QM Talks@CBPF: Víctor Montenegro — 28.06, 16h00

Next week in our QM Talks@CBPF series we’ll have a talk by Víctor Montenegro, from the Pontificia Universidad Catolica de Chile. Victor holds a postdoc position at the PUC-Chile in the group lead by Miguel Orszag — a group which has contributed a lot to the development of the quantum optics area.

Victor is on vacation in Rio, and he was very kind to contact us and to accept to give a talk at CBPF. See the details of the talk below, and be sure to be there!

Title: Macro-mechanical quantum state superposition via spin post-selection in dispersive systems

Speaker: Víctor Montenegro (PUC- Chile)

Coordinates: room 601C, CBPF. 28.06, 16h00

Abstract: Macroscopic quantum superposition states are fundamental to test the classical-quantum boundary and present suitable candidates for quantum technologies. Although the preparation of such states have already been realized, the existing setups commonly consider external driving and resonant interactions, which might limit scalability for quantum computation purposes. Motivated by these, we present a scheme to prepare non-classical states of a macroscopic mechanical object. The protocol comprises a probabilistic qubit (0 and 1 phononic states) superposition, and the generation of mechanical Schroedinger’s cat states. To realize this, we have considered an open spin-mechanical quantum system via conditional displaced interaction Hamiltonian in the dispersive regime without any need for adjusting resonances. Therefore, in comparison with previous works on the matter, our proposal does not rely on any non-linearity, energy exchange nor external pumping —which could be an advantage for scalability purposes. Our probabilistic preparation protocol is uniquely based on two steps. Firstly, we weakly evolve the spin-mechanical system for a time t, allowing us to truncate the oscillator Hilbert space up to a single phonon excitation. Subsequently, we then proceed to post-select the spin system. The latter step aims to prepare (probabilistically) any mechanical qubit superposition. Our results can be understood within the clear connection between the quantum coherence of the mechanics and the amplification of the position and momentum quadratures on average.

QM Talks@CBPF: Alexandre Baron Tacla – 14.01, 16h00

Happy new year! 2016 seems like a very promising year, with many projects getting into more consistent shape, and others about to finish! Of course we need to start new projects as well, and that is one more reason for welcoming Alexandre Baron Tacla, from the University of Strathclyde, Glasgow, in our qig@CBPF. We’ve been chatting a lot these days, specially together with our postdoc Alex, and I’m sure something very interesting is coming out soon. Before that, however, Alexandre (don’t get confused with the names!) will tell us about his most recent results in our first QM Talks@CBPF of the year. Look up the details of the talk below, and stay tuned for a great 2016!

Speaker: Alexandre Baron Tacla (University of Strathclyde)

Title: Controllable electron interactions in quantum dots coupled to nanowires

Coordinates: room 601D, CBPF. 14.01, 16h00

Abstract: We theoretically study transport properties in quantum dot devices proximity coupled to superconducting nanowires. In particular, we investigate the controllable transition from resonant pair tunneling to Andreev bound states, which has been recently observed in nanodevices fabricated at the interface of the oxide heterostructure LaAlO3/SrTiO3. We show that such a transition in transport features can signify a Lifshitz transition, at which electron interactions change from attractive to repulsive. The ability to tune interactions on a reconfigurable nanoelectronic device could be a useful ingredient for solid state quantum simulation. Lastly, we discuss an alternate description for this transition in terms of magnetic impurities.

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: Bruno Juliá-Díaz — 28.04, 16h

Update (29.04.14): Here are the slides from the excellent talk given by Bruno. Lot’s of results and new ideas to work on!


Bruno, thank you for your talk and visit!

Our next QM Talk@CBPF will be given by Bruno Juliá-Diaz, from the University of Barcelona. Bruno is currently visiting the qig@CBPF, more specifically he is collaborating with Itzhak Roditi, and will stay here up to 05.05. You can find the details of his talk below.

See you there!

Speaker: Bruno Juliá-Diaz (U. Barcelona)

Coordinates: auditorium 5th floor, CBPF. 28.04, 16h

Title: Bosonic Josephson junctions: Spin-squeezing, fragmentation and shortcuts to adiabaticity

Abstract: I will consider a gas of ultracold atoms at zero temperature trapped in a double-well potential. In the simplest case the system can be described using just two modes. I ll go through the different quantum many-body correlations present in the ground state of the system depending on the character of the atom-atom interactions.
Afterwards, I ll discuss the presence of pseudo-spin squeezed states in the spectrum and a way to produce them in fast times using a protocol to shortcut the adiabatic following useful for the Hamiltonian describing the system.


Shortcut to adiabaticity in internal bosonic Josephson junctions
A. Yuste, B. Juliá-Díaz, E. Torrontegui, J. Martorell, J. G. Muga, A. Polls
Phys. Rev. A 88, 043647 (2013)

Dynamic generation of spin-squeezed states in bosonic Josephson junctions
B. Juliá-Díaz, T. Zibold, M. K. Oberthaler, M. Melé-Messeguer, J. Martorell, A. Polls
Phys. Rev. A 86, 023615 (2012)

Macroscopic self trapping in BECs: analysis of a dynamical quantum phase transition
B. Julia-Diaz, D. Dagnino, M. Lewenstein, J. Martorell, A. Polls
Phys. Rev. A 81, 023615 (2010)

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.”


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!

QM Talks@CBPF: Pablo B. Blostein (UNAM) — 24.07, 16h

Continuing with our series of seminars, we have Pablo B. Blostein from Universidad Nacional Autónoma de México (UNAM). He’ll show us how to control decoherence by postselection. Be sure to be there! Details below.

Notice the time change! Our QM Talks@CBPF are now 16h!!

Speaker: Pablo B. Blostein (IIMAS-UNAM)
Title: Using postselection to control ground state quantum beats in Cavity QED
Coordinates: 24.07, 16h @ CBPF Seminar room 601C
Abstract: Ground state quantum beats observed in the second order intensity correlation from a continuously driven atomic ensemble inside a two mode optical cavity are subject to a frequency shift and decoherence. While driving the cavity with light of linear polarization (π transitions) the second order autocorrelation function is measured in the undriven mode (orthogonal polarization): a first photon detection prepares a superposition of atomic ground state Zeeman sublevels and the second measures the ground state beats. Between these two detections, the atoms can become excited and return to the ground state, emitting most of the photons into modes other than the cavity modes. Depending on the drive strength this process can happen several times. Each time there is a relative phase advance between the Zeeman sublevels. The information of this phase advance and its associated decoherence is then leaked into the modes that are not the cavity modes, which form the environment. It is possible to get information about the number of photons leaked into the environment by monitoring the driven mode. Here we propose a scheme to manipulate the loss of amplitude of the beats (decoherence) and the beat frequency shift, by postselecting on the basis of information gathered through measurement of the driven cavity mode. This proposal is a new strategy compared with controlling the decoherence and light shift through turning off the driven field.