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: 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: Fernando Nicácio — 07.06, 16h00

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).

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: 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: 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.

QM Talks@CBPF: Ivan S. Oliveira – 25.05, 16h00

Now that Ivan, the leader of our NMR experimental effort here at the qig@CBPG, is out of bureaucratic duty, he’s full power back to research. This week he will tell us about some ideas he has been playing with concerning the quantum thermodynamics of NMR experiments. Be sure to check that! Details follow.

Title: Termodinâmica em experimentos de RMN

Speaker: Ivan S. Oliveira (qig@CBPF)

Coordinates: Auditorium 5th floor, CBPF. 25.05, 16h00

Abstract: A RMN tem contribuído significativamente para a compreensão de processos microscópicos na chamada “Termodinâmica Quântica”. No entanto, não existe na literatura (artigos ou livros) uma discussão dos processos termodinâmicos evolvidos nos experimentos de RMN em si. Dois fatores parecem contribuir para esta ausência na literatura especializada: 1) Experimentos de RMN pulsada são naturalmente fora do equilíbrio e, 2) O fenômeno da relaxação (uma forma paramétrica de considerar as interações entre os spins nucleares e o ambiente) dificulta muito uma descrição quântica completa. Neste seminário estas questões serão abordadas, em particular as relações entre trabalho, calor, energia interna e relaxação na RMN clássica. 5