Próximos seminarios

Multifractal metal in a disordered Josephson Junctions Array
Manuel Pino (IFF,CSIC)

Date & location:   Fri 05 May  2017 12:00:00 GMT+0200 (CEST), Meeting Room, Serrano 113-bis (CFMAC)

Abstract: We show that quantum chaotic dynamic may not result in thermalization in certain bosonic models that can be realized as an array of Josephson junctions. This model exhibits a many-body localization transition which separates insulating and metallic phases. Localization prevents the system to thermalize in the insulating phase. We show that there is a intermediate region in the phase diagram, between Many-Body localized and ergodic phases, in which the system behaves as a metal but it is not described by the laws of Statistical Mechanics.

Probing quantum correlations with multiple atomic impurities
Jordi Mur-Petit (Clarendon Laboratory, University of Oxford)

Date & location:  Thu 04 May  2017 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: Experimental advances in the control and measurement of quantum systems are driving the development of quantum technologies across multiple experimental platforms, from trapped ions and cold atoms, to superconducting circuits and nanomechanical setups [1]. Among the most promising practical applications of these devices lies quantum-enhanced sensing, where individual or quantum-correlated particles are used to accurately measure observables, from magnetic fields to gravity. Extending this paradigm, recent work has highlighted the potential of single quantum systems to probe strongly-correlated quantum systems [2].


In this talk, I will present a quantum protocol that uses multiple atomic impurities to measure N-point correlations in strongly-correlated quantum systems [3], and discuss ongoing experimental efforts at Oxford to implement it with a two-species cold-atom setup [4].


[1] G. Kurizki et al., PNAS 110, 3866-3873 (2014); K. Bongs et al., Proc. SPIE 9900, 990009 (2016).

[2] See, e.g., D. Hangleiter et al., Phys. Rev. A 91, 013611 (2015); T.H. Johnson et al., Phys. Rev. A 93, 053619 (2016).

[3] M. Streif, A. Buchleitner, D. Jaksch & J. Mur-Petit, Phys. Rev. A 94, 053634 (2016).

[4] E. Bentine et al., J. Phys. B 50 094002 (2017).




Huygens principle and Dirac equation
Saverio Pascazio (Universita di Bari/INFN)

Date & location:  Tue Apr 25 2017 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: Every point on the wave front of a propagating wave is a source of secondary wavelets, which spread forward at the same speed as the source wave. The wave front at later times is then given by the surface tangent to the secondary wavelets.  This principle was proposed by Christiaan Huygens in 1678, to explain the laws of reflection and refraction. It was used again more than a century later, in 1816, by Augustin-Jean Fresnel, to intepret the diffraction effects that occur when visible light encounters slits, edges and screens. 

The principle provides crucial insight into the nature of wave propagation and it is a milestone in the physics of ondulatory phenomena. For this reason, its universal validity is usually taken for granted. However, yet one century later, Jacques Hadamard noticed that Huygens' principle is valid only when waves propagate in an odd number n>1 of spatial dimensions.
Both quantum mechanics and quantum field theory make use of wave equations in their formulation. It is therefore interesting to ask whether Huygens' principle holds for the seminal equations that are the backbone of these theories. The Schrodinger equation, being non-relativistic, does not admit a satisfactory formulation of this question. What about the Dirac equation? 

We discuss the validity of Huygens' principle for the massless Dirac-Weyl equation. We find that the principle holds for odd space dimension n, while it is invalid for even n. We explicitly discuss the cases n=1,2 and 3.


New insights into metallic nanoparticles for enhancement of particle therapy in hypoxic tumors

Marta Bolsa (Institut des Sciences Moléculaires d’Orsay, CNRS)

Date & location:  Wed Apr 19 2017 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: Low oxygen concentration in tumors results in lower cell death after exposure to radiation. The oxygen effect is expressed by the Oxygen Enhancement Ratio (OER) which depends, among others, on the oxygen level and the linear energy transfer (LET) of the radiation. Particle therapy benefits the treatment of hypoxic tumors compared to radiotherapy due to a decrease of OER when LET increases [Scifoni et al., 2013]. However, the irradiation of healthy tissues at the entrance channel remains a major limitation.
Nanotechnology brought new perspectives of using high-Z nanoparticles (NPs) to increase local radiation-effect. Previous studies performed by the group demonstrated that the radio-enhancement due to PtNPs is mostly related to the production of water radicals (OH) produced in the vicinity of the NPs [Porcel et al., 2010]. In parallel, it has been observed that the contribution of OH-mediated cell damage is strongly influenced by the presence of oxygen [Hirayama et al., 2013]. 
In the collaborative works performed at the Heavy Ion Medical Accelerator in Chiba (HIMAC, Japan) and at GSI-HIT (Germany), we investigated the effect of metallic NPs on human cancer cells incubated in oxic and hypoxic conditions and irradiated by carbon and helium ions. We have studied the variation of alpha and beta parameters in the presence of NPs. The presence of NPs might increase the direct lethal damage. This first study shows an attenuation of the amplification effect by NPs in the absence of oxygen. This gives new insights in the processes involved in the radio-enhancement by NPs.
This work is part of the European Commission Framework 7 Programme (grant number EC FP7 MC - ITN - 608163 – ARGENT) (

Decoherence produced by spin bath

Erik Torrontegui (IFF, CSIC)

Date & location: Tue Apr 18 2017 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: Any quantum system is never isolated and represents an open system. As result, the environment that surrounds the system introduces decoherence spoiling its quantum properties. Employing an alternative formalism to study open quantum systems, the Stochastic Surrogate Hamiltonian, we analyze dissipation and dephasing processes produced by a spin bath. For dissipation, we show that the Stochastic Surrogate Hamiltonian enables the simulation of thermalization beyond the weak and Markovian regimes. For dephasing, the relation with temperature is studied, showing that the effect depends on the dephasing mechanism itself. 
[1] E. Torrontegui and R. Kosloff, New J. Phys. 18, 093001 (2016).
[2] R. Baer and R. Kosloff, J. Chem. Phys. 106, 21 (1997).
[3] G. Katz, M. A. Ratner, and R. Kosloff, J. Phys. Chem. C 118, 21798 (2014).2017


Agregados de agua en nanoestructuras de carbono

José Bretón (Departamento de Física, Universidad de la Laguna)

Date & location:  Wed Mar 22 2017 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: En   este Seminario presentamos   un   estudio  teórico sistemático   de   las configuraciones  de  mínima  energía  que  presentan  pequeños  agregados  de agua  (de  hasta  20  moléculas)  en  interacción  con  nano  estructuras  formadas básicamente por átomos de carbono. Las nanoestructuras de carbono que se analizan son:  hidrocarburos policíclicos  aromáticos,  como  el coroneno  y  el coranuleno, grafeno  y  grafito,  fulerenos  y  nanotubos  de  carbono.  Para  estos dos  últimos  también presentamos  los  efectos  asociados  al confinamiento  de estos  agregados  de  agua en  el  interior  de  las cavidades  huecas asociadas  a estas estructuras.Todos  estos  estudios  pretenden  contribuir a  la  comprensión de los fundamentos  nanoscópicos de la hidrofobicidad de la interacción agua-estructuras de carbono.

Classical and semiclassical energy conditions

Prado Martín Moruno (Departamento de Física Teórica I, Universidad Complutense de Madrid)

Date & location: Tue Mar 21 2017 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: The standard energy conditions usually associated to general relativity are (mostly) linear in the stress-energy tensor, and have clear physical interpretations in terms of geodesic focussing, but suffer the significant drawback that they are often violated by semi-classical quantum effects. In contrast, it is possible to develop non-standard energy conditions that are intrinsically non-linear in the stress-energy tensor, and which exhibit much better well-controlled behaviour when semi-classical quantum effects are introduced, at the cost of a less direct applicability to geodesic focussing. In this talk I will review the standard energy conditions and their various limitations. Then, I will briefly introduce the averaged energy conditions, and present in detail the nonlinear and the semi-classical energy conditions.

Edge transport over sub-millimeter distance in the 2D Topological Insulator InAs/GaSb

Enrique Díaz (Departamento de Física Fundamental, Universidad de Salamanca )

Date & location: Fri Mar 14 2017 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: The InAs/GaSb double quantum well (DQW), a tunable two dimensional (2D) electron-hole system, was recently proposed as a 2D topological insulator (TI) [1]. While convincing evidence for transport by edge states in micrometer-sized devices has been reported in several experiments [2, 3, 4], the topological origin of these transport  properties is still under debate [5]. In the present work we test the transport by edge states in several DQW’s with diferrent sizes InAs/GaSb to cover from normal insulator (NI) and TI samples up to unprecedented length scales and high magnetic fields. First of all, we address the low-temperature electrical transport in a local measurement configuration (Fig.1 (a)), both in zero-field and in the integer quantum Hall regime (Fig.1 (c)), with the latter indicating the realization of a non-trivial system [6]. Successively, making use of non-local transport measurements (Fig.1 (b), (d)), we consistently find evidence for transport by edge states over sub-millimeter distances, as tested using the methodology of Refs.[2, 3] and applying a resistor network model analogous to the one of Ref.[4]. Under high magnetic fields (up to B = 30 T), we detect persistent transport by edge states, with a clear suppression of backscattering in the case of perpendicular orientation. We analyse the response of the edge states to systematic inversions of the current contacts  and/or of the polarity of the magnetic field. Many-body interactions can produce exotic novel ground states in these systems. An interacting electron and hole can spontaneously form excitons, i.e. a neutral bound state provided that the exciton binding energy exceeds the energy separation between the single particle states.
 [1] C.X. Liu, T.L Hughes, X.L. Qi, K. Wang and S.C Zhang, Phys. Rev. Lett. 100, 236601 (2008). [2] K. Suzuki, Y. Harada, K. Onomitsu, and K. Muraki, Phys. Rev. B  87, 235311 (2013). [3] K. Suzuki, Y. Harada, K. Onomitsu, and K. Muraki, Phys. Rev. B  91, 245309 (2015).
[4] S. Mueller et al:, Phys. Rev. B 92, 081303(R) (2015).
[5] F. Nichele et al:, arXiv:1511.01728.
[6] B. Büttner et al:, Nature Phys. 7, 418 (2011).

Not so normal modes

J. Gonzalo Muga (Dpto. de Química-Física, UPV/EHU )

Date & location: Mon Mar 06 2017 16:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: I will present the theory of time-dependent point transformations to find independent dynamical normal modes for two-dimensional systems subjected to time-dependent control in the limit of small oscillations. The condition that determines if the independent modes can indeed be defined is identified, and a geometrical analogy is put forward. The results explain and unify recent work to design fast operations on trapped ions, needed to implement a scalable quantum-information architecture: Transport,expansions, and the separation of two ions,two-ion phase gates, as well as the rotation of an anisotropic trap for an ion, are treated and shown to be analogous to a mechanical system of two masses connected by springs with time-dependent stiffness.
REF: PRA 95 022130 (2017)


Quantum walks gravity simulation

Giuseppe di Molfetta (Laboratoire d'Informatique Fondamentale, CNRS/Université Aix-Marseille )

Date & location: Fri Feb 24 2017 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: As we know, spacetime is not flat at the cosmological scale. In order to describe spacetime, in General Relativity theory (GR), we need a continuous and differentiable manifold and a formal way to account for the continuous distortion of the metrics. The main point is that changing coordinate systems should not affect physics  laws (General Covariance). However at the Planck length, matter is not continuous and obeys Quantum Theory (QT). Although one century has passed, finding an intrinsically discrete counterpart of GR is still an open question. In fact, discretized GR does not turn out in just a mere finite difference scheme of the old formula.
I recently showed that one way to describe a discrete curved spacetime is by using Quantum Walks. From a physical perspective a QW describes situations where a quantum particle is taking steps on a discrete grid conditioned on its internal state (say, spin states). The particle dynamically explores a large Hilbert space associated with the positions of the lattice and allows thus to simulate a wide range of transport phenomena.
It is surprising that this unitary and local dynamics, defined on a rigid space-time lattice coincides in the continuous limit with the dynamical behavior of a quantum spinning-particle spreading on a curved spacetime. This could really turn out to be a powerful quantum numerical method to discretize GR.

Quantum optics in low dimensions: from fundamentals to applications

Alejandro González Tudela (Max-Planck-Institut für Quantenoptik)

Date & location: Fri Feb 10 2017 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: Recent experimental developments in nanophotonics [1], circuit QED [2] and cold atoms [3] allow to engineer systems where quantum emitters couple to low dimensional photon-like reservoirs with non-trivial energy dispersion. Compared to three-dimensional and structureless baths, the interactions induced by such structured environments can be strongly enhanced and have long-range character.

In this talk, I will show several phenomena that can emerge in these scenarios such as the existence of multi-photon bound states around single quantum emitters [4], the generation of tuneable long-range coherent interactions [5], or how one can boost the fidelities and efficiencies of non-classical states of light [6].

[1] Nature 508, 241–244 (2014), Nature Communications 5, 3808 (2014), Rev. Mod. Phys. 87, 347 (2015)
[2] Nature Physics 13 (1), 48-52 (2017)
[3] Nature Physics 8, 267–276 (2012),Physical Rev Lett. 101 (26), 260404 (2010)
[4] Physical Review X 6 (2), 021027 (2016)
[5] Nature Photonics 9 (5), 320-325 (2015), PNAS, 201603777 (2016)

[6] Physical Review Letters 115 (16), 163603 (2015), New Journal of Physics 18 (4), 043041 (2016) arXiv:1603.01243


Anomalies (in) matter

Karl Landsteiner (IFT, UAM-CSIC)

Date & location: Tue Jan 24 2017 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: The concept of symmetry is one cornerstone of modern theoretical physics, quantum mechanics is another. Sometimes they are incompatible with each other. These incompatibilities are called anomalies. 
They constrain possible fermion spectra of gauge theories and explain otherwise forbidden processes such as the decay of the neutral pion into two photons. In the recent years however anomalies play an ever bigger role in a totally different realm of physics: condensed matter. In particular anomalies induce exotic new transport phenomena such as the chiral magnetic and the chiral vortical effects.
I will review of chiral anomalies, anomaly induced transport phenomena and discuss some of its applications in a new exciting class of materials: the Weyl semimetals.


Machine Intelligence and quantum information processing

Peter Wittek (ICFO)

Date & location: Fri Jan 13 2016 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: Statistical learning theory is rich in results that changed the way we process and think about data. Furthermore, the theory offered a fresh perspective on artificial intelligence. Applications are countless and much progress has been made in applying learning schemes in quantum control problems. Quantum information processing and quantum computing are the next frontier for machine learning, but benefits work both ways: quantum-enhanced learning is also a promising research direction, but we must generalize known classical results to the quantum case to fully understand the limits and possibilities. This talk gives an introduction to the core concepts in learning theory, and then looks at the major challenges in the intersection of machine learning, artificial intelligence, and quantum information processing.



Epitaxial III-V Quantum Dots for Quantum Optical Information S&T:   Pros and Cons

 Benito Alén (MBE: Quantum Nanostructures Group (IMM-CSIC))

Date & location: Tue Dec 13 2016 15:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)



Abstract: Quantum optical experiments performed on single atoms and ions had inspired the rapid development of solid-state Quantum Optical Information S&T. In the study and exploitation of quantum properties of semiconductor nanostructures, much of the state of the art is defined by experiments done with epitaxial III-V quantum dots (QDs). There are many properties of III-V QDs which make them especially appealing compared with other systems. Either stand alone or embedded in optical microcavities, III-V QDs naturally emit single photons and entangled photon pairs in the relevant telecomm spectral ranges [1-4]. Moreover, just as any other semiconductor, they can be electrically driven from a low power battery, both to emit quantum light [5,6] or to manipulate quantum states [7,8].

These and other proofs of concept make of III-V QDs amenable for highly integrated quantum optical information technologies, yet, to bring expectations to reality, rather important technical and fundamental problems must be solved first. In this talk, I will introduce some of the current challenges in the field and describe how our activities at the MBE: Quantum Nanostructures Group (IMM-CSIC) try to address them [9-10].


[1] P. Michler et al, «A Quantum Dot Single-Photon Turnstile Device», Science, 290 2282 (2000)
[2] C. Santori et al, «Indistinguishable photons from a single-photon device», Nature 419, 594 (2002)
[3] M. Birowosuto et al, «Fast Purcell-Enhanced Single Photon Source in 1,550-mm Telecom Band from a Resonant Quantum Dot-Cavity Coupling». Scientific Reports 2, 321 (2012)
[4] J. Kim et al «Two-Photon Interference from a Bright Single-Photon Source at Telecom Wavelengths». Optica 3, 577 (2016)
[5] Z. Yuan et al, «Electrically Driven Single-Photon Source», Science 295, 102, (2002)
[6] C. L. Salter et al, «An entangled-light-emitting diode», Nature 465, 594, (2010)
[7] E. D. Kim et al., «Fast Spin Rotations by Optically Controlled Geometric Phases in a Charge-Tunable InAs Quantum Dot», Phys. Rev. Lett. 104, 167401 (2010)
[8] W. Liu et al., «In situ tunable g factor for a single electron confined inside an InAs quantum dot», Phys. Rev. B  84 121304 (2011)
[9] J. Herranz et al «Role of re-growth interface preparation process for spectral linewidth reduction of single InAs site-controlled quantum dots». Nanotechnology 26 195301 (2015)
[10] J. M. Llorens et al., «Type II InAs/GaAsSb quantum dots: Highly tunable exciton geometry and topology», Applied Physics Letters 107, 183101 (2015)


Quo vadis, Física? Los límites del conocimiento

MIguel Ángel Sanchís Lozano  (UPV, IFIC/CSIC)

Date & location: Thu Dec 01 2016 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: Desde el reduccionismo de los antiguos griegos aplicado a la explicación de la complejidad de la Naturaleza, la "navaja de Occam" (la explicación más simple ha de ser la verdadera) y el  extraordinario éxito de la ciencia para entender el universo, se ha buscado con ahínco una Teoría de Todo (Theory of Everything) en Física.

En Matemáticas, a principios del siglo XX, Hilbert propuso buscar una base axiomática a partir de la cual, y mediante una serie de argumentos lógicos, se pudiera demostrar la verdad o falsedad de cualquier enunciado matemático bien planteado (es decir un sistema formal, completo y consistente). Los teoremas de incompletitud de Gödel formulados en 1931 (y la posterior aportación de Turing), supusieron echar un inesperado jarro de agua fría a tales aspiraciones.

A finales del siglo XX hubo científicos que creyeron haber encontrado, en la Teoría de Cuerdas, la ansiada Teoría de Todo en Física, con un extraordinario carácter unificador de partículas y fuerzas (aunque de una gran complejidad matemática y formal). Y sin embargo, se ha hallado un conjunto ("landscape") de al menos 10500 posibles soluciones, cualquiera de ellas posible, que determinaría un multiverso donde las leyes y constantes físicas pueden tomar prácticamente cualquier valor en los distintos universos.


Edge states at phase boundaries and their stability

Juan Manuel Pérez Pardo  (Dpto. Matemáticas, UC3M)

Date & location: Wed Nov 30 2016 15:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: Quantum field theories can be used to construct effective modells to describe condensed matter systems. One of the main differences between condensed matter systems and fundamental particle physics is that, in the former, the materials where the effective field theory lives are of finite size and have boundaries. This difference is crucial and brings extra features like the appearance of edge states.

I will show that the appearance of edge states is related with the type of boundary conditions describing the effective model and the difficulties that might arise when one considers different boundary conditions. For doing that I will consider two different situations, scalar theories and fermionic theories. Surprisingly, in the latter case, there is a threshold size for the sample below which the edge states disappear. 

Modifications of molecular structure and reactions under strong

light-matter coupling

Johannes Feist  (Dpto. Física Teórica de la Materia Condensada, UAM)

Date & location: Tue Nov 22 2016 15:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: Strong coupling is achieved when the coherent energy exchange between a confined electromagnetic field mode and material excitations becomes faster than the decay and decoherence of either constituent. This creates a  paradigmatic hybrid quantum system with eigenstates that have mixedlight-matter character (polaritons). Organic molecules are a particularly useful system to achieve strong coupling at room temperature, since they possess excitons (bound electron-hole pairs) with large transition dipole moments and binding energies.

While most models of strong coupling are based on two-level systems, this is far from a realistic description for molecules with many nuclear (rovibrational) degrees of freedom. The influence of strong coupling on these internal degrees of freedom has only come into focus recently.

Pioneering experiments have shown modifications of material properties and chemical reaction rates under strong coupling, which cannot be explained by simple two-level models. In order to address this mismatch, we developed a first-principles model combining the tools of cavity QED with well-known molecular models in order to fully take into account electronic, nuclear and photonic degrees of freedom. 

I will first discuss the applicability of the Born-Oppenheimer approximation, which is challenged by the introduction of the new intermediate timescale of energy exchange between the molecule(s) and the field. We then show how photochemical reactions such as photoisomerization can be almost completely suppressed under strong coupling. Surprisingly, this suppression works more efficiently when many molecules are coupled to a single light mode due to a “collective protection” effect within the delocalized polaritonic state.



Dissipative long-range entanglement generation between electronic spins

Mónica Benito  (Instituto de Ciencias Materiales de Madrid, CSIC)

Date & location: Fri Oct 28 2016 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: We propose a scheme for deterministic preparation of steady-state entanglement between remote qubits, defined by electron spins confined in spatially separated quantum dots. Our approach relies on an electronic quantum bus, consisting either of quantum Hall edge channels or surface acoustic waves, that can mediate long-range coupling between localized spins over distances of tens of micrometers. Since the entanglement is actively stabilized by dissipative dynamics, our scheme is inherently robust against noise and imperfections.


Symmetry-Protected Heat Transport in Quantum Hall Physics

Ángel Rivas (Dpto. de Física Teórica I, UCM)

Date & location: Wed Oct 19 2016 15:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: The study of non-equilibrium properties in topological quantum systems is of practical and fundamental importance. Here, we will discuss stationary properties of a two-dimensional boson topological insulator coupled to two thermal baths in the quantum open-system formalism [1]. Novel phenomena appear like chiral edge heat currents that are the out-of-equilibrium
counterparts of the zero-temperature edge currents.  A new set of discrete symmetries protect these topological heat currents, differing from the zero-temperature limit, and with a purely dissipative origin. Remarkably, one of these currents flows opposite to the decreasing external temperature gradient. As the starting point, we will review some basics about quantum Hall physics and consider the case of a single external reservoir showing prominent results like thermal erasure effects and topological thermal currents. Finally we will comment about the possibility to experimentally observe these new phenomenology with platforms like photonics chips and
optical lattices.

[1] A. Rivas and M. A. Martin-Delgado, Topological Heat Transport and Symmetry-Protected Boson Currents, arXiv:1606.07651


Zero-Mode Rotating Surface States in 3D Dirac and Weyl Semimetals under Radiation

Rafael Molina (Instituto de Estructura de la Materia, CSIC)

Date & location: Wed Sep 28 2016 15:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: Topological semimetals are exciting new materials whose discovery became possible thanks to the refined understanding of exotic phenomena in topological band theory. We investigate the development of novel surface states when 3D Dirac or Weyl semimetals are placed under circularly polarized electromagnetic radiation. We find that the hybridization between inverted Floquet bands opens, in general, a gap, which closes at so-called exceptional points found for complex values of the momentum. This corresponds to the appearance of midgap surface states in the form of evanescent waves decaying from the surface exposed to the radiation. We observe a phenomenon reminiscent of Landau quantization by which the midgap surface states get a large degeneracy proportional  to the radiation flux traversing the surface of the semimetal. We show that all of these surface states carry angular current, leading to an angular modulation of their charge that rotates with the same frequency of the radiation, which should manifest in the observation of a macroscopic chiral current in the irradiated surface.


J. González, R.A. Molina, PRL 116, 156803 (2016).


Polaritons: classical and/or quantum aspects

Fabrice P. Laussy (Dpto. de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid)

Date & location: Wed Sep 14 2016 15:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Abstract: Polaritons arise as the eigenstates of light-matter coupling hamiltonians (1). They have enjoyed considerable attention since their discovery in 1992, since they bring together solid state, condensed-matter, atomic and cavity QED physics. Interest has been growing exponentially with subsequent reports of their Bose–Einstein condensation in (Nature 2006), of their superfluid propagation (Nature 2009), of their quantum-hydrodynamics and solitonic attributes (2010-2012), and, more recently, exotic phases, band-engineering, exceptional points (Nature 2015) and other topological features. They are a fantastic playground for theorists and experimentalists alike allowing a wide breadth of exploration in a variety of topics. In this talk, I will survey some of the most striking polaritonic feasts and review the efforts to bring them down to the single-particle quantum regime, culminating with our recent announcement of polariton entanglement (2). I will also take advantage of this system to present our proposal of quantum spectroscopy, based on sweeping continuously varying quantum statistics from a quantum source driving a target (3).

(1) Microcavities, Kavokin et al., Oxford University Press 2011
(2) Entangling a polariton with one photon: effect of interactions at the single-particle level, Cuevas et al., arXiv:1609.01244
(3) Exciting Polaritons with Quantum Light, López Carreño et al., Phys. Rev. Lett. 115:196402 2015


Simulating spin-boson models with trapped ions

Andreas Lemmer Institute of Theoretical Physics, University of Ulm (Germany)
Date & location: Fri Jul 22 2016 12:00:00 GMT+0200 (CEST), 
Seminar Room, Serrano 121 (CFMAC)

The spin-boson model considers a single spin coupled to a bath of harmonic oscillators. It is a paradigmatic model for the emergence of dissipation and decoherence in quantum systems. Although the model is seemingly simple, no closed analytic solution is known to date. On the other hand, a number of numerical methods have been developed to study the dynamics of spin-boson models. However, also numerics are challenged for environments that lead to highly non-Markovian dynamics, e.g. highly structured environments with longlived vibrational modes. Therefore a physical simulator with a high degree of control is desirable.
Trapped atomic ions provide a clean and highly controllable system where dynamical quantities are directly accessible. In this talk I will present a method to simulate the dynamics of spin-boson models with macroscopic and
non-Markovian environments with trapped ions.

Machine Learning: Between the hype and the new possibilities

Emilio Alba CTO of Quantum Bussiness Analytics
Date & location: Wed Jul 06 2016 12:00:00 GMT+0200 (CEST), 
Seminar Room, Serrano 121 (CFMAC)

Machine Learning is at the forefront of a renewed interest in (big) data analytics and statistical analysis. In this talk we try to briefly describe what it is, make the case for its increasing role in technological applications, and open a discussion on its bright and dark effects on physics as a whole - and the realm of Quantum Information/Simulations in particular.


Quantum simulation with a boson sampling circuit

Diego González Olivares, Instituto de Física Fundamental (CSIC)
Date & location: Wed Jun 29 2016 15:00:00 GMT+0200 (CEST), 
Seminar Room, Serrano 121 (CFMAC)

Boson sampling is a model of quantum computation that was proposed by S. Aaroson and A. Arkhipov as a restricted model for post-classical computation whose architecture should be experimentally achievable with current, state of the art technologies.

We have studied a system that consists of 2M matter qubits that interact through a boson sampling circuit, i.e., an M-port interferometer, embedded in two di erent architectures. We have proven that, under the conditions required to derive a master equation, the qubits evolve according to e ective bipartite XY spin Hamiltonians, with or without local and collective dissipation terms. This opens the door to the simulation of any bipartite spin or hard-core boson models and exploring dissipative phase transitions as the competition between coherent and incoherent exchange of excitations. We have also shown that, in the purely dissipative regime, this model has a large number of exact and approximate dark states, whose structure and decay rates can be estimated analytically. We argue that this system may be used for the adiabatic preparation of boson sampling states encoded in the matter qubits.
Chiral Quantum Optics with Spins, Photons, and Phonons

Tomás Ramos, Institute for Theoretical Physics, University of Innsbruck and Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences
Date & location: Thu Jun 16 2016 14:30:00 GMT+0200 (CEST), Small Meeting Room, Serrano 121 (CFMAC)

Quantum optics studies the interaction between light and matter on the most fundamental level, namely, as energy exchanges between single photons and single quantum emitters. In the case of an excited atom interacting with the electromagnetic vacuum, this coupling is fundamentally isotropic, leading to a spontaneously emitted photon with no preferred direction. Nevertheless, recent advances in trapping atoms close to nanophotonic waveguides have shown that the strong confinement of light can naturally lead to situations in which excited atoms decay asymmetrically into left- and right-moving photons along the waveguide. This directional light-matter interaction has been recently called ‘chiral’, and opens fascinating perspectives for realizing directional quantum networks with photons as quantum information carriers, as well as novel many-body quantum phases of light and matter. In this context, we study the implications of chiral interactions on the driven-dissipative dynamics of many quantum emitters coupled via a one-dimensional waveguide. In particular, we determine how the interplay between coherent drive, chiral interactions and collective decay can lead a chain of two-level atoms to a steady state that is pure and multi-partite entangled, which can be interpreted as a novel non-equilibrium magnetic phase of matter. In addition, we propose various purely atomic realizations of this model, where not only the emitters but also the waveguides are realized with atomic degrees of freedom such as Rydberg atoms, trapped ions, or Bose condensed atoms. Therefore, instead of photons, phonons or spin excitations mediate the chiral interactions, giving a high degree of control over the resulting open many-body dynamics for the emitters. These engineered atomic setups also provide a route to controllably access physics beyond the Markovian quantum optics paradigm. For instance, using modern many-body numerical methods, we include the full dynamics of the atomic waveguide on the same footing as the emitters and thereby describe non-Markovian effects such as retardation in the exchange of excitations between emitters and non-linear dispersive effects. On the other hand, the same framework allows for the realization of `on-chip’ chiral quantum networks, which we illustrate with basic building blocks for quantum information applications, such as state transfer protocols or time-reversal of wave-packets.

Majorana bound states from exceptional points in non-topological superconductors

Ramón Aguado, Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC)
Date & location: Mon Jun 06 2016 12:00:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)

Recent experimental efforts towards the detection of Majorana bound states have focused on creating the conditions for topological superconductivity. In this talk, I will discuss an alternative route, which achieves zero-energy Majorana bound states when a topologically trivial superconductor is strongly coupled to a helical normal region. Such a junction can be experimentally realised by e.g. proximitizing a finite section of a nanowire with spin-orbit coupling, and combining electrostatic depletion and a Zeeman field to drive the non-proximitized portion into a helical phase. Majorana zero modes emerge in these junctions without fine-tuning as a result of charge-conjugation symmetry, and can be ultimately linked to the existence of `exceptional points' (EPs) in parameter space (non-hermitian degeneracies extensively studied in photonics [1-3], but seldom discussed in electronic systems), where two quasibound Andreev levels bifurcate into two quasibound Majorana zero modes. After the EP, one of the latter becomes non-decaying and fully localised as the junction approaches perfect Andreev reflection. As I will show, these Majoranas generated through EPs exhibit the full range of properties associated to conventional closed-system Majorana bound states, while not requiring topological superconductivity [4].
[1]The physics of exceptional points, W. D. Heiss, J. Phys. A 45, 444016 (2012).
[2]Spawning rings of exceptional points out of Dirac cones, Bo Zhen et al, Nature 525, 354 (2015)
[3]Topologically protected defect states in open photonic systems with non-hermitian charge-conjugation and parity-time symmetry, Simon Malzard, Charles Poli, Henning Schomerus, Phys. Rev. Lett. 115, 200402 (2015)
[4] Majorana bound states from exceptional points in non-topological superconductors, P. San-Jose, J. Cayao, E. Prada and R. Aguado, Scientific Reports, 6, 21427 (2016)

Entropy/information flux in Hawking radiation
Ana Alonso Serrano, Instituto de Física Fundamental (IFF, CSIC)
Date & location: Wed Jun 01 2016 15:30:00 GMT+0200 (CEST), Seminar Room, Serrano 121 (CFMAC)


Blackbody radiation contains (on average) an entropy of 3.9+-2.5 bits per photon. This applies not only to provervial case of “burning a lump of coal”, but also to the Hawking radiation from black holes. The flip side of this observation is the information budget: If the emission process is unitary, as it certainly is for normal physical burning, then this entropy is exactly compensated by the “hidden information” in the correlations. We shall now extend this argument to the Hawking radiation from black holes, demonstrating that the assumption of unitarity leads to a perfectly reasonable entropy/information budget. The key technical aspect of our calculation is the “average subsystem” approach, but applied to a tripartite pure system consisting of the (black hole)+(Hawking radiation)+(rest of the universe).