New article: Emerging Dynamics Arising From Quantum Mechanics

Title: Emerging Dynamics Arising From Quantum Mechanics

Authors: Cristhiano Duarte (UFMG), Gabriel Dias Carvalho (CBPF), Nadja K. Bernades (UFMG), Fernando de Melo (CBPF)

Link: https://arxiv.org/abs/1705.01604

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.

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.

New article: Vestiges of quantum oscillations in the open evolution of semiclassical states

Title: Vestiges of quantum oscillations in the open evolution of semiclassical states

Author: Alfredo M. Ozorio de Almeida (qig@CBPF)

Link: https://arxiv.org/abs/1703.08533

Abstract: A single wave component of a quantum particle can in principle be detected by the way that it interferes with itself, that is, through the local wave function correlation. The interpretation as the expectation of a local translation operator allows this measure of quantum wavyness to be followed through the process of decoherence in open quantum systems. This is here assumed to be Markovian, determined by Lindblad operators that are linear in position and momentum. The limitation of small averaging windows and even smaller correlation lengths simplifies the semiclassical theory for the evolving local correlation. Its spectrum has a peak for each classical momentum, subjected to Gaussian broadening with decoherence. These spectral lines can be clearly resolved even after the Wigner function has become positive: The correlations located far from caustics seem to be the last vestige of quantum oscillations.

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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Open postdoc positions

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.

Interested candidates can find information about the call at http://portal.cbpf.br/chamada-publica-pci-2016.

Contact us if you are willing to join our quantum information group as postdoc!

New Article: Experimental rectification of entropy production by a Maxwell’s Demon in a quantum system

Our NMR “quantum computer” is once again used to probe the foundations of quantum thermodynamics. The qig@CBPF, in collaboration with the quantum information group from the UFABC, were now able to create a quantum Maxwell’s Demon! See the details below, and check the full article here.

Title: Experimental rectification of entropy production by a Maxwell’s Demon in a quantum system

Authors: P. A. Camati, J. P. S. Peterson, T. B. Batalhão, K. Micadei, A. M. Souza, R. S. Sarthour, I. S. Oliveira, R. M. Serra

Abstract: Maxwell’s demon explores the role of information in physical processes. Employing information about microscopic degrees of freedom, this “intelligent observer” is capable of compensating entropy production (or extracting work), apparently challenging the second law of thermodynamics. In a modern standpoint, it is regarded as a feedback control mechanism and the limits of thermodynamics are recast incorporating information-to-energy conversion. We derive a trade-off relation between information-theoretic quantities empowering the design of an efficient Maxwell’s demon in a quantum system. The demon is experimentally implemented as a spin-1/2 quantum memory that acquires information, and employs it to control the dynamics of another spin-1/2 system, through a natural interaction. Noise and imperfections in this protocol are investigated by the assessment of its effectiveness. This realization provides experimental evidence that the irreversibility on a non-equilibrium dynamics can be mitigated by assessing microscopic information and applying a feed-forward strategy at the quantum scale.

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