# Talks

Older:

Steve Simon. 12 June 2015 | 15:00 | Room 9.90

#### Topological Matter and Why You Should be Interested

Abstract: In two dimensional topological phases of matter, processes depend on gross topology rather than detailed geometry. Thinking in 2+1 dimensions, particle world lines can be interpreted as knots or links, and the amplitude for certain processes becomes a topological invariant of that link. While sounding rather exotic, we believe that such phases of matter not only exist, but have actually been observed in quantum Hall experiments, and could provide a uniquely practical route to building a quantum computer. Possibilities have also been proposed for creating similar physics in systems ranging from superfluid helium to strontium ruthenate to semiconductor-superconductor junctions to quantum wires to spin systems to cold atoms.

Dr Dmitry Kovrizhin, TCM, Cavendish Laboratory, Cambridge

#### Dynamics of fractionalization in quantum spin liquids

Abstract: We provide a complete and exact theoretical study of the dynamical structure factor of a two-dimensional quantum spin liquids in gapless and gapped phases, as realized in Kitaev’s honeycomb model. We show that there are direct signatures —qualitative and quantitative— of the Majorana fermions and gauge fluxes emerging in this model. These include counterintuitive manifestations of quantum number fractionalization, such as a neutron scattering response with a gap even in the presence of gapless excitations, and a sharp component despite the fractionalization of electron spin. Our analysis identifies new varieties of the venerable X-ray edge problem and explores connections to the physics of quantum quenches.

B.Beri. 19 May 2015 | 15:00 | Room 8.60

#### Topological Kondo effect with Majorana fermions

The Kondo effect is a striking consequence of the coupling of itinerant electrons to a quantum spin with degenerate levels. While degeneracies usually arise from symmetries or fine-tuning of parameters, the discovery of Majorana fermions introduced a new possibility: a “topological degeneracy” arising from the nonlocal character of the Majoranas. In this talk I shall describe how nonlocal quantum spins formed from these degrees of freedom give rise to a novel “topological Kondo effect”. Focusing on mesoscopic superconductor devices, I shall discuss several transport signatures which would demonstrate the emerging non-Fermi liquid behaviour, the key strong correlation feature of the effect; and probe the non-local quantum dynamics of Majorana fermions, a key feature for their use in topological quantum computation.

www.birmingham.ac.uk/staff/profiles/physics/beri-benjamin.aspx

Felix Flicker, Bristol. 2 April 2015 | 10:00

#### Quasicrystals, Charge Ordering and the Quantum Hall Effect

Recent experimental observations have been argued to demonstrate the equivalence of one-dimensional quasicrystals to two-dimensional quantum Hall systems. If correct this would lend strong evidence to the claim that three-dimensional quasicrystals such as certain Ho-Mg-Zn alloys are, in a very real sense, higher dimensional objects living in our world. The interpretation is controversial, but the objections mathematically subtle, based on the topological classification of free fermion systems. In recent work we showed that the mathematics of both quasicrystals and the quantum Hall effect can be considered as different limits of a third problem: incommensurate charge order. We use the equivalence, and a couple of properties of fractal patterns, to resolve the debate by means of a simple analogy.

Zlatko and Konstantinos. 27 March 2015 | 11:00 | Theoretical Physics Corridor

#### How universal is the entanglement spectrum

We will review the following paper regarding the universality of the entanglement spectrum and the ambiguity about the information one can infer from it about the physical energy spectrum. This ambiguity stems from the fact that the entanglement Hamiltonian He, defined by H=exp(-He), can undergo phase transitions while the physical Hamiltonian H is unaffected. The related paper is arxiv.org/abs/1311.2946

Alexander Bullivant. 20 March 2015 | 10:00 | Theoretical Physics Corridor

#### Entropic topological invariants in (3+1) dimensions

Giandomenico Palumbo. 3 March 2015 | 11:00 | Theoretical Physics Corridor

#### Cartan’s legacy in modern physics

Chris & Konstantinos. 27 Feb 2015 | 15:00 | Theoretical Physics Coridor

#### Driven-dissipative Ising model

We review a paper by Chamon et al in which the Ising model rapidly driven and coupled to a heat bath is studied. For a carefully selected bath, it is possible to tune the driving in order to change the order of the phase transition, the critical exponent, the value of the critical temperature, and the magnetic order FM <-> AFM.

Zoltan Kadar. 13 Feb 2015 | 10:00

#### Some new knowledge about representations of the loop braid group

I will try to present a consistent bunch of facts from 1401.1774 about the representations the motion group of n loops in the 3-ball, which are supposed to describe possible 3-dimensional topological phases of matter.

Duncan Haldane (Princeton). 2 Feb 2015 | 16:00 | EC Stoner 9.90

#### Quantum geometrodynamics of the incompressible fractional quantum Hall fluid

Courtney Brell. 26 Nov 2014 | 15:00

#### Self-correcting stabilizer quantum memories in 3 dimensions (or slightly less)

Self-correcting quantum memories are systems that allow for the long-term storage of quantum information without the need for active error-correction, in analogy to a classical hard drive. We propose a family of local CSS stabilizer codes in 3D as candidates for self correcting quantum memories. The construction is inspired by the classical Ising model on a Sierpinski carpet fractal, which acts as a classical self-correcting memory. Our models are naturally de fined on fractal subsets of a 4D hypercubic lattice which may be embedded into 3-space with finite distortion. The X and Z sectors of the code are dual to one another, and we show that there exists a finite temperature phase transition associated with each of these sectors, providing evidence that the system may robustly store quantum information at finite temperature.

Konstantinos. 09 Oct 2014 | 13:00

#### Entanglement Deconstructionism

Jiannis. 08 Oct 2014 | 14:00

#### Introduction to topological states of matter

James. 26 Sept 2014

#### Topological meta-materials (topo part)

Giandomenico. 25 Sept 2014 | 12:30

#### Torsion, string vortices, and exotic gravity (non-topo part)

Giandomenico. 19 Sept 2014 | 12:00

#### Torsion, string vortices, and exotic gravity (topo part)

James. 18 Sept 2014 | 16:00