11 - 15 Aug 2015

ALBA MARCELA HERRERA TRUJILLO | UNIVERSIDADE FEDERAL DO ABC | Super-adiabatic Quantum Heat Engine | In our work we exploit the implementation of a non-transitional evolution which no stick to the quantum adiabatic dynamics. The quantum adiabatic dynamics is governed by the adiabatic theorem, imposing a speed limit on the evolution. Our deployment was achieved using the super-adiabatic evolution that is a useful tool to find a shortcut to the adiabatic one. Specifically, we study the performance of the quantum Otto cycle and we implement it in a NMR setup. With this result we show that it is possible to obtain a improved quantum engine. |

Alencar José de Faria | Universidade Federal de Alfenas | Quantum non-demolition verification of entanglement | It is proposed a optical procedure in context of continuous variables to verify the entanglement between two light modes without destroying both modes and their entanglement. The method relies on beam-splitter and quantum non-demolition interactions of the signal modes with two ancillary probe modes. The probe modes are measured by homodyne detection systems, and the detector signals are used to feedforward modulation of signal modes, concluding the procedure. It is shown that the classical information obtained from homodyne measurements is sufficient to verify the entanglement of the signal modes. The processing impact on the entanglement is analyzed. |

Alexandre M. Souza | CBPF | High-fidelity gate operations for quantum computing beyond dephasing time limits | The implementation of quantum gates with fidelities that exceed the threshold for reliable quantum computing requires robust gates whose performance is not limited by the precision of the available control fields. The performance of these gates also should not be affected by the noisy environment of the quantum register. Here we use randomized benchmarking of quantum gate operations to compare the performance of different families of gates that compensate errors in the control field amplitudes and decouple the system from the environmental noise. We obtain average fidelities of up to 99.8%, which exceeds the expected limit imposed by the dephasing environment. |

ALVARO ANDRES CIFUENTES GARCIA | UNIVERSIDADE FEDERAL DO ABC | Quantum Thermodynamics and Trapped Ions | We study the work statistics and irreversibility when a classical electromagnetic field interacts with a trapped ion. The interaction time is taken to be infinitesimally small i.e., a “quench” process. Our motivation comes from the emergence of Quantum Thermodynamics, in addition to the well established understanding of trapping and control of charged particles. Thus, the thermodynamical properties of an important candidate to perform quantum computation are characterized. Three different figures of merit are used, and contrasted, in this work to quantify the degree of irreversibility in our protocol. The first one is related to the “Relative Entropy”, which accounts for the difference between the actual system state and the reference thermal state. The second quantifier is a quantum version of a classical entropy, which follows from a mechanical model of equilibrium thermodynamics, the so-called Volume Entropy, and is valid for closed systems. Finally, the third one is related with a comparison between the actual and an ideal work process, done along an ideal reversible adiabatic transformation. |

Ana Majtey | IF-UFRJ | Dynamics of entanglement in systems of identical fermions undergoing decoherence | Information that is stored in quantum-mechanical systems can be easily lost because of the interaction with the environment in a process known as decoherence. Possible physical implementations of many processes in quantum information theory involve systems of identical particles, whence comprehension of the dynamics of entanglement induced by decoherence processes in identical-particle open systems becomes relevant. Here [1] we study the effects and concomitant entanglement evolution arising from the interaction between a system of two identical fermions and the environment for two paradigmatic quantum channels. Entanglement measures are introduced to quantify the entanglement between the different parties, and a study of the dynamics of entanglement for some particular examples is carried out. Our analysis, which includes also the evolution of an entanglement indicator based on an entropic criteria, offers insights into the dynamics of entanglement in open systems of identical particles, involving the emergence of multipartite genuine entanglement. The results improve our understanding of the phenomenon of decoherence and will provide strategies to control it. |

Andreas Ketterer | Laboratoire MPQ, Université Paris Diderot | Encoding discrete quantum information in continuous variables: A modular variables approach | Quantum information can be processed in two fundamentally different ways, namely, using discrete or continuous variable representations, respectively. However, only some quantum information protocols can be adapted from discrete to continuous variables, providing several practical advantages/drawbacks. In the present work we use modular variables to address the problem of bridging the world of continuous and discrete quantum information protocols and operations. We establish clear correspondences between the universal sets of operations defined in both realms by expressing them in terms of eigenstates of the modular position and momentum operator. Our results shed new light on the well known work of D. Gottesman et al., ``Encoding a qubit in an oscillator'', by revealing naturally discrete structures of operators and states defined in the continuous variable regime. Finally, we garnish our findings with a novel detection scheme allowing for a readout of the discrete quantum information from the continuous variable states in terms of inventively chosen observables. |

Andres Felipe Estrada Guerra | Universidad de Antioquia | Quantum limit for driving linear non-Markovian open quantum systems | The interplay between non-Markovian dynamics and driving fields in the survival of entanglement between two non-degenerate oscillators is considered here. Based on exact analytical results for the non-Markovian dynamics of two parametrically coupled non-degenerate oscillators in contact with non-identical independent thermal baths, the out-of-equilibrium quantum limit derived in [Phys. Rev. Lett. 105 180501 (2010)] is generalised to the non-Markovian regime. Specifically, it is shown that non-Markovian dynamics, when compared to the Markovian case, allow for the survival of stationary entanglement at higher temperatures, with larger coupling strength to the baths and at smaller driving rates. The effect of the asymmetry of the (i) coupled oscillators, (ii) coupling strength to the baths at equal temperature, and (iii) temperature at equal coupling strength is discussed. In particular, it is shown that the non-Markovian character of the dynamics is capable of beating the resonant condition that states that the driving frequency equals the sum of the natural frequencies for the maximum rate of squeezing generation; hence, squeezing generation is more robust under non-Markovian dynamics. |

Barbara Amaral | UFOP | Maximum Contextuality Allowed by Quantum Theory | Contextuality has been identified as a basic resource for quantum information and computation. Here we address the problem of which is the maximum contextuality allowed by quantum theory. While a traditional approach based on computing a measure of contextuality for all possible scenarios would suggest that this problem is unaffordable, here we show that a connection between quantum contextuality and graph theory allows us to prove that the ratio between the quantum violation and the noncontextual bound of a noncontextuality inequality can be lager than $n^{1- \epsilon}$, where $n$ is the number of events and $\epsilon$ is positive and arbitrarily small, while in general probabilistic theories this ratio cannot be larger than $n$. Remarkably, this maximum is much larger than the one exhibited in the violation of Mermin inequalities, which goes as $n^{\frac{1}{4}} . |

Baris Cakmak | UNICAMP | Quantum coherence and uncertainty in the anisotropic XY chain | We explore the local quantum coherence and the local quantum uncertainty, based on Wigner-Yanase skew information, in the ground state of the anisotropic spin-1/2 XY chain in transverse magnetic field. We show that the skew information, as a figure of merit, supplies the necessary information to reveal the occurrence of the second order phase transition and the completely factorized ground state in the XY model. Additionally, in the same context, we also discuss the usefulness of a simple experimentally friendly lower bound of local quantum coherence. Furthermore, we demonstrate how the connection between the appearance of non-analyticities in the local quantum uncertainty of the ground state and the quantum phase transitions does not hold in general, by providing explicit examples of the situation. Lastly, we discuss the ability of the local quantum coherence to accurately estimate the critical point of the phase transition, and investigate the robustness of the factorization phenomenon at low temperatures. |

Carlos Alberto Parra Murillo | Universidade Federal de Minas Gerais | Diffusion-driven quantum thermalization at a resonant tunneling scenario | We present recent results on relaxation dynamics and thermalization in a many-body implementation of the Wannier-Stark system [1]. It is hown that the induced delocalization of instantaneous eigenstates via cascade of Landau-Zener transition triggers an effective thermalization process in finite time evolution. We show that not only single particle observables but two-particle ones approach their thermal average values and a way to define the effective temperature of the system based on the mixing properties of the quantum spectrum. |

Carlos Ivan Henao Osorio | Universidade Federal do ABC | ROBUSTNESS TO NOISE OF TWO-WAY QUANTUM KEY DISTRIBUTION (TWQKD) PROTOCOLS | In this work we study the security of TWQKD protocols that use qubits prepared in non orthogonal states as carriers of information. In particuar, we present a new security proof for a recently proposed TWQKD protocol and investigate the possibility of outperforming it. We find that within our framework there is only one TWQKD protocol that could have a larger secret fraction. Moreover, we propose some techniques to potentially reduce the information leaked to an eavesdropper in TWQKD, which we term "pre-encoding operations". Such operations might increase the robustness to noise of a given protocol, but at the expense of a reduced efficiency of the communication. |

Carlos Mario Rivera Ruiz | UFSCAR | Quantum dot implementation of the quantum permutation algorithm | Title: Quantum dot implementation of the quantum permutation algorithm Authors: C. M. Rivera, B. Çakmak, E. F. de Lima, F. F. Fanchini, and L. K. Castelano Abstract The scientific community and technological industries are putting a great effort on the pursuit of physical systems capable of implementing quantum computing. Such a challenge is related to the efficiency of quantum algorithms, which offer an exponential speedup in the calculation time with respect to the classical counterpart. To achieve such advantages, entanglement was considered as a fundamental source for the power of the quantum computation. Nevertheless, it has been shown that the quantum efficiency can be achieved by quantum algorithms where entanglement is out of table. A recent quantum algorithm [1] that uses only a single qutrit or qudits, allows to determine the parity of permutations of a set of three numbers by employing only one measure, in contrast to the classical case, where two measures are needed. Such an algorithm has been implemented in a NMR quantum system [2]. In this work, we present an implementation of another platform that could realize the permutation algorithm in a three quantum dot system. Such a system is very interesting due to its potential scalability and miniaturization [3-5]. Moreover, we find the electric pulses capable of producing the necessary quantum gates that implement the permutation algorithm. Our results give the route to a possible experimental realization of the quantum permutation algorithm in three coupled quantum dots. References: [1] Z. Gedik, arXiv:1403.5861. [2] I. A. Silva, B. Çakmak, G. Karpat, E. L. G. Vidoto, D. O. Soares-Pinto, E. R. deAzevedo, F. F. Fanchini, Z. Gedik, arXiv:1406.3579. [3] A. Imamoglu, D. D. Awschalom, G. Burkard, D. P. DiVincenzo, D. Loss, M. Sherwin, and A. Small, Phys. Rev. Lett. 83, 4204 (1999). [4] Danny Kim, Samuel G. Carter, Alex Greilich, Allan S. Bracker, and Daniel Gammon, Nature Physics 7, 223229 (2011). [5] J. H. Jefferson, M. Fearn, D. L. J. Tipton, and T. P. Spiller, Phys. Rev. A 66, 042328 (2002). |

Cristhiano Duarte | Universidade Federal de Minas Gerais | Multigraph approach to quantum non-locality | Non-contextuality (NC) and Bell inequalities can be expressed as bounds Ω for positive linear combinations S of probabilities of events, S≤Ω. Exclusive events in S can be represented as adjacent vertices of a graph called the exclusivity graph of S. In the case that events correspond to the outcomes of quantum projective measurements, quantum probabilities are intimately related to the Gr\"otschel-Lov\'asz-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 optimizing S over the theta body and calculating the Lov\'asz 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. Here 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 party's parts of the events. Then, the maximum quantum violation of the Bell inequality is exactly given by a refinement of the Lov\'asz number that applies to these edge-coloured multigraphs. We show how to calculate upper bounds for this number using a hierarchy of semi-definite programs and calculate upper bounds for I3, I3322 and the three bipartite Bell inequalities whose exclusivity graph is a pentagon. The multigraph-theoretical approach introduced here may remove some obstacles in the program of explaining quantum correlations from first principles. |

Cyntia Vanessa Henrique Bezerra Uhlig | Centro Brasileiro de Pesquisas Físicas | ||

David Velasco Villamizar | UFSC | Quantum Speed Limit for relativistic electron in an uniform magnetic field | How fast a physical system can process information? It is an extremely important question in the field of computation. The answer to this question is the central focus of this study, to investigate the minimum time required for a relativistic quantum system changes to an orthogonal state respected to the initial one. In the present study we analysed the relativistic dynamics according to the Dirac equation an electron in the presence of an uniform magnetic field. The initial state of the system was chosen as a superposition of two states with equal weight, each of them being associated with a different Landau levels. Analysing the speed in which the electron moves from its initial mean position to its mean final position, it was found that in the case relativistic electron description never reach a speed greater than the speed of light. On the other hand, in non-relativistic description obtained by the Schrödinger equation, the electron will reach a higher rate of displacement greater than c when it is in a very strong magnetic field. Therefore, to realize a correct description of this problem of quantum speed limit is necessary to treat it according to the relativistic quantum mechanics. |

Diogo O. Soares-Pinto | IFSC / USP | Distributed correlations and information flows within a hybrid multipartite quantum-classical system | Understanding the non-Markovian mechanisms underlying the revivals of quantum entanglement in the presence of classical environments is central in the theory of quantum information. Tentative interpretations have been given by either the role of the environment as a control device or the concept of hidden entanglement. We address this issue from an information-theoretic point of view. To this aim, we consider a paradigmatic tripartite system made of two independent qubits and a random classical field locally interacting with one qubit alone. We study the dynamical relationship between the two-qubit entanglement and the genuine tripartite correlations of the overall system, finding that collapse and revivals of entanglement correspond, respectively, to raise and fall of the overall tripartite correlations. Interestingly, entanglement dark periods can enable plateaux of nonzero tripartite correlations. We then explain this behavior in terms of information flows among the different parties of the system. Besides showcasing the phenomenon of the freezing of overall correlations, our results provide new insights on the origin of retrieval of entanglement within a hybrid quantum-classical system. |

Eduardo da Costa Paul | UFRJ | Witnessing continuous-variable entanglement with the use of a set of three mutually unbiased bases | In a finite-dimensional Hilbert space, if the quantum state of a physical system can be expressed as an eigenvector of a given basis and simultaneously as a balanced superposition of every eigenvector of another basis, we say that the two bases are mutually unbiased bases (MUBs). That means that the measurement of the state in one basis gives no information about the possible measurement outcomes in the other basis. For the infinite-dimensional continuous-variable case, this concept can be generalized in the natural way. In this case, the most well known example are the position and momentum bases: perfect knowledge of the position of a quantum particle means that a measurement of its momentum could yield any result with equal probabilities. Interestingly, position and momentum do not constitute the only possible set of MUBs for this system. It can be shown that a third basis, x-p, can be added to that set, although making it asymmetric among the bases. Nevertheless, a completely symmetric set of MUBs can be achieved if we forego the momentum basis and instead use the bases equivalent to rotations of 0º, 120º and 240º in phase space, which correspond to fractional Fourier transforms of position. In this work, we experimentally study the transverse spatial variables of twin photons using the set of three MUBs as described. Each MUB can be measured with the use of non-confocal lens systems. We check the validity of uncertainty relations, and verify the existence of entanglement between the two photons. |

Emanuel Cardozo Diniz | Federal University of São Carlos | Quantum correlations in superconducting qubits in ultra strong regime under the action of reservoir collective markovian | We investigated the generation of quantum correlations between a system consisting of two superconducting qubits interacting with an electromagnetic field in a regime named ultra-strong coupling under the action of a collective Markovian reservoir for the qubits and an independent one for the field, at zero as well at finite temperature. Using the results obtained previously by the author and collaborators in [1, 2] we have found that in this situation the system displays the absence of thermalization, providing the possibility of generating a high degree of correlation between the qubits. We have shown that quantum discord, a type of quantum correlation between different subsystems, which appears in this system is much more robust to dissipative effects caused by the environment than the traditional entanglement, quantified in our work by the entanglement of formation. Our results can be useful for the implementation of possible architectures of quantum computing using superconducting qubits and also for the implementation of quantum communication protocols based on highly entangled states. References: [1] E. C. Diniz, “Termalização de qubits sujeitos à ação de reservatórios coletivos Markovianos”, Master Dissertation, Physics Department, Federal University of São Carlos, São Carlos, Brazil, (2014). [2] E. C. Diniz, D. Z. Rossato, T. Werlang, C. J. Villas-Boas. Termalization in Rabi Model in the Open Quantum System Context, In preparation (2015). |

FATEMEH | UFRGS | NMR Implementation of the two-dimensional YANG-BAXTER Equation | The Yang-Baxter Equation (YBE) is a sufficient condition for the integrability of a model. This means that if a model is constructed from the Yang-Baxter Equation, it will be called an integrable model, which means that it can be solved exactly, i.e., we will know its eigenvalues and eigenfunctions. This mathematical construction was introduced by C.N. Yang and R. Baxter in diffierent contexts. We tested the two-dimensional (2D) Yang-Baxter Equation (YBE) using Nuclear Magnetic Resonance (NMR) and we presented a practical scheme to test the YBE in the framework of quantum information. The equality of the two sides of the Yang-Baxter Equation is directly verified. |

Federico Cerisola | University of Buenos Aires | Work Measurement as a Generalized Quantum Measurement | We present a new method to measure the work w 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 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. |

Flaminia Giacomini | University of Vienna | Infinite-dimensional quantum systems on indefinite causal structures | Standard Quantum Mechanics assumes that events are embedded in a global causal structure. The process matrix framework keeps the local validity of standard Quantum Mechanics while relaxing the assumption on the global causal structure. This leads to multipartite correlations which lie outside the usual causally ordered framework, and opens interesting perspectives on our understanding of the nature of space and time. So far, the formalism has been developed only for finite-dimensional systems. Here we address the generalization to infinite-dimensional Hilbert spaces, with the long-term goal of formulating Quantum Fields on indefinite causal structure. |

Frank E. S. Steinhoff | Universität Siegen | Hypergraph states | Hypergraph states are multi-qubit states that form a subset of the locally maximally entangleable states and a generalization of the well--established notion of graph states. Mathematically, they can conveniently be described by a hypergraph that indicates a possible generation procedure of these states; alternatively, they can also be phrased in terms of a non-local stabilizer formalism. In this paper, we explore the entanglement properties and nonclassical features of hypergraph states. First, we identify the equivalence classes under local unitary transformations for up to four qubits, as well as important classes of five- and six-qubit states, and determine various entanglement properties of these classes. Second, we present general conditions under which the local unitary equivalence of hypergraph states can simply be decided by considering a finite set of transformations with a clear graph-theoretical interpretation. Finally, we consider the question whether hypergraph states and their correlations can be used to reveal contradictions with classical hidden variable theories. We demonstrate that various noncontextuality inequalities and Bell inequalities can be derived for hypergraph states. |

Frederico Brito | IFSC/USP | Testing time reversal symmetry in artificial atoms | Over the past several decades, a rich series of experiments has repeatedly verified the quantum nature of superconducting devices, leading some of these systems to be regarded as artificial atoms. In addition to their application in quantum information processing, these ‘atoms’ provide a test bed for studying quantum mechanics in macroscopic limits. Regarding the last point, we present here a feasible protocol for directly testing time reversal symmetry through the verification of the microreversibility principle in a superconducting artificial atom. Time reversal symmetry is a fundamental property of quantum mechanics and is expected to hold if the dynamics of the artificial atom strictly follow the Schrödinger equation. However, this property has yet to be tested in any macroscopic quantum system. In the end, as an application of this work, we outline how the successful implementation of the protocol would provide the first verification of the quantum work fluctuation theorems with superconducting systems. |

Gabriel Fagundes Camargo | UFMG | Memory Cost for Simulating Sequential Quantum Correlations | It is well known that contextual properties of quantum measurements cannot be reproduced by classical models. But in an implementation with sequential measurements, it may still be possible to achieve the task by using some classical, deterministic automatons with internal memory. The particular model we use are Mealy machines. We thus characterize contextuality by the number of internal states needed by the machine to reproduce quantum predictions. Following this line, we calculate the amount of memory for the scenario of the Peres-Mermin square. In the analysis so far, the only requirement on the automaton was that it merely must not produce events that are forbidden according to QM. One could expect that it is much more expensive to reproduce the exact probabilities. We find that this is not the case: mixtures of deterministic automata with three internal states are sufficient to simulate all quantum correlations. |

Gláucia Murta | Universidade Federal de Minas Gerais | Algebraic bounds on the quantum value of XOR games | Quantum non-locality is a surprising phenomena which reveals us some counter-intuitive characteristics of quantum theory. As surprising as the fact that quantum theory can violate local realism, expressed in the form of Bell inequalities, is the fact that it cannot violate as much as the non-signalling principle allows. The study of how much quantum mechanics can violate a Bell inequality has fundamental and practical applications. Fundamentally, it can help us understand the physical and information-theoretic principles that defines quantum theory. On the practical side, Bell violations has innumerous applications as for example to cryptography, device independent protocols and entanglement witnessing. Bell inequalities can also be phrased in a game scenario: the parties who are suppose to make a Bell test are now the players of a game. When the game starts the parties are not allowed to communicate anymore. A referee is responsible for distributing inputs among the players. Up to receiving an input each party is suppose to answer with an output. The goal of the game is that the outputs of all the parties together satisfy some function previously defined by the game. We study bounds on the maximum performance that quantum players can achieve in a XOR-d game. XOR-d games are a particular class of games where the winning condition only depends on the sum modulo d of the outputs. They are a natural generalization for d-outputs of the binary XOR games, where the winning condition is defined by the XOR of the outputs. XOR games are also referred as correlation Bell inequalities. We propose an algebraic bound to the quantum value of these games and use it to derive several interesting properties of them. As an example, we re-derive in a simple manner a recently discovered bound on the quantum value of the CHSH-d game for prime d. We then study the principle of no quantum advantage for the distributed computation of binary functions (Non-Local Computation) which is a well-known information-theoretic principle designed to pick out quantum correlations from amongst general no-signaling ones. We prove a large-alphabet generalization of this principle, showing that quantum theory provides no advantage in the task of non-local computation of a restricted class of functions with d outcomes for prime d, while general no-signaling boxes do. Finally we consider extension of previous results for the multiparty scenario (work in progress in collaboration with R. Ramanathan, N. Móller and M. Terra Cunha). As preliminary results we derive an algebraic upper bound for n-players XOR games and discuss its possible applications. |

Gonzalo Carvacho | Università degli Studi di Roma "La Sapienza" | Experimental entanglement using vector vortex beams | Gonzalo Carvacho (1), Vincenzo D'Ambrosio (1), Chiara Vitelli (1), Francesco Graffitti (1), Giulia Rubino (1), Bruno Piccirillo (2), Lorenzo Marrucci (2), Fabio Sciarrino (1). (1) Dipartimento di Fisica, Sapienza Universita di Roma, Roma, Italy (2) Dipartimento di Fisica, Universit a Federico II di Napoli, Napoli, Italy We can exploit the vectorial nature of the light in order to generate vector vortex beams (VVB), also referred as spirally polarized beams, using a recent device call q-plate which allow us manipulate the spatial distribution of the polarization in the transversal section of the beam in a quantum regime. Because of the coupling of the polarization with the orbital angular momentum of the photons, we can use an infinite-dimensional Hilbert space which is useful to perform different kinds of protocols. In our experiment we violated the Bell inequalities to demostrate the entangled nature of these particular beams; we also performed quantum state tomography to fully characterize the vector vortex beams for each pair of q-plates with different topological charges (that implies an entanglement of different Hilbert spaces between two interlocutors). |

Halyne Borges | Federal University of São Carlos | Quantum memory based on Electromagnetically Induced Transparency in Optical Cavities | An essential element for quantum computing to be possible is a quantum memory. In this work we theoretically studied the implementation of quantum memory in a system composed by a single atom trapped inside a high finesse cavity. In order to store and map the quantum state of an input pulse onto an internal state of the single atom, we based on the electromagnetically induced transparency (EIT) phenomenon where the information was transferred to the dark state of the atom modelled by a three-level system in the lambda-type configuration. In our model we consider a suitable temporal shape to the control field that ensure the adiabaticity in the storage process and retrieval of the probe pulse. The dynamic of the field inside the cavity was obtained by master equation approach, while the outside field was calculated by input-output formalism. Our results shows that for the appropriate setup and commonly used to observe the cavity-EIT in the transmission spectrum, the memory efficiency value is very low. We also discuss the differences between an input-output and a master equation approach considering different setups of the physical system. Finally we showed that considering a single-sided cavity, the quantum memory efficiency increases considerably and can reach almost 100% in the strong coupling regime. In this way our work provide a full theoretical description of this system for quantum memory applications, analyzing parameters regimes accessible experimentally. |

Ibrahim SAIDEH | Institut des Sciences Moléculaires d'Orsay (ISMO) | Mapping all separable qudits to separable qubits to detect entanglement | We present a general strategy to build entanglement criteria which consists in performing a mapping from qudits to qubits that preserves the separability of the parties and SU(2) rotational invariance.Consequently,it is possible to apply the well known positive partial transpose criterion for the qudits in terms of the correlations between the spins of each party. Consequently, it is possible to apply the well known positive partial transpose criterion to reveal the existence of quantum correlations between qudits. We discuss some examples of entangled states that are detected using the proposed strategy. Finally, we demonstrate using our scheme how some variance based entanglement witnesses can be generalized from qubits to higher dimensional spin systems. |

Igor Konieczniak | LMCAL - Universidade de São Paulo | Bicolour Quantum Entanglement for Teleportation in Continuous Variables | Several proposals for quantum computation devices are been implemented around the world. As these devices process information over different physical basis, it is expected that, when using light as a medium, their light will not necessarily be of the same wavelength. On the other hand, quantum teleportation is a fundamental part in many quantum computation protocols. So, the ability to quantum teleport between fields of different colours becomes a very desirable one. Bicolour quantum teleportation requires production and noise measurement abilities of entangled light fields in different wavelengths. Optical Parametric Oscilators (OPOs) operating in non-degenerate configuration have been used for producing such fields in the continuous variables regime. In these systems, entanglement is found between the phase an intensity quadratures of the two fields and expresses itself in noise reduction below the Standard Quantum Limit (SQL) in the sum phase quadrature and difference intensity quadrature of the two fields. Previous entanglement produced by a doubly resonant OPO resulted in 5 dB noise reduction below SQL in the sum intensity quadrature, but for the phase sum quadrature we found noise higher than expected. This excess noise was attributed to the pump field, as previous experiments had already shown. To reduce the phase noise in the pump field, a filter cavity was installed. As the filter cavity reduces the available pump power, a triply resonant OPO was deemed necessary for having a lower power oscillation threshold. A triply resonant OPO with asymmetrical mirror configuration was built and its noise covariance matrix was measured via the cavity noise ellipse rotation method. We report the production of quantum entanglement in continuous variables, a step further towards the completion of the quantum teleportation protocol. |

Irati Alonso Calafell | University of Vienna | Searching for Nonlinearities in Graphene at the Single Photon Level | Single-photon sources are an essential tool for experiments in quantum information, and it is known that nonlinear effects can be used to create such a source. To date, however, available optical materials require high intensities and long interaction times to induce strong enough nonlinearities. In this poster, we introduce a theoretical proposal in which nanostructured graphene shows strong nonlinear effects at the single photon level. In this proposal, graphene’s strong nonlinear effects are used to implement the photon blockade. The predicted anti-bunched emission indicates that graphene could be an extraordinary candidate to produce high quality single photons with an extremely high efficiency. However, the predicted wavelength of the single photons is longer than 2um – much beyond the range of current single photon detectors. In addition to presenting an overview of the quantum picture of light, and the Hanbury Brown-Twiss interferometer, this poster will discuss our efforts to implement super-conducting nanowire detectors for very long wavelength single photons in order demonstrate this exciting effect. |

Jessica Bavaresco | Universidade Federal de Minas Gerais | ||

JORDANA TORRICO FERREIRA | UNIVERSIDADE FEDERAL DE ALAGOAS | Thermal entanglement and frustration temperature in the Ising-Hubbard diamond chain | In this paper, we study the Ising-Hubbard model for a inﬁnity diamond chain with mobile eletrons in the presence of an external magnetic field. In this spin-chain model, the nodal sites are occupied by located Ising spins whereas the chain interstitial sites can be considered as single orbitals with one electron per site. In particular, electrons are allowed to hop between two interstitial sites providing the dynamical hopping terms of the model. Using the decoration-iteration transformation and transfer matrix methods obtain the exactly solution. The eﬀect of hopping term, magnetic ﬁeld and temperature dependences of the functions of correlations and magnetization are studied in several phase diagrams that the model provides us with we analytically and numerically calculate the frustration temperature and thermal entanglement, via concurrence, as a function of the external magnetic field. |

José Ferraz de Moura Nunes Filho | Dipartimento di Fisica, Sapienza Università di Roma, Italy e Departamento de Física, Universidade Federal Rural de Pernambuco, Recife, Brasil | Diluted Quantum Walk via Integrated Photonics | Abstract The transport of wave energy in disordered media has stirred a tremendous interest in different classical and quantum media, including optical, acoustic, electronic and matter wave systems. When waves propagate in a "static" disordered material, its propagation is normally arrested due to the presence of exponentially localized modes, which are spatio-temporally localized and inhibit any propagation of energy. This effect was analyzed for entangled photonic states in a quantum walk via integrated photonics [Nature Photonics 7, 322-320 (2013)]. On the other hand, when disorder is "evolving" (i.e., changing upon propagation), localization might breaks and a new type of diffusive behavior can settle in, where the transport rate might be even higher than the classical ballistic transport. In this work, we study theoretically the variance of entangled photons as a function of the quantum walk number of steps and show that a superdiffusive regime is obtained and is intrinsically correlated to the photonic state symmetry (bosonic or fermionic). |

Jose Raul Gonzalez Alonso | University of Southern California | The Long and Winding Road towards Error Mitigation for Photons with Orbital Angular Momentum in a Turbulent Atmosphere | It is not very often that a completely new property of electromagnetic waves is discovered. However, this was exactly what happened in 1993 when Les Allen in the United Kingdom reported that light with spiral phase fronts could carry orbital angular momentum (OAM). Since then, there is a growing interest in the applications of the OAM of light in multiple fields. In particular, OAM photons are very attractive for quantum information because they can carry arbitrarily large amounts of information per photon. Since communication with OAM photons happens over free space, it is important to understand the loss of coherence due to the interaction between a turbulent atmosphere and quantum states of light with OAM. In this work, I will review some of the alternatives to describe the noise processes OAM photons undergo when traveling in a turbulent atmosphere. In particular, I will discuss both the Kraus and the Lindblad representations of the decoherence process by assuming different noise models for the turbulence. Additionally, I will analyze possible applications of quantum error correcting codes and decoherence-free subspaces to the problem of protecting the desirable quantum properties of photons with OAM. |

Luciano Soares da Cruz | UFABC | Squeezing and Entanglement on quantum states of light polarization. | The preparation of optical fields with strictly quantum properties is an essential requirement for several Quantum Computation and Quantum Information protocols. Among the tangible experimental realizations, the use of atomic ensembles as non-linear media and continuous variables of light as transmitter has received special attention in the scientific community due to many quantum phenomena that can be reached with this matter-light interaction. We studied an optically thin two level atomic ensemble (including its complete Zeeman degeneracy) interacting with a classical field, the generation of states exhibiting squeezing and entanglement in the polarization degree of freedom. We determined the power spectra to the polarization variables, showing the system’s feasibility to induced quantum properties as squeezing and entanglement in the optical fields. |

Marcio Fernando Cornelio | Universidade Federal de Mato Grosso | Entanglement and Discord in pure multipartite systems | We extend the conservation law for the distribution of entanglement of formation and discord (Fanchini et al, PRA 84 012313, 2011) to four-partite and five-partite systems. We also obtain generalised n-partite conservation relations. An interesting difference between systems of even and odd parts shows up. For systems with odd number of parts, we obtain equalities like in the three-partite system. However, for systems with even number of parts, we obtain inequalities. We also show that there are basically two types of conservation laws: one is based in a key system and the other one is based in a loop over all the systems. |

Márcio Mendes Taddei | Federal University of Rio de Janeiro | Quantum brachistochrone | (Summary submitted to both School and Workshop applications) The quantum brachistochrone problem consists of finding the fastest way of driving a quantum system from a given initial state to a desired final state under a set of restrictions to the operator(s) governing evolution (i.e. to the Hamiltonian, if the evolution is unitary). The question has a wide range of applications especially because the restrictions to the evolution are suitable to mimic experimental or practical limitations of a control system. If the restrictions to the evolution are minimal -- only limiting the amount of energy available to the system --, the problem reduces to that of finding the geodesic between two states. In the general case, there is an interplay between the path traveled by the state of a system and the structure of the operator(s) governing evolution. Here we discuss variational approaches to the quantum brachistochrone problem based either on the Euler-Lagrange equation or on Pontryagin's theorem. |

Marco Cerezo | Instituto de Física la Plata (IFLP)-Facultad de Ciencias Exactas, Departamento de Física, Dpto. Cs. Bs. & Facultad de Ingeniería, Universidad Nacional de la Plata (UNLP). | Entanglement and non-transverse factorizing fields in spin chains | We examine the entanglement of quantum spin systems with anisotropic XYZ Heisenberg couplings of arbitrary range at transverse and non-transverse factorizing magnetic fields. The conditions for which the system presents an exactly separable eigenstate were determined for both cases. Previous results showed that at the transverse factorizing magnetic field, the system exhibits a degenerate parity-breaking separable ground state (GS) resulting from the crossing of two levels of opposite spin-parity, but at non-transverse factorizing magnetic fields, spin-parity is broken and the GS is no longer degenerate (although the system still presents an exactly separable GS). We also examine the pairwise entanglement of cyclic spin-1/2 chains in the vicinity of the factorizing magnetic field, showing that it acquires full range also in the non-transverse case. Related aspects of the magnetization and entropy of entanglement between two spins and the rest of the system are also discussed. |

Martin Drechsler | Departamento de Física, facultad de ciencias exactas y naturales, Universidad de Buenos Aires | ||

Moises Porfirio Rojas Leyva | Universidade Federal de Lavras | Quantum teleportation via Ising-XXZ diamond chain structure | Most investigations in entanglement teleportation are focused on two-qubits Heisenberg chain as a quantum channel to teleport an unknown state. The entanglement for one infinite chain structure is a considerably cumbersome task. The quantum entanglement properties involving an infinite chain structure is quite important, not only because the mathematical calculation is cumbersome but also because real materials are well represented by an infinite chain. Recently, thermal average was obtained for two-qubit density operator immersed in the Ising-XXZ diamond chain structure [13]. In this paper, we explore the quantum teleportation, using the average thermal of the two-qubit density operator as a quantum channel for the standard teleportation of two-qubit state. Using standard teleportation protocol, we have derived the analytical expressions for output concurrence, fidelity and average fidelity. We study in detail the effects of coupling parameters, external magnetic field and temperature over quantum teleportation. Finally, we study the relations between entanglement of the quantum channel, the output entanglement and the average fidelity of the system. |

Murray Olsen | University of Queensland | NonGaussian correlations and entanglement in Kerr media | We show how Kerr media such as optical fibres can be used to generate non-Gaussian states of the electromagnetic field. We further show how these can be simply manipulated to provide a source of entangled fields and Einstein-Podolsky-Rosen steering. Given the requirement of non-Gaussian states for central quantum information tasks, this will be a useful resource. |

Norma Canosa | IFLP-CONICET. Departamento de Física-Universidad Nacional de La Plata | Evolution and control of entanglement between two harmonic modes in stable and unstable regimes | The exact evolution of the entanglement between two harmonic modes generated by an angular momentum coupling is examined. Such system arises when considering a particle in a rotating anisotropic harmonic trap or a charged particle in a fixed harmonic potential in a magnetic field, and exhibits a rich dynamical structure, with stable, unstable and critical regimes according to the values of the rotational frequency or field and trap parameters. Consequently, it is shown that the entanglement generated from an initially separable gaussian state can exhibit quite distinct evolutions, ranging from quasiperiodic behavior in stable sectors to different types of unbounded increase in critical and unstable regions. The latter lead respectively to a logarithmic and linear growth of the entanglement entropy with time. It is also shown that entanglement can be controlled by tuning the frequency, such that it can be increased, kept constant or returned to a vanishing value just with stepwise frequency variations. Exact analytic expressions for the entanglement entropy in the different dynamical regimes are provided. |

Pablo Barberis Blostein | UNAM | Optomechanical laser cooling with mechanical modulations | We theoretically study the laser cooling of optomechanical cavities when the mechanical resonance frequency and damping depend on time. In the regime of weak optomechanical coupling we extend the theory of laser cooling using an adiabatic approximation. We discuss the modifications of the cooling dynamics and compare it with numerical simulations in a wide range of modulation frequencies. |

Pablo Gonzalez | Universidad de Concepción | Quantum key distribution with untrusted detectors | Side-channel attacks currently constitute the main challenge for quantum key distribution (QKD) to bridge theory with practice. So far two main approaches have been introduced to address this problem, (full) device-independent QKD and measurement-device-independent QKD. Here we present a third solution that might exceed the performance and practicality of the previous two in circumventing detector side-channel attacks, which arguably, is the most hazardous part of QKD implementations. We prove its security in the high-loss regime against a particular class of attacks, and we present a proof-of-principle experiment that demonstrates the feasibility of the protocol. |

Patrice Camati | UFABC | Observing fluctuations under feedback controlled protocols | Much research has been done in the last decades studying thermal fluctuations in classical systems both theoretically and experimentally. More recently, the study of quantum fluctuation theorems reached the experimental level, first in a NMR setup and then in an ion-trap setup. Also, the study of fluctuation relations under feedback control (Maxwell's demon-like) has been the subject of research both in classical and quantum systems. Up until now, quantum fluctuation relations under feedback controlled protocols has not been tackled experimentally. We present a quantum circuit which enables the measurement of the characteristic function of the probability work distribution under feedback controlled protocols and discuss its implementation in a NMR setup. |

Paul Erker | UAB, USI | ||

Peter Alexander Bouvrie Morales | Centro Brasileiro de Pesquisas Físicas | Composite bosons: entangled parts or bosonic whole? | Most bosons in nature are composites made of more elementary bosons and fermions. Still, from hadrons to ultracold molecules, these composites behave very similarly to elementary bosons, because the statistics of the underlying constituents is negligible. The deviation from ideal bosonic behavior is quantified by the normalization ratio of the quantum state of N composites. Using tools from quantum information science, the normalization ratio for two-boson and two-fermion composites can be bound efficiently in terms of entanglement measures [1,3,4]. Using these results, we predict an abrupt transition between ordinary and exaggerated bosonic behavior in a condensate of twoboson composites [3], and show how the entanglement between the parts becomes observable in the collective interference pattern of the bosonic whole [2]. [1] M.C.Tichy, P.A. Bouvrie & K. Mølmer: Bosonic behavior of entangled fermions. Phys. Rev. A 86, 042317 (2012) [2] M.C.Tichy, P.A. Bouvrie & K. Mølmer: Collective Interference of Composite Two-Fermion Bosons. Phys. Rev. Lett. 109, 260403 (2012) [3] M.C.Tichy, P.A. Bouvrie & K. Mølmer: Two-boson composites. Phys. Rev. A 88, 061602(R) (2013) [4] M.C.Tichy, P.A. Bouvrie & K. Mølmer: How bosonic is a pair of fermions? |

Pietro Liuzzo-Scorpo | University of Nottingham | Efficiency and correlation in coherent feedback cooling | The study of a measurement-based feedback protocol applied to a, initially uncorrelated, system consisting of two qubits (identified as principal system and auxiliary respectively) leaded us to investigate the relation between correlations and efficiency of the feedback protocol. In particular we studied the nature of correlations at each step of the protocol, i.e. the amount of classical and quantum correlations builded up and consumed. |

R. Rossignoli | Depto. de Física-IFLP-CIC, Universidad Nacional de La Plata | Generalized conditional entropy in quantum systems | We analyze, for general concave entropic forms, the conditional entropy of a quantum system A+B obtained as a result of a local measurement on one of the subsystems (B). This quantity measures the average conditional mixedness of A after such measurement, and its minimum over all local measurements is the associated entanglement of formation between A and a purifying third system C. In the standard case, it also determines the quantum discord. We show that for certain states, the minimizing measurement can be determined analytically and is universal, i.e.,the same for all concave forms. While these properties no longer hold for general states, we also show that in the special case of the linear entropy, an explicit expression for the conditional entropy can be obtained, whose minimum in a general qudit-qubit state can be determined analytically in terms of the eigenvalues of a simple 3x3 matrix. Such minimum determines the maximum conditional purity of A, and the associated minimizing measurement is shown to be universal in the vicinity of maximal mixedness. An approximate analytic solution for general entropies is also derived, together with a simple geometrical picture in terms of a correlation ellipsoid. Some illustrative results are as well discussed. |

Raphael Campos Drumond | Universidade Federal de Minas Gerais | Violations of general Bell inequalities for multipartite pure random states | We estimate the probability of random $N$-qudit pure states violating Bell Inequalities at the most general scenario, \ie $m$ observables, $v$ possible outcomes per observables, and $N$ parts. In fact, we claim that if the local dimension $d$ of a $N$-party quantum system satisfy $d>mv(2m-1)^2$, then typically we will no able to see any degree of violation for any Bell inequality. |

Raul Oscar Vallejos | CBPF | Irreversibility versus entanglement spectrum in chaotic maps | We study the connection between chaotic spectrum of the asymptotic density matrix and irreversibility. The asymptotic states are obtained by iterative application of certain unitary chaotic quantum maps. |

Renato M. Angelo | Federal University of Parana (UFPR) | A measure of physical reality | From the premise that an observable is real after it is measured, we envisage a tomography-based protocol that allows us to propose a quantifier for the degree of indefiniteness of an observable given a quantum state. Then we find that reality can be inferred locally only if there is no quantum correlation in the system, i.e., quantum discord prevents Einstein's notion of separable realities. Also, by monitoring changes in the local reality upon measurements on remote systems, we are led to define a measure of nonlocality. Proved upper-bounded by discord-like correlations and requiring indefiniteness as a basic ingredient, our measure signals nonlocality even for separable states, thus revealing nonlocal aspects that are not captured by Bell inequality violations. |

Romeu Rossi Junior | Universidade Federal de Viçosa | Optomechanical device as a quantum detector | We show that the optomechanical device composed by a Fabry-Perot cavity divided in two by a non-transmissive membrane, subject to radiation pressure, can be used as meter. The system is composed by two optical modes, one in each side of the cavity, and the membrane is assumed to behave as a quantum mechanical oscillator. The effect of the radiation pressure on the membrane will record information about the a field mode quadrature. The membrane plays the role of a quantum detector that allows for homodyne measurement of the optical mode. |

Sanah Altenburg | Department Physics, University Siegen | Fisher information, multiparameter estimation and entanglement | In quantum metrology, entanglement is used as a resource to enhance experiments for high precision phase estimation, such as atomic clocks or gravitational wave detectors. The quantum Fisher information is a quantity that allows us to decide whether a state is useful in order to overcome classical limits in precision for such experiments. Besides, the quantum Fisher information can also detect entanglement: It has an upper bound for separable states, so that overcoming classical limits in precision implies entanglement. These ideas have been extended to multipartite entanglement, but so far only to systems of qubits [1,2]. For a more general framework we use the Fisher information for multiparameter estimation. We will consider dynamics generated by arbitrary local generators in systems of qudits. In our case, the Fisher information for multiparameter estimation is a matrix. We investigate the Fisher information matrix and derive a criterion for detecting entanglement. [1] P. Hyllus et al., PRA 85, 022321 (2012). [2] G. Tóth, PRA 85, 022322 (2012). |

Saulo Vicente Moreira | Université Paris Diderot- Paris 7 | Modelling Invasiveness in a Leggett-Garg Inequality Test | We propose a theoretical model to test Leggett-Garg inequalities thats aims to determine the effects of the invasiveness of the measurement in its violation. This is done by investigating a specific measurement model where invasiveness is related in an intuitive way to one parameter introduced in the model. We also relate our invasiveness model to ones where violation disappears when the "size" of the system is increased, suggesting that even in this case, the invasiveness is the only element determining the violation (or not) of Legget Garg inequalities. |

Thiago O. Maciel | Universidade Federal de Minas Gerais - UFMG | Quantum process tomography with informational incomplete data of two $J$-coupled heterogeneous spins relaxation | We reconstruct the time dependent quantum map corresponding to the relaxation process of a two-spin system in liquid-state NMR at room temperature. By means of quantum tomography techniques that handle informational incomplete data, we show how to properly post-process and normalise the measurements data for the simulation of quantum information processing, overcoming the unknown number of molecules prepared in a non-equilibrium magnetisation state ($N_j$) by an initial sequence of radiofrequency (RF) pulses. From the reconstructed quantum map, we infer both longitudinal ($T_1$) and transversal ($T_2$) relaxation times, and introduce the $J$-coupling relaxation times ($T^J_1$, $T^J_2$), which are relevant for quantum information processing simulations. |

Tiago Barbin Batalhão | UFABC and University of Vienna | Irreversibility and the arrow of time in a quenched quantum system | We address the issue of testing experimentally the thermodynamic arrow of time by using a nuclear magnetic resonance set-up that allows for the determination of the nonequilibrium entropy produced in an isolated spin-1/2 system following fast quenches of an external magnetic field. We demonstrate that the macroscopic average entropy production equals the entropic distance between a microscopic process and its time-reversal. This thus establishes a microscopic foundation of irreversibility beyond the linear response regime and both elucidates and quantifies the physical origin of the arrow of time in a quantum setting. |