# Seminars

### From GICC

## Contents |

## Seminars in 2008

## Géza Tóth

*Instituto de Ciencias Fotónicas*

**Título:**
Entanglement and its experimental detection PDF

**Lugar y hora:**
21 de Noviembre, 14:30 en Aula 11

**Resumen:**

In the first part of the talk I will review basic facts about quantum entanglement and entanglement measures. Beside bipartite entanglement, I will also consider multipartite entanglement and discuss how the multipartite scenario is fundamentally different from the bipartite case.

In the second part of the talk I will discuss methods for entanglement detection in experiments. After motivating the topic, I will show some concrete examples, which appeared in experiments. These demonstrate how to construct entanglement criteria that need few measurements.

## Diego Porras

*Instituto Max Planck de Óptica Cuántica, Garching (Munich), Alemania*

**Título:**
Simulacion cuantica con iones atrapados

**Lugar y hora:**
6 de Febrero, 14:15 en la sala de grados

**Resumen:**

Un simulador cuantico es un dispositivo experimental en el cual se puede controlar el estado cuantico del sistema, asi como las interacciones entre particulas, de forma que su dinamica siga un modelo determinado. En esta charla discutire una propuesta teorica para realizar esta idea con iones atrapados, donde el estado cuantico de un sistema puede controlarse al nivel de un solo atomo.

## Seminars in 2007

### Martin Plenio

*Imperial College, London, UK*

**Title:**
Quantum Phase Transitions with Photons and Polaritons

**Place:**
Aula 11, Fac. C.C.Físicas UCM

**Date:**
Tuesday 24th of April, 14:15

**Abstract:**

The experimental observation of quantum phenomena in strongly correlated many particle systems is difficult because of the short length- and timescales involved. Obtaining at the same time detailed control of individual constituents appears even more challenging and thus to date inhibits employing such systems as quantum computing devices. Substantial progress to overcome these problems has been achieved with cold atoms in optical lattices, where a detailed control of collective properties is feasible but it is very difficult to address and hence control or measure individual sites. Here we show, that polaritons, combined atom and photon excitations, in an array of cavities such as a photonic crystal or coupled toroidal micro-cavities, can form a strongly interacting many body system and permits the demonstration of quantum phase transitions, while individual particles can be controlled and measured. All building blocks of the proposed setting have already been experimentally realised individually, thus demonstrating the potential of this device as a quantum simulator.

### Jiannis Pachos

*Quantum Information Group, University of Leeds*

**Title:**
An Index Theorem for Graphene in Various Topologies

**Place:**
Aula 11, Fac. C.C.Físicas UCM

**Date:**
Tuesday 2nd of February, 14:15

**Abstract:**

In the talk we will consider a graphene sheet folded in a compact geometry with arbitrary topology given by a genus g. In the continuous limit, the Hamiltonian that describes graphene takes the form of the Dirac operator, which provides a good description of the low energy spectrum of the lattice system. After presenting an overview of the index theorem we will employ it to relate the zero energy modes of the graphene sheet with the genus, g, of the compact lattice. The result coincides with analytical and numerical studies for the known cases of fullerene molecules and carbon nanotubes and it extend to more complicated molecules. Potential applications to topological quantum computation are discussed.

## Seminars in 2006

### Emili Bagan

*Univ. Autónoma de Barcelona*

**Title:**
Multiple-copy mixed state discrimination: the quantum Chernoff bound

**Place:**
SALA DE GRADOS. Fac.C.C.Físicas UCM

**Date:**
Thursday 30 th November, 14:15.

**Abstract:**

The error probability of discriminating two states when N copies of one of them is given, provides an operational definition of distance in the state space. We find a lower-bound on the trace distance between arbitrary positive semi-definite operators, and use it to give a quantum version of the Chernoff bound and calculate the asymptotic value of the error probability, thus solving a long standing open problem. The bound reduces to the classical Chernoff bound when the quantum states under consideration commute.

### Francisco Guinea

*Instituto de Ciencia de Materiales de Madrid. CSIC. *

**Title:**
Models for the electronic properties of graphene and graphite

**Place:**
SALA DE GRADOS. Fac.C.C.Físicas UCM

**Date:**
Thursday 22nd November, 14:15.

**Abstract:**

Graphene, a graphite layer one atom thick, shows unusual electronic properties, as the band structure is approximated by the Dirac equation. The material behaves as a two dimensional semimetal. Electron interactions and disorder induce novel effects, different from those observed in metals and semiconductors. We review models for the electronic properties of single layer graphene, and multilayered materials.

### Javier Rodríguez Laguna

*SISSA Trieste*

**Title:**
Quantum mechanics and optimization: quantum annealing and spin glasses

**Place:**
SALA DE GRADOS. Fac.C.C.Físicas UCM

**Date:**
Thursday 14th September, 14:15.

**Abstract:**

Simulated annealing, with its more than 20 years of history, is one of the most used methods in optimization. Thermal fluctuations allow the system to escape the metastable states and reach the absolute minimum with higher probability. The experiments of Brooke et al. opened a very interesting alternative way: to use the quantum fluctuations as escape route. The method is still under development, and is known as "quantum annealing". Along the talk we will describe its main features, advantages and handicaps. Afterwards, we will explain the alternative implementation we have carried out, based on the density matrix renormalization group, and its application to the analysis of the quantum spin glass phase transition.

## Seminars in 2005

### Luigi Amico

*Universita di Catania*

**Title:**
Entanglement in spin chains

**Place:**
Aula Magna. Físicas UCM

**Date:**
Thursday 21st June, 14:15.

**Abstract:**

We review some of the critical and non critical properties of the entanglement encoded in the low temperature states of various one dimensional quantum spin models.

At T=0, near quantum phase transitions, entanglement can be classified in the framework of the standard scaling theory; however, analyzing its range, profound differences between classical correlations and non-local correlations emerge. At low temperature the entanglement sensitivity to thermal and to quantum fluctuations obeys universal $T\neq 0$--scaling behaviour. We show that the entanglement, together with its criticality, exhibits a peculiar universal crossover behaviour.

Far from criticality, we follow up the entanglement in the phenomenon leading to the so called 'separable ground states' in XYZ spin systems. In particular we evidence that the range of the pairwise entanglement diverges while approaching separable ground states, that therefore are reached by a long range reshuffling of the entanglement in the system. The effect of a finite temperature in such phenomenon are also considered.

### Alberto Galindo Tixaire

* Académico numerario de la RAC*

**Title:**
EL ARTE DE DISFRAZAR LA INFORMACIÓN: DE LA C A LA Q

**Place:**
Sede de la Real Academia de Ciencias
Valverde, 22
28004 Madrid

**Date:**
Thursday 1st June, 19:00.

### Ramón Muñoz-Tapia

*Univ. Autonoma de Barcelona-IFAE *

**Title:**
Quantum Directions

**Place:**
Aula Magna. Físicas UCM

**Date:**
Tuesday 30th May, 14:15.

**Abstract:**

Imagine that Bob went into an intergalactic journey. Because of a classical computer failure, he is completely lost in space. At the earth base, Alice still shares a one way quantum channel with Bob. In the absence of a common reference frame, the only way to indicate Bob the direction to go back home is to send a physical object pointing into that direction. In the quantum realm these objects are spin systems. I review the optimal protocols for encoding and decoding this analogical information. I will also present other related problems in the general framework of of quantum state estimation.

### Carlos Tejedor

* Universidad Autonoma de Madrid *

**Title:**
Nanoestructuras de semiconductores como componentes para informacion optica cuantica.

**Place:**
Sala de Grados. Físicas UCM

**Date:**
Wednesday 24th May

**Abstract:**

Discutiremos como las nanoestructuras de semiconductores pueden cumplir los requisitos necesarios para hacer óptica cuántica aplicada a la información. Nos centraremos en puntos cuánticos y microcavidades para explorar las posibilidades que ofrecen. Fotones individuales o en pares son un buen candidato para realizar algunas operaciones de información cuántica. Por ello, presentaremos un análisis teórico de su emisión y de los diferentes mecanismos que controlan la dinámica de éstos sistemas. Trataremos de extraer conclusiones sobre las posibilidades reales de usar nanoestructuras de semiconductores en procesos de información cuántica.

### Miguel Ángel Martín-Delgado

* Depto. Fisica Teorica I UCM *

**Title:**
Topological quantum computation and condensed matter

**Place:**
Sala de Grados. Físicas UCM

**Date:**
Wednesday 10th May

**Abstract:**

An introduction to topological quantum computation as a promising instance of fault-tolerant quantum computation and its relation to condensed matter models. Summary:

I. Quantum Communication Briefing

II. Quantum Computation Briefing

III. Quantum Error Correction and Topology

IV. Topological Orders in Condensed Matter

V. Topological Quantum Computation and Condensed Matter Systems