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 )

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

Our next seminar from the series QM Talks@CBPF will be given by Fernando Nicácio, aka, Boiúna (or is it the other way around?).
See the details below. See you there!

Title: Determinando propriedades de estados estacionários diretamente
das interações do sistema com o ambiente

Speaker: Fernando Nicácio (UFRJ)

Coordinates: room 601C, CBPF. 07.06, 16h00

Abstract: Considerando estados estacionários de um sistema de variáveis contínuas evoluindo sob uma dinâmica não-unitária, revelamos a conexão entre propriedades e simetrias do sistema com os parâmetros dinâmicos da evolução. Em particular, estabelecemos uma relação entre a equação de Lyapunov para sistemas dinâmicos não-Hamiltonianos e as soluções estacionárias de uma equação mestra de Lindblad independente do tempo para modos bosônicos. Explorando relações de “bona fide”, as quais são utilizadas para caracterizar propriedades quânticas genuínas (emaranhamento, “steerabilidade”, classicalidade), obtemos condições sobre os parâmetros dinâmicos da equação de Lindblad que fazem com que o sistema seja conduzido a um estado estacionário que detém tais propriedades. Desenvolveremos também um método para capturar as simetrias do estado estacionário baseado nas simetrias da equação de Lyapunov. E por fim, apresentamos um método teórico simples para engenharia de reservatórios baseado nos resultados preliminares.

Referência:
F. Nicacio, M. Paternostro, & A. Ferraro,
Determining stationary-state quantum properties directly from system-environment interactions,
Phys. Rev A 94, 052129 (2016).

Abstract: Physics dares to describe Nature from elementary particles all the way up to cosmological objects like cluster of galaxies and black holes. Although a unified description for all this spectrum of events is desirable, an one-theory-fits-all would be highly impractical. To not get lost in unnecessary details, effective descriptions are mandatory. Here we analyze what are the dynamics that may emerge from a fully quantum description when one does not have access to all the degrees of freedom of a system. More concretely, we describe the properties of the dynamics that arise from Quantum Mechanics if one has only access to a coarse grained description of the system. We obtain that the effective channels are not necessarily of Kraus form, due to correlations between accessible and non-accessible degrees of freedom, and that the distance between two effective states may increase under the action of the effective channel. We expect our framework to be useful for addressing questions such as the thermalization of closed quantum systems, and the description of measurements in quantum mechanics.

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).

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.

The Brazilian Center for Research in Physics (CBPF), a research unit of the Ministry of Science, Technology, Innovations and Communications (MCTIC), has openings for 13 postdoctoral scholar positions in various research topics. The topics include quantum information, both theory and experiment.

The Paraty Quantum Information School and Workshop is over… or is it?
The actual conference it is over, but its effects are still strongly felt. Our next seminar of the series QM Talks@CBPF will be delivered by Nadja K. Bernardes, a researcher from UFMG. She gave such a nice talk in Paraty, that we invited her over to hear more details… She was faster, however, and is already collaborating with the qig@CBPF in an experiment to see some non-Markovian effect. So she’ll no only give a talk here, but also fine tune the details of the experiment they are performing.

These are not the only consequences of the last Paraty in Nadja’s life… She will be in the organization of the Paraty 2017!

The details of her talk are below. See you there!

Speaker: Nadja K. Bernardes (UFMG)

Title: Experimental observation of weak non-Markovianity

Coordinates: room 601D, CBPF. 16.09, 16h00

Abstract: Non-Markovianity has recently attracted large interest due to significant advances in its characterization and its exploitation for quantum information processing. However, up to now, only non-Markovian regimes featuring environment to system backflow of information (strong non-Markovianity) have been experimentally simulated. Here, we report an all-optical observation of the so-called weak non-Markovian dynamics. Through full process tomography, we experimentally demonstrate that the dynamics of a qubit can be non-Markovian despite an always increasing correlation between the system and its environment. We also show the transition from the weak to the strong regime by changing a single parameter in the environmental state, leading us to a better understanding of the fundamental features of non-Markovianity.

Edit 27.08.2015. The colloquium given by Steve is already online!

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Tomorrow (25.09.15) CBPF will award the “CBPF prize” to Stephen Patrick Walborn… I mean, to Steve. Actually, you should pronounce his name as “Xtiiive”, as dictated by the carioca accent that he speaks with mother-tongue fluency. Stephen was born in USA; he then became Steve, when he moved to Belo Horizonte to do his PhD; and finally he turned into “Xtiiive”, spending his time in between the lab at the Federal University of Rio de Janeiro (UFRJ) and the Posto 6 in Copacabana… with occasional stops at Pavão Azul.

The award to Steve is more than deserved, it is natural. I’ve known Steve for more than 10 years by now, and if something changed during this time was… that I got older. Steve didn’t. As I wrote in the acknowledgement of my PhD thesis in 2006, and which is still true nowadays, Steve is tireless. The text I wrote said something along the lines “Steve is an inexhaustible source of ideas, causing me to be almost embarrassed when he would tell me at ten o’clock in the morning (after he had gone to the beach, to the gym, made an experiment, written an article and revised another – phew!) that the idea he had told me in the previous day had already been published by someone else but he had already thought of two other possibilities, for which he had already made some small calculations”. This is so true, that we made it into a framed picture which now hangs over his table in the lab… (okay, there is a Rocky Balboa picture in his office as well)

And that is just one aspect of Steve. Because he is not only a great physicist, with more than 80 articles, collecting more than 3300 citations (see his Google Scholar and cv), member of the Brazilian Academy o Sciences, professor at the physics department of UFRJ, head of the quantum optics lab, advisor of various students, mentor for many of us… but he does all that with style. Seriously, the guy is cool. Check it out.

Steve taking part in a stand up paddle competition in 2014, when he became “o Rei do Rio”.

Sure he is the main author of the first direct measurement of entanglement (by the way this is the reason he will be given the CBPF prize), but he is also “o Rei do Rio” (the king of Rio)! Sure he is the brain behind the use of the Talbot effect to do quantum computation, but he is also the founding father of the WhatsApp group “Os Fulanos de Talbot” and inventor of the already worldwide known expression “Talbôtimo”.

Steve combines in a unique way a profound knowledge of physics, with an almost childish passion for new ideas and for enjoying life. It is a great privilege for us to have Steve around, specially as such a good friend.

Now that is clear why awarding Steve the CBPF prize is natural, don’t miss the chance of getting inspired by Steve and, of course, to congratulate him. The details of this special colloquium are below. See you there.

Congratulations and thank you very much Xtiiive!

SPECIAL COLLOQUIUM: AWARD OF THE CBPF PHYSICS PRIZE OF 2015

Awardee: Stephen P. Walborn (IF-UFRJ)

Seminar title: Medida Direta de Emaranhamento Quântico

Coordinates: 25th of August, 16h. In the auditorium located at CBPF’s 6th floor.

Summary: O emaranhamento quântico tem sido um aspecto controverso da Física Quântica desde 1935, quando Einstein, Podolsky e Rosen postularam que esta “fantasmagórica ação à distância” era apenas uma consequência teórica de uma teoria incompleta. Na década de 1960, John Bell mostrou que a existência (ou não) de correlações mais fortes do que as correlações clássicas, tais como emaranhamento, poderia ser testada experimentalmente. Desde então, quase todos os resultados experimentais parecem confirmar a existência de emaranhamento na natureza.

Nos últimos vinte anos o emaranhamento vem sendo visto não só como um aspecto fundamental da teoria quântica, como também um recurso que poderia ser utilizado para realizar tarefas que não podem ser executadas dentro das leis da física clássica. A criptografia quântica e o teletransporte prometem revolucionar o modo de transmissão de informações secretas. Enquanto os computadores quânticos permitirão a solução mais rápida de vários importantes problemas computacionais. Estas aplicações estão vinculadas frequentemente a conhecimentos fundamentais sobre o papel da informação e das correlações no mundo quântico.

Já que o emaranhamento é um recurso, será que poderia ser quantificado? Esta pergunta foi respondida positivamente e várias métricas têm sido propostas. Em geral, os quantificadores de emaranhamento são funções da matriz de densidade e não correspondem diretamente a um observável físico que pode ser medido no estado quântico. No entanto, em 2004, foi mostrado que o emaranhamento pode ser medido diretamente se temos acesso a duas cópias do estado quântico simultaneamente. Em 2006, a primeira medição direta de emaranhamento quântico foi realizada no Laboratório de Ótica Quântica do Instituto de Física da UFRJ. Nesta palestra, discutirei este experimento e mencionarei brevemente alguns resultados importantes que têm sido obtidos desde então.