# Video Library

Since 2002 Perimeter Institute has been recording seminars, conference talks, public outreach events such as talks from top scientists 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 and 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.

Accessibly by anyone with internet, Perimeter aims to share the power and wonder of science with this free library.

## Spin liquid and Deconfined Criticality in a Kagome Lattice Bose-Hubbard Model

Thursday May 03, 2012
Speaker(s):

We present large-scale quantum Monte Carlo simulations on a sign-problem free Bose-Hubbard model on the kagome lattice. This model supports a quantum Z2 spin liquid phase with fractional excitations and topological order, which can be characterized definitively through calculation of the topological entanglement entropy.  I will outline how the entanglement entropy can be measured in general using a direct implementation of the "replica trick", which allows for the study of entanglement scaling in a variety of other models amenable to study by QMC.

## Molecular rotation in doped superfluid clusters

Thursday May 03, 2012
Speaker(s):

Experiments where impurities were incorporated into helium nanodroplets have shown that the impurity freely rotates, and this has been attributed to the superfluidity of the nanodroplet [1]. Results from experiments with smaller helium clusters suggest that the onset of superfluidity is linked to system size and bosonic exchange effects [2]. We have used path integral techniques to investigate these systems and predict their spectroscopic behaviour in the microwave and the infrared regions of the spectrum.

## Classical Wigner Crystals on Flat and Curved Surfaces, Topological Defects, Pleats ‘ and Particle Fractionalization

Thursday May 03, 2012
Speaker(s):

Charged colloidal particles present a controllable system for study a host of condensed matter/many body problems such as crystallization. 2D crystals are invariably hexagonal. Hexagons perfectly tile a flat plane but a soccer ball requires exactly 12 pentagons dispersed among the hexagons on its curved surface. Pentagons and hexagons are positive and negative topological charges, disclinations, sources for positive and negative curvature.

## Cosmology of Axions and Moduli

Thursday May 03, 2012
Speaker(s):

I will discuss string cosmology and the dynamics of multiple scalar fields in potentials that can become negative, and their features as (Early) Dark Energy models. The point of departure is the `String Axiverse'', a scenario that motivates the existence of cosmologically light axion fields as a generic consequence of string theory. These fields can constitute part of the Dark Matter, suppressing structure formation in a manner similar to massive neutrinos. Future observations will constrain their existence to percent level accuracy.

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## Chiral Mott Insulator of Bosons in a Fully Frustrated Bose Hubbard Model

Thursday May 03, 2012
Speaker(s):

Recent experiments have demonstrated that it is possible to create a synthetic magnetic field for neutral atoms in optical lattices using two-photon (Raman) processes. Motivated by exploring the interplay of such artificial magnetic fields and strong correlations for bosons, we have studied the Bose Hubbard model in the presence of pi-flux per plaquette. Using a variety of techniques, this model is shown to support a remarkable chiral Mott insulator phase on a 2-leg ladder. This state is a fully gapped insulator with staggered loop currents.

## Effective Spin-1/2 Hamiltonians Determined for Er2Ti2O7 & Yb2Ti2O7 Through Inelastic Neutron Scattering

Thursday May 03, 2012
Speaker(s):

We used time-of-flight inelastic neutron scattering to measure the excitation spectra from field-polarized states of exotic frustrated magnets. A knowledge of these spin-wave excitations in various directions in reciprocal space allows a robust determination of exchange parameters in suitable model Hamiltonians.  We have taken this approach with two pyrochlores, Er2Ti2O7 and Yb2Ti2O7, whose magnetic properties have until this point been somewhat puzzling.  The model we use is an effective spin-1/2 exchange Hamiltonian that incorporates the full anisotropy allowed by symmetry at the rare ear

## Quantum Spin Liquids: Life After the Drought

Thursday May 03, 2012
Speaker(s):

The quantum spin liquid state is a prime example of an emergent phenomenon. Theory predicts that new particles such as spinons and gauge fields may emerge at low temperatures. However, for many years there have not been any examples in nature. The situation has changed in recent years in that a number of candidate materials have been discovered which may exhibit these exotic phenomena.

## Songs of the Stars: the Real Music of the Spheres

Wednesday May 02, 2012
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## Quantum Reed-Muller Codes and Magic State Distillation in All Prime Dimensions

Wednesday May 02, 2012
Speaker(s):

Joint work with Earl Campbell (FU-Berlin) and Hussain Anwar (UCL)   Magic state distillation is a key component of some high-threshold schemes for fault-tolerant quantum computation [1], [2]. Proposed by Bravyi and Kitaev [3] (and implicitly by Knill [4]), and improved by Reichardt [4], Magic State Distillation is a method to broaden the vocabulary of a fault-tolerant computational model, from a limited set of gates (e.g.

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## In Search of Majorana

Wednesday May 02, 2012
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Majorana disappeared under mysterious circumstances in 1938 and the particle that bears his name remains elusive to experiments. There is growing interests in realizing the Majorana bound state in the Laboratory because it is expected to possess unusual properties such as non-abelian statistics. I shall discuss various proposals to produce  Majorana bound states and the associated topological superconductors which support them.

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## Next Public Lecture

Check back for details on the next lecture in Perimeter's Public Lectures Series

## LECTURES ON-DEMAND

### Roger Melko: Perimeter Institute and University of Waterloo

Speaker: Roger Melko