order is a generic phenomenon or whether it relies on the discrete Ising symmetry. To begin answering this question, we introduce a circuit model with continuous symmetry where ferromagnetic order arises in the steady state. Notably, our model requires feedback based on measurement results in order to generate this ferromagnetic order

Black hole echoes have been considered as new probes to standard gravitational waveforms. Here, I consider reflections of scalar waves around a black hole as a model of black hole echoes arising from scalar fields. This problem is difficult due to the need for a proper understanding of the characteristic fields that propagate in numerical relativity. Using the "Einstein-Christoffel" system, I model the characteristic fields and the boundary conditions in such a way as to properly reflect scalar waves at a boundary using the full power of Einstein’s equations.

The core collapse of rapidly rotating massive 10Msun stars (“collapsars”), and resulting formation of hyper-accreting black holes, are a leading model for the central engines of long-duration gamma-ray bursts (GRB) and promising sources of r-process nucleosynthesis. In this talk, I will explore the signatures of collapsars from progenitors with extremely massive helium cores >= 130Msun above the pair-instability mass gap. While rapid collapse to a black hole likely precludes a prompt explosion in these systems, we demonstrate that disk outflows can generate a large quantity (up to >= 50Msun) of ejecta, comprised of >= 5 10Msun in r-process elements and 0.1 1M of 56Ni, expanding at velocities 0.1 c. Radioactive heating of the disk-wind ejecta powers an optical/infrared transient, with a characteristic luminosity 1042 erg s1and spectral peak in the near-infrared (due to the high optical/UV opacities of lanthanide elements) similar to kilonovae from neutron star mergers, but with longer durations >= 1 month. These “super-kilonovae” (superKNe) herald the birth of massive black holes >= 60M, which— as a result of disk wind mass-loss—can populate the pair-instability mass gap “from above” and could potentially create the binary components of GW190521. SuperKNe could be discovered via wide-field surveys such as those planned with the Roman Space Telescope or via late-time infrared follow-up observations of extremely energetic GRBs. Gravitational waves of frequency 0.1 50 Hz from non-axisymmetric instabilities in self-gravitating massive collapsar disks are potentially detectable by proposed third-generation intermediate and high-frequency observatories at distances up to hundreds of Mpc; in contrast to the “chirp” from binary mergers, the collapsar gravitational-wave signal decreases in frequency as the disk radius grows (“sad trombone”).

In this talk we will discuss the notion of thermality for quantum field theories in curved spacetimes, and how it relates to the Unruh effect and Hawking radiation. Then we will argue that particle detectors are physical systems which can act as thermometers, thermalizing to the temperature of the field. We will show that any non-relativistic quantum system undergoing appropriate trajectories can probe the field’s temperature, regardless of how they are coupled to the field.

Quantum error correction is necessary for scalable quantum computation. Topological quantum error correcting codes have exceptional properties that make them ideal for future experiments. Massive gains can be achieved if the code is optimized for the noise, which we demonstrate for some 3D codes under biased noise.

Process matrices are objects that comprehensively describe multipartite quantum processes and their correlations. These matrices simultaneously play the roles of quantum states and quantum channels, and, to ensure a well-defined probability distribution, they are constrained to be positive. A key advantage of the process matrix formalism is that it can be used to describe processes with an indefinite causal order, which obey local quantum mechanics but cannot take place within a global causal structure. In this talk, i will give a complete introduction to the process matrix formalism and share preliminary results about the geometric positivity constraints of a special class of parameterised process matrices.

A fundamental area in statistical analysis is the study of which causal structures connecting the events of interest can explain the correlations that are observed between them. This is done through the falsification of invalid causal models. Our causal structure might posit the existence of hidden (unobserved) causes between the observed events. For example, if we see a positive correlation between the numbers of shark attacks and ice cream sales, we do not expect to explain it by a direct causal influence between these two things; instead, there should be a hidden common cause (for example, the Summer) that explains the correlation. Physicists also have a vested interest in falsifying causal hypotheses involving hidden variables. Bell's Theorem, for example, highlights the failure of many such classical causal hypotheses to explain the correlations predicted by quantum theory. In the scenario which Bell considered, if instead of treating the unobserved causes of classical random variables we treat them as potentially entangled quantum systems, we can explain a strictly larger set of correlations. Out project explores a simple but difficult question: In what other causal structures this also happens? In other words, for a given causal hypothesis, would the set of correlations it can explain expand if we relax our assumptions regarding posited unobservable systems to allow for shared entanglement? By a series of tricks developed during the PSI Winter School, we found that allowing for quantum causes makes an operational difference in a large number of causal hypotheses involving four observed variables. This work is of general interest as it generalizes Bell's Theorem: it exposes (qualitatively novel?!) advantages afforded by quantum theory over classical models. Bell's Theorem has proven crucially insightful in efforts to provide a causal accounting of quantum theory, and has inspired a plethora of quantum information theoretic protocols; similar dividends may be implicitly suggested by this work.