We continue our series of seminars with a talk by Daniel S. Tasca (IF-UFF). Daniel and co-authors have recently performed an experiment where they verify that a coarse graining procedure might break the mutual unbiasedness between conjugate variables. That sounds interesting! To know more about the topic, check out Daniel’s article here, and attend to his talk. See you there.
Title: Mutual Unbiasedness in Coarse-grained Continuous Variables
Speaker: Daniel S. Tasca (IF-UFF)
Coordinates: room 601C, CBPF. 27.09, 16h00
Abstract: The notion of mutual unbiasedness for coarse-grained measurements of quantum continuous variable systems is considered. It is shown that while the procedure of “standard” coarse graining breaks the mutual unbiasedness between conjugate variables, this desired feature can be theoretically established and experimentally observed in periodic coarse graining. We illustrate our results in an optics experiment implementing Fraunhofer diffraction through a periodic diffraction grating, finding excellent agreement with the derived theory. Our results are an important step in developing a formal connection between discrete and continuous variable quantum mechanics.
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.
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.
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.
F. Nicacio, M. Paternostro, & A. Ferraro,
Determining stationary-state quantum properties directly from system-environment interactions,
Phys. Rev A 94, 052129 (2016).
Title: Emerging Dynamics Arising From Quantum Mechanics
Authors: Cristhiano Duarte (UFMG), Gabriel Dias Carvalho (CBPF), Nadja K. Bernades (UFMG), Fernando de Melo (CBPF)
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).
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.
Title: Vestiges of quantum oscillations in the open evolution of semiclassical states
Author: Alfredo M. Ozorio de Almeida (qig@CBPF)
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.