Device Nanofabrication and Quantum Electronic Transport in Twisted van der Waals Heterostructures

In this project I will learn how to fabricate twisted van der Waals heterostructures using mechanical exfoliation and pick up & transfer methods. I will also learn how to perform device nanofabrication using modern electron beam lithography techniques. The devices will consist of heterostructures built by assembling two van der Waals materials on top of each other with a small angle of rotation between the two crystalline lattices. These exhibit a range of interesting quantum phases and electronic behaviors, such as correlated insulating states, superconductivity, nematicity, etc. I will investigate this behavior via quantum electronic transport experiments at low temperature and variable magnetic field.Outgoin

Superradiance and subradiance in a gas of two-level atoms

Cooperative effects describe atomic ensembles with exchange of photonic excitations, such as dipole-dipole interactions. As a particular example, superradiance arises from spontaneous emission when this exchange leads to constructive interference of the emitted photons. Here, we introduce an integrated method for studying cooperative radiation in many-body systems. This method, which allows to study extended systems with arbitrarily large number of particles can be formulated by an effective, nonlinear, two-atom master equation that describes the dynamics using a closed form which treats single- and many-body terms on an equal footing. We apply this method to a homogeneous gas of initially inverted two-level atoms, and demonstrate the appearance of both superradiance and subradiance, identifying a many-body coherence term as the source of these cooperative effects. We describe the many-body induced broadening - which is analytically found to scale with the optical depth of the system - and light shifts, and distinguish spontaneous effects from induced ones. In addition, we theoretically predict the time-dependence of subradiance, and the phase change of the radiated field during the cooperative decay.Comment: 30 pages, 11 figure

Characterizing superradiant dynamics in atomic arrays via a cumulant expansion approach

Ordered atomic arrays with subwavelength lattice spacing emit light collectively. For fully inverted atomic arrays, this results in an initial burst of radiation and a fast build up of coherences between the atoms at initial times. Based on a cumulant expansion of the equations of motion, we derive exact analytical expressions for the emission properties and numerically analyze the full many-body problem resulting in the collective decay process for unprecedented system sizes of up to a few hundred atoms. We benchmark the cumulant expansion approach and show that it correctly captures the cooperative dynamics resulting in superradiance. For fully inverted arrays, this allows us to extract the scaling of the superradiant peak with particle number. For partially excited arrays where no coherences are shared among atoms, we also determine the critical number of excitations required for the emergence of superradiance in one- and two-dimensional geometries. In addition, we study the robustness of superradiance in the case of non-unit filling and position disorder.Comment: 13 pages, 7 figure

Build your own optical amplifier: design, implementation and physical model of an Erbium Doped-Fiber Amplifier

2017/201

Strange metal in magic-angle graphene with near Planckian dissipation

Recent experiments on magic-angle twisted bilayer graphene have discovered correlated insulating behavior and superconductivity at a fractional filling of an isolated narrow band. In this paper we show that magic-angle bilayer graphene exhibits another hallmark of strongly correlated systems --- a broad regime of $T-$linear resistivity above a small, density dependent, crossover temperature--- for a range of fillings near the correlated insulator. We also extract a transport "scattering rate", which satisfies a near Planckian form that is universally related to the ratio of $(k_BT/\hbar)$. Our results establish magic-angle bilayer graphene as a highly tunable platform to investigate strange metal behavior, which could shed light on this mysterious ubiquitous phase of correlated matter.Comment: 7 pages, 3 figures. (Supplementary material: 3 pages, 2 figures

Lazlo Lovász i Avi Widergson: premis Abel 2021

El dia 17 de març l’Acadèmia de Ciències Noruega va anunciar que el Premi Abel 2021 s’atorgava a Laszló Lovász i Avi Widgerson per, segons es llegeix de la laudatio del premi, “. . . les seves contribucions fonamentals a la informàtica teòrica i les matemàtiques discretes, i el seu paper principal en la seva configuració en camps centrals de les matemàtiques modernes.Peer ReviewedPostprint (published version

Seguiment de la biodiversitat marina al Parc Natural de Cap de Creus i al Parc Natural del Montgrí, les Illes Medes i el Baix Ter. Informe 2014

Informe 2014. Contracte nº AG-2014-654 amb la Generalitat de Catalunya. Departament d‘Agricultura, Ramaderia, Pesca, Alimentació i Medi Natural. Servei d'Espais Naturals Protegits.Aquesta memòria presenta els resultats del grup de treball del Departament d’Ecologia de la UB respecte el seguiment de l’any 2014, tal i com consta al plec prescripcions expedient AG-2014-654, en compliment de la llei 19/1990 de 10 de desembre del Parlament de Catalunya, i amb les millores proposades a l’oferta tècnica Seguiment del medi marí als Parcs Naturals marins de Catalunya. Aquests resultats tenen l’objectiu d’avaluar les poblacions i hàbitats marins en relació tant amb les activitats humanes que hi tenen lloc com amb els factors ambientals; analitzar l’estat de les espècies i les comunitats, la seva evolució temporal i l’efecte que hi produeix la protecció, així com detectar altres situacions de risc pel patrimoni natural com podrien ser les espècies introduïdes o invasores o bé els possibles efectes del canvi climàtic

Device Nanofabrication and Quantum Electronic Transport in Twisted van der Waals Heterostructures

In this project I will learn how to fabricate twisted van der Waals heterostructures using mechanical exfoliation and pick up & transfer methods. I will also learn how to perform device nanofabrication using modern electron beam lithography techniques. The devices will consist of heterostructures built by assembling two van der Waals materials on top of each other with a small angle of rotation between the two crystalline lattices. These exhibit a range of interesting quantum phases and electronic behaviors, such as correlated insulating states, superconductivity, nematicity, etc. I will investigate this behavior via quantum electronic transport experiments at low temperature and variable magnetic field.Outgoin

Device Nanofabrication and Quantum Electronic Transport in Twisted van der Waals Heterostructures

In this project I will learn how to fabricate twisted van der Waals heterostructures using mechanical exfoliation and pick up & transfer methods. I will also learn how to perform device nanofabrication using modern electron beam lithography techniques. The devices will consist of heterostructures built by assembling two van der Waals materials on top of each other with a small angle of rotation between the two crystalline lattices. These exhibit a range of interesting quantum phases and electronic behaviors, such as correlated insulating states, superconductivity, nematicity, etc. I will investigate this behavior via quantum electronic transport experiments at low temperature and variable magnetic field.Outgoin

Photon control and coherent interactions via lattice dark states in atomic arrays

Ordered atomic arrays with subwavelength spacing have emerged as an efficient and versatile light-matter interface, where collective interactions give rise to sets of super- and subradiant lattice states. Here, we demonstrate that highly subradiant states, so-called lattice dark states, can be individually addressed and manipulated by applying a spatial modulation of the atomic detuning. More specifically, we show that lattice dark states can be used to store and retrieve single photons with near-unit efficiency, as well as to control the temporal, frequency, and spatial degrees of freedom of the emitted electromagnetic field. Furthermore, we demonstrate how to engineer arbitrary coherent interactions between multiple dark states and thereby manipulate information stored in the lattice. These results pave the way towards quantum optics and information processing using atomic arrays