QM Talks@CBPF: Alexandre B. Tacla — 13.11, 16h00

Our next talk in the series QM Talks@CBPF will be delivered by Alexandre B. Tacla (Glasgow). Alexandre has many interests, and a broad knowledge. In this talk he will tell us about his recent results on how to deal with complex many-body systems in an efficient way.

Note that this week, due to the holiday celebrating the Proclamation of the Republic in Brazil on Wednesday, the talk will be on Monday. See the full info below, and be sure to not miss this talk!

Title: Particle-correlated states: A non-perturbative treatment beyond mean field

Speaker: Alexandre B. Tacla (Glasgow)

Coordinates: room 601C, CBPF. 13.11 (Monday), 16h00

Abstract: Many useful properties of dilute Bose gases at ultralow temperatures are predicted precisely by the (mean-field) product-state Ansatz, in which all particles are in the same single-particle state. However, in situations where particle-particle correlations become important, this technique fails and more sophisticated methods are required. In this talk, I will introduce a new set of states that include quantum correlations nonperturbatively: The particle-correlated state (PCS) of N = l × n particles is derived by symmetrizing the n-fold product of an l-particle quantum state. Quantum correlations of the l-particle state “spread out” to any subset of the N bosons by symmetrization. Specifically, I will present the PCS theory for the ground-state of bosonic systems constructed from a two-particle pure state (l=2) [Phys. Rev. A 96, 023621 (2017)]. In particular, I will show (i) how to simulate PCS efficiently for large systems and (ii) how to calculate analytically the reduced density matrices (correlation functions) directly from the PCS normalization factor. Lastly, I will discuss the efficacy of PCS when applied to the two-site Bose-Hubbard model. The key result is that the PCS Ansatz can faithfully represent the exact ground-state over the entire parameter region from a superfluid to a Mott insulator.

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QM Talks@CBPF: Marcelo F. Santos — 08.11, 16h00

This week we have the pleasure to receive Marcelo F. Santos (UFRJ) as a speaker in our series QM Talks@CBPF. Marcelo and co-authors have recently put in the arXiv an intriguing paper: Photonic Counterparts of Cooper Pairs. This article, already accepted for publication in Physical Review Letters, attracted quite some attention (see here the Nature News feature on the article) for proposing that photons can interact inside a medium in a way very similar to that of electrons in a superconducting material, forming the so-called Cooper pairs. Got interested?! Then do not miss Marcelo’s talk. The info follows:

Title: Photonic Cooper pairs

Speaker: Marcelo F. Santos (UFRJ)

Coordinates: room 601C, CBPF. 08.11, 16h00

Abstract: Photons are the elementary particles of light. Contrary to most particles, photons do not interact directly with each other in vacuum. However, when propagating in a material, e.g. water, photon pairs may interact through the medium. In the Raman effect, for example, it is possible that a photon creates or absorbs a vibrational excitation of the material. In this work, we demonstrate theoretically and experimentally that photon pairs may interact via a virtual vibration, meaning that the energy exchanged in the process does not correspond to a quantum of vibrational energy. The same process occurs in a metal at very low temperatures, where virtual vibrations of the medium create an effective attractive interaction between electrons, forming the so-called Cooper pairs. This phenomenon changes a normal metal into a superconductor – a zero-resistance state. We have shown theoretically and experimentally the analogue of this phenomenon with light, namely an effective photon-photon interaction mediated by a virtual vibration, i.e, a photonic Cooper pair. An important next step is to test how far the analogy with superconductivity extends.

QM Talks@CBPF: Thiago Guerreiro — 01.11, 16h00

Following with our series of seminars QM Talks@CBPF, the next talk will be given by Thiago Guerreiro (PUC-RJ). Thiago has just returned to Brazil after postdoc and PhD in the group of Nicolas Gisin. In this “welcome back” talk, Thiago will tell us about his recent results and also about his research plans. Be sure to be there!

Title: Table-top high-energy quantum physics

Speaker: Thiago Guerreiro (PUC-RJ)

Coordinates: room 601C, CBPF. 01.11, 16h00

Abstract: Often in history, important measurements and discoveries were preceded by long periods of technical development. Today, fundamental physics may be at the edge of a new exciting age which will exploit the development of so-called quantum technologies. In this talk I will discuss examples of how precise control over quantum matter can lead to new developments in fundamental physics, from gravitational waves to the search for new particles and interactions of nature.

QM Talks@CBPF: Pedro C. da Silva — 11.10, 16h00

Next in our series QM Talks@CBPF is a talk by Pedro C. da Silva, PhD student here at CBPF. In this talk Pedro will show some interesting results he got during his stay in Maryland, collaborating with the group of Prof. Stephen P. Jordan. Be sure to be there! Details follow.

Title: Quantum Algorithm for Simulating the Wave Equation

Speaker: Pedro C. da Silva (CBPF)

Coordinates: room 601C, CBPF. 11.10, 16h00

Abstract: We present a quantum algorithm for simulating the wave equation under Dirichlet and Neumann boundary conditions. The algorithm uses Hamiltonian simulation and quantum linear system algorithms as subroutines. It relies on factorizations of discretized Laplacian operators to allow for improved scaling in truncation errors and improved scaling in state preparation relative to general purpose linear differential equation algorithms. We also consider using Hamiltonian simulation for Klein-Gordon equations and Maxwell’s equations.

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).