COTEO@CBPF: Giuseppe Di Molfetta — 25.10, 14h30

From this Friday (20.10) up to the end of the month we have the pleasure to receive Giuseppe Di Molfeta at CBPF. Giuseppe has many contributions to the topic of quantum walks. More specifically he employs quantum walks to simulate all sort of systems: from neutrino oscillations and Dirac equation, all the way up to gravity! The latter is the subject of the talk he will deliver in the Theory Seminar. See the details below, and be sure to be there!

Title: Quantum walking in curved spacetime

Speaker: Giuseppe Di Molfetta (Université Aix-Marseille )

Coordinates: seminar room 6th floor, CBPF. 25.10, 14h30

Abstract:In the framework of Quantum Simulation, a crucial topic for the exploration of physical situations where experiments are currently hard or impossible to setup (e.g. quantum gravity), Quantum Walks (QW) are increasingly recognized as prominent models. A discrete-time QW is essentially a unitary operator driving the evolution of a single particle on the lattice. Some QWs admit a continuum limit, leading to familiar PDEs (e.g. the Dirac equation). We introduce Grouped QWs, a generalization of the usual QWs where (i) the input is allowed a simple prior encoding and (ii) the local unitary coin is allowed to act on larger than usual neighborhoods. In [1] it was shown that the continuum limit of this class of QWs leads to an entire class of PDEs, encompassing the Hamiltonian form of the massive Dirac equation in (1 + 1) curved spacetime [2]. Therefore a certain QW provides us with a unitary discrete toy model of a test particle in curved spacetime, in spite of the fixed background lattice.
Here we take a step further and discretize the coin operator itself, only allowing, as elementary local unitary operator, the identity (no propagation) or the Pauli X operator (full-speed propagation). This discretization has the practical advantage of allowing easier experimental implementation, as well as of being of interest for studying the quantization of the metric. We prove that we can obtain the Dirac equation in the case of constant background metric. We also thoroughly analyze the non-constant metric case showing how, due to a non-differentiability issue in the discrete model, a new term arises in the differential equation, deviating from the usual Dirac equation.

[1] P. Arrighi, S. Facchini, M. Forets, Quantum Inf. Process. (2016) 15: 3467
[2] G. Di Molfetta, F. Debbasch, M. E. Brachet, Phys. Rev. A 88.4 (2013): 042301


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: Yelena Guryanova — 30.11, 16h00

Title: Thermodynamics of quantum systems with multiple conserved quantities

Speaker: Yelena Guryanova (IQOQI- Institute for Quantum Optics and Quantum Information)

Coordinates: room 601D, CBPF. 30.11, 16h00

Abstract: We consider a generalisation of thermodynamics that deals with multiple conserved quantities at the level of individual quantum systems. Each conserved quantity, which, importantly, need not commute with the rest, can be extracted and stored in its own battery. Unlike in standard thermodynamics, where the second law places a constraint on how much of the conserved quantity (energy) that can be extracted, here, on the contrary, there is no limit on how much of any individual conserved quantity that can be extracted. However, other conserved quantities must be supplied, and the second law constrains the combination of extractable quantities and the trade-offs between them which are allowed. We present explicit protocols which allow us to perform arbitrarily good trade-offs and extract arbitrarily good combinations of conserved quantities from individual quantum systems.

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.

EXP Seminar: Dieter Suter — 10.09, 15h

As I said in the previous post, the next days we have the visit of Prof. Marcos Saraceno (Tandar Laboratory, CNEA) and Prof. Dieter Suter (Dortmund University). Both of them will be giving talks on subjects related to quantum information; Marcos on the theory side, and Dieter on the experimental front. Below is the info on Dieter’s talk, which is organized by CBPF’s experimental department (see here for Marcos’ talk). See you there!

Speaker: Dieter Suter (Dortmund University)

Coordinates: auditorium Oliveira Castro, 1st floor, CBPF. 10.09, 15:00h

Title: Magnetic resonance of nanoscale samples: principles and applications

Abstract: Magnetic resonance provides valuable information on virtually every type of material, in physics, chemistry, medicine and other fields. In many applications, its usefulness is only limited by its inherently low sensitivity. If the amount of available sample is limited, it becomes difficult to acquire a signal that exceeds the thermal noise of the detector. In these cases, it is often possible to increase the signal by several orders of magnitude by adapting the resonator to the available sample. We show examples from NMR and EPR, where miniaturized resonators allow one to measure samples with dimensions in the nanometer to micrometer range. Apart from increasing the sensitivity, these microresonators also generate microwave fields that are orders of magnitude stronger than those in conventional resonators for the same amount of available microwave power.

QM Talks@CBPF: Stefan Boettcher – 14.08, 16h

Our series of quantum mechanics related talks continues at full power. This week we have Stefan Boettcher (Emory) visiting us, and we have been discovering a lot of overlapping research lines! Stefan has a Science without Borders project with Renato Portugal, from the quantum information group at LNCC, and was very kind to drop by and deliver a talk here at CBPF. See the details of the talk below. See you there!

Speaker: Stefan Boettcher (Emory)

Coordinates: room 601D, CBPF. 14.08, 16:00h

Title: Renormalization Group for Quantum Walks

Abstract: Grover proposed a quantum algorithm to accomplish the search for an entry in an unstructured list that can succeed quadratically faster than any classical search algorithm. This has raised significant interest in designing theoretical and experimental implementations of such an algorithm in real, structured systems. There has been less focus on the physical engine that drives the quantum search, namely the quantum version of a random walk. Here, we analyze the asymptotic properties of such a quantum walk by developing the real-space renormalization group (RG), which we introduce for the simple walk on a line. Unitarity precludes the stochastic properties familiar from Markovian random walks. Thus, the canonical analysis of the RG-recursions at a real fixed-point is replaced by a study of chaotic attractors in the complex plane. In turn, we are rewarded with a rich phenomenology, such as localization effects, and some exact results.