Talks

I will discuss constraints on the S-matrix of gapped, Lorentz invariant quantum field theories due to crossing symmetry, analyticity and unitarity.  In particular I will bound cubic couplings, quartic couplings and scattering lengths relevant for the elastic scattering amplitude of two identical scalar particles. After a warm-up in 1+1 dimensions I will move to 3+1 dimensions.  In the cases where the results can be compared with results in the older S-matrix literature they are in excellent agreement.

Gravitational waves from the mergers of five binary black holes and one binary neutron star were detected in the past two years by the advanced LIGO and Virgo detectors. These detections allowed our Universe to be observed in gravitational waves for the first time, and they have tested the predictions of general relativity for dynamical and strongly gravitating systems. I will discuss these results and also highlight a few additional examples of ways in which gravitational waves can shed light on open questions in theoretical physics and astrophysics. One involves the gravitational-wave memory effect, which is a constant change in the gravitational-wave strain produced by the energy that gravitational waves and matter carry away from an isolated system. I will describe the challenges in detecting the memory with LIGO and Virgo, and how the memory is related to the symmetries and conserved quantities of isolated gravitating systems. A second involves using precision astrometry to detect a stochastic background of gravitational waves from astrophysical, and potentially even cosmological, sources. With the upcoming data release from the Gaia mission, it will likely be possible to place improved constraints on the stochastic background at frequencies higher than those coming from the cosmic microwave background, but below those of pulsar timing searches. I will show how these constraints can be determined. Finally, I will discuss gravitational waves from a compact object orbiting an intermediate mass black hole surrounded by a dark matter spike. I will describe how details about the dark matter distribution can be imprinted in the emitted gravitational waves.

Science is like puzzle-solving. Making sense of quantum theory is a particularly thorny kind of brain-twister, with more than its fair share of mysteries. If you are stuck on a puzzle, it may be because you have made a false assumption about the nature of some entity that is absolutely central to the whole business. If so, you have made a category mistake: you are not just wrong about what this entity is, but about what sort of thing it is.

In his Public Lecture at Perimeter Institute, Robert Spekkens will explain why he believes that many quantum mysteries are a result of a category mistake concerning the nature of quantum states. Along the way, he will address some idiosyncratic questions, such as: What did Plato have to say about Heisenberg’s uncertainty principle? What do poorly implemented clinical drug trials have to do with "spooky action at a distance"? And, most importantly, what did the successful deciphering of Egyptian hieroglyphs teach us about the interpretation of quantum theory? 

Spekkens is a faculty member at Perimeter Institute whose research examines the foundations of quantum theory. He co-edited the book Quantum Theory: Informational Foundations and Foils, and he is a Project Leader of the international research collaboration "Quantum Causal Structures.” In 2012, he won first prize in the Foundational Questions Institute (FQXi) essay contest "Questioning the Foundations: Which of Our Assumptions Are Wrong?" He lives in Waterloo with his wife and three-year-old son.

Stabilizer states are a rich class of quantum states which can be efficiently classically represented and manipulated. In this talk I will describe some ways in which they can help us to represent and manipulate more general quantum states. I will discuss classical simulation algorithms for quantum circuits which are based on expressing a quantum state as a superposition of (as few as possible) stabilizer states.

Based on arXiv:1601.07601 (with Sergey Bravyi) and work in progress with Sergey Bravyi, Dan Browne, Padraic Calpin, Earl Campbell and Mark Howard.

One of the most enduring mysteries in particle physics is the nature of the non-baryonic dark matter that makes up 85% of the matter in the universe. For several decades, most searches for this mysterious substance have focused on Weakly Interacting Massive Particles (WIMPs). Recently, there has been a surge in theoretical interest in ultra-light-field dark matter candidates, including QCD axions (spin 0 bosons) and hidden photons (spin 1 bosons), which can be probed through their coupling to electromagnetism or nuclear spin. I will discuss general principles of efficiently searching for the direct detection of the electromagnetic coupling of these candidates, how the sensitivity of these experiments can be improved by the exploitation of quantum correlations in the electromagnetic signals that they produce, and describe the Dark Matter Radio, and experiment searching for axions and hidden photons in the frequency range of 1 kHz through 100 MHz.

In this talk I will discuss recently-identified classes of quantum correlations that go beyond nonlocal classical hidden-variable models equipped with communication. First, in the bipartite scenario, I will focus on so-called instrumental causal networks, which are a primal tool in causal inference. There, I will show that it is possible to “fake” classical causal influences with quantum common causes, in a formal sense quantified by the average causal effect (ACE). Furthermore, I will show that it is possible to violate instrumental inequalities with quantum resources, both in the device-independent and in the 1-sided device-independent settings. Second, in the multipartite setting, I will present a causal hierarchy of multipartite nonlocality. I will make special emphasis on quantum correlations in the upper classes of the hierarchy that define stronger forms of genuinely multipartite quantum non-locality than those previously known. The seminar will touch upon concepts like Bell nonlocality and quantum steering as well as Bayesian nets and causal inference.