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    Secure quantum communication using classical correlated channel
    (2016) Costa, D.; Almeida, Norton Gomes de; Villas-Bôas, Celso Jorge
    We propose a secure protocol to send quantum information from one part to another without a quantum channel. In our protocol, which resembles quantum teleportation, a sender (Alice) and a receiver (Bob) share classical correlated states instead of EPR ones, with Alice performing measurements in two different bases and then communicating her results to Bob through a classical channel. Our secure quantum communication protocol requires the same amount of classical bits as the standard quantum teleportation protocol. In our scheme, as in the usual quantum teleportation protocol, once the classical channel is established in a secure way, a spy (Eve) will never be able to recover the information of the unknown quantum state, even if she is aware of Alice’s measurement results. Security, advantages, and limitations of our protocol are discussed and compared with the standard quantum teleportation protocol.
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    Does “cooling by heating” protect quantum correlations?
    (2016) Villas-Bôas, Celso Jorge; Cardoso, Wesley Bueno; Avelar, Ardiley Torres; Xuereb, André; Almeida, Norton Gomes de
    The connection between nonequilibrium quantum correlations, such as entanglement and quantum discord, and cooling by heating is investigated for a system composed by two atoms interacting with a single electromagnetic mode of a lossy cavity. This Hamiltonian model is experimentally feasible in the quantum optics domain and presents the occurrence of both nonequilibrium quantum correlations and cooling by heating for a range of parameters. Since in the cooling by heating phenomenon the effective temperature of the system decreases even increasing the environments temperature, it could be expected that quantum correlations could be enhanced. Interestingly, for some parameters we show that, contrary to this expectation, in the case studied here the lowering of the system effective temperature leads to no enhancement in the quantum correlations. In addition, we found that at both zero and finite temperature, depending on the parameters used, quantum correlations can be enhanced even when increasing the damping rates, a somewhat counterintuitive result.
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    Unambiguous discrimination of nonorthogonal quantum states in cavity QED
    (2017) Assis, Rogério Jorge de; Sales, José de Souza; Almeida, Norton Gomes de
    We propose an oversimplified scheme to unambiguously discriminate nonorthogonal quantum field states inside high-Q cavities. Our scheme, which is based on positive operator-valued measures (POVM) technique, uses a single three-level atom interacting resonantly with a single mode of a cavity-field and selective atomic state detectors. While the single three-level atom takes the role of the ancilla, the single cavity mode field represents the system we want to obtain information. The efficiency of our proposal is analyzed considering the nowadays achievements in the context of cavity QED. We also analyze the effect of a thermal environment to discrimination of nonorthogonal states.
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    Reconstruction of Bohmian trajectories via weak measurement for bipartite states
    (2018) Almeida, Agnaldo Rosa de; Cardoso, Wesley Bueno; Avelar, Ardiley Torres; Almeida, Norton Gomes de
    Here we propose a protocol to reconstruct Bohmian trajectories for bipartite states of massive particles moving in an arbitrary one-dimensional potential. The proposal employs weak measurements and generalizes recent proposals presented in the literature (Kocsis et al., 2011). Finally, our work opens perspectives to experimentally study quantum correlation (primary resource for quantum information tasks) using the hydrodynamic formulation of quantum mechanics as suggested by Madelung and Bohm.
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    Universal two-level quantum Otto machine under a squeezed reservoir
    (2020) Assis, Rogério Jorge de; Sales, José de Souza; Cunha, Jefferson Adriany Ribeiro da; Almeida, Norton Gomes de
    We study an Otto heat machine whose working substance is a single two-level system interacting with a cold thermal reservoir and with a squeezed hot thermal reservoir. By adjusting the squeezing or the adiabaticity parameter (the probability of transition) we show that our two-level system can function as a universal heat machine, either producing net work by consuming heat or consuming work that is used to cool or heat environments. Using our model we study the performance of these machine in the finite-time regime of the isentropic strokes, which is a regime that contributes to make them useful from a practical point of view.
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    Efficiency of a quantum Otto heat engine operating under a reservoir at effective negative temperatures
    (2019) Assis, Rogério Jorge de; Mendonça, Taysa Mendes de; Villas-Bôas, Celso Jorge; Souza, Alexandre Martins de; Sarthour Junior, Roberto Silva ; Oliveira Júnior, Ivan dos Santos; Almeida, Norton Gomes de
    We perform an experiment in which a quantum heat engine works under two reservoirs, one at a positive spin temperature and the other at an effective negative spin temperature, i.e., when the spin system presents population inversion. We show that the efficiency of this engine can be greater than that when both reservoirs are at positive temperatures. We also demonstrate the counterintuitive result that the Otto efficiency can be beaten only when the quantum engine is operating in the finite-time mode.
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    Quantum Otto thermal machines powered by Kerr nonlinearity
    (2021) Mendes, Udson Cabral; Sales, José de Souza; Almeida, Norton Gomes de
    We study the effect of Kerr nonlinearity in quantum thermal machines having a Kerr-nonlinear oscillator as working substance and operating under the ideal quantum Otto cycle. We first investigate the efficiency of a Kerr-nonlinear heat engine and show that by varying the Kerr-nonlinear strength the efficiency surpasses in up to 2.5 times the efficiency of a quantum harmonic oscillator (QHO) Otto engine. Moreover, the Kerr-nonlinearity makes the coefficient of performance of the Kerr-nonlinear refrigerator to be as large as three times the performance of QHO Otto refrigerators. These results were obtained using realistic parameters from circuit quantum electrodynamics devices formed by superconducting circuits and operating in the microwave regime.
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    Feasible platform to study negative temperatures
    (2019) Assis, Rogério Jorge de; Villas-Bôas, Celso Jorge; Almeida, Norton Gomes de
    We afford an experimentally feasible platform to study Boltzmann negative temperatures. Our proposal takes advantage of well-known techniques of engineering Hamiltonian to achieve steady states with highly controllable population inversion. Our model is completely general and can be applied in a number of contexts, such as trapped ions, cavity-QED, quantum dots coupled to optical cavities, circuit-QED, and so on. To exemplify, we use Hamiltonian models currently used in optical cavities and trapped ions domain, where the level of precision achieved the control of the freedom degrees of a single atom inside a cavity as well as a single trapped ion. We show several interesting effects such as absence of thermalization between systems with inverted population and cooling by heating in these unconventional systems.
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    Two-level quantum Otto heat engine operating with unit efficiency far from the quasi-static regime under a squeezed reservoir
    (2021) Assis, Rogério Jorge de; Sales, José de Souza; Mendes, Udson Cabral; Almeida, Norton Gomes de
    Recent theoretical and experimental studies in quantum heat engines show that, in the quasi-static regime, it is possible to have higher efficiency than the limit imposed by Carnot, provided that engineered reservoirs are used. The quasi-static regime, however, is a strong limitation to the operation of heat engines, since an infinitely long time is required to complete a cycle. In this paper we propose a two-level model as the working substance to perform a quantum Otto heat engine surrounded by a cold thermal reservoir and a squeezed hot thermal reservoir. Taking advantage of this model we show a striking achievement, that is to attain unity efficiency even at non-null power.
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    Entanglement dynamics for a conditionally kicked harmonic oscillator
    (2016) Cavalcante, Eric Gomes Arrais; Sales, José de Souza; Almeida, Norton Gomes de
    The time evolution of the quantum kicked harmonic oscillator (KHO) is described by the Floquet operator which maps the state of the system immediately before one kick onto the state at a time immediately after the next. Quantum KHO is characterized by three parameters: the coupling strength V0, the so-called Lamb–Dicke parameter η whose square is proportional to the effective Planck constant , and the ratio T of the natural frequency of the oscillator and the kick frequency. To a given coupling strength and depending on T being a natural or irrational number, the phase space of the classical kicked oscillator can display different behaviors, as for example, stochastic webs or quasicrystal structures, thus showing a chaotic or localized behavior that is mirrored in the quantum phase space. On the other hand, the classical limit is studied letting  become negligible. In this paper we investigate how the ratio T, considered as integer, rational or irrational, influences the entanglement dynamics of the quantum KHO and study how the entanglement dynamics behaves when varying either V0 or  parameters.
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    Negative response with an optical cavity and traveling wave fields
    (2017) Assis, Rogério Jorge de; Villas-Bôas, Celso Jorge; Almeida, Norton Gomes de
    We present a feasible protocol using a traveling wave field to experimentally observe a negative response, i.e., to obtain a decrease in the output field intensity when the input field intensity is increased. Our protocol uses one beam splitter and two mirrors to direct the traveling wave field into a lossy cavity in which there is a three-level atom in a Λ configuration. In our scheme, the input field impinges on a beam splitter and, while the transmitted part is used to drive the cavity mode, the reflected part is used as the control field to obtain a negative response of the output field. We show that the greater the cooperativity of the atom-cavity system, the more pronounced the negative response. The system we are proposing can be used to protect devices sensitive to intense fields, since the intensity of the output field, which should be directed to the device to be protected, is diminished when the intensity of the input field increases.
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    Li4C4H2N2: a molecule with large hyperpolarizabilities and electride characteristic
    (2018) Oliveira, Igor M.; Castro, Marcos Antônio de; Fonseca, Tertius Lima da; Leão, Salviano de Araujo; Pontes, Renato Borges
    We design and systematically investigate the structural stability, electrical, and optical properties of a novel molecule named Li4C4H2N2 with electride characteristic. This molecule is obtained from pyridazine by replacing the two hydrogens opposed to the nitrogens by two lithium atoms and introducing other two lithium atoms bonded to the nitrogens. Dipole moment, polarizability, first and second hyperpolarizabilities, binding energies, excitation energies, and oscillator strengths of the Li4C4H2N2 molecule were computed at the coupled cluster with single and double excitations level through response theory. The binding energies of the lithium atoms to the nitrogens are larger than 70 kcal/mol, evidencing the high stability of the molecule investigated. The most representative results for β and γ are urn:x-wiley:00207608:media:qua25661:qua25661-math-0001 a.u. and urn:x-wiley:00207608:media:qua25661:qua25661-math-0002 a.u., respectively, indicating that this molecule has potential for application in nonlinear optical devices.
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    Electronic and vibrational hyperpolarizabilities of lithium substituted (aza)benzenes and (aza)naphthalenes
    (2018) Silva Filho, Suélio Marques da; Castro, Marcos Antônio de; Leão, Salviano de Araujo; Fonseca, Tertius Lima da
    In this article we report results for the electronic and vibrational hyperpolarizabilities of ten molecules: Li@benzene, Li@pyridine, Li@pyrimidine, and Li@pyrazine; Li@naphthalene, Li@quinoline, Li@isoquinoline, Li@cinnolin, Li@quinazoline, and Li@quinoxaline. An electron correlation study shows that second-order many-body perturbation theory and density functional theory with CAM-B3LYP and M05-2X functionals give results for the electronic hyperpolarizabilities in good agreement with coupled cluster with single and doubles reference values. Static and dynamic vibrational corrections were computed through the perturbation theoretical method of Bishop and Kirtman and using a variational approach. In general, we obtained notable discrepancies between the results obtained by the two methods for the pure vibrational corrections because of the deficiency of the perturbation method to properly treat low-frequency normal modes present in the investigated systems. However, both methods give results similar to the zero-point vibrational average corrections.
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    Assessing the structure and first hyperpolarizability of Li@B 10 H 14 in solution: a sequential QM/MM study using the ASEC-FEG method
    (2020) Leite, Idney Resplandes Brandão; Fonseca, Tertius Lima da; Georg, Herbert de Castro; Castro, Marcos Antônio de; Pontes, Renato Borges
    The structure and electronic properties of the lithium decahydroborate (Li@B10H14) complex in chloroform and water in normal thermodynamic conditions have been investigated using sequential QM/MM calculations by means of the average solvent electrostatic configuration (ASEC) and the Free Energy Gradient (FEG) methods. To obtain the structure of the Li@B10H14 complex in each of the solvents considered, we have performed geometry optimizations in solution using the ASEC–FEG method. The results show, for the first time with a realistic model of the molecular environment, that this alkali–metal–borane cluster is stable in chloroform but unstable in water. We have also explored the role of the electronic polarization of the solute due to solvent in the static first hyperpolarizability. The results show that, despite the reduction due to the effect of electrostatic polarization in chloroform, the Li@B10H14 complex still exhibits a large electronic first hyperpolarizability, with potential for application as a second-order nonlinear optical (NLO) material. In water, in contrast, the contribution of the excess electron for NLO responses is significantly affected by the electrostatic polarization effects. Therefore our results reveal that the influence of the environment must be considered in the design of new stable NLO materials.
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    On the calculation of magnetic properties of nucleic acids in liquid water with the sequential QM/MM method
    (2019) Oliveira, Guilherme Colherinhas de; Oliveira, Leonardo Bruno Assis; Castro, Marcos Antônio de; Fonseca, Tertius Lima da; Coutinho, Kaline Rabelo; Canuto, Sylvio Roberto Accioly
    We present calculations of nuclear magnetic shieldings of the nucleic acids adenine, cytosine, guanine e thymine in liquid water environment in normal thermodynamic condition. The study is based on a sequential approach, where a classical Monte Carlo simulation of the solvated system is performed to generate the different solute– solvent configurations to be used in the following quantum mechanics calculations of magnetic properties. We find that the environment polarization effect shields the unprotonated nitrogens, with solvent shifts for the magnetic shieldings between +10.52 and +48.95 ppm. Protonated and amino nitrogens are less shielded by amounts of −17.77 and +0.22 ppm. The results obtained from supermolecule calculations show that protonated and amino nitrogen magnetic shieldings are particularly sensitive to the hydrogen bond interactions, with solvent shifts between −5.48 and −33.42 ppm, when the first microsolvation shell is explicitly considered. In addition, the supermolecular calculations also show a marked deshielding effect for the hydrogens of the protonated nitrogens, with solvent shifts between −2.43 and −5.12 ppm. There is an overall agreement between the theoretical predictions for the 1 H and 15N chemical shifts and the available experimental results.
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    Electronic and vibrational second hyperpolarizabilities of (MgO) n clusters
    (2017) Feitoza, Luan da Silva; Castro, Marcos Antônio de; Leão, Salviano de Araujo; Fonseca, Tertius Lima da
    In this work, we report results for the static second hyperpolarizability of magnesium oxide clusters including electronic and vibrational contributions. The comparison between second-order Møller-Plesset (MP2) perturbation theory and coupled cluster results to the electronic contribution points out that MP2 is a suitable method to compute this property. When computed at the MP2 level, the electronic contribution per atom converges to approximately 5000 a.u. Vibrational corrections were computed at the MP2 level through the perturbation theoretical method of Bishop and Kirtman. Results obtained showed that the term represents around 20% of the electronic counterpart while the term is comparable to it. Modes that contribute significantly to are those in which all or part of the bond lengths simultaneously increase and decrease, leading to large polarizability derivatives. In turn, modes that provide relevant contributions to are those in which oxygen anions move in opposite directions to the magnesium cations yielding large derivatives of the dipole moment and first hyperpolarizability.
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    Confirming the relationship between first hyperpolarizability and the bond length alternation coordinate for merocyanine dyes
    (2017) Leite, Idney Resplandes Brandão; Franco, Leandro Rezende; Fonseca, Tertius Lima da; Castro, Marcos Antônio de; Georg, Herbert de Castro
    We investigated the first electronic hyperpolarizability of a typical merocyanine dye in several solvents in a wide range of dielectric constants. The equilibrium geometry of the molecule was obtained in each solvent by employing an optimization technique allied to atomistic simulations. The results confirm, for the first time with a realistic model of the molecular environment, the relationship between the first electronic hyperpolarizability (β) and the bond length alternation (BLA) coordinate, with a maximum value of β for intermediate positive BLA and a vanishing β when the BLA goes to zero.
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    Electronic and vibrational hyperpolarizabilities of alkali- and alkaline-earth-doped boron nitride nanotubes
    (2019) Silva Filho, Suélio Marques da; Castro, Marcos Antônio de; Pontes, Renato Borges; Leão, Salviano de Araujo; Fonseca, Tertius Lima da
    This work reports the results of the vibrational corrections and frequency dependence to the first hyperpolarizabilities of the alkali- and alkaline-earth-doped boron nitride nanotubes. The electronic contributions were computed by means of the density functional theory with the M06-2X functional, and the vibrational corrections were calculated using the perturbation theoretical method and the field-induced coordinates methodology. The results for the electronic contribution show that such materials exhibit large first hyperpolarizabilities and electride characteristic. We also show that the distribution of the excess electron, which originates from the doping atoms, plays an important role in the large electronic hyperpolarizabilities (βel). Moreover, our findings strongly indicate that the effect of vibrations on the hyperpolarizabilities can be quite important and can even be much larger than the electronic counterpart.
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    Applicability of DFT functionals for evaluating the first hyperpolarizability of phenol blue in solution
    (2021) Leite, Idney Resplandes Brandão; Fonseca, Tertius Lima da; Franco, Leandro Rezende; Georg, Herbert de Castro; Castro, Marcos Antônio de
    The first electronic hyperpolarizability (β) of phenol blue (PB) in several solvents in a wide range of dielectric constants is investigated using the density functional theory (DFT). The reliability of various exchange-correlation functionals is assessed by a comparison to reference Møller–Plesset second-order perturbation theory (MP2) calculations. The equilibrium geometry of PB in each solvent is obtained by using the average solvent electrostatic configuration/free energy gradient method, which performs optimizations on the free energy hyper-surface by employing iteratively the sequential quantum mechanics/molecular mechanics methodology. The dependence of β on the bond length alternation (BLA) coordinate is rationalized by means of the two-level model. Within the employed exchange-correlation functionals, the LC-BLYP functional shows the best performance for describing the static and dynamic MP2 results of β, which increases as the BLA diminishes, reaching a maximum in an intermediate value of BLA. The results also illustrate the role played by the difference between the ground- and excited-state dipole moments (Δμ) in determining the hyperpolarizability behavior in solution. Particularly, in the aqueous solution case, Δμ goes to around zero when BLA is near zero, leading to an abrupt decline in the β value. The DFT results of this study, therefore, indicate a clear relationship between the first hyperpolarizability and the BLA coordinate for the PB in solution, in agreement with experiment.
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    Solvent effects on the absorption spectrum and first hyperpolarizability of keto-enol tautomeric forms of anil derivatives: a Monte Carlo/quantum mechanics study
    (2016) Adriano Junior, Luizmar; Fonseca, Tertius Lima da; Castro, Marcos Antônio de
    Theoretical results for the absorption spectrum and electric properties of the enol and keto tautomeric forms of anil derivatives in the gas-phase and in solution are presented. The electronic properties in chloroform, acetonitrile, methanol, and water were determined by carrying out sequential Monte Carlo simulations and quantum mechanics calculations based on the time dependent density functional theory and on the second-order Møller–Plesset perturbation theory method. The results illustrate the role played by electrostatic interactions in the electronic properties of anil derivatives in a liquid environment. There is a significant increase of the dipole moment in solution (20%-100%) relative to the gas-phase value. Solvent effects are mild for the absorption spectrum and linear polarizability but they can be particularly important for first hyperpolarizability. A large first hyperpolarizability contrast between the enol and keto forms is observed when absorption spectra present intense lowest-energy absorption bands. Dynamic results for the first hyperpolarizability are in qualitative agreement with the available experimental results.