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.

Reference:  1. 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).