The QM Talks@CBPF are back! The kickoff of the series’ year will be given next Wednesday (07.02) by Fabricio Toscano (UFRJ). Fabricio will give us an overview about uncertainty inequalities, with special focus on entropic inequalities for coarse-grained continuous variables systems. The talk will be based on Fabricio’s recent works in collaboration with Stephen P. Walborn (UFRJ), Lukasz Rudnicki (Polish Academy of Sciences), and Daniel S. Tasca (UFF). Be sure to be there.
Title: Quantum Uncertainty Relations for Coarse-grained Measurements
Speaker: Fabricio Toscano (UFRJ)
Coordinates: room 601C, CBPF. 07.02, 16h00
Abstract: We review recent developments in quantifying quantum uncertainty when coarse-grained measurements are performed on quantum systems. Particular attention is given to coarse-graining of continuous variable systems, for which many well-known uncertainty relations break down under strong coarse graining. The use of these uncertainty relations in quantum information schemes is also discussed.
The year is getting to the end, but we are still full power! To finish the 2017 series of QM Talks@CBPF we have pleasure to receive Jean-Pierre Gazeau.
Prof. Gazeau is a emeritus professor from Université Paris Diderot, and a visiting professor at CBPF. Jean-Pierre has many contributions to theoretical physics, most specially to the theory of quantum mechanics, both in applied and fundamental aspects — some of his contributions can be found in his book Coherent States in Quantum Physics, or in his long list of publications. In this talk he will focus on some foundational aspects of quantum mechanics. Check out the details of the talk below, and see you there!
Title: Orientations in the plane as quantum states
Speaker: Jean-Pierre Gazeau (Diderot/CBPF)
Coordinates: room 601C, CBPF. 20.12, 16h00
Abstract: I will introduce and to discuss some of the most basic fundamental concepts of quantum physics by using orientations or angles in the plane, illustrated through linear polarizations. Starting with the Euclidean plane, which is certainly a paradigmatic example of a Hilbert space, orientations in it are identified with the pure states. Associating these quantum orientations with linear polarizations of light in the plane normal to its propagation constitutes the most appealing physical example of the presented formalism. The pure states form the unit circle (actually a half of it) and the mixed states form the unit disk (actually a half of it). Rotations in the plane rule time evolution through Majorana-like equations involving only real quantities for closed and open systems. Interesting probabilistic aspects are developed. Since the tensor product of two planes, their direct sum, and their cartesian product, are isomorphic (2 is the unique solution to x^x= x X x = x+x), and they are also isomorphic to C^2, and to the quaternion field H (as a vector space), I will describe an interesting relation between entanglement of real states, one-half spin cat states, and unit-norm quaternions which form the group SU(2). Finally, I will present an example of quantum measurement with pointer states lying also in the Euclidean plane.
Title: Reversing the thermodynamic arrow of time using quantum correlations
Authors: Kaonan Micadei, John P. S. Peterson, Alexandre M. Souza, Roberto S. Sarthour, Ivan S. Oliveira, Gabriel T. Landi, Tiago B. Batalhão, Roberto M. Serra, Eric Lutz
Abstract: The second law permits the prediction of the direction of natural processes, thus defining a thermodynamic arrow of time. However, standard thermodynamics presupposes the absence of initial correlations between interacting systems. We here experimentally demonstrate the reversal of the arrow of time for two initially quantum correlated spins-1/2, prepared in local thermal states at different temperatures, employing a Nuclear Magnetic Resonance setup. We observe a spontaneous heat flow from the cold to the hot system. This process is enabled by a trade off between correlations and entropy that we quantify with information-theoretical quantities.
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.
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.
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.
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 )
Coordinates: seminar room 6th floor, CBPF. 25.10, 14h30
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  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 . 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.
 P. Arrighi, S. Facchini, M. Forets, Quantum Inf. Process. (2016) 15: 3467
 G. Di Molfetta, F. Debbasch, M. E. Brachet, Phys. Rev. A 88.4 (2013): 042301