Since 2002 Perimeter Institute has been recording seminars, conference talks, and public outreach events using video cameras installed in our lecture theatres. Perimeter now has 7 formal presentation spaces for its many scientific conferences, seminars, workshops and educational outreach activities, all with advanced audio-visual technical capabilities. Recordings of events in these areas are all available On-Demand from this Video Library and on Perimeter Institute Recorded Seminar Archive (PIRSA). PIRSA is a permanent, free, searchable, and citable archive of recorded seminars from relevant bodies in physics. This resource has been partially modelled after Cornell University's arXiv.org.
A fundamental theorem of quantum field theory states that the generating functionals of connected graphs and one-particle irreducible graphs are related by Legendre transformation. An equivalent statement is that the tree level Feynman graphs yield the solution to the classical equations of motion. Existing proofs of either fact are either lengthy or are short but less rigorous. Here we give a short transparent rigorous proof. On the practical level, our methods could help make the calculation of Feynman graphs more efficient.
Some years ago Valiant introduced a notion of \'matchgate\' and \'holographic algorithm\', based on properties of counting perfect matchings in graphs. This provided some new poly-time classical algorithms and embedded in this formalism, he recognised a remarkable class of quantum circuits (arising when matchgates happen to be unitary) that can be classically efficiently simulated. Subsequently various workers (including Knill, Terhal and DiVincenzo, Bravyi) showed that these results can be naturally interpreted in terms of the formalism of fermionic quantum computation.
We give an overview of several connections between topics in quantum information theory, graph theory, and statistical mechanics. The central concepts are mappings from statistical mechanical models defined on graphs, to entangled states of multi-party quantum systems. We present a selection of such mappings, and illustrate how they can be used to obtain a cross-fertilization between different research areas.
The idea of pseudo-randomness is to use little or no randomness to simulate a random object such as a random number, permutation, graph, quantum state, etc... The simulation should then have some superficial resemblance to a truly random object; for example, the first few moments of a random variable should be nearly the same. This concept has been enormously useful in classical computer science. In my talk, I\'ll review some quantum analogues of pseudo-randomness: unitary k-designs, quantum expanders (and their new cousin, quantum tensor product expanders), extractors.
Certain structures arising in Physics (mub\'s and sic-povm\'s) can be viewed as sets of lines in complex space that are as large as possible, given some simple constraints on the angles between distinct lines. The analogous problems in real space have long been of interest in Combinatorics, because of their relation to classical combinatorial structures. In the complex case there seems no reason for any combinatorial connection to exist.
Based on a U(1) gauge theory of the Hubbard model on the triangular lattice, it is argued that a spin liquid phase may exist near the Mott transition in the organic compound κ-(BEDT-TTF)2Cu2(CN)3. In the spin liquid state, low energy excitations are fermionic spinons and an emergent U(1) gauge boson. Highly unusual transport properties are predicted due to the presence of a spinon Fermi surface.
There are a few examples in the literature of metals that, in the T 0 K limit, show a resistivity that rises with decreasing temperature without any sign of either saturation or a gap. Well known cases include underdoped cuprates in high magnetic fields and some doped uranium heavy fermion compounds. I will review these and some less-well-known cases, before describing the behaviour of FeCrAs [1], in which we find a continuously rising resistivity from 900 K down to below 50 mK, with a brief interruption due to an antiferromagnetic transition at about 100 K.
Calculating universal properties of quantum phase transitions in microscopic Hamiltonians is a challenging task, made possible through large-scale numerical simulations coupled with finite-size scaling analyses. The continuing advancement of quantum Monte Carlo technologies, together with modern high-performance computing infrastructure, has made amenable a new class of quantum Heisenberg Hamiltonian with four-spin exchange, which may harbor a continuous Néel-to-Valence Bond Solid quantum phase transition.
Responding electrically to magnetic stimuli and vise versa, multiferroics offer exciting possibilities for applications and challenge our understanding of coupled lattice and spin degrees of freedom in solids. I discuss how multiferroic properties can develop in frustrated magnets where competing interactions produce non-collinear spin order and symmetry breaking lattice distortions. Our experiments in TbMnO3, Ni3V2O8, and RbFe(MoO4)2 show that when the low temperature magnetic order breaks spatial inversion symmetry it is accompanied by ferroelectricity [1-3].