Master dissertation defense @ CBPF: Pedro Correia — 05.05, 14h00

This Friday, Pedro Correia, student at the qig@CBPF, will defend his master dissertation.
The dissertation title is “Entanglement in Coarse-grained Systems”, and it contains results on: i) coarse-grained entanglement dynamics in spin-chains, and ii) coarse-grained entanglement in micro-macro systems, with an application to the measurement problem. The details of the defense talk are below. Everyone is invited to attend it.
Boa defesa, Pedrinho!

Title: Entanglement in Coarse-grained Systems

Candidate: Pedro Correia (qig@CBPF)

Dissertation Committee: Marcelo Sarandy (UFF), Roberto Sarthour (CBPF), Gabriel Aguilar (UFRJ), Raul Vallejos (CBPF), and Fernando de Melo (CBPF).

Coordinates: Auditorium 6th floor, CBPF. 05.05, 14h00.

Abstract: In the present work we investigate the behavior of entanglement in coarse-grained systems. Our approach is basically composed of two parts.
In the first, we construct a coarse graining map that describes the entanglement dynamics in a spin-chain considering a “blurred” detection of the system. In the second part we derive an equation of motion for entanglement in 2×D systems, when the second subsystem undergoes an arbitrary channel. Finally, considering as the channel in this equation the coarse-graining map created in the first part, we are able to investigate the measurement process, when a detector (macroscopic object) interacts with a quantum system. Then we see how entanglement behaves as the detector increases.

QM Talks@CBPF: Alex Bouvrie — 29.03, 16h00

We resume our series of seminars with a talk by Alex Bouvrie, a postdoc here in the qig@cbpf. Indeed, Alex has just being awarded a new postdoc fellowship (PCI), which will allow him to stay a little longer with us. Luckly for us!

This time he’ll tell us about his latest results on composite fermions, and how the entanglement between them explains some features of experiments producing  Bose-Einstein condensates with fermions… got confused? Check out the details of his talk below,  and see you there!

 

Title: Quantum information in ultracold interacting Fermi gases

Speaker: Alex Bouvrie (CBPF)

Coordinates: room 601C, CBPF. 29.03, 16h00

Abstract: Recently the quantum information group of the CBPF showed that the application of the composite bosons theory [1] to ultracold  interacting Fermi gases is remarkable [2,3]. The effects of the underlying fermionic structure of composite bosons (molecules made from two fermions) formed in two-component Fermi gases, are well described by this theory in the strong binding regime and are reflected in experimentally measurable observables  [2]. For example, the fraction of ground state molecules in an interacting Fermi gas, i.e. the (Bose-Einstein) condensate fraction, depends on the entanglement created by the Feshbach induced interaction between the fermions that make up the molecules. Different fermion species interact via Feshbach resonance and fermion pairs interact among them via Pauli principle or fermion exchange interaction. Ultracold interacting Fermi gases are, therefore, strongly correlated (entangled) systems. In this seminar we will present our latests results [1] and show that the theory of composite bosons can be a useful tool to theoretically describe these quantum correlations. We will also show that Pauli correlations between fermion pairs (molecules) are essential to preserve the quantum coherence of the condensate in beam-splitter dynamics and how to generate entangled Bose-Einstein condensates  with these dynamical processes [3].

[1] M. Combescot, O. Betbeder-Matibet, and F. Dubin, Phys. Rep. 463, 215 (2008)
[2] P. Alexander Bouvrie, Malte C. Tichy, and Itzhak Roditi, Phys. Rev. A 95, 023617 (2017)
[3] P. Alexander Bouvrie, Malte C. Tichy, and Klaus Mølmer, Phys. Rev. A 94, 053624 (2016)

 

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QM Talks@CBPF: David Jennings – 30.08, 16h00

The Olympic games are over, but Rio is still receiving many visitors. Among them, next week, we welcome David Jennings, from Imperial College London. David’s interests are really broad, ranging from foundational issues in quantum mechanics up to cosmology. From the micro up to the macro, his research also crosses quantum thermodynamics, and this is the subject he’ll tell us about in our next QM Talks@CBPF. See the description below, and see you there!

Title: Thermodynamics and quantum information theory

Speaker: David Jennings (Imperial College London)

Coordinates: room 601D (tentative), CBPF. 30.08, 16h00

Abstract: How do we separate finite-sized effects and stochasticity from genuinely non-classical features in thermodynamics? In the past two decades, quantum information science has developed a range of results designed to perform precisely this type of separation. While these results were originally motivated by computational, information-processing and foundational concerns, more recently there is increasing work that applies such techniques to thermodynamics.
Here I will describe such approaches and discuss their strengths and weaknesses. I will argue that present approaches are poorly suited to handling such topics as quantum phase transitions, however I will also argue that such approaches do provide new perspectives on the interplay between coherence and time-dependent processes, shed light on the role of non-commutativity and emphasize structural relations between thermodynamics and the theory of entanglement.

New Article: Multipartite concurrence for identical-fermion systems

Peter A. Bouvrie, aka Alex, a postdoc from the qig@CBPF, has recently published a PRA in collaboration with the quantum information group from UFRJ, and colleagues from Mexico e Argentina — a Latin America collaboration! See the details below, and read the article here

Title: Multipartite concurrence for identical-fermion systems

Authors: A. P. Majtey, P. A. Bouvrie, A. Valdés-Hernández, and A. R. Plastino

Abstract: We study the problem of detecting multipartite entanglement among indistinguishable fermionic particles. A multipartite concurrence for pure states of N identical fermions, each one having a d-dimensional single-particle Hilbert space, is introduced. Such an entanglement measure, in particular, is optimized for maximally entangled states of three identical fermions that play a role analogous to the usual (qubit) Greenberger-Horne-Zeilinger state. In addition, it is shown that the fermionic multipartite concurrence can be expressed as the mean value of an observable, provided two copies of the composite state are available.

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.

O Pote de Ouro de Bell

Neste ano comemoramos as bodas de ouro, ou seja, os 50 anos da publicação do artigo “On the Einstein-Podolsky-Rosen Paradox”, por J. S. Bell [Physics 1, 195 (1964)]. Para celebrar esse evento, que segundo Henry Stapp é a “descoberta mais importante da ciência”, estamos organizando um pequeno encontro para discutir a essência e as implicações do teorema de Bell. Veja (e distribua!) o cartaz do evento.

Cada um dos palestrantes foi especialmente escolhido para analisar uma das várias perspectivas do teorema de Bell e suas implicações. Dessa forma esperamos que o evento seja acessível para uma grande gama de pessoas e com diversos interesses.

O evento é aberto e franqueado a todos! Pedimos, no entanto, que os interessados em participar do encontro “O pote de ouro de Bell” mandem e-mail para
bell50anos [at] gmail.com
com nome completo para que possamos liberar a entrada no CBPF.

Os seminários serão realizados no Auditório Ministro João Alberto, a partir de 13h da tarde do dia 5 de dezembro. Veja abaixo a programação:

Horário Palestrante Título
13:00 – 13:45 Ruynet M. Filho (UFRJ) Algumas considerações sobre a realidade da função de onda
13:45 – 14:30 Nelson Pinto (CBPF) A perspectiva de de Brolie-Bohm: teoria realista não-local
14:30 – 15:15 Lucas C. Céleri (UFG) Desafios experimentais das desigualdades de Bell
15:15 – 15:45 Coffee-break discussões livres!
15:45 – 16:30 Ivan S. Oliveira (CBPF) Emaranhamento e desigualdades de Bell com sistemas macroscópicos
16:30 – 17:15 Rafael Rabelo (UFMG) Mecânica quântica fora de contexto?
17:15 – 18:00 Ernesto Galvão (UFF) Não-localidade além da mecânica quântica

Nos vemos lá!!

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.