Physics

Collisions at the Large Hadron Collider (LHC) are dominated by jets, collimated sprays of particles that arise from quantum chromodynamics (QCD) at high energies.  With the remarkable performance of the ATLAS and CMS detectors, jets can now be characterized not just by their overall direction and energy but also by their substructure.  In this talk, I highlight the increasingly important role that jet substructure is playing in searches for dark matter and other new physics at the LHC, especially when exploring extreme kinematic regimes involving large Lorentz boosts.

I also explain how innovative theoretical studies of jet substructure have taught us surprising lessons about QCD, revealing new probes of hot dense matter and universal features of gauge theories.

In the past few years we have witnessed a flurry of activity in the field of topological phases of matter. An outstanding theme in the field is the study of the interplay between geometry and topology of many-body wave functions, which has attracted the attention of condensed matter and high-energy physicists. In this talk, I will present the quantum field-theoretic descriptions of the fascinating novel phenomena emergent from intertwined geometry and topology, which are vividly exemplified by the geometric responses in fractional quantum Hall systems. For the strict topological limit, where only the global topology of space matters, the fractional quantum Hall systems are characterized by their universal properties such as fractional quantum Hall conductivity and a degeneracy on surfaces with the topology of a torus. Quite surprisingly, these topological fluids also couple to the geometry and have universal responses to the adiabatic deformations of the background geometry. These responses are given by a Wen-Zee term, Hall viscosity term, and gravitational Chern-Simons term. Through the field-theoretic approaches of the topological fluids, I will for the first time show how to derive all the universal geometric responses. To account for the coupling to the background geometry, I show that the concept of “flux attachment” must be modified in the curved space and use it to derive the responses from Chern-Simons theories for all the known fractional quantum Hall states. I also apply these results to the theory of the anisotropic quantum Hall systems, where the geometric responses play a central role in understanding their universal physics.

Many-body entanglement can lead to exotic phases of matter beyond conventional symmetry breaking paradigm. Those exotic phases may contain fractionalized quasiparticles and emergent gauge fields. In this talk,  I will focus on a wide class of long-range entangled phases—quantum spin liquid. In quantum spin liquids, the spins are entangled in some intricate fashion giving rise to interesting physics such as emergent topological field theory and QED3 theory. I will show in detail how such exotic physics can emerge in simple spin systems.

It is commonly believed that quantum information is not lost in a black hole. Instead, it is encoded into non-local degrees of freedom in some clever way; like a quantum error-correcting code. In this talk, I will discuss how one may resolve some paradoxes in quantum gravity by using the theory of quantum error-correction. First, I will introduce a simple toy model of the AdS/CFT correspondence based on tensor networks and demonstrate that the correspondence between the AdS gravity and CFT is indeed a realization of quantum codes. I will then show that the butterfly effect/scrambling in black holes can be interpreted as non-local encoding of quantum information and can be quantitatively measured by out-of-time ordered correlations. Finally I will describe a simple decoding protocol for reconstructing a quantum state from the Hawking radiation and suggest a physical interpretation as a traversable wormhole in an AdS black hole. The decoding protocol also provides an attractive platform for laboratory experiments for measuring out-of-time ordered correlation functions as it clearly distinguishes unitary scrambling from non-unitary decoherence. 

For a family of finite rate stabilizer codes, one can define two distinct error correction thresholds: the usual "block" threshold for the entire code, and the single-qubit threshold, where we only care about the stability of a single encoded qubit corresponding to a randomly chosen conjugate pair of logical X and Z operators.  Our main result is that in the case of erasures, for hyperbolic surface codes related to a {p,q} tiling of the hyperbolic plane, it is the latter threshold that coincides exactly with the infinite-graph edge percolation transition.  I will also discuss likely generalizations to more general codes and other error models. This is joint work with Nicolas Delfosse.

General relativity taught us that space time is dynamical and quantum theory posits that dynamical objects are quantum. Hence the Newtonian notion of space time as a passive stage where physics takes place needs to be replaced by a notion of quantum space time which is interacting with all other quantum matter fields.

I will present recent developments that aim at constructing quantum space time. In particular I will explain how  topological quantum field theories give rise to new quantum geometry realizations and how these serve as starting points  for the construction of a  dynamics  of quantum gravity, which is consistent over all scales.  Such a dynamics will then determine the properties of quantum space time.  

Topological phases of matter serve as one of the most striking examples of the richness and novelty of quantum systems with many degrees of freedom.  In contrast to conventional matter, they are characterized by both non-local properties and non-classical notions such as entanglement.  I will discuss two broad categories of topological phases, distinguished by whether or not they possess fractionalized “anyon” excitations that are neither bosons nor fermions.  I will demonstrate that entanglement not only provides an understanding of such phases but also enables the transmutation between these two categories of topological phases. 

We normally think of large accelerators and large-scale cosmic events when we consider the frontiers of elementary particle physics, pushing to understand the universe at higher and higher energy scales. However, several tabletop low-energy experiments are posed to discover a wide range of new physics beyond the Standard model, where feeble interactions require precision measurements rather than high energies.  In our experiments, high-Q resonant sensors enable ultra-sensitive force and field detection. In this talk I will describe two applications of these sensors in searches for new physics, based on techniques in atomic-molecular-and optical (AMO) physics. First, I will discuss an experiment which uses laser-cooled optically trapped silica nanospheres to search for corrections to Newtonian gravity at micron distances with zeptonewton sensitivity.  Finally, I will discuss the Axion Resonant InterAction Detection Experiment (ARIADNE), a new precision magnetometry experiment using laser-polarized 3-He gas to search for a notable dark-matter candidate: the QCD axion.  

We prove that constant-depth quantum circuits are more powerful than their classical counterparts. We describe an explicit (i.e., non-oracular) computational problem which can be solved with certainty by a constant-depth quantum circuit composed of one- and two-qubit gates. In contrast, we prove that any classical probabilistic circuit composed of bounded fan-in gates that solves the problem with high probability must have depth logarithmic in the input size. This is joint work with Sergey Bravyi and Robert Koenig (arXiv:1704.00690).

We apply the recently suggested strategy to lift state spaces and operators for (2+1)-dimensional topological quantum field theories to state spaces and operators for a (3+1)-dimensional TQFT with defects. We start from the (2+1)-dimensional Turaev-Viro theory and obtain a state space, consistent with the state space expected from the Crane-Yetter model with line defects. This work has important applications for quantum gravity as well as the theory of topological phases in (3+1) dimensions.

I present three possible non-standard additions to cosmology.  First I show that a very long early period of inflation could exist in which parameters evolve, or 'relax', to seemingly fine-tuned values.  Next, I show that even if cosmic inflation existed, a period after inflation with anisotropic stress can dramatically affect super-horizon modes and thus the imprint on the cosmic microwave background.  Finally, I show that cosmological singularities can be avoided by a bounce without using exotic matter that violates the Null Energy Condition, but by the addition of vorticity in compact extra dimensions.

Quantum annealing with superconducting qubits: status and prospects
Adrian Lupascu, Institute for Quantum Computing

Quantum-enhanced Gibbs sampling in statistical relational learning
Peter Wittek, Institute of Photonic Sciences, University of Boras