Forthcoming events in this series


Tue, 23 Apr 2013

12:00 - 13:00
L3

The search for Intrinsic Decoherence

Philip Stamp (Vancouver)
Abstract

              Conventional decoherence (usually called 'Environmental

Decoherence') is supposed to be a result of correlations

established between some quantum system and the environment.

'Intrinsic decoherence' is hypothesized as being an essential

feature of Nature - its existence would entail a breakdown of

quantum mechanics. A specific mechanism of some interest is

'gravitational decoherence', whereby gravity causes intrinsic

decoherence.

I will begin by discussing what is now known about the mechanisms of

environmental decoherence, noting in particular that they can and do

involve decoherence without dissipation (ie., pure phase decoherence).

I will then briefly review the fundamental conflict between Quantum

Mechanics and General Relativity, and several arguments that suggest

how this might be resolved by the existence of some sort of 'gravitational

decoherence'.  I then outline a theory of gravitational decoherence

(the 'GR-Psi' theory) which attempts to give a quantitative discussion of

gravitational decoherence, and which makes predictions for

experiments.

The weak field regime of this theory (relevant to experimental

predictions) is discussed in detail, along with a more speculative

discussion of the strong field regime.

Tue, 15 Jan 2013

12:00 - 13:00
L3

Supersymmetric loop space

Mir Faizal
Abstract

We will first review the construction of N =1

supersymmetric Yang-Mills theory in three dimensions. Then we will

construct a superloop space formulation for this super-Yang-Mills

theory in three dimensions.Thus, we will obtain expressions for loop

connection and loop curvature in this superloop space. We will also

show that curvature will vanish, unless there is a monopole in the

spacetime. We will also construct a quantity which will give the

monopole charge in this formalism. Finally, we will show how these

results hold even in case of deformed superspace.

Tue, 30 Oct 2012

12:00 - 13:00
L3

Protecting Topological Encodings Against Local Perturbations

Alastair Kay
Abstract

Topological quantum error correcting codes, such as the Toric code, are
ideal candidates for protecting a logical quantum bit against local noise.
How are we to get the best performance from these codes when an unknown
local perturbation is applied? This talk will discuss how knowledge, or lack
thereof, about the error affects the error correcting threshold, and how
thresholds can be improved by introducing randomness to the system. These
studies are directed at trying to understand how quantum information can be
encoded and passively protected in order to maximise the span of time between successive rounds of error correction, and what properties are
required of a topological system to induce a survival time that grows
sufficiently rapidly with system size. The talk is based on the following
papers: arXiv:1208.4924 and Phys. Rev. Lett. 107, 270502 (2011).

Tue, 15 May 2012

12:00 - 13:00
L3

BPS state counting on singular varieties

Elizabeth Gasparim (UNICAMP-Brazil)
Abstract

This is a report of joint work with T. Koppe, P. Majumdar, and K.

 Ray.

I will define new partition functions for theories with targets on toric

singularities via

products of old partition functions on  crepant resolutions. I will

present explicit examples 

and show that the  new partition functions turn out to be homogeneous on

MacMahon factors.

Tue, 31 Jan 2012

12:00 - 13:00
L3

An Update on Dark Matter

Malcolm Fairbairn (KCL)
Abstract

We still don't know what dark matter is but a class of leading candidates

are weakly interacting massive particles or WIMPs. These WIMP models are

falsifiable, which is why we like them. However, the epoch of their

falsifiability is upon us and a slew of data from different directions is

placing models for WIMPs under pressure. I will try and present an updated

overview of the different pieces of evidence, false (?) alarms and

controversies that are making this such an active area of research at the

moment.

Tue, 29 Nov 2011

12:00 - 13:00
L3

Quantum Field Theory: Where do we come from? What are we? Where are we going?

Nazim Bouatta (DAMTP)
Abstract

In recent decades, quantum field theory (QFT) has become the framework for

several basic and outstandingly successful physical theories. Indeed, it has

become the lingua franca of entire branches of physics and even mathematics.

The universal scope of QFT opens fascinating opportunities for philosophy.

Accordingly, although the philosophy of physics has been dominated by the

analysis of quantum mechanics, relativity and thermo-statistical physics,

several philosophers have recently undertaken conceptual analyses of QFT.

One common feature of these analyses is the emphasis on rigorous approaches,

such as algebraic and constructive QFT; as against the more heuristic and

physical formulations of QFT in terms of functional (also knows as: path)

integrals.

However, I will follow the example of some recent mathematicians such as

Atiyah, Connes and Kontsevich, who have adopted a remarkable pragmatism and

opportunism with regard to heuristic QFT, not corseted by rigor (as Connes

remarks). I will conceptually discuss the advances that have marked

heuristic QFT, by analysing some of the key ideas that accompanied its

development.  I will also discuss the interactions between these concepts in

the various relevant fields, such as particle physics, statistical

mechanics, gravity and geometry. 

Tue, 15 Nov 2011

12:00 - 13:00
L3

Review on Lifshitz type quantum field theories in Particle Physics

Jean Alexandre (KCL)
Abstract

Attractive features of Lifshitz type theories are described with different

examples,

as the improvement of graphs convergence, the introduction of new

renormalizable

interactions, dynamical mass generation, asymptotic freedom, and other

features

related to more specific models. On the other hand, problems with the

expected

emergence of Lorentz symmetry in the IR are discussed, related to the

different

effective light cones seen by different particles when they interact.

Tue, 25 Oct 2011

12:00 - 13:00
L2

Six-dimensional space-time from quaternionic quantum mechanics

Dorje C. Brody (Brunel University)
Abstract

Quaternionic quantum Hamiltonians describing nonrelativistic spin particles

require the ambient physical space to have five dimensions. The quantum

dynamics of a spin-1/2 particle system characterised by a generic such

Hamiltonian is described. There exists, within the structure of quaternionic

quantum mechanics, a canonical reduction to three spatial dimensions upon

which standard quantum theory is retrieved. In this dimensional reduction,

three of the five dynamical variables oscillate around a cylinder, thus

behaving in a quasi one-dimensional manner at large distances. An analogous

mechanism exists in the case of octavic Hamiltonians, where the ambient

physical space has nine dimensions. Possible experimental tests in search

for the signature of extra dimensions at low energies are briefly discussed.

(Talk based on joint work with Eva-Maria Graefe, Imperial.)

Tue, 17 May 2011

12:00 - 13:00
L3

123 TQFTs

Jamie Vicary (Comlab)
Abstract

I will present some new results on classifying 123 TQFTs,
using a 2-categorical approach. The invariants defined by a TQFT are
described using a new graphical calculus, which makes them easier to
define and to work with. Some new and interesting physical phenomena
are brought out by this perspective, which we investigate. I will
finish by banishing some TQFT myths! This talk is based on joint work
with Bruce Bartlett, Chris Schommer-Pries and Chris Douglas.