11 - 15 Aug 2015

Name | Institution | Title | Abstract |

Alfredo Ozório | CBPF | Representation of superoperators in double phase space | Operators in quantum mechanics - either observables, unitary propagators or density matrices - can be represented by c-numbers in operator bases. Such bases are in one to one correspondence with lagrangian surfaces in double phase space. These representations include, among others, the well known Wigner-Weyl and chord correspondence. We extend these phase space methods to the representation of superoperators. The main result is to show that the Choi-Jamiolkowsky isomorphism between the dynamical matrix and the linear action of the superoperator constitutes a "double" Wigner or chord transform when represented in double phase space. As a byproduct several previously unknown integral relationships between different representations are derived. |

Andre Carvalho | The Australian National University | Ignorance is bliss: General and robust cancellation of decoherence via no-knowledge quantum feedback | A “no-knowledge” measurement of an open quantum system yields no information about any system observable; it only returns noise input from the environment. Surprisingly, measuring nothing is most advantageous. We prove that a system undergoing no-knowledge monitoring has reversible noise, which can be cancelled by directly feeding back the measurement signal. We show how no-knowledge feedback control can be used to cancel decoherence in an arbitrary quantum system coupled to a Markovian reservoir that is being monitored. Since no-knowledge feedback does not depend on the system state or Hamiltonian, such decoherence cancellation is guaranteed to be general, robust and can operate in conjunction with any other quantum control protocol. |

Breno Marques Teixeira p/SEBASTIAO PADUA | Departamento de Física - Universidade Federal de Minas Gerais | Experimental simulation of decoherence in photonics qudits | We experimentally perform the simulation of open quantum dynamics in single-qudit systems. Using a spatial light modulator as a dissipative optical device, we implement dissipative-dynamical maps onto qudits encoded in the transverse momentum of spontaneous parametric down-converted photon pairs. We show a well-controlled technique to prepare entangled qudits states as well as to implement dissipative local measurements; the latter realize two specific dynamics: dephasing and amplitude damping. Our work represents a new analogy-dynamical experiment for simulating an open quantum system. |

Christian Schmiegelow | Physics Institute Mainz University | Exciting atoms in the dark or: how Ions Learned to Stop Worrying and Love the Light-twist. |
Results will be presented where an electronic transition of a single ion is excited with a beam with orbital angular momentum. We excite an electric quadrupole transition in ^{40}Ca^+ by placing the ion if the center of a beam with orbital angular momentum. We observe, as predicted, that the ion can be excited by such beam, even if it stands in the center where it is dark. In fact, the excitation is maximal at this point, where, though there is no field intensity, the field gradient is maximal. This transverse gradient, is exactly what drives the transition. We also see how transition rules change accordingly to this extra amount of angular momentum. We believe this is the first experimental demonstration where single quanta of orbital angular momentum is transfered from such structured light beams to single atoms. Details, implications and future directions will be discussed. |

Daniel Jost Brod | Perimeter Institute for Theoretical Physics | Computational complexity of constant-depth BosonSampling | BosonSampling is a restricted model of quantum computation where a non-adaptive linear-optical network is used to solve a sampling problem that seems hard for classical computers. Here we show that, even if the linear-optical network has a constant number (greater than four) of beam splitter layers, the exact version of the BosonSampling problem is still classically hard, unless the polynomial hierarchy collapses to its third level. |

Daniel S. Tasca | Universidade Federal do Rio de Janeiro | Testing for entanglement with periodic images | We introduce a periodic discretisation of continuous variables that allows for the construction of entanglement criteria based on the measurements of d-outcome observables in conjugate variables of the quantum system. Using photon pairs from spontaneous parametric down-conversion, we perform an experiment to test for spatial entanglement according to our procedure. The discretised d-outcome measurements are performed by employing a set of amplitude spatial masks as mode analysers, each of which selects a given region of the detection plane that is orthogonal to the others. Our entanglement criteria are computable from 2d2 joint projective measurements, required to characterise the correlation of the photons over the spatial masks positioned in the image plane and the far-field. We study the spatial correlations and entanglement detection as a function of the dimensionality d and periodicity of the spatial masks. Our results should be useful for characterising spatial entanglement, and have applications in quantum cryptography and imaging. |

Dario Gerace | University of Pavia | Quantum information processing and simulation with hybrid spin-photon qubits | A novel circuit quantum electrodynamics scheme is introduced to perform scalable quantum information processing based on a dual-rail encoding of hybrid qubits consisting of spin ensembles coherently coupled to microwave photons in superconducting resonators. Within this framework, quantum gates are implemented simply by shifting the resonance frequencies of the resonators on nanosecond time scales. An additional cavity containing a transmon qubit is exploited as an auxiliary degree of freedom to implement two-qubit gates. The scheme is shown to be robust against the main decoherence sources, i.e. mainly cavity photon loss. Finally, it is shown that a universal, scalable and locally tunable digital quantum simulator can be implemented with such a two-dimensional architecture of superconducting resonators with unprecedented efficiency. |

Eduardo Mascarenhas | (EPFL) Ecole polytechnique fédérale de lausanne | Direct determination of Nonequilibrium Steady States: Matrix Product State Ansatz | We present an efficient implementation of a variational approach for the determination of nonequilibrium steady states of open quantum systems governed by Lindblad master equations without resorting to real time dynamics. Through purification and super-operator techniques we show that standard matrix product state (MPS) methods can be directly applied in the variational determination of the stationary state. We show that the method converges efficiently to the nonequilibrium steady state without enforcing Hermiticity nor positivity to the MPS describing the density matrix. We compare our results with both standard MPS dynamics and quantum trajectories which are currently used methods for large driven dissipative quantum systems. We show how the method surpasses MPS Trotter-dynamics since it allows for addressing arbitrarily long ranged interactions. We also discuss the advantages of our implementation of the method which is more efficient and stable with respect to recent developments. |

Elizabeth Agudelo | Universitaet Rostock | Multimode nonclassicality of light in phase-space | We report on the recent advances of the nonclassicality quasiprobability (NQP) technique, its experimental applicability and relevance. Negativities of the Glauber-Sudarshan $P$~function are indicators of the quantumness of general quantum states of light. The resulting impossibility of describing optical field correlations in terms of classical electrodynamics rules this definition. Due to the strong singularities occurring typically in the $P$~function for many physical systems, e.g., squeezed light, this definition is not experimentally applicable. As a consequence the Wigner function had gained an increasing recognition, due to its regularity and operational relevance in quantum state tomography. Despite this benign behavior, this distribution is less sensitive than the $P$~function to probe quantum effects. These conflicting properties, regularity and sensitivity to nonclassicality, can be jointly realized using novel NQPs, whose negativities visualize nonclassicality. The NQPs, which are regularized versions of the highly singular $P$~function, have been first introduced for the single-mode case. For any nonclassical single-mode state, they show negativities and can be directly reconstructed from experimental data. For characterizing nonclassicality in any radiation field, multimode NQPs were established and they were demonstrated to be necessary and sufficient to certify nonclassicality of any multipartite $P$~function. The method uncovers any simultaneous quantum correlation among the modes. We consider an experimentally accessible bipartite state, with classical single-mode marginals, no entanglement, zero quantum discord, and a positive Wigner function. Still, our NQPs clearly reveal the quantum correlations, even when other methods fail. Using a pattern function approach, experimental reconstructions of NQPs are straightforward from experimental data obtained by balanced homodyne detection. As a proof of principle, we analyze an experimentally generated squeezed state using NQPs. Such states of light have a positive Wigner function and, at the same time, a highly irregular $P$~function. The sampled NQP is shown to be well-behaved and exhibits significant negativities. Furthermore, an analysis has been conducted to compare measurements with a discrete set of phases and continuous phase measurements. Whereas the former requires phase interpolations, the latter is favorable to display highly significant negativities even with a rather small amount of data. In conclusion, the NQP technique is shown to be a powerful, universal, and easily implementable method to visualize both single-mode quantum effects and multimode quantum correlations. |

Esteban Castro Ruiz | University of Vienna | Measuring time with physical clocks: loss of coherence due to gravitational interaction | In general relativity, the picture of space-time assigns an ideal clock to each space-time point. Being ideal, gravitational effects due to these clocks are ignored and the flow of time according to one clock is independent of the flow of time according to any other. However, if time is defined operationally, as a pointer position of a physical clock that obeys the laws of quantum mechanics and general relativity, such a picture is at most a convenient fiction. We show that the requirement that a clock is dynamical (i.e. is in an energy superposition) and the mass-energy equivalence imply gravitational interaction between the clocks, which entangles them through time dilation effect and leads to a loss of coherence of a single clock. Hence, the time as measured by a single clock is not well-defined. However, the general relativistic notion of time is recovered in the classical limit of clocks. |

Esteban Sepulveda Gomez | Center for Optics and Photonics, Universidad de Concepcion | Applying the Simplest Kochen-Specker Set for Quantum Information Processing | Kochen-Specker (KS) sets are key tools for proving some fundamental results in quantum theory and also have potential applications in quantum information processing. However, so far, their intrinsic complexity has prevented experimentalists from using them for any application. The KS set requiring the smallest number of contexts has been recently found. Relying on this simple KS set, here we report an input state-independent experimental technique to certify whether a set of measurements is actually accessing a preestablished quantum six-dimensional space encoded in the transverse momentum of single photons. |

Felipe Fernandes Fanchini | UNESP | Non-Markovianity and the flow of information | Exchange of information between a quantum system and its surrounding environment plays a fundamental role in the study of the dynamics of open quantum systems. Here, we present the relationship between the non-Markovian behavior and the flow of information between the system and the environment through entropic measures. We discuss the role of the information exchange in the non-Markovian behavior of dynamical quantum processes following the decoherence approach, where we consider a quantum system that is initially correlated with its measurement apparatus, which in turn interacts with the environment. We introduce different ways of looking at the information exchange between the system and environment using the accessible information and the quantum loss for quantifying the backflow of information from the environment to the system. Our results give an interpretation, in terms of flow of information, to the entanglement and mutual information based measures of non-Markovianity and reveal a clear conceptual relation between them. We also present experimental results on the investigation of the quantum loss and accessible information for a two-level system undergoing a zero temperature amplitude damping process. We use an optical approach that allows full access to the state of the environment. |

Frederico Brito | IFSC/USP | Is entanglement enough to quantum annealing? | We present experimental results in an NMR system demonstrating that a quantum annealing can be overcome by its companion classical annealing, even when presenting high levels of entanglement during the system evolution. Such a result raises the question about which features one should seek in order to assure the best success of a quantum annealing, owning to the fact that highly entangled states do not seem to be a sufficient condition. |

Graciana Puentes | Universidad de Buenos Aires | Weak Measurements with Orbital Angular Momentum Pointer States | Weak measurements are a unique tool for accessing information about weakly interacting quantum systems with minimal back action. Joint weak measurements of single-particle operators with pointer states characterized by a two-dimensional Gaussian distribution can provide, in turn, key information about quantum correlations which can be of relevance for quantum information applications. Here we demonstrate that by employing two-dimensional pointer states endowed with orbital angular momentum (OAM), it is possible to extract weak values of the higher order moments of single-particle operators, an inaccessible quantity with Gaussian pointer states only. We provide a specific example that illustrates the advantages of our method both, in terms of signal enhancement, and information retrieval. |

Jan Kolodynski | ICFO - The Institute of Photonic Sciences | Truly beating the Standard Quantum Limit in atomic spectroscopy | Quantum systems when employed as probes of physical parameters allow to beat classical limits imposed on the precision of estimation, in particular, the Standard Quantum Limit: 1/N with N being the number of constituent particles. In an ideal noiseless scenario, by preparing highly entangled probes the Heisenberg Limit, 1/N^2, may be attained yielding thus quantum enhancement that grows linearly with N. However, typical uncorrelated noise-types asymptotically restrict the enhancement to a constant factor, even though the SQL can then be still significantly exceeded for finite N. In atomic spectroscopy scenarios, in which one possesses the freedom to optimise the sensing-time of experimental shots, such constant can yet be further increased. Moreover, in case of non-Markovian environments and transversal dephasing noise this allows to truly exceed the SQL, so that despite the uncorrelated noise the quantum enhancement diverges with N. In this work, we establish a common framework that allows us to explicitly prove that only systems with dynamics exhibiting the so-called "quantum Zeno" regime at short time-scales may allow for such a behaviour. Furthermore, we show that also only such systems may thus let the error-correction techniques be beneficial in fully restoring the HL. |

Jonatan Bohr Brask | University of Geneva | An autonomous quantum thermal machine for generating steady-state entanglement | We first discuss a simple quantum thermal machine for the generation of steady-state entanglement between two interacting qubits. The machine is autonomous in the sense that it uses only incoherent interactions with thermal baths, but no source of coherence or external control. By weakly coupling the qubits to thermal baths at different temperatures, inducing a heat current through the system, steady-state entanglement is generated far from thermal equilibrium. We then go on to discuss two possible implementations, using superconducting flux qubits or a semiconductor double quantum dot. Experimental prospects for steady-state entanglement are promising in both systems. |

Joseph Hope | The Australian National University | Non-adiabatic schemes for controlling ultracold gases and trapped ions | Both ultracold gases and systems of trapped ions offer exceptional control opportunities via optical and magnetic fields. Adiabatic schemes are typically extremely robust, and are used in cooling and manipulating Bose-Einstein condensates, and for performing quantum gate operations on trapped ions. The disadvantage of an adiabatic process is that it is necessarily limited in speed. This forms a fundamental constraint in using trapped ions for quantum information processing, where the figure of merit is the number of gate operations per decoherence lifetime. It also constrains options for controlling ultracold gases, which are used in precision inertial measurements and in quantum emulation. We discuss the implementation and scaling possibilities for fast gates in trapped ions, open-loop control of Bose-Einstein condensates to produce arbitrary phase imprints, and closed-loop measurement feedback on Bose-Einstein condensates, which may be able to cool an atomic gas through the BEC transition itself. If such feedback cooling could be realised, it has the potential to avoid the evaporative cooling step, which could in principle increase atom numbers by three orders of magnitude. |

Juan Pablo Paz | UBA | Work measurement as a generalized quantum measurement | I will discuss a new method to measure the work performed on a driven quantum system and to sample its probability distribution P(w). The method is based on a simple fact that remained unnoticed until now: Work on a quantum system can be measured by performing a generalized quantum measurement at a single time. Such measurement, which technically speaking is denoted as a POVM (positive operator valued measure) reduces to an ordinary projective measurement on an enlarged system. This observation not only demystifies work measurement but also suggests a new quantum algorithm to efficiently sample the distribution P(w). This can be used, in combination with fluctuation theorems, to estimate free energies of quantum states on a quantum computer |

Leandro Aolita | IF-UFRJ | The resource theory of steering | We present an operational framework for Einstein-Podolsky-Rosen steering as a physical resource. We show that local operations assisted by one-way classical communication (1W-LOCCs) from the quantum part to the black box cannot create steering. Based on this, we build a resource theory of steering with 1W-LOCCs as the free operations. We introduce the notion of convex steering monotones as the fundamental axiomatic quantifiers of steering. As a convenient example thereof, we present the relative entropy of steering. In addition, we prove that two previously proposed quantifiers, the steerable weight and the robustness of steering, are also convex steering monotones. To end up with, for minimal-dimensional systems, we establish, on the one hand, neces- sary and sufficient conditions for pure-state steering conversions under stochastic 1W-LOCCs and prove, on the other hand, the non-existence of steering bits, i.e., measure-independent maximally steerable states from which all states can be obtained by means of the free operations. Our findings reveal unexpected aspects of steering and lay foundations for further research, with potential implications in Bell non-locality. |

Lee Rozema | University of Viena | Experimental Superposition of Orders of Quantum Gates | Quantum computers achieve a computational speed-up by placing quantum bits (qubits) in superpositions of different states. However, it has recently been appreciated that quantum mechanics also allows one to superimpose different operations. Furthermore, it has been shown that using a qubit to coherently control the gate order allows one to accomplish a task---determining if two quantum gates commute or anti-commute---with fewer gate uses than any known quantum algorithm. I will present our experimental demonstration of this advantage in a photonic experiment, and will discuss how controllable superpositions of gate orders can be implemented by making use of additional degrees of freedom of the physical system encoding the qubits. The new resource that our experiment exploits can be interpreted as a superposition of causal orders, and it could allow quantum algorithms to be implemented with an efficiency unlikely to be achieved on a fixed-gate-order quantum computer. |

Liliana Sanz de la Torre | Instituto de Física, Universidade Federal de Uberlândia | Quantum phenomena in quantum molecules | In this talk, we present a review of our work on several aspects of quantum behavior in quantum dots and quantum molecules, considering excitonic and charge states. We demonstrate the existence of robust states in a GaAsAl quantum molecule [1], the apparition of induced transparency controlled by tunneling [2,4] and the optical response of multilevel system in a quantum molecule [3]. The entanglement properties and generation of Bell states are discussed in the context of coupled charged quantum molecules [5]. We also discussed new results concerning the interaction between quantum molecules inside cavities, as well as entanglement behavior between electron and hole of the excitonic state inside a quantum molecule. We thanks Fapemig, CNPq and Brazilian National Institute of Science and Technology for Quantum Information (INCT-IQ) for financial support. |

Marcelo Terra Cunha | UFMG / Unicamp | Coloured Graph Approach to Quantum Nonlocality | The study of contextuality received great impact from the perception of its connection with graph theory (Cabello, Severini, and Winter 2010 and 2014). In these remarkable papers, the authors establish the connection between non-contextuality inequalities, written as weighted sums of probabilities, and three graph invariants: the weighted versions of the independence number (for non-contextual theories), the Lovász number (for quantum theory), and the fractional packing number (for generalised probability theories obeying the exclusivity principle). The most striking manifestation of contextuality is certainly Bell nonlocality, but in the original contribution of Cabello, Severini, and Winter, the weighted Lovász number could only work as an upper bound, not necessarily tight, when Bell constraints were imposed. Such a gap started to be closed by Rabelo, Duarte, López-Tarrida, Terra Cunha, and Cabello, in 2014, by introducing coloured (multi)graphs and their corresponding invariants into this study. My aim at Paraty 2015 is to discuss this contribution and some work in progress originated by it. |

Marco Túlio Quintino | Université de Genève | Joint Measurability, Einstein-Podolsky-Rosen Steering, and Bell Nonlocality | Here we investigate the relation between the incompatibility of quantum measurements and quantum nonlocality. We show that any set of measurements that is not jointly measurable (i.e. incompatible) can be used for demonstrating EPR steering, a form of quantum nonlocality. This implies that EPR steering and (non) joint measurability can be viewed as equivalent. Moreover, we discuss the connection between Bell nonlocality and joint measurability, and give evidence that both notions are inequivalent. This suggest the existence of incompatible quantum measurements which are Bell local, similarly to certain entangled states which admit a local hidden variable model. Finally, we discuss applications of these results to problems in joint mesurability, and for EPR steering using randomly chosen measurements. |

Maria Jose Sánchez | Centro Atómico Bariloche and Instituto Balseiro | Mesoscopic fluctuations in driven artificial atoms: Environmental effects | We investigate the effect of time reversal symmetry broken in artificial atoms in contact with an environmental bath. Using as a test system a flux qubit driven by a biharmonic magnetic signal of period τ with a phase lag, it is possible to establish a direct analogy between interference effects at avoided level crossings and scattering events in mesoscopic disordered media. In the regime of large relaxation times, accounting for decoherence as a classical noise, the transition rate between ground and excited states plays the role of an effective transmitance. In addition, the phase lag acts as an effective time reversal control parameter. In this way we have a test bed to study mesoscopic signatures , like weak localization and conductance fluctuations-like effects, in driven qubits. Our study shows that it is decoherence -and not the driving protocol as previous interpretations suggest- what limits the experimental detection of weak localization effects. |

Mateus Araújo | Universität Wien | Witnessing causal nonseparability | Our common understanding of the physical world relies deeply on the notion that events are ordered with respect to some time parameter, with past events serving as causes for future ones. Nonetheless, it was found recently that it is possible to give a formulation of quantum mechanics without any reference to a global time or causal structure. The resulting framework includes new kinds of quantum resources that allow performing tasks -- in particular, the violation of causal inequalities -- which are impossible for events ordered according to a global causal order; however, no physical interpretation for such resources was known. Here we show that a recently demonstrated resource for quantum computation -- the quantum switch -- is in fact a genuine example of ``indefinite causal order'', even though it cannot violate any causal inequality. Its lack of definite causal order is instead demonstrated trough a new tool -- a causal witness -- which can be thought of as a device-dependent version of a causal inequality. |

Matthew Lahaye | University of Syracuse | Superconducting Circuitry for Quantum Electromechanical Systems | Mechanical quantum systems are being actively developed both for fundamental studies of quantum coherence and related topics at the macroscopic scale and to serve as elements in quantum information networks and quantum sensing applications. An important class of these systems includes electromechanical devices that incorporate superconducting qubits and circuitry into their design for the read-out and manipulation of nanomechanical elements. It is expected that such hybrid quantum systems should enable the production and measurement of a variety of non-classical states of nanostructures, making these systems a potentially versatile new element for quantum processing architectures and for testing decoherence in new limits. In this talk, I will give a brief overview of the burgeoning field of mechanical quantum systems and then highlight ongoing work in my group at Syracuse to integrate superconducting quantum devices with nanoelectromechanical systems. |

Matthias Kleinmann | University of the Basque Country/Bilbao/Spain | Is nature merely dichotomic? | Quantum theory is not particularly complicated when it comes to the question of admissible measurements: Whenever a measurement outcome can occur in some measurement, it can appear in any measurement, as long as it does not contradict the rules of probability. While this might seem like an innocent assumption, we show that as a consequence, the number of outcomes is for itself a quantum phenomenon which can be tested in a Bell-type experiment. |

Michal Oszmaniec | ICFO, Barcelona / CFT PAS, Warsaw | Almost all symmetric states are useful for quantum metrology | In this work we use powerful concentration of measure techniques to systematically study which and how many states are useful for quantum metrology, i.e., give a precision in parameter estimation surpassing fundamental limits in the classical case. First, we show that most pure random states drawn from the Haar measure of the whole multiparticle space do not lead to quantum enhancement. Conversely, we prove that generic pure states on the symmetric subspace achieve Heisenberg scaling with probability approaching unity for any fixed Hamiltonian encoding. We generalise our results to the random mixed states having the fixed spectrum. This allows us to model the impact of noise on the generic meteorological usefulness of quantum states. |

Nadja K. Bernardes | Universidade Federal de Minas Gerais | Experimental observation of weak non-Markovianity | 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. |

Osvaldo Jimenez Farias | CBPF | Quantum information processing by weaving Quantum Talbot Carpets | Single photon interference due to passage through a periodic grating is considered in a novel proposal for processing D-dimensional quantum systems (quDits) encoded in the spatial degrees of freedom of light. We show that free space propagation naturally implements basic single quDit gates by means of the Talbot effect: an intricate time-space carpet of light in the near field diffraction regime. Adding a diagonal phase gate, we show that a complete set of single quDit gates can be implemented. We then introduce a spatially-dependent beam splitter that allows implementation of controlled operations between two quDits. A new form of universal quantum information processing can then be implemented with linear optics and ancilla photons. Though we consider photons, our scheme should be directly applicable to a number of other physical systems. Interpretation of the Talbot effect as a quantum logic operation provides a beautiful and interesting way to visualize quantum computation through wave propagation and interference. |

Pablo Lima Saldanha | Universidade Federal de Minas Gerais | Hidden momentum in a hydrogen atom and the Lorentz force law | By using perturbation theory, we show that a hydrogen atom with magnetic moment due to the orbital angular momentum of the electron has "hidden momentum" in the presence of an external electric field. This means that the atomic electronic cloud has a nonzero linear momentum in its center of mass rest frame due to a relativistic effect. This is completely analogous to the hidden momentum that a classical current loop has in the presence of an external electric field. We discuss that this effect is essential for the validity of the Lorentz force law in quantum systems. We also connect our results to the secular Abraham-Minkowski debate about the momentum of light in material media. |

Rafael Chaves | University of Freiburg | Revisiting Quantum Nonlocality from a Causal Inference Perspective | It is a relatively new insight of classical statistics that empirical data can contain information about causation rather than mere correlation. Within that context, during the past year we have developed and formalized a new research program for the study of causal relations in both the classical and quantum cases. In this presentation we will discuss two recent developments that highlight the very fruitful interplay between the causal inference literature and problems in quantum information, a connection which is increasingly appreciated among quantum physicists. We will describe a general algorithm for computing information–theoretic constraints on the correlations that can arise from a given causal structure, where we allow for quantum systems as well as classical random variables. As an application we will derive a strengthened version of the information causality principle, that furthermore can be extended to multipartite scenarios. In a second part, we will revisit Bell's theorem from a causal inference perspective and provide an unifying framework to solve the following question: How much do we need to relax the causal assumptions entering in Bell's theorem to classically explain nonlocal correlations? |

Rafael Rabelo | Universidade Federal de MInas Gerais | Multigraph approach to quantum nonlocality | Non-contextuality and Bell inequalities can be expressed as bounds for positive linear combinations of probabilities of events. Exclusive events can be represented as adjacent vertices of a graph called the exclusivity graph of the inequality. In the case that events correspond to the outcomes of quantum projective measurements, quantum probabilities are intimately related to the Grötschel–Lovász–Schrijver theta body of the exclusivity graph. Then, one can easily compute an upper bound to the maximum quantum violation of any NC or Bell inequality by optimising the inequality over the theta body and calculating the Lovász number of the corresponding exclusivity graph. In some cases, this upper bound is tight and gives the exact maximum quantum violation. However, in general, this is not the case. The reason is that the exclusivity graph does not distinguish among the different ways exclusivity can occur in Bell-inequality (and similar) scenarios. An interesting question is whether there is a graph-theoretical concept which accounts for this problem. We show that, for any given N-partite Bell inequality, an edge-coloured multigraph composed of N single-colour graphs can be used to encode the relationships of exclusivity between each partyʼs parts of the events. Then, the maximum quantum violation of the Bell inequality is exactly given by a refinement of the Lovász number that applies to these edge-coloured multigraphs. |

Roberto M. Serra | Universidade Federal do ABC (UFABC) | Spin based quantum heat engines | We will present an experimental implementation of a quantum engine, where the work substance is a spin-1/2 system in a molecular sample. Employing Nuclear Magnetic Resonance (NMR) techniques, we have implemented and completely characterized all energy fluctuations in the non-equilibrium operation of a spin quantum engine performing an Otto like cycle. The full statistics of work and heat as well as the operational limits of such an engine will be discussed. We will also show an optimized (frictionless) version of the engine employing a super-adiabatic driving which suppress the entropy production along the thermodynamic cycle. Finally, we will present the perspectives of future experiments employing quantum thermodynamic cycles in our NMR setup. |

Stefan Boettcher | Physics Dept, Emory University | Renormalization Group Solution of Quantum Walks | I will describe the renormalization group method (RG) from statistical physics to solve master equations with a unitary propagator. It allows to determine many asymptotic properties of quantum walks, although I will focus here on the walk dimension $d_w$, which describes the similarity solution, $\rho(x,t)\sim f(|x|^{d_w}/t)$, for the probability density function $\rho$. We can calculate $d_w$ to arbitrary accuracy for a number of networks, such as the dual Sierpinksi gasket, small-world Hanoi networks, or Migdal-Kadanoff lattices, which we have verified with direct simulations. Based on the exact RG for those fractal networks, we can conjecture a few general conclusions, for instance, that $d_w$ for a discrete-time quantum walk is always half of that for the random walk on the same $r$-regular network, when driven with the Grover coin. (This is joint work with Stefan Falkner at Emory and Renato Portugal at LNCC, see for example, http://dx.doi.org/10.1103/PhysRevA.90.032324 or http://arxiv.org/abs/1410.7034.) |

Steve Walborn | UFRJ | Entanglement redistribution under decoherence channels: experimental study | When an initially entangled pair of qubits undergoes a local decoherence process, there are a number of ways in which the original entanglement can spread throughout the entire multipartite system, consisting of the two qubits and their environments. Here we report theoretical and experimental results regarding the dynamics of the distribution of entanglement in this type of system. The experiment employs an all optical set-up that we have used for some time to study dynamics of open quantum systems. The qubits are encoded in the polarization degrees of freedom of two photons, and each local decoherence channel is implemented with an interferometer that couples the polarization to the path of each photon, which acts as an environment. We monitor the dynamics and distribution of entanglement and observe the transition from bipartite to multipartite entanglement and back, and show how these transitions are intimately related to the sudden death and sudden birth of entanglement. The multipartite entanglement is further analyzed in terms of three- and four-partite entanglement contributions, and genuine four-qubit entanglement is observed at some points of the evolution. |

Thiago R de Oliveira | UFF | Entanglement of Formation is Monogamous | It is well know that a particle can not freely share entanglement with two or more particles. This restriction is generally called monogamy. However the formal quantification of such restriction is only known for some measures of entanglement and for two-level systems. The first and broadly known monogamy relation was established by Kundu, Coffman and Wootters for the square of the concurrence. Since then, it is usually said that the entanglement of formation is not monogamous, as it does not obey the same relation. We show here that despite that, the entanglement of formation can not be freely shared and therefore should be said monogamous. Furthermore, the square of the entanglement of formation does obey the same relation of the squared concurrence, a fact recently noted for three particle and extend here for N particles. We also numerically study how the entanglement is distributed in pure states of three qubits and the relation between the sum of the bipartite entanglement and the classical correlation. |

Vincenzo D'Ambrosio | Sapienza, Università di Roma | Rotation-invariant single photon qubits | Quantum information bits are often encoded in photon polarization since this degree of freedom is easy to manipulate via standard birefringent optical elements. However, in a polarization based quantum communication scenario, users need to have knowledge of their mutual orientation in order to correctly communicate. Such limitation can be overcome with rotation-invariant single photon states obtained as a proper combination of polarization and orbital angular momentum (OAM) of photons. Here we present a series of experiments that have been carried out to test rotation-invariant qubits for quantum communication. By exploiting birefringent plates able to couple together polarization and OAM (q-plates) it is possible to easily generate, manipulate and measure rotation-invariant qubits. These states have been tested for long-distance quantum communication in free-space over 210 meters. Thanks to their hybrid structure in polarization and OAM, rotational invariant qubits result to be robust against atmospheric turbulences. Furthermore these states have been recently stored in a multiple degree of freedom quantum memory. Finally, by further exploiting the polarization and OAM joint action, it is possible to generate single photon states that allow to increase the sensitivity in polarization based roll angles measurement. |

Yasser Omar | Physics of Information Group, IT & University of Lisbon | Robustness of Spatial Quantum Search | A continuous-time analogue of Grover's algorithm can be formulated as a quantum walk on a complete graph of N vertices. We analyse the robustness of this algorithm against the loss of edges in the graph, as well as against static disorder. Firstly, when a few edges are removed from a complete graph, the dynamics of the algorithm is restricted to a three dimensional subspace and its optimality is retained. Subsequently, in the more general scenario where each edge of the graph remains with probability p, the underlying structure can be treated as an Erdos-Renyi random graph. We show that in the asymptotic limit, when N goes to infinity, search is optimal on almost all graphs as long as p = c N^(-z), where c is a constant and 0<z<1. Secondly, we analyze the robustness of this algorithm to random static disorder in the diagonal and off-diagonal elements of the search Hamiltonian. We show that the algorithm retains its optimality as long as the standard deviation of the disorder is of O(N^(-1/2)). |