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
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.
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.
This week we have many guests visiting the qig@CBPF: Roberto Serra (UFABC), Thiago Batalhão (UFABC), and Cristhiano Duarte (UFMG). Serra and Thiago are here to fine tune some details about an experiment our NMR crew is performing; while Cristhiano is here to work in a project on quantum channels from coarse-grained dynamics in collaboration with Fernando de Melo (that’s me).
We take this chance and asked Serra to deliver a talk on his latest results (which by the way involve some collaboration with the qig@CBPF). See the details below. See you there.
Title: Irreversibility and Maxwell’s Demons in quantum systems
Speaker: Roberto M. Serra – UFABC
Coordinates: Auditorium 6th floor, CBPF. 18.05, 16h00
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 into fluctuation theorems. Endeavors to incorporate information into thermodynamics acquire a pragmatic applicability within the recent technological progress, where information just started to be manipulated at the micro and nano-scale. In this seminar, we will discuss the panorama of the Thermodynamics of Information at Quantum Scales. We will introduce a trade-off relation between information-theoretic quantities empowering the design of an efficient Maxwell’s demon in a quantum system. Moreover, an experimental implementation of the Demon will also be presented. Such a creature is materialised 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 in a NMR setup. This realisation provides experimental evidence that assessing microscopic information and applying a feed-forward strategy at the quantum scale can mitigate the irreversibility on a non-equilibrium dynamics.
Happy new year! 2016 seems like a very promising year, with many projects getting into more consistent shape, and others about to finish! Of course we need to start new projects as well, and that is one more reason for welcoming Alexandre Baron Tacla, from the University of Strathclyde, Glasgow, in our qig@CBPF. We’ve been chatting a lot these days, specially together with our postdoc Alex, and I’m sure something very interesting is coming out soon. Before that, however, Alexandre (don’t get confused with the names!) will tell us about his most recent results in our first QM Talks@CBPF of the year. Look up the details of the talk below, and stay tuned for a great 2016!
Speaker: Alexandre Baron Tacla (University of Strathclyde)
Title: Controllable electron interactions in quantum dots coupled to nanowires
Coordinates: room 601D, CBPF. 14.01, 16h00
Abstract: We theoretically study transport properties in quantum dot devices proximity coupled to superconducting nanowires. In particular, we investigate the controllable transition from resonant pair tunneling to Andreev bound states, which has been recently observed in nanodevices fabricated at the interface of the oxide heterostructure LaAlO3/SrTiO3. We show that such a transition in transport features can signify a Lifshitz transition, at which electron interactions change from attractive to repulsive. The ability to tune interactions on a reconfigurable nanoelectronic device could be a useful ingredient for solid state quantum simulation. Lastly, we discuss an alternate description for this transition in terms of magnetic impurities.
The Paraty Quantum Information School and Workshop is over… or is it?
The actual conference it is over, but its effects are still strongly felt. Our next seminar of the series QM Talks@CBPF will be delivered by Nadja K. Bernardes, a researcher from UFMG. She gave such a nice talk in Paraty, that we invited her over to hear more details… She was faster, however, and is already collaborating with the qig@CBPF in an experiment to see some non-Markovian effect. So she’ll no only give a talk here, but also fine tune the details of the experiment they are performing.
These are not the only consequences of the last Paraty in Nadja’s life… She will be in the organization of the Paraty 2017!
The details of her talk are below. See you there!
Speaker: Nadja K. Bernardes (UFMG)
Title: Experimental observation of weak non-Markovianity
Coordinates: room 601D, CBPF. 16.09, 16h00
Abstract: Non-Markovianity has recently attracted large interest due to significant advances in its characterization and its exploitation for quantum information processing. However, up to now, only non-Markovian regimes featuring environment to system backflow of information (strong non-Markovianity) have been experimentally simulated. Here, we report an all-optical observation of the so-called weak non-Markovian dynamics. Through full process tomography, we experimentally demonstrate that the dynamics of a qubit can be non-Markovian despite an always increasing correlation between the system and its environment. We also show the transition from the weak to the strong regime by changing a single parameter in the environmental state, leading us to a better understanding of the fundamental features of non-Markovianity.
As the last talk of the year, we have Clemens Gneiting, from the Quantum Optics and Statistics group of Freiburg university. I have a special relation with this group, as I was postdoc there for about 3 years with Andreas Buchleitner. Great physics, loads of fun!
Clemens is an expert on modular variables, specially in connection with violations of Bell inequalities using continuous-variables systems. But of course he has other research lines, and this time he’ll tell us about optimal coherent control of noisy quantum systems. A subject that will definitely resonate with both experimentalists and theorists. So please be sure to show up for this talk! See info below.
Speaker: Clemens Gneiting (Freiburg Univesity)
Title: Prospects of coherent control in the presence of dissipation
Coordinates: room 601D, CBPF. 11.12, 16:00h
Abstract: Genuine quantum features such as entanglement or coherence are resources as precious as fragile, and their uncovering usually requires strong efforts in isolating and controlling quantum systems. Without thorough measures, decoherence efficiently shields the quantum world from our access and hides it behind its classical guise. While there has been unprecedented progress in the quantum control of various model systems, e.g. ions, quantum dots, or cold atoms, it is impossible to completely decouple these systems from their environment and thus to fully suppress the detrimental effect of decoherence. Standard optimal control techniques therefore focus on accessing quantum features in the transient regime, and the exploration and exploitation of quantum properties is consequently confined to a finite, generically short time window. We investigate to what extent coherent Hamiltonian control can enduringly counteract the detrimental effect of decoherence. Explicitly, we determine Hamiltonians that optimally uphold desired control objectives (e.g., coherence, entanglement, or fidelity w.r.t. a target state) in the presence of dissipation. As we show, our method is applicable to both static and periodically time-dependent Hamiltonians. Finally, we also discuss modifications of the scheme due to continuous measurement and feedback.