Past Quantum Field Theory Seminar

The talk is based on my paper with E. Beggs appearing in Class. Quantum

Gravity.

Working within a bimodule approach to noncommutative geometry, we show that

even a small amount of noncommutativity drastically constrains the moduli

space of

noncommutative metrics. In particular, the algebra [x,t]=x is forced to have

a geometry

corresponding to a gravitational source at x=0 so strong that even light

cannot

escape. This provides a non-trivial example of noncommutative Riemannian

geometry

and also serves as an introduction to some general results.

The modular invariant partition functions for SU(2) and SU(3)

conformal field theories have been classified. The SU(2) theory is closely

related to the preprojective algebras of Coxeter-Dynkin quivers. The

analogous finite dimensional superpotential algebras, which we call almost

Calabi-Yau algebras, associated to the SU(3) invariants will be discussed.

Cutting-edge experiments in quantum communications are reaching regimes

where relativistic effects can no longer be neglected. For example, there

are advanced plans to use satellites to implement teleportation and quantum

cryptographic protocols. Relativistic effects can be expected at these

regimes: the Global Positioning System (GPS), which is a system of

satellites that is used for time dissemination and navigation, requires

relativistic corrections to determine time and positions accurately.

Therefore, it is timely to understand what are the effects of gravity and

motion on entanglement and other quantum properties exploited in quantum

information.

In this talk I will show that entanglement can be created or degraded by

gravity and non-uniform motion. While relativistic effects can degrade the

efficiency of teleportation between moving observers, the effects can also

be exploited in quantum information. I will show that the relativistic

motion of a quantum system can be used to perform quantum gates. Our

results, which will inform future space-based experiments, can be

demonstrated in table-top experiments using superconducting circuits.

              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.

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.

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).

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.

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.