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South American optics research has seen remarkable growth over the past 50 years, with significant contributions in areas such as quantum optics, holography, spectroscopy, nonlinear optics, statistical optics, nanophotonics and integrated photonics. The research has driven economic development in sectors like telecom, biophotonics, biometrics, and agri-sensing. This joint feature issue between JOSA A and JOSA B exhibits cutting-edge optics research from the region, fostering a sense of community and promoting collaboration among researchers.

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

We present a first-principles model for a non-volatile atomic memory based on recoil-induced resonances (RIR) and directly compare it to experimental data obtaining a good agreement on all phases of the experiment.

By combining genetic algorithm and a spatial light modulator we demonstrate theoretically how to improve a two-photon cascade absorption in atomic ensembles using optimized phase masks. At low atomic densities, we compare the cases of sequential transitions with the two photons coming from the same pulse and from two different pulses. We achieved an enhancement by a factor of 9.4 ± 0.2, about 30% higher than previously reported. For the later, we obtained 20 ± 3 for the enhancement factor. This absorption of two photons by different pulses is of particular interest for the storage of ultra-broadband single photons by atomic ensembles, in which case the second photon would come from a control pulse. We investigate this process as a function of the atomic density, demonstrating still enhancements by factors on the order of 6 for the two-photon absorption after propagating through large optical depths, on the order of hundreds.

In spite of recent conceptual and experimental advances, the ultimate nature of the wave function in quantum theory remains nebulous. Is it a mathematical device that describes our knowledge on a physical system or is it a physical field in any tenable sense? In this work we report on significant experimental steps toward a test on the reality of the wave function, which does not address the duality “epistemic versus ontic” in the usual way. Instead of considering inequalities derived from the partial indistinguishability of non-orthogonal states, we focus on the possibility of discriminating instantaneous reduction from finite-time wave function collapse. The former can only be associated with a Bayesian update, while the latter would be compatible with realistic interpretations. We employ non-maximally entangled photons which must be finely synchronized before detection. The required partial entanglement of the pairs is characterized by the violation they produce in the Clauser–Horne–Shimony–Holt inequality and the synchronization is obtained via time-scan measurements. This realization allows us to better understand the necessary instrumental conditions to the execution of a final, conclusive test.

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 two-photon 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. Finally, we provide a road map with different strategies to achieve larger absorptions.

The bunching of light resulting from the spontaneous emission of an atomic ensemble is a well known effect, shared with various sources of thermal fields. It is, on the other hand, also an integral part of recently developed quantum memories for various applications. Here we theoretically and experimentally investigate the statistical correlations in light spontaneously scattered from a cold ensemble of two level atoms. We demonstrate that the bunching of light in this situation is both long-lived and robust to various parameters of the excitation process. This memory comes from constructive interference of the emitted light for particular distributions of positions of the atoms, being washed out by their movement. We offer both a first-principles quantum theory for the process and a corresponding classical theory. We also introduce various methods to treat long-term fluctuations of the experimental data in order to be able to compare it to our simple theories. As a result, we obtain a detailed picture of such spontaneous scattering processes in cold atomic ensembles, which may result in better control of quantum memories heralded by these processes.

In this work, we explore both the internal and external atomic degrees of freedom to demonstrate the observation of giant gain and parametric oscillation in multiple four-wave mixing (FWM) processes in a sample of cold cesium atoms. We employ a standard backward FWM beam configuration to achieve parametric probe-beam gains exceeding 2000. This giant gain is accompanied by the generation of three other beams of equivalent power emitted along the directions satisfying the phase-matching conditions for multiple cascade forward and backward FWM triggered by the incident probe and the counterpropagating pumping beams. Moreover, we have also observed a simultaneous threshold for parametric oscillation of the four coupled optical fields with the pump intensity. These results point to an alternative pathway to generate multipartite correlated optical fields associated with long-lived atomic systems for applications in quantum information.

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.

An experimental test for the wave function collapse is realized. The experimental results reaffirms the epistemic interpretation for the state vector and opposes the ontic view.

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

Zero area single photons are generated through the resonant interaction of a parametric-down-conversion single photon with a dense atomic ensemble. This indicates a new pathway for developing a quantum memory for broadband single photons.

Electromagnetically induced transparency (EIT) has led to several quantum optics effects such as lasing without inversion or squeezed light generation. More recently quantum memories based on EIT have been experimentally implemented in different systems such as alkali metal atoms. In this system the excited state of the optical transition splits into several sublevels due to the hyperfine interaction. However, most of the theoretical models used to describe the experimental results are based on a Λ-system with only one excited state. In this article, we present a theoretical model for the Λ-type interaction of two light, fields and an atomic system with multiple excited state. In particular we show that if the control and probe fields are orthogonally circularly polarized the EIT effect in an alkali-metal vapor can almost disappears. We also identify the reasons of this reduction and propose a method to recover the transparency via velocity selective optical pumping.

The present work explores new possibilities for the selective storage of optical information through higher-order nonlinearities in a sample of cold cesium atoms obtained from a magneto-optical trap. Particularly, we demonstrate the storage of a third-, fifth-, and seventh-order nonlinear atom–light interaction into the coherences and populations of the Zeeman structure of the hyperfine cesium 6S1/2, F=3 ground state. Two writing beams create coherence and population gratings associated with high-order nonlinear processes, which can be selectively retrieved by a reading beam after a controllable storage time. Each kind of grating can store only specific orders of the nonlinear interaction, leading to special features in each signal. Applications of this nonlinear memory to herald multiphotons are discussed.

We report a new type of optical memory using a pure two-level system of cesium atoms cooled by the magnetically assisted Sisyphus effect. The optical information of a probe field is stored in the coherence between quantized vibrational levels of the atoms in the potential wells of a 1-D optical lattice. The retrieved pulse shows Rabi oscillations with a frequency determined by the reading beam intensity and are qualitatively understood in terms of a simple theoretical model. The exploration of the external degrees of freedom of an atom may add another capability in the design of quantum-information protocols using light.

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

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.

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 present a series of experiments measuring intensity fluctuations and correlations of laser beams after propagation through an atomic vapour at room temperature in situations where coherent effects play a major role. In particular, we investigate these properties in the condition of electromagnetically induced transparency (EIT) in three-level lambda systems and in degenerate two-level systems. In a similar way, we also study the Hanle-EIT effect and electromagnetically induced absorption (EIA) in degenerate two-level systems. These results show the great influence of coherent interaction with the atomic system over the noise properties of the macroscopic laser fields, and indicate new sensitive ways of probing an atomic sample.

We investigate the problem of a two-level atom excited by a single photon source with variable statistics. Analytical expressions are obtained and compared to Monte Carlo simulations considering the correlation between absorption and stimulated emissions. Photon number fluctuations and time correlations are calculated showing the influence of the excitation statistics.

In this paper we consider the interaction of inhomogeneously broadened two-level atoms with a train of pulses from a femtosecond mode-locked laser. Analytical results are obtained for the case in which the atomic relaxation times are longer then the laser repetition period, leading to accumulation in both population and coherence. These results are adequate for arbitrary pulse areas, and saturation effects are discussed. We illustrate our analytical results with a comparison to a direct numerical integration of the Bloch equations.

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

We report the experimental observation of Coherent Population Oscillation (CPO) based light storage in thermal helium and cesium vapors.

We consider a new model for invasion in which the mechanism is also dependent on the fluid's filling of neighboring pores. During the invasion process, it is assumed that the resistance of a pore for invasion may be weakened depending on the fluids’ configurations of its nearest-neighbor pores. In case of nontrapping, the cluster generated by the present correlated invasion percolation is connected to the critical percolated cluster of the site-bond-correlated percolation model. A surprising result emerging from our calculations consist of a double-plateau structure observed in the acceptance profile for the invader cluster. The discussion about the origin of the profiles allows us to relate the occurrence of a second plateau to the strength of correlation.

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

We report on the demonstration of a nonlinear optical memory (NOM) for storage and on-demand manipulation of orbital angular momentum (OAM) of light via higher-order nonlinear processes in cold cesium atoms. A spatially resolved phase-matching technique is used to select each order of the nonlinear susceptibility associated, respectively, with time-delayed four-, six-, and eight-wave mixing processes. For a specific configuration of the stored OAM of the incident beams, we demonstrated that the OAM of the retrieved beam can be manipulated according to the order of the nonlinear process chosen by the operator for reading out the NOM. This demonstration indicates new pathways for applications in classical and quantum information processing where OAM of light is used to encode optical information.

We analyze the efficiency and scalability of the Duan-Lukin-Cirac-Zoller (DLCZ) protocol for quantum repeaters focusing on the behavior of the experimentally accessible measures of entanglement for the system, taking into account crucial imperfections of the stored entangled states. We calculate then the degradation of the final state of the quantum-repeater linear chain for increasing sizes of the chain, and characterize it by a lower bound on its concurrence and the ability to violate the Clausner-Horne-Shimony-Holt inequality. The states are calculated up to an arbitrary number of stored excitations, as this number is not fundamentally bound for experiments involving large atomic ensembles. The measurement by avalanche photodetectors is modeled by ''ON/OFF'' positive operator-valued measure operators. As a result, we are able to consistently test the approximation of the real fields by fields with a finite number of excitations, determining the minimum number of excitations required to achieve a desired precision in the prediction of the various measured quantities. This analysis finally determines the minimum purity of the initial state that is required to succeed in the protocol as the size of the chain increases. We also provide a more accurate estimate for the average time required to succeed in each stepmore » of the protocol. The minimum purity analysis and the new time estimates are then combined to trace the perspectives for implementation of the DLCZ protocol in present-day laboratory setups.« less

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 analyze the efficiency and scalability of the DLCZ protocol for quantum repeaters through experimentally accessible measures of entanglement (concurrence; CHSH inequality) providing a better time estimate to succeed in each step of the protocol.

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 introduce a theory for the interaction of multi-level atoms with well-stabilized pulse trains, which is general enough to take into account arbitrarily-shaped frequency combs. It is applied to the excitation of rubidium-87 atoms.

We investigate the transmission of a cw laser interacting with rubidium vapor and a frequency comb. The results reveal various regimes of competition and the importance of optical pumping and power broadening of the lasers.

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 investigate the generation of correlated light beams in a coherently prepared atomic system consisting of cold cesium atoms. Double Bragg diffraction into a Zeeman coherence grating is employed to conditionally extract the stored information.

We present experimental results on the Two-Beam Coupling of femtosecond beams in atomic rubidium vapor. Propagation and accumulation effects due to the modelocked pulse train are investigated.

Direct frequency-comb spectroscopy is a technique that employs a train of well-stabilized ultrashort pulses to study the spectral properties of atomic or molecular systems. In this way, it opens the possibility of incorporating various coherent-control techniques for such spectral investigations. Here we introduce a theory for the interaction of a multilevel atom with such pulse trains, which is general enough to take into account an arbitrarily shaped frequency comb. We illustrate its application by studying the interaction of $^{87}\text{R}\text{b}$ atoms with trains of pulses of various shapes, resonant with the $5S\text{\ensuremath{-}}5D$ two-photon transition of rubidium. More specifically, we treat the interaction with hyperbolic-secant pulses, chirped pulses, and 0-$\ensuremath{\pi}$ pulses, respectively. The theory is designed to work at an arbitrary perturbation order. For the results presented here, we mostly used a 12th-order perturbation series at the pulse's electric field. Due to the large number of levels involved, such modeling may be quite complex computationally, and an important point of the present work is then to introduce the required numerical approach to treat this problem efficiently.