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We report the experimental verification of nonclassical correlations for a four-wave-mixing process in an ensemble of cold two-level atoms, confirming theoretical predictions by Du et al. in 2007 for the violation of a Cauchy-Schwarz inequality in the system, and obtaining $R = (1.98\pm0.03) \nleq 1$. Quantum correlations are observed in a nano-seconds timescale, in the interference between the central exciting frequency and sidebands dislocated by the detuning to the atomic resonance. They prevail without filters over the noise background coming from linear scattering from the same optical transition. These correlations are fragile with respect to processes that disturb the phase of the atomic excitation, but are robust to variations in number of atoms and to increasing light intensities., Comment: 8 pages, 6 figures

The two-photon transition $5S\ensuremath{-}5P\ensuremath{-}5D$ in rubidium vapor is investigated by detecting the fluorescence from the $6{P}_{3/2}$ state when the atomic system is excited by the combined action of a cw diode laser and a frequency comb. The cw laser plays a role as a velocity-selective filter and allows for sub-Doppler spectroscopy over a large spectral range including the $5{D}_{3/2}$ and $5{D}_{5/2}$ states. For a counterpropagating beam configuration, the response of each atomic velocity group is well characterized within the Doppler profile and the excited hyperfine levels are clearly resolved. The contribution of the optical pumping to the direct two-photon process is also revealed. The results are well described in a frequency domain picture by considering the interaction of each velocity group with the cw laser and the modes of the frequency comb.

Broadband single photons are usually considered not to couple efficiently to atomic gases because of the large mismatch in bandwidth. Contrary to this intuitive picture, here we demonstrate that the interaction of ultrashort single photons with a dense resonant atomic sample deeply modifies the temporal shape of their wavepacket mode without degrading their non-classical character, and effectively generates zero-area single-photon pulses. This is a clear signature of strong transient coupling between single broadband (THz-level) light quanta and atoms, with intriguing fundamental implications and possible new applications to the storage of quantum information., 5 pages, 4 figures

We report on the amplification of an optical vortex beam carrying orbital angular momentum via induced narrow Raman gain in an ensemble of cold cesium atoms. A 20\% single-pass Raman gain of a weak vortex signal field is observed with a spectral width of order of 1 MHz, much smaller than the natural width, demonstrating that the amplification process preserves the phase structure of the vortex beam. The gain is observed in the degenerated two-level system associated with the hyperfine transition $6S_{1/2}(F=3)\leftrightarrow 6P_{3/2}(F^{\prime}=2)$ of cesium. Our experimental observations are explained with a simple theoretical model based on a three-level $\Lambda$ system interacting coherently with the weak Laguerre-Gauss field and a strong coupling field, including an incoherent pumping rate between the two degenerate ground-states., Comment: 9 pages, 4 figures

We report on the storage of light via the phenomenon of recoil-induced resonance in a pure two- level system of cold cesium atoms. We use a strong coupling beam and a weak probe beam to couple different external momentum states of the cesium atom via two-photon Raman interaction which leads to the storage of the optical information of the probe beam. We have also measured the probe transmission spectrum, as well as the light storage spectrum which reveals very narrow subnatural resonance features showing absorption and gain. We have demonstrated that this memory presents the unique property of being insensitive to the reading process, which does not destroy the stored information leading to a memory lifetime limited only by the atomic thermal motion., Comment: 5 pages, 5 figures. To appear in Physical Review A

We report on the storage of Orbital Angular Momentum (OAM) of light via the phenomenon of Coherent Population Oscillation (CPO) in cold cesium atoms. The experiment is performed using a delayed four wave mixing configuration where the transverse optical information of a probe field carrying OAM associated with its azimuthal phase dependence is stored in the CPO of Zeeman sublevels of the hyperfine transition $F=3\rightarrow F^{\prime}=2$ of cesium $D_{2}$ line. We also demonstrate experimentally the simultaneous storage and retrieval of different OAM states propagating along different directions in space, leading to algebric operations with OAM and therefore opening the possibility of multiplexing OAM states., Comment: 4 pages, 4 figures

High degrees of intensity correlation between two independent lasers were observed after propagation through a rubidium vapor cell in which they generate Electromagnetically Induced Transparency (EIT). As the optical field intensities are increased, the correlation changes sign (becoming anti-correlation). The experiment was performed in a room temperature rubidium cell, using two diode lasers tuned to the $^{85}$Rb $D_2$ line ($\lambda = 780$nm). The cross-correlation spectral function for the pump and probe fields is numerically obtained by modeling the temporal dynamics of both field phases as diffusing processes. We explored the dependence of the atomic response on the atom-field Rabi frequencies, optical detuning and Doppler width. The results show that resonant phase-noise to amplitude-noise conversion is at the origin of the observed signal and the change in sign for the correlation coefficient can be explained as a consequence of the competition between EIT and Raman resonance processes., Comment: Accepted for publication in EPJ D

We report on the storage of orbital angu- lar momentum (OAM) of light of a Laguerre-Gaussian mode in an ensemble of cold cesium atoms and its re- trieval along an axis different from the incident light beam. We employed a time-delayed four-wave mixing configuration to demonstrate that at small angle (2o), after storage, the retrieved beam carries the same OAM as the one encoded in the input beam. A calculation based on mode decomposition of the retrieved beam over the Laguerre-Gaussian basis is in agreement with the experimental observations done at small angle values. However, the calculation shows that the OAM retrieving would get lost at larger angles, reducing the fidelity of such storing-retrieving process. In addition, we have also observed that by applying an external magnetic field to the atomic ensemble the retrieved OAM presents Larmor oscillations, demonstrating the possibility of its manipulation and off-axis retrieval., Comment: 9 pages, 4 figures

We report on the storage of light via the phenomenon of Coherent Population Oscillation (CPO) in an atomic cesium vapor at room temperature. In the experiment the optical information of a probe field is stored in the CPO of two ground states of a Lambda three-level system formed by the Zeeman sublevels of the hyperfine transition F = 3 - F' = 2 of cesium D2 line. We show directly that this CPO based memory is very insensitive to stray magnetic field inhomogeneities and presents a lifetime which is mainly limited only by atomic motion. A theoretical simulation of the measured spectra was also developed and is in very good agreement with the experiment., 6 pages, 6 figures

The interaction of an ensemble of atoms with common vacuum modes may lead to an enhanced emission into these modes. This phenomenon, known as superradiance, highlights the coherent nature of spontaneous emission, resulting in macroscopic entangled states in mundane situations. The complexity of the typical observations of superradiance, however, masks its quantum nature, allowing alternative classical interpretations. Here we stress how this picture changed with the implementation ten years ago of a new process for single-photon generation from atomic ensembles. We present then the last piece of evidence for the superradiant nature of such process, reporting the observation of an accelerated emission of the photon with a rate that may be tuned by controllably changing the number of atoms in the ensemble. Such investigation paves the way to a new, bottom-up approach to the study of superradiance., Comment: 8 pages, 9 figures

The mechanism of extraction of information stored in a quantum memory is studied here in detail. We consider memories containing a single excitation of a collective atomic state, which is mapped into a single photon during the reading process. A theory is developed for the wavepacket of the extracted photon, leading to a simple analytical expression depending on the key parameters of the problem, like detuning and intensity of the read field and the number of atoms in the atomic ensemble. This theory is then compared to a large set of experimental situations and a satisfactory quantitative agreement is obtained. In this way, we are able to systematically study the saturation and spectrum of the reading process, as well as clarify the role of superradiance in the system., 25 pages (one-column format), 8 figures

We report a detailed investigation on the properties of correlation spectra for cold atoms under the condition of Electromagnetically Induced Transparency (EIT). We describe the transition in the system from correlation to anti-correlation as the intensity of the fields increases. Such transition occurs for laser frequencies around the EIT resonance, which is characterized by a correlation peak. The transition point between correlation and anti-correlation is independent of power broadening and provides directly the ground-state coherence time. We introduce a method to extract in real time the correlation spectra of the system. The experiments were done in two distinct magneto-optical traps (MOT), one for cesium and the other for rubidium atoms, employing different detection schemes. A simplified theory is introduced assuming three-level atoms in $\Lambda$ configuration interacting with a laser with stochastic phase fluctuations, providing a good agreement with the experimental observations., Comment: 8 pages, 10 figures, submitted to Physical Review A

Electromagnetically induced transparency (EIT) has mainly been modeled for three-level systems. In particular, considerable interest has been dedicated to the $\ensuremath{\Lambda}$ configuration, with two ground states and one excited state. However, in the alkali-metal atoms, which are commonly used, the hyperfine interaction in the excited state introduces several levels which simultaneously participate in the scattering process. When the Doppler broadening is comparable with the hyperfine splitting in the upper state, the three-level $\ensuremath{\Lambda}$ model does not reproduce the experimental results. Here we theoretically investigate the EIT in a hot vapor of alkali-metal atoms and demonstrate that it can be strongly reduced by the presence of multiple excited levels. Given this model, we also show that well-designed optical pumping enables us to significantly recover the transparency.

We report a detailed investigation on the generation of pulse pairs during the readout of a coherence grating stored in a cold atomic ensemble. The pulse shapes and the split of the retrieved energy between the two pulses are studied as a function of the relative intensities of the two reading fields, and a minimum is observed for the total retrieved energy. We introduce a simplified analytical theory for the process, considering a three-level atomic system, which explains all the most striking experimental features., 9 pages, 6 figures

We investigate the evolution of a Zeeman coherence grating induced in a cold atomic cesium sample in the presence of an external magnetic field. The gratings are created in a three-beam light storage configuration using two quasi-collinear writing laser pulses and reading with a counterpropagating pulse after a variable time delay. The phase conjugated pulse arising from the atomic sample is monitored. Collapses and revivals of the retrieved pulse are observed for different polarizations of the laser beams and for different directions of the applied magnetic field. While magnetic field inhomogeneities are responsible for the decay of the coherent atomic response, a five-fold increase in the coherence decay time, with respect to no applied magnetic field, is obtained for an appropriate choice of the direction of the applied magnetic field. A simplified theoretical model illustrates the role of the magnetic field mean and its inhomogeneity on the collective atomic response., Comment: To appear in J. Phys. B

We report on the storage of orbital angular momentum of light in a cold ensemble of cesium atoms. We employ Bragg diffraction to retrieve the stored optical information impressed into the atomic coherence by the incident light fields. The stored information can be manipulated by an applied magnetic field and we were able to observe collapses and revivals due to the rotation of the stored atomic Zeeman coherence for times longer than 15 $\mu s$., Comment: Submitted to Physical Review A

We report on a detailed investigation of the dynamics and the saturation of a light grating stored in a sample of cold cesium atoms. We employ Bragg diffraction to retrieve the stored optical information impressed into the atomic coherence by the incident light fields. The diffracted efficiency is studied as a function of the intensities of both writing and reading laser beams. A theoretical model is developed to predict the temporal pulse shape of the retrieved signal and compares reasonably well with the observed results.

We demonstrate entanglement distribution between two remote quantum nodes located 3 meters apart. This distribution involves the asynchronous preparation of two pairs of atomic memories and the coherent mapping of stored atomic states into light fields in an effective state of near maximum polarization entanglement. Entanglement is verified by way of the measured violation of a Bell inequality, and can be used for communication protocols such as quantum cryptography. The demonstrated quantum nodes and channels can be used as segments of a quantum repeater, providing an essential tool for robust long-distance quantum communication., 10 pages, 7 figures. Text revised, additional information included in Appendix. Published online in Science Express, 5 April, 2007

The ability to distribute quantum resources over long distances is a key element for the development of several practical applications in the field of quantum information. In this way, major efforts have been made in the past decade to extend the maximum distances for the observation of entanglement or for the realization of quantum cryptography. Using optical fibers as the quantum channel, entanglement has already been achieved between photons separated by 50 km of fiber, and quantum cryptography is now a commercial technology that allows quantum key distributon over distances up to around 100 km. Progress has also been made towards entanglement distribution in free space, with the achieved distances of about 10 km on ground atmosphere being a substantial step toward the development of satellite-based quantum communication.

Violations of a Bell inequality are reported for an experiment where one of two entangled qubits is stored in a collective atomic memory for a user-defined time delay. The atomic qubit is found to preserve the violation of a Bell inequality for storage times up to 21 microseconds, 700 times longer than the duration of the excitation pulse that creates the entanglement. To address the question of the security of entanglement-based cryptography implemented with this system, an investigation of the Bell violation as a function of the cross-correlation between the generated nonclassical fields is reported, with saturation of the violation close to the maximum value allowed by quantum mechanics., 4 pages, 3 figures. Minor changes. Published version

We report significant improvements in the retrieval efficiency of a single excitation stored in an atomic ensemble and in the subsequent generation of strongly correlated pairs of photons. A 50% probability of transforming the stored excitation into one photon in a well-defined spatio-temporal mode at the output of the ensemble is demonstrated. These improvements are illustrated by the generation of high-quality heralded single photons with a suppression of the two-photon component below 1% of the value for a coherent state. A broad characterization of our system is performed for different parameters in order to provide input for the future design of realistic quantum networks.

Quantum networks hold the promise for revolutionary advances in information processing with quantum resources distributed over remote locations via quantum-repeater architectures. Quantum networks are composed of nodes for storing and processing quantum states, and of channels for transmitting states between them. The scalability of such networks relies critically on the ability to perform conditional operations on states stored in separated quantum memories. Here we report the first implementation of such conditional control of two atomic memories, located in distinct apparatuses, which results in a 28-fold increase of the probability of simultaneously obtaining a pair of single photons, relative to the case without conditional control. As a first application, we demonstrate a high degree of indistinguishability for remotely generated single photons by the observation of destructive interference of their wavepackets. Our results demonstrate experimentally a basic principle for enabling scalable quantum networks, with applications as well to linear optics quantum computation., Comment: 10 pages, 8 figures; Minor corrections. References updated. Published at Nature Physics 2, Advanced Online Publication of 10/29 (2006)

We report an investigation to establish the physical mechanisms responsible for decoherence in the generation of photon pairs from atomic ensembles, via the protocol of Duan et. al for long distance quantum communication [Nature (London) 414, 413 (2001)] and present the experimental techniques necessary to properly control the process. We develop a theory to model in detail the decoherence process in experiments with magneto-optical traps. The inhomogeneous broadening of the ground state by the trap magnetic field is identified as the principal mechanism for decoherence. In conjunction with our theoretical analysis, we report a series of measurements to characterize and control the coherence time in our experimental setup. We use copropagating stimulated Raman spectroscopy to access directly the ground state energy distribution of the ensemble. These spectroscopic measurements allow us to switch off the trap magnetic field in a controlled way, optimizing the repetition rate for single-photon measurements. With the magnetic field off, we then measure nonclassical correlations for pairs of photons generated by the ensemble as a function of the storage time of the single collective atomic excitation. We report coherence times longer than 10 microseconds, corresponding to an increase of two orders of magnitude compared to previous results in cold ensembles. The coherence time is now two orders of magnitude longer than the duration of the excitation pulses. The comparison between these experimental results and the theory shows good agreement. Finally, we employ our theory to devise ways to improve the experiment by optical pumping to specific initial states., Comment: 16 pages, 11 figures, submitted for publication

A critical requirement for diverse applications in Quantum Information Science is the capability to disseminate quantum resources over complex quantum networks. For example, the coherent distribution of entangled quantum states together with quantum memory to store these states can enable scalable architectures for quantum computation, communication, and metrology. As a significant step toward such possibilities, here we report observations of entanglement between two atomic ensembles located in distinct apparatuses on different tables. Quantum interference in the detection of a photon emitted by one of the samples projects the otherwise independent ensembles into an entangled state with one joint excitation stored remotely in 10^5 atoms at each site. After a programmable delay, we confirm entanglement by mapping the state of the atoms to optical fields and by measuring mutual coherences and photon statistics for these fields. We thereby determine a quantitative lower bound for the entanglement of the joint state of the ensembles. Our observations provide a new capability for the distribution and storage of entangled quantum states, including for scalable quantum communication networks ., 13 pages, 4 figures Submitted for publication on August 31 2005

A phase-stabilized femtosecond laser comb is directly used for high-resolution spectroscopy and absolute optical frequency measurements of one- and two-photon transitions in laser-cooled \rb atoms. Absolute atomic transition frequencies, such as the 5S$_{1/2}$ F=2 \ra 7S$_{1/2}$ F"=2 two-photon resonance measured at 788 794 768 921(44) kHz, are determined without \textit{a priori} knowledge about their values. Detailed dynamics of population transfer driven by a sequence of pulses are uncovered and taken into account for the measurement of the 5P states via resonantly enhanced two-photon transitions., Comment: 5 pages, 4 figures, submitted

Ultrashort laser pulses have thus far been used in two distinct modes. In the time domain, the pulses have allowed probing and manipulation of dynamics on a subpicosecond time scale. More recently, phase stabilization has produced optical frequency combs with absolute frequency reference across a broad bandwidth. Here we combine these two applications in a spectroscopic study of rubidium atoms. A wide-bandwidth, phase-stabilized femtosecond laser is used to monitor the real-time dynamic evolution of population transfer. Coherent pulse accumulation and quantum interference effects are observed and well modeled by theory. At the same time, the narrow linewidth of individual comb lines permits a precise and efficient determination of the global energy-level structure, providing a direct connection among the optical, terahertz, and radio-frequency domains. The mechanical action of the optical frequency comb on the atomic sample is explored and controlled, leading to precision spectroscopy with an appreciable reduction in systematic errors.

The time dependence of nonclassical correlations is investigated for two fields (1,2) generated by an ensemble of cold Cesium atoms via the protocol of Duan et al. [Nature Vol. 414, p. 413 (2001)]. The correlation function R(t1,t2) for the ratio of cross to auto-correlations for the (1,2) fields at times (t1,t2) is found to have a maximum value Rmax=292(+-)57, which significantly violates the Cauchy-Schwarz inequality R, 5 pages, 5 figures

Nonlinear magneto-optical (NMO) resonances occurring for near-zero magnetic field are studied in Rb vapor using light-noise spectroscopy. With a balanced detection polarimeter, we observe high contrast variations of the noise power (at fixed analysis frequency) carried by diode laser light resonant with the 5S$_{1/2}(F=2) \to 5$P$_{1/2}(F=1) $ transition of $^{87}$Rb and transmitted through a rubidium vapor cell, as a function of magnetic field $B$. A symmetric resonance doublet of anti-correlated noise is observed for orthogonal polarizations around $B=0 $ as a manifestation of ground state coherence. We also observe sideband noise resonances when the magnetic field produces an atomic Larmor precession at a frequency corresponding to one half of the analysis frequency. The resonances on the light fluctuations are the consequence of phase to amplitude noise conversion owing to nonlinear coherence effects in the response of the atomic medium to the fluctuating field. A theoretical model (derived from linearized Bloch equations) is presented that reproduces the main qualitative features of the experimental signals under simple assumptions., 10 pages, 8 figures, submitted to Physical Review A Revised version

The spatial ring-shaped distribution of atoms in a magneto-optical trap, produced by misaligned Gaussian laser beams, is investigated with respect to the increase in the number of trapped atoms. Experimental observation of the bifurcation of a single ring into a double-ring structure is reported and explained using a model that includes radiation force due to reabsorption of photons together with the conventional radiative force.

We report on the combined action of a cw diode laser and a train of ultrashort pulses when each of them drives one step of the 5S-5P-5D two-photon transition in rubidium vapor. The fluorescence from the 6P_{3/2} state is detected for a fixed repetition rate of the femtosecond laser while the cw-laser frequency is scanned over the rubidium D_{2} lines. This scheme allows for a velocity selective spectroscopy in a large spectral range including the 5D_{3/2} and 5D_{5/2} states. The results are well described in a simplified frequency domain picture, considering the interaction of each velocity group with the cw laser and a single mode of the frequency comb., Comment: 4 pages, 4 figures

We report on the observation of enhanced four-wave mixing via crossover resonance in a Doppler-broadened cesium vapor. Using a single laser frequency, a resonant parametric process in a double-Λ level configuration is directly excited for a specific velocity class. We investigate this process in different saturation regimes and demonstrate the possibility of generating intensity correlation and anticorrelation between the probe and conjugate beams. A simple theoretical model is developed that accounts qualitatively well to the observed results.

We report on the simultaneous observation, by delayed Bragg diffraction, of four- and six-wave mixing processes in a coherently prepared atomic ensemble consisting of cold cesium atoms. For each diffracted order, we observe different temporal pulse shapes and dependencies with the intensities of the exciting fields, evidencing the different mechanisms involved in each process. The various observations are well described by a simplified analytical theory, which considers the atomic system as an ensemble of three-level atoms in {\Lambda} configuration.

Superradiance in an ensemble of atoms leads to the collective enhancement of radiation in a particular mode shared by the atoms in their spontaneous decay from an excited state. The quantum aspects of this phenomenon are highlighted when such collective enhancement is observed in the emission of a single quantum of light. Here we report a further step in exploring experimentally the nonclassical features of superradiance by implementing the process not only with single excitations, but also in a two-excitations state. Particularly we measure and theoretically model the wave-packets corresponding to superradiance in both the single-photon and two-photons regimes. Such progress opens the way to the study and future control of the interaction of nonclassical light modes with collective quantum memories at higher photon numbers., 5 pages, 4 figures. V2: substantial changes on the introductory part of the paper

Quantum networks hold the promise for revolutionary advances in information processing with entanglement distributed over remote locations via quantum repeaters. We report two milestones in this direction: the conditional control of memories and the implementation of functional nodes.

The phenomenon called Electromagnetically Induced Transparency (EIT) may induce different types of correlation between two optical fields interacting with an ensemble of atoms. It is presently well known, for example, that in the vicinity of an EIT resonance the dominant correlations at low powers turn into anti-correlations as power increases. Such correlation spectra present striking power-broadening-independent features, with the best condition for measuring the characteristic linewidth occurring at the highest powers. In the present work we investigate the physical mechanisms responsible for this set of observations. Our approach is first to reproduce these effects in a better controlled experimental setup: a cold atomic ensemble, obtained from a magneto-optical trap. The results from this conceptually simpler system were then compared to a correspondingly simpler theory, which clearly relates the observed features to the interplay between two key aspects of EIT: the transparency itself and the steep normal dispersion near two-photon resonance.

BARBOSA, Daniel Felinto Pires, também é conhecido(a) em citações bibliográficas por: FELINTO, Daniel CAPES Nesta dissertação são apresentadas minhas contribuições na construção de uma Armadilha Magneto-Óptica (AMO) e na implementação experimental do protocolo DLCZ no Laboratório de Redes Quânticas da UFPE. Entre elas estão a montagem da cabeça de um laser de diodo (Teco), um controlador de temperatura digital em Arduino, bobinas para geração de campos magnéticos, entre outras. A AMO se baseia na força por pressão de radiação que a luz exerce sobre a matéria. Ao combinar um campo magnético inomogêneo com três pares de feixes de laser contrapropagantes de polarizações circulares opostas, conseguimos criar uma força viscosa e restauradora que resfria e aprisiona os átomos em torno da origem definida pelo campo magnético, formando uma nuvem fria de átomos. Como parte do esforço coletivo para sua construção, montei ainda um sistema de absorção saturada do laser Teco, o feixe de rebombeio e as bobinas para os campos magnéticos de compensação e armadilhamento. A temperatura que a nuvem atômica atinge é da ordem de 1mK, sua dimensão é de ~ 3mm e conseguimos armadilhar cerca de 10⁸ átomos. Uma vez formada a nuvem de átomos frios e armadilhados, desligamos os feixes que sustentam a AMO e realizamos o experimento de geração de pares de fótons individuais seguindo o protocolo de informação quântica DLCZ, onde contribuí com a montagem do feixe de leitura. Com a ajuda do Time-Multiplexed Detector que implementei, fazemos a deteção dos fótons gerados e obtemos as estatísticas necessárias para sua caracterização. A partir dos dados obtidos, determinamos se os pares de fótons gerados estão em um regime não-clássico. In this dissertation we present my contributions in the setup of a Magneto-Optical Trap (MOT) and in the experimental implementation of the DLCZ protocol in UFPE’s Quantum Network Laboratory. Among them, it is the assembly of the head of a diode laser (Teco), a digital temperature controller in Arduino, coils for the generation of magnetic fields, and others. The MOT is based on the radiation pressure force that light exerts on matter. By combining an inomogeneous magnetic field with three pairs of counter-propagating laser beams with opposing circular polarizations, we are able to create a viscous and restoring force that cools and traps the atoms around the origin defined by the magnetic field, forming a cold cloud of atoms. As part of the collective effort for its construction, I set up the saturated absorption of the laser Teco, the repump beam and the coils for the magnetic fields of compensation and trapping. The atomic cloud reached temperatures of the order of 1mK, its size is ~ 3mm and we trapped about 10⁸ atoms. Once the cold cloud of atoms is formed, we turn off the MOT beams and begin the experiment of generation of pairs of individual photons following the DLCZ quantum information protocol, where I contributed with the assembly of the read beam. With the help of the Time-Multiplexed Detector that I set up, we make the detection of the generated photons and we obtain the necessary statistics for its characterization. From the obtained data, we determine whether the generated pair of photons are in a non-classical regime.

BARBOSA, Daniel Felinto Pires, também é conhecido em citações bibliográficas por: FELINTO, Daniel O armazenamento de fótons individuais “ultra banda larga” derivados de uma fonte baseada em conversão paramétrica descendente apresenta grande potencial para promover um desenvolvimento significativo no campo da informação quântica. O desafio, no entanto, é a incompatibilidade entre a duração do fóton (ultracurto) e o tempo de vida longo requerido para a memória, que se traduzem em faixas de frequência bastante diferentes para os dois sistemas. Essa dificuldade pode ser mapeada para o problema de como um meio de banda estreita pode absorver eficientemente um pulso de luz banda larga. Nesta tese, é apresentada uma abordagem detalhada para esse problema, focando na absorção de fótons com ~800 nm, uma escolha comum para fontes de conversão paramétrica descendente, por um vapor de átomos de Rubídio aquecido. Para isso, é empregado um campo de controle intenso para conduzir uma transição sequencial de dois fótons nos átomos, juntamente com um campo de sinal fraco consistentes com pulsos de luz de femtosegundos limitados por transformada de Fourier. Portanto, é realizada uma descrição detalhada mostrando como medir pequenas absorções do pulso do sinal e como melhorar essa absorção através dos vários parâmetros do problema. Os resultados são modelados por uma teoria perturbativa adequada ao atual regime de absorção fraca. O uso sistemático de máscaras espectrais no pulso de sinal favoreceu o aumento de absorção por um fator de ~1,6 em comparação com o pico de absorção obtido no regime de pulsos limitados por transformada de Fourier. Por fim, é fornecido um roteiro com estratégias diferentes para obter absorções maiores. The storage of broadband single photons from a parametric-down-conversion source is a capability with the potential to foster significant development in the field of quantum information. A particular challenge to this problem, however, is the mismatch between the short-lived photon and the long-lived memories, which translates into quite different frequency bands for the two systems. Ultimately, this difficulty can be mapped into the problem of how a narrowband medium can efficiently absorb a broadband pulse of light. Here we present a detailed approach to this problem focusing on the absorption of photons at 800 nm, a common choice for parametric-down-conversion sources, by hot vapors of rubidium atoms. For this, we employ a stronger control field to drive a sequential twophoton transition on the atoms, which is intrinsically broadband, together with a weak signal field consisting of a femtosecond pulse of light.We describe then how to measure small absorptions of the signal pulse and how to improve this absorption through the various parameters of the problem. Our results are modeled by a perturbative theory suitable to our present weak-absorption regime. The systematic use of spectral masks in the signal pulse favored the enhancement in absorption by a factor of ~1.6 compared to the peak absorption obtained in the regime of pulses limited by transform. Finally, we provide a roadmap with different strategies to achieve larger absorptions.

FACEPE Neste trabalho serão apresentadas as minhas contribuições na montagem de uma armadilha magneto-óptica (AMO), focadas no cancelamento de campos magnéticos espúrios na região da armadilha. Para que os átomos sejam armadilhados usamos um campo magnético inomogêneo que, junto com três feixes de laser contrapropagantes de polarizações circulares opostas, resfriam e aprisionam os átomos graças a uma força viscosa e restauradora. Para gerar esse campo inomogêneo contruímos um par de bobinas circulares que gera um campo nulo em sua região central e um circuito de chaveamento de vários ampéres, que garante que ao ser desligado o campo magnético gerado por essas bobinas decaia rapidamente e não interfira nos níveis de energia dos átomos. Para prevenir descoerência precisamos cancelar os campos magnéticos espúrios na região da armadilha e para isso utilizamos três pares de bobinas quadradas e um circuito de corrente que as alimenta. Esse circuito muda a corrente em cada par de bobinas compensando campos DC. Conseguimos cancelar campos magnéticos indesejáveis com esse circuito até o nível de 23 mG, mas os resultados de medidas de espectroscopia de micro-ondas no sistema nos sugeriu uma melhora para esse circuito. Então montamos um novo modelo de fonte que agora muda a corrente em cada bobina de um par de forma independente. This work will present my contributions for the building of a magneto-optical trap (MOT), focused on the anullment of the spurious magnetic fields in the region of the trap. For atoms to be trapped we use an inhomogeneus magnetic field, together with three counter-propagating beams of opposing circular polarizations, that cool and trap atoms through a viscous and restorative force.In this work my contributions will be presented in the assembly of a magneto-optical trap (AMO). For atoms to be trapped we use an inomogeneous magnetic field which, together with three counterpropagating laser beams of opposing circular polarizations, cool and trap atoms through a viscous and restoring force. To generate this inomogeneous field we construct a pair of circular coils which generates a null field in its central region and a switching circuit of several amperes, which guarantees that when the magnetic field generated by these coils is switched off it decays rapidly and does not interfere with the levels of energy of the atoms. To prevent decoherence we need to cancel the spurious magnetic fields in the trap region and for this we use three pairs of square coils and a current circuit that feeds them. This circuit changes the current in each pair of coils compensating DC fields. We were able to cancel undesirable magnetic fields with this circuit until the level of 23 mG, but the result of the microwave spectroscopy measesures suggested us an improvement for this circuit and then we set up a new model of source that now changes the current in each coil of a pair in an independent way.

BARBOSA, Daniel Felinto Pires, também é conhecido em citações bibliográficas por: FELINTO, Daniel. FACEPE Neste trabalho, investigamos o mecanismo de uma memória no espalhamento de fótons individuais por um conjunto de átomos frios bombeados para uma transição cíclica. Através do uso de funções de correlação, identificamos um tempo de vida para essa memória que é muito maior que o tempo de vida do estado excitado dos átomos e é resiliente ao processo de leitura. O trabalho de interpretação dos dados releva a necessidade de cuidado ao pressupor simplicidade à estatística dos processos envolvidos para que artefatos experimentais não se manifestem de forma relevante nos resultados finais. Por fim, propomos um modelo teórico para o mecanismo dessa memória que possibilita uma interpretação mais concreta da natureza da memória observada, atribuindo sua origem a um efeito coletivo nos graus de liberdade externos dos átomos. In this work, we investigate the mecanism of a memory in the scattering of individual photons by an ensemble of cold atoms pumped to a cyclic transition. Through the use of correlation functions, we identify a lifetime for this memory that is much larger than the lifetime of the exicted state of the atoms and is resilient to the reading process. The task of interpretation of the results shows the necessity of care in making assumptions about the simplicity of the statistics of the relevant processes so that experimental artifacts do not manifest themselves in a relevant way in the final results. Finally, we propose a theoretical model for the mecanism of this memory that allows a more concrete interpretation of the nature of the memory we observe, assigning its origin to a collective effect in the external degrees of freedom of the atoms.

Em 1993, Bennett propôs um protocolo de teletransporte de um estado quântico por meio de dois canais de comunicação de natureza distinta: Um canal clássico, formado por duas vias clássicas de informação, e um canal quântico, formado por um par de sistemas entrelaçados. Após algumas implementações bem-sucedidas no domínio de variáveis discretas, em 1998 surge uma versão de variáveis contínuas do protocolo [1]. No mesmo ano, Furusawa et al. realizam o teletransporte quântico incondicional baseado em variáveis contínuas do campo [2]. Recentemente, o emaranhamento no domínio de variáveis contínuas entre feixes intensos de luz com diferentes cores foi medido [3, 4, 5]. Neste trabalho relatamos os avanços na implementação do protocolo de teletransporte entre campos intensos de cores distintas. Depois de alguns esforços, conseguimos construir um oscilador paramétrico óptico (OPO) que se apresentou como uma fonte razoável de feixes intensos emaranhados. Os equipamentos necessários para a execução do protocolo foram construídos e instalados. Seguiram-se vários testes para garantir o funcionamento combinado de todas as partes. Entre eles, a teleportação de um estado de vácuo não deslocado, onde obtivemos resultados promissores. Após ajustes de configuração e desenvolvimento de técnicas de análise de dados para fornecer a produção, detecção e análise de estados deslocados do campo, o protocolo foi executado com esse tipo de estados. Alcançamos uma fidelidade de 45% para um estado de entrada deslocado, enquanto que a fidelidade considerando apenas as variâncias chegou a 60%. Para comprovar sucesso de nosso protocolo de teletransporte quântico, estão previstas algumas otimizações e alterações na operação do OPO, através das quais esperamos superar em breve a marca de 50% de fidelidade, correspondente ao limite clássico. In 1993, Bennett et al. [6] proposed a teleportation protocol of a quantum state by means of two communication channels of distinct natures: A classical channel, formed by two classical information pathways, and a quantum channel, formed by a pair of entangled systems. After some successful implementations in the discrete variables domain, in 1998 arises a continuous variables version of the teleportation protocol [1]. In the same year, Furusawa et al. accomplish the unconditional quantum teleportation based on continuous variables of the field. [2]. Recently, entanglement in the continuous variables domain between bright beams of light with different colors was measured [3, 4, 5]. In this work we report the advances on the teleportation protocol implementation between bright fields of distinct colors. After some struggle, we managed to build an Optical Parametric Oscillator (OPO) witch presented it self as a reasonable source of entangled bright beams. The setup required for the execution of the protocol was built and installed. This was followed by several tests to ensure the combined working of all parts. The teleportation of a nondisplaced vacuum state was one of the performed tests, on wich we achieved promising results. After setup adjustments and the developing of data analysis techniques to provide the production, detection and analysis of displaced states of the field, the protocol was run with this kind of states. We reached a fidelity of 45% for a displaced input state, while the fidelity considering only the variances was up to 60%. To prove the success of our quantum teleportation protocol, some optimizations and changes in the operation of the OPO are in plan, whereby we expect soon to surpass the 50% fidelity benchmark, corresponding to the classical limit.

O emaranhamento é uma propriedade intrinsecamente quântica, que pode ser empregada em protocolos de comunicação quântica de informação, como criptografia quântica e teletransporte, e um recurso importante no processamento quântico de informação, visando a implementação de algoritmos mais eficientes que os atualmente empregados, de uso limitado em problemas ditos insolúveis, nos quais, as etapas de processamento crescem exponencialmente com o tamanho dos dados de entrada. O Processo de Mistura de Quatro Ondas [4WM: Four Wave Mixing] na configuração duplo- em vapor de Rubídio tem-se mostrado como uma fonte robusta de estados fortemente emaranhados [Boyer et al., 2008b]. No presente trabalho é caracterizado o ganho paramétrico G dos campos Stokes e anti-Stokes no processo de 4WM associado à linha D 1 do Rubídio. Os parâmetros estudados foram a dessintonia do campo de bombeio , sua intensidade I B e a temperatura da célula de Rubídio T Rb . Além da caracterização do ganho, estudaram-se também a absorção eletromagneticamente induzida e as intensidades dos feixes conjugados. Obtiveram-se ganhos de até 450% ajustáveis via sintonização destes parâmetros. Os elevados ganhos em meios atômicos permitem geração de emaranhamento sem uso de cavidade e aceitam operação multimodo. Consequentemente podem ser usados na realização de medidas ultraprecisas e ensejam a possibilidade de explorar regimes até agora inatingíveis pelos Osciladores Paramétricos Óticos [OPO] baseados em cristais (nos quais os ganhos são da ordem de 4%). O OPO tem sido estudado em situações acima ou abaixo do limiar, mas na transição rareiam resultados experimentais. Os resultados obtidos foram comparados com a literatura, e o próximo passo é usá-los em cavidades abertas para construir um OPO que seja facilmente operado acima ou abaixo do limiar, e estudar o limite entre vácuo enmaranhado e feixes intensos. Espera-se observar estados emaranhados não gaussianos gerados neste sistema e estudar suas aplicações em processamento de informação com variáveis contínuas da luz. Entanglement is an intrinsically quantum property that can be used in quantum communication protocols, such as quantum cryptography and teleportation. It is a basic piece for quantum information processing, aiming at the implementation of more efficient algorithms than those currently used. These new algorithms are important to attack the so called nonsolvable problems, in which, the processing steps grow exponentially with the size of the input data. On the other hand, the Four Wave Mixing Process in the double- level configuration in Rubidium vapor has been shown to be a robust source of strongly entangled fields [Boyer et al., 2008b]. In the present work the parametric gain G of the Stokes and anti-Stokes fields in the 4WM process associated with the D 1 line of Rubidium is characterized. The parameters studied were the detuning and intensity I B of the pumping field and the temperature of the Rubidium cell T Rb. Besides the gain characterization, the electromagnetically induced absorption and the intensities of the conjugated fields were also studied. Adjustable gains up to 450% were achieved via the tuning of those parameters. The high gains in atomic media allow the generation of entanglement without cavity, and permit multi-mode operation. Therefore, they can be used to carry out ultra-precise measurements and allow the possibility of exploring schemes hitherto unattainable by crystal-based Optical Parametric Oscillators (OPO) (with gains of 4 %). The OPO has been studied in situations above or below the threshold, but in the transition experimental results are rare. The results we obtained were compared with literature, and the next step is to use them in open cavities to construct an OPO that can be easily operated above or below the threshold, and to study the boundary between entangled vacuum and intense beams. In this system, it is expected to be observed non-Gaussian entangled states, and to be able to study its applications in information processing with continuous variables states of light.

BARBOSA, Daniel Felinto Pires, também é conhecido(a) em citações bibliográficas por: FELINTO, Daniel CNPq Nesta tese, exploramos técnicas de óptica quântica para abordar dois problemas distintos. Primeiro, relatamos passos experimentais significativos em direção a um teste sobre a realidade da função de onda, destacando a possibilidade de diferenciar o colapso instantâneo da função de onda de sua redução em tempo finito. Para isto, foi necessária a caracterização de uma fonte capaz de gerar estados de fótons parcialmente emaranhados em polarização. Essa fonte é baseada em interferômetro de Sagnac e em um cristal de PPKTP que, pelo processo de conversão paramétrica descendente espontânea, gera pares de fótons colineares e com polarização ortogonais. O sistema é bombeado pelo trem de pulsos de um laser de femtossegundos pulsado. O emaranhamento parcial requerido dos estados de pares de fótons é caracterizado pela violação que produzem na desigualdade de Clauser- Horne-Shimony-Holt (CHSH) e a sincronização é obtida através de medições de varreduras temporais. Isto nos permite compreender melhor as condições instrumentais necessárias para a execução de um teste final da hipótese de um tempo finito de colapso da função de onda. Para nosso segundo problema, investigamos o espalhamento espontâneo de fótons de um conjunto de átomos frios de dois níveis a um certo ângulo. Para isto, utilizamos uma armadilha magneto-óptica para átomos de Rubídio-87. Relatamos a observação de correlação entre fótons espalhados por pulsos consecutivos do laser de excitação, com uma dessintonia muito maior do que o inverso do tempo de vida do estado excitado. Tais correlações mostram um aumento da probabilidade de detectar outro fóton na mesma direção, uma vez que o primeiro foi observado, e modificações na estatística dos fótons da luz espalhada condicionada. Elas duram consideravelmente mais do que o tempo de vida do estado excitado e são robustas ao processo de leitura, apontando o papel fundamental das ressonâncias induzidas por recuo no sistema. In this thesis, we explore quantum optics techniques to address two distinct problems. First, we report significant experimental steps towards a test on the reality of the wave function, highlighting the possibility of differentiating the instantaneous collapse of the wave function from its reduction in finite time. For this, it was necessary to characterize a source able to generation photon states partially entangled in polarization. This source is based on Sagnac interferometer and a PPKTP crystal which, by the spontaneous parametric down conversion process, generates orthogonal polarizing and collinear photon pairs. The system is pumped by the pulse train of a pulsed femtosecond laser. The required partial entanglement of the photon pair states is characterized by the violation they produce in the Clauser-Horne-Shimony-Holt (CHSH) inequality and synchronization is obtained via time-scan measurements. This allows us to better understand the instrumental conditions necessary for the execution of a final test of the hypothesis of a finite time of wave function collapse. For our second problem, we investigate the spontaneous photon scattering of an ensemble of two-level cold atoms at a certain angle. For this, we used a magneto-optical trap for Rubidium-87 atoms. We report the observation of correlation between photons scattered by consecutive pulses of the excitation laser, with a much greater detuning than the inverse of the average lifetime of the excited state. Such correlations show an increased probability of detecting another photon in the same direction, once the first was observed, and changes in the photon statistic of the conditioned scattered light. They last considerably longer than the excited state lifetime and are robust to the reading process, pointing to the crucial role of recoil induced resonances in system.

BARBOSA, Daniel Felinto Pires, também é conhecido em citações bibliográficas por: FELINTO, Daniel CNPq In this thesis, two optical systems that generate non-classical states of light were studied: a triply resonant Optical Parametric Oscillator (OPO) operating above the threshold and a cold atomic ensemble of neutral 87Rb atoms obtained from a magneto-optical trap. The main idea in both cases is to prepare entangled states for future use in quantum information protocols. In relation to the OPO, experimental measurements of the complete quantum state for six modes of the electromagnetic field produced by this system are theoretically explained. The investigation involves the sidebands of the intense pump, signal and idler fields generated by stimulated parametric down-conversion inside the resonator. The model takes into account the coupling of the field modes with the phonon bath of the nonlinear crystal, clearly showing the roles of different physical effects in shaping the structure of the quantum correlations between the six optical modes. Moreover, it is theoretically and experimentally studied how these modes are entangled to one another using the positive partial transpose criterion in the continuous variables regime. It was found that the hexapartite entanglement in this system can be thought of as being generated by a combination of two-mode squeezers and beam splitter Hamiltonians acting on six different colors of light. On the other hand, in relation to the cold atomic ensemble, a "write-read" scheme inspired by the Duan-Lukin-Cirac-Zoller (DLCZ) protocol for long-distance quantum communication was implemented in order to experimentally generate an entangled state between individual photons of a mode of the electromagnetic field and atomic excitations in a particular collective mode. By performing photon statistics analysis it was possible to determine that the quantum state of the system corresponds to an entangled two-mode squeezed vacuum state, as expected from the theory. In the experiments, either one or two excitations are initially stored in the atomic medium, which acts as a quantum memory, and are subsequently mapped to one or two photons, respectively. Moreover, measurements and theoretical modeling of the photonic wave packets were realized, observing an acceleration in the photon emission due to the collective nature of the atomic state, a phenomenon known as superradiance. Such progress opens the way to the study and future control of the interaction of non-classical light modes with collective quantum memories at higher photon numbers. Nesta tese foram estudados dois sistemas óticos geradores de estados da luz não clássicos: um oscilador paramétrico ótico (OPO) triplamente ressonante que opera acima do limiar e um ensamble atômico frio de átomos de 87Rb neutros obtido a partir de uma armadilha magneto-ótica. A ideia principal em ambos os casos é preparar estados emaranhados para uso futuro em protocolos de informação quântica. Em relação ao OPO, medições experimentais do estado quântico completo, para os seis modos do campo eletromagnético produzido por este sistema, são explicadas teoricamente. A investigação envolve as bandas laterais dos campos de bombeio, sinal e complementar gerados por conversão paramétrica descendente espontânea dentro do ressonador. O modelo leva em consideração o acoplamento dos modos do campo com o banho de fônons do cristal não-linear, mostrando claramente os papéis dos diferentes efeitos físicos na formação da estrutura das correlações quânticas entre os seis modos ópticos. Além disso, é estudado teórica e experimentalmente como esses modos estão emaranhados entre si fazendo uso do critério de transposição parcial positiva no regime de variáveis contínuas. Verificou-se que o emaranhamento hexapartite neste sistema pode ser pensado como sendo gerado por uma combinação de Hamiltonianas de compressão de dois modos e de divisores de feixes que atuam sobre seis cores diferentes da luz. Por outro lado, em relação ao ensemble atômico frio, foi implementado um esquema de “leitura-escrita” inspirado no protocolo de Duan-Lukin-Cirac-Zoller (DLCZ) para comunicação quântica de longas distâncias com o fim de gerar experimentalmente um estado emaranhado entre fótons individuais de um modo do campo eletromagnético e excitações atômicas em um modo coletivo particular. Fazendo uma análise da estatística dos fótons foi possível determinar que o estado quântico do sistema corresponde a um estado de vácuo comprimido de dois modos, como esperado pela teoria. Nos experimentos, uma ou duas excitações são inicialmente armazenadas no meio atômico, que atua como memória quântica, e posteriormente são mapeadas para um ou dois fótons, respectivamente. Além disso, foram realizadas medições e uma modelagem teórica dos pacotes de ondas fotônicos, observando-se uma aceleração na emissão de fótons devido à natureza coletiva do estado atômico, um fenômeno conhecido como superradiância. Tal progresso abre o caminho para o estudo e controle futuro da interação de modos da luz não-clássica com memórias quânticas coletivas em números de fótons mais altos.

Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-05-16T23:56:29Z No. of bitstreams: 1 EDMAR JOSÉ DO NASCIMENTO – TESE (PPGEE) 2017.pdf: 1146136 bytes, checksum: 66fa0c285fd895d4aa000dd5ad1d1eef (MD5) Made available in DSpace on 2018-05-16T23:56:29Z (GMT). No. of bitstreams: 1 EDMAR JOSÉ DO NASCIMENTO – TESE (PPGEE) 2017.pdf: 1146136 bytes, checksum: 66fa0c285fd895d4aa000dd5ad1d1eef (MD5) Previous issue date: 2018-04-18 Protocolos para a distribuição quântica de chaves (DQC) permitem que duas partes (Alice e Bob) compartilhem uma chave secreta que pode ser usada para fins criptográficos. A segurança do protocolo é baseada em propriedades da mecânica quântica, ao invés de hipóteses computacionais. Na distribuição quântica de chaves com variáveis contínuas (DQCVC), a informação é codificada nas amplitudes de quadratura do campo eletromagnético quantizado. Quando implementado com variáveis contínuas, o aparato usado na DQC é consideravelmente mais simples que nas implementações convencionais com variáveis discretas, já que se pode utilizar a medição do tipo homódina, ao invés da detecção de fótons. Uma vez realizada a medida, ainda se faz necessária uma etapa de processamento clássico, denominada de reconciliação da informação, a fim de que Alice e Bob possam compartilhar uma cadeia comum de bits. Para que a DQCVC possa ser realizada em distâncias razoáveis (superiores a 30 km), o processo de reconciliação precisa ser feito com eficiências elevadas (superiores a 90%). Entretanto, eficiências dessa ordem para baixas SNRs (signal-to-noise ratio - razão sinal ruído) requerem o uso de códigos clássicos de comprimento bastante elevado e, assim, são difíceis de serem alcançadas. Nesta tese, se propõe o uso dos mapas de Shannon-Kotel’nikov na preparação dos estados quânticos que são usados na DQCVC. Com a utilização desses mapas, é possível aumentar a SNR entre Alice e Bob sem aumentar a variância da modulação de Alice. Dessa forma, o processo de reconciliação se torna mais simples, pois eficiências de reconciliação mais altas são mais facilmente alcançadas em SNRs maiores. Como contribuições desta tese têm-se: a proposição de um protocolo; a definição de um cenário de simulação e a análise do protocolo para dois tipos de mapas (a espiral uniforme de Arquimedes e as curvas geodésicas em um toro planar). Quantum key distribution (QKD) protocols allow two parties, Alice and Bob, to share a secret key that may be used for cryptographic purposes. The security of QKD is based on quantum mechanics properties instead of computational assumptions. In continuous-variable quantum key distribution (CVQKD), the information is encoded in the quadrature amplitudes of the quantized electromagnetic field. When QKD is implemented with continuous variables, hardware components are much simpler than their discrete variables equivalents. This is mainly due to homodyne detection instead of photon detection. After measuring the transmitted states, it is still necessary to carry out a classical processing stage known as information reconciliation. This stage allows Alice and Bob to share a common sequence of bits. In order to deploy CVQKD over reasonable distances (over 30 km), reconciliation must be done at high efficiencies (over 90%). However, such high efficiencies for low SNRs (signal-to-noise ratio) require long length classical codes and are difficult to be reached. In this thesis, we propose to use Shannon-Kotel’nikov maps for preparing quantum states in CVQKD. By using these maps, it is possible to increase the SNR between Alice and Bob, without increasing Alice’s variance. Thus, reconciliation becomes easier because higher reconciliation efficiencies are more easily reached for higher SNRs. The contributions of this theses are: the proposal of a CVQKD protocol; the statement of a simulation scenario; the analysis of the proposed protocol for two kinds of maps (uniform Archimedes’ spiral and geodesic curves on a flat torus).

BARBOSA, Daniel Felinto Pires, também é conhecido em citações bibliográficas por: FELINTO, Daniel FACEPE The ability to coherently manipulate quantum systems enables a wide range of applications in the field of quantum information. In this thesis we are going to address two applications of quantum interaction between light and an atomic medium. First, in the case of a single charged atom (ion), we propose a set of logical gates for quantum computation. Second, we use a macroscopic ensemble of neutral atoms as quantum memory for the storage of quantum entanglement to be used in fundamental studies of the process of superradiance. The first part of this thesis is a theoretical work with experimental perspectives. The traditional platform for the development of quantum computing with a chain of ions confined in a Paul trap is based on the model introduced in 1995 by Cirac and Zoller (CZ). In this model the electronic levels describe the qubits and the collective vibrational modes act as mediators between the qubits. In order to help mitigate the problems of scalability, we have proposed a new architecture that is a true reversal of the CZ scheme. Our approach consists in using the internal degrees of freedom of a single ion as a mediator (bus) and harnessing the motion of ions as the unit of quantum information, which leads us to a continuous variables picture of the problem. In addition we developed a toolbox of quantum operations complete enough to achieve computational universality. In the second part of the thesis, we develop a study not only theoretical but also experimental using quantum memories in an atomic ensemble. The idea here is to use a set of atoms to store quantum information over a certain period of time. This stored information is originally packaged in a collective atomic mode, which could subsequently be extracted on demand in a light field, presenting non-classical correlations with a previously detected heralding field. The atomic medium used is a cloud of cold Rubidium atoms obtained from a magnetooptical trap, when the trap is off. The generation of photon pairs, correlated through interaction with the atoms, is done by implementing a "write-read" scheme inspired by the Duan-Lukin-Cirac-Zoller protocol for long-distance quantum communication. A characteristic of the photons that mediate these interactions is that they are narrowband enough to allow a direct study of the temporal shape of their wave packet. Through the analysis and theoretical modeling of this wave packet, we investigate the dynamics of extraction of the quantum information stored in the quantum memory of the atomic ensemble. We report an experimental finding of non-classical characteristics of superradiance by implementing the process not only with a single excitation, but also with two-photon Fock states. Specifically, this work presents the experimental measurement and the theoretical modeling of wave packets in both single-photon and two-photon superradiance regimes. This new step opens the way for the study of interactions between collective quantum memories and light modes in a regime of higher-order components of the electromagnetic field. A capacidade de manipular coerentemente os sistemas quânticos permite uma ampla gama de aplicações no campo da informação quântica. Nesta tese vamos abordar duas aplicações da interação quântica entre luz e um meio atômico. Primeiro, no caso de um único átomo carregado (íon), propomos um conjunto de portas lógicas para a computação quântica. Em segundo lugar, usamos um ensemble macroscópico de átomos neutros como memória quântica para o armazenamento de emaranhamento quântico a fim de ser usado em estudos fundamentais do processo de superradiância. A primeira parte desta tese é um trabalho teórico com perspectivas experimentais. A plataforma tradicional para o desenvolvimento da computação quântica com uma cadeia de íons aprisionados em uma armadilha de Paul é baseada no modelo introduzido em 1995 por Cirac e Zoller (CZ). Neste modelo os níveis eletrônicos descrevem os qubits e os modos vibracionais coletivos atuam como mediadores entre os qubits. Para ajudar a mitigar os problemas de escalabilidade, temos proposto uma nova arquitetura que é uma verdadeira inversão do esquema CZ. Nossa abordagem consiste em usar os graus de liberdade internos de um único íon como um mediador (bus) e aproveitar o movimento dos íons como a unidade de informação quântica, o que nos leva a um quadro de variáveis contínuas do problema. Além disso, desenvolvemos uma caixa de ferramentas de operações quânticas completa o suficiente para alcançar a universalidade computacional. Na segunda parte da tese, desenvolvemos um estudo não só teórico, mas também experimental usando memórias quânticas em um ensemble atômico. A ideia aqui é usar um conjunto de átomos para armazenar informação quântica durante um certo período de tempo. Esta informação armazenada é originalmente anunciada por um modo atômico coletivo, o qual poderia ser posteriormente extraído ‘a pedido’ em um campo de luz, apresentando correlações não clássicas com um campo ‘anunciante’ previamente detectado. O meio atômico é uma nuvem de átomos frios de Rubídio obtida a partir de uma armadilha magneto-óptica, quando a armadilha está desligada. A geração de pares de fótons, correlacionados através da interação com os átomos, é feita através da implementação de um esquema de “escrita-leitura” inspirado no protocolo Duan-Lukin-Cirac-Zoller para comunicação quântica de longa distância. Uma característica dos fótons que mediam essas interações é que eles são banda estreita o suficiente para permitir um estudo direto da forma temporal de seu pacote de onda. Através da análise e modelagem teórica deste pacote de onda, investigamos a dinâmica de extração da informação quântica armazenada na memória quântica do ensemble atômico. Reportamos um achado experimental das características não-clássicas da superradiância, implementando o processo não apenas com uma única excitação, mas também com estados de Fock de dois fótons. Especificamente, este trabalho apresenta a medição experimental e a modelagem teórica de pacotes de onda em ambos os regimes de superradiância, de um único fóton e dois fótons. Este novo passo abre o caminho para o estudo das interações entre memórias quânticas coletivas e modos de luz em um regime de componentes de mais alta ordem do campo eletromagnético.

FACEPE Neste trabalho, apresentamos a construção e caracterização de uma Armadilha Magneto-Óptica (AMO) para átomos neutros de Rubídio. Esta AMO usa fundamentalmente a força de radiação de três pares de feixes contrapropagantes, que se interceptam na origem de uma região onde um campo magnético quadrupolar é aplicado. Implementamos uma montagem para a geração de pares de fótons individuais a partir de ensembles atômicos como proposto no protocolo de informação quântica DLCZ. No desenvolvimento deste trabalho, montamos o sistema de vácuo e a câmara de vácuo onde é gerada a nuvem de átomos aprisionados. Fizemos a construção, sintonia e chaveamento dos feixes de resfriamento, e dos feixes usados no experimento de geração de fótons; chamados de escrita e leitura. Para a caracterização da AMO, fizemos medidas do número de átomos, e medidas da densidade da nuvem, calculando a profundidade óptica. Utilizando um método baseado em espectroscopia de micro-ondas, o qual foi implementado pela primeira vez em nosso grupo, fizemos o cancelamento de campos magnéticos espúrios atuando sobre a AMO. In this work we present the construction and characterization of a Magneto-Optical Trap (MOT) of rubidium neutral atoms. This MOT basically uses the radiation pressure force of three pairs of counterpropagating beams which intersect each other at the origin of a region where a quadrupole magnetic field is applied. We implement an experimental setup for single photon pair generation from atomic ensembles as proposed in DLCZ quantum information protocol. In the development of this work, we mounted the vacuum system and the vacuum chamber in which a cloud of trapped atoms is generated. We carried out the construction, tuning and switching of the cooling beams and the beams used in the photon generation experiment; called writing and reading beams. For the MOT characterization, we measured the number of atoms and cloud density calculating the optical depth. Using a method based on microwave spectroscopy, which was first implemented in our group, we made the spurious magnetic fields cancellation acting on the MOT.

Neste trabalho, apresentamos uma montagem experimental capaz de aprisionar átomos de K e Rb simultaneamente em uma armadilha óptica de dipolo cruzada. Contudo, para atingir este feito foi necessário, durante o desenvolvimento do projeto, estudar cada espécie atômica de forma isolada. Assim, desenvolvemos uma técnica inédita para carregar uma armadilha de dipolo diretamente de uma armadilha magneto-óptica de 39K, constatando a importância da estrutura hiperfina do estado 4P3/2 no processo de carregamento. Também observamos e explicamos o processo de fotoassociação de Rb devido ao laser em 1071 nm, inclusive a diferença isotópica entre 85Rb e 87Rb. Na sequência, realizamos o carregamento duplo com K e Rb na armadilha e observamos a fotoassociação de KRb. Um modelo teórico simples prevê a distribuição dos níveis vibracionais obtidos. O mesmo modelo abre caminho para um possível bombeamento óptico para a produção e acúmulo de moléculas de KRb no estado X1Σ+ (v = 0). In this paper, we present an experimental setup to simultaneously trap atoms of K and Rb in a crossed optical dipole trap. This accomplishment required the individual study of each atomic species during the project development. This allowed the development of a new technique for loading a dipole trap directly from a magneto-optical trap of 39K, providing evidence of the importance of the hyperfine structure of the state 4P3/2 in the loading process. We also observe and explain the photoassociation process of Rb2 due to the dipole trap laser at 1071 nm, including an isotopic difference between 85Rb and 87Rb. In the sequence, we performed dual species loading of K and Rb and observed the photoassociation process of KRb. A simple theoretical model predicts the final distribution of vibrational levels obtained. The same model provides a potential path to optically pump the molecules and accumulate them in the molecular ground state X1Σ+ (v = 0) for KRb.