QM Talks@CBPF: Tobias Micklitz — 21.06, 16h00

Our series of seminars continues this week with Tobias Micklitz (CBPF). Tobias is an expert on many-body problems within condensed matter, especially on issues related to Anderson’s location. Recently, we’ve been discussing some ideas at the interface between condensed matter and quantum information. I’m sure something nice will come out of this interaction.

See the details of the talk below, and be sure to not miss it. See you there!

Title: Disordered Quantum Systems from Anderson- to many-body localization

Speaker: Tobias Micklitz (CBPF)

Coordinates: room 601C, CBPF. 21.06, 16h00

Abstract: Disorder is known to have dramatic effects on single particle-dynamics in low dimensional quantum systems. The absence of diffusion in dimensions smaller than three emerges within a single-particle picture where non-interacting particles, scattering off disorder, interfere with themselves and effectively get localized to a finite region in space. This ‘Anderson localization’ originates from the quantum-mechanical wave-nature of particles and is fundamentally different from classical trapping in deep valleys of a disorder potential. The impact of weak interactions on the single-particle localization problem can be subsumed as a fluctuating bath. The bath induces decoherence and thus suppresses localization. More strikingly, it has been recently proposed that (isolated) disordered quantum systems of interacting particles undergo a finite-temperature phase-transition which can be thought of as a many-body localization transition. The ‘many-body localized’ phase is characterized by the absence of ergodicity and the vanishing of transport coefficients. In the talk I will give a brief introduction into the phenomenon of (quantum) localization in disordered systems emphasizing recent trends, and then discuss a field-theory approach to the many-body localization problem.


QM Talks@CBPF: Clemens Gneiting – 11.12, 16h

As the last talk of the year, we have Clemens Gneiting, from the Quantum Optics and Statistics group of Freiburg university. I have a special relation with this group, as I was postdoc there for about 3 years with Andreas Buchleitner. Great physics, loads of fun!

Clemens is an expert on modular variables, specially in connection with violations of Bell inequalities using continuous-variables systems. But of course he has other research lines, and this time he’ll tell us about optimal coherent control of noisy quantum systems. A subject that will definitely resonate with both experimentalists and theorists. So please be sure to show up for this talk! See info below.

Speaker: Clemens Gneiting (Freiburg Univesity)

Title: Prospects of coherent control in the presence of dissipation

Coordinates: room 601D, CBPF. 11.12, 16:00h

Abstract: Genuine quantum features such as entanglement or coherence are resources as precious as fragile, and their uncovering usually requires strong efforts in isolating and controlling quantum systems. Without thorough measures, decoherence efficiently shields the quantum world from our access and hides it behind its classical guise. While there has been unprecedented progress in the quantum control of various model systems, e.g. ions, quantum dots, or cold atoms, it is impossible to completely decouple these systems from their environment and thus to fully suppress the detrimental effect of decoherence. Standard optimal control techniques therefore focus on accessing quantum features in the transient regime, and the exploration and exploitation of quantum properties is consequently confined to a finite, generically short time window. We investigate to what extent coherent Hamiltonian control can enduringly counteract the detrimental effect of decoherence. Explicitly, we determine Hamiltonians that optimally uphold desired control objectives (e.g., coherence, entanglement, or fidelity w.r.t. a target state) in the presence of dissipation. As we show, our method is applicable to both static and periodically time-dependent Hamiltonians. Finally, we also discuss modifications of the scheme due to continuous measurement and feedback.

Colloquium @CBPF: Paulo Henrique Souto Ribeiro — 30.09, 16h

Paulo H. S. Ribeiro, known to everyone as Paulão, is one of the most important figures for the quantum information community in Brazil. Paulão is the head (the arm, the leg, the heart!) of the Quantum Optics Laboratory at Federal University of Rio de Janeiro. His lab draws from close collaboration with the theoretical part of the quantum information group at UFRJ to be one of most well known groups in Brazil and worldwide.

Actually, one of the “secrets” of the quantum info group at UFRJ is that there is no such sharp distinction between theory and experiment. Of course at the end of the day there are those who will go align the mirrors and detectors, and those who will solve integrals and apply Cauchy-Schwartz whereever they can, but most of the times they are all mingled together, and the discussions fly high!

I can say that from first eye witness, as I did my PhD in this group. It was really exciting for a kid like me (at that time, at least) to be able to take part and contribute to the design of an experiment where we saw for the first time the dynamics of entanglement in an open quantum system in a very controlled way (article here). I still remember one day when I was showing Paulão the results we were getting for a two-qubits tomography and which were giving strange results. Paulão, without caring too much about my fancy Mathematica program for tomography, asked me to see the raw data file. These were just a bunch of numbers, I thought, with no real meaning… but for Paulão these numbers are the real thing, and he immediately realized that those results were not possible and suggested to swap two columns. When I did what he suggested, the reconstruction just popped out exactly as we expected. I couldn’t hold my self and shouted at Paulão: “Vai tomar no %$#” (something like “go f#$% yourself”), to which I immediately apologized, but that was out of a profound feeling of respect (and envy, I must confess) for his knowledge and understanding how things really work (this repeated many times, and still does, but I can hold myself better nowadays). We went back to the lab with Marcelo and Stephen and realized that we indeed had mixed up the label of some basis elements. After performing the experiment again, all results were perfect. Below is a picture I got during the realization of this experiment.

From left to right:  Stephen Walborn, Marcelo P. Almeida, Paulão, and Luiz Davidovich. Experimental setup to measure the entanglement dynamics for an open quantum system.

From left to right: Stephen Walborn, Marcelo P. Almeida, Paulão, and Luiz Davidovich. Experimental setup to measure the entanglement dynamics for an open quantum system.

I should emphasize that Paulão didn’t get the lab ready for him to “just” work out his ideas. He built everything from scratch. And I have the feeling that this is what he likes to do: to get a lab just starting and build it into a world class research facility. That might explain why he’s now moving from Rio (leaving the Lab at Steve’s very capable hands) to Florianópolis. I’m sure we won’t stop hearing from Paulão and his accomplishments… and we have a great excuse to go to Floripa every now and then!

It is thus more than timing that we invite Paulão for a colloquium at CBPF. It’s both an opportunity to learn once more from Paulão, and to thank him for being such unstoppable force pushing the Physics of (all) Brazil to ever better levels. See the information about Paulão’s colloquium below, and be sure to show up!

Thank you Paulão!!
(P.s.: If you have any story involving Paulão that you want/can share, or just want to thank him, please leave a comment below!)

Palestrante: Paulo Henrique Souto Ribeiro (UFRJ)

Título: Experimentos com Fótons Gêmeos: dos Fundamentos da Mecânica Quântica à Termodinâmica Quântica

Coordenadas: 30.09, 16h no auditório do 6º andar – CBPF

Resumo: Os fótons gêmeos produzidos na conversão paramétrica descendente espontânea foram produzidos pela primeira vez no início dos anos 1970. Inicialmente foram observadas correlações entre eles, das quais a correlação temporal é a mais marcante e leva à denominação “gêmeos” para o par de fótons. Eles possuem correlações quânticas naturais em seus graus de liberdade espaciais e de energia e tempo, podendo ser também preparados em estados emaranhados de polarização. Esta é a fonte de estados emaranhados mais simples e versátil que se conhece até hoje. Com estes pares de fótons e suas propriedades quânticas sem análogo clássico, foram feitos experimentos para estudar os fundamentos da Mecânica Quântica, testar algoritmos quânticos, implementar esquemas de computação e comunicação quântica e mais recentemente para a simulação de outros sistemas quânticos. Neste caso, eles começam a ter utilidade para o estudo da chamada Termodinâmica Quântica. Neste seminário, será feito um breve resumo das principais aplicações de fótons gêmeos e discutiremos experimentos recentes realizados na UFRJ para estudar correlações quânticas e para simulação de sistemas quânticos, abordando alguns aspectos termodinâmicos.

QM Talks@CBPF: Osvaldo Jiménez Farías — 18.03, 14:30h

Dear All,

Our next QM Talk@CBPF will be given by Osvaldo J. Farías. Osvaldo is joining the qig@CBPF as a postdoc to work with noisy cellular automata. That is a subject that he and me have been willing to work on for a long time, as we discovered chatting over a coffee sometime ago, and we finally got this going. Stay tuned for the results that will come out of this collaboration. I’m sure they will be plenty and very interesting!

To introduce himself to the group and to everyone willing to know more about his research, Osvaldo will give a QM Talk@CBPF covering his previous results and also telling a bit what he plans to do here at CBPF. A sneaky preview can be found here. More info below.

Very welcome Osvaldo!

Speaker: Osvaldo Jiménez Farías

Coordinates: room 601D, CBPF. 18.03, 14:30h

Title: Entanglement Dynamics: experiment and theory

Abstract: The estimation of the entanglement of multipartite systems undergoing decoherence usually requires access to the final state and its full reconstruction through quantum tomography. We found that when one of the parties of an initially entangled two-qubit system is subject to a noisy channel, a single universal curve describes the dynamics of entanglement for both pure and mixed states. Our result, is experimentally demonstrated using a linear optics setup. We also present a novel experimental setup which allows access to the noisy environment degrees of freedom. This new technique opens the possibility to study new phenomena in the dynamics of entanglement revealed by full tomography of the global system including the environment.

Review article: “Open-System Dynamics of Entanglement”

Leandro Aolita (former PhD student at IF-UFRJ and now Marie Curie fellow at Freie Universität Berlin), Luiz Davidovich (IF-URFJ), and I (Fernando) (qig@CBPF), finally put online a preliminary version of our review paper on the dynamics of entanglement in open quantum systems. You can find the review at arXiv:1402.3713 .

It took us six years of intensive work to get this done. It was almost painful, but we are really happy with the outcome! The preliminary version of the review is 76 pages long, contains 32 pictures, and 500 citations! And it’s still growing!! We tried to be very careful in not only to compile a bunch of interesting results, but also to give the necessary tools and insights as to make the results clear. We hope that it’s going to be useful and accessible both for experts and beginners, theoreticians and experimentalists. Comments are welcome!

Without further ado:

Title: Open-System Dynamics of Entanglement

Authors: Leandro Aolita, Fernando de Melo, Luiz Davidovich

Available at: arXiv:1402.3713

Abstract: “One of the greatest challenges in the fields of quantum information processing and quantum technologies is the detailed coherent control over each and all of the constituents of quantum systems with an ever increasing number of particles. Within this endeavor, the harnessing of many-body entanglement against the detrimental effects of the environment is a major and pressing issue. Besides being an important concept from a fundamental standpoint, entanglement has been recognised as a crucial resource for quantum speed-ups or performance enhancements over classical methods. Understanding and controlling many-body entanglement in open systems may have strong implications in quantum computing, quantum simulations of many-body systems, secure quantum communication or cryptography, quantum metrology, our understanding of the quantum-to-classical transition, and other important questions of quantum foundations.

In this paper we present an overview of recent theoretical and experimental efforts to underpin the dynamics of entanglement under the influence of noise. Entanglement is thus taken as a dynamic quantity on its own, and we survey how it evolves due to the unavoidable interaction of the entangled system with its surroundings. We analyse several scenarios, corresponding to different families of states and environments, which render a very rich diversity of dynamical behaviors.

In contrast to single-particle quantities, like populations and coherences, which typically vanish only asymptotically in time, entanglement may disappear at a finite time. In addition, important classes of entanglement display an exponential decay with the number of particles when subject to local noise, which poses yet another threat to the already-challenging scaling of quantum technologies. Other classes, however, turn out to be extremely robust against local noise. Theoretical results and recent experiments regarding the difference between local and global decoherence are summarized. Control and robustness-enhancement techniques, scaling laws, statistical and geometrical aspects of multipartite-entanglement decay are also reviewed; all in order to give a broad picture of entanglement dynamics in open quantum systems addressed to both theorists and experimentalists inside and outside the field of quantum information.”


Frederico Brito visits qig@CBPF — 13.01 to 22.01

The year has just started and we are happy to host Frederico Brito (IFSC-USP) one more time at the qig@CBPF!
The last time Fred was here, he gave a wonderful talk about all the (q)bits and pieces of D-Wave’s computer. Click on the picture below for his slides.
Fred's slides 2013
Unfortunately, we didn’t record his talk, but if the slides are not enough to you, then you can watch a similar talk by Matthias Troyer (ETH) following this link.

During Fred’s visit we’ll be discussing (guess what) simulation of noisy quantum computation. If you happen to be in Rio during this time and want to chat with Frederico, please just drop me a line (fmelo [at] cbpf.br).

A great and productive 2014 for all of us!


QM Talks@CBPF: Slides online!


the last two talks at our QM Talks@CBPF were just great!

Process of parameter estimation

Bruno showed us how to obtain the minimum error in the estimation of a parameter when the dynamical process that the probe undergoes, which depends on the parameter we want to estimate, is a noisy quantum one. This was an open question for almost 30 years! Congratulations to Bruno, Ruynet, and Luiz for a great series of works in quantum metrology!
Here are the slides for Bruno’s talk: Bruno’s slides

Pablo’s talk concerned the theoretical description of the beautiful cQED experiments carried out in the group of Luis Orozco (Maryland).

Experimental setup - cQED group@Maryland

In his very didactic talk, Pablo showed us how to theoretically describe the quantum beats that were measured by Orozco’s team via the two-point correlation function. More than that, he also showed us that by post-selecting some states that leak out the cavity they can suppress the decoherence in the quantum beats. And this is not only a proposal, they measured it!
The slides for Pablo’s talk are here: Pablo’s slides

Thanks again for Bruno and Pablo for the great talks!

…And stay tuned for the next QM Talks@CBPF!