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ABSTRACT: 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.
Optics Express 01/2013; 21(2):1512-9. · 3.59 Impact Factor
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ABSTRACT: We report a spectroscopic investigation of the reading process of a cold atomic ensemble coherently prepared in a superposition of its degenerate ground states. Specifically we measure the spectra of the generated signal for different frequencies of the reading laser pulse and for intensities below saturation. The spectra present a double-peaked structure, with a decrease of the signal around the atomic resonance. A simple theory using the density matrix formalism and accounting for propagation effects qualitatively describes the experimentally observed results.
Phys. Rev. A. 07/2012; 86(1).
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ABSTRACT: 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.
Optics and Spectroscopy 04/2012; 111(4):583-588. · 0.61 Impact Factor
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ABSTRACT: 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.
The European Physical Journal D 04/2012; 60(2):373-382. · 1.48 Impact Factor
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ABSTRACT: 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
85Rb D2 line (λ= 780nm). 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.
The European Physical Journal D 04/2012; 41(3):531-539. · 1.48 Impact Factor
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ABSTRACT: 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-$\Lambda$ 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 anti-correlation between the probe and conjugate
beams. A simple theoretical model is developed that accounts qualitatively well
to the observed results.
07/2011;
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ABSTRACT: Electromagnetically induced transparency (EIT) has mainly been modelled for
three-level systems. In particular, a considerable interest has been dedicated
to the Lambda-configuration, with two ground states and one excited state.
However, in the alkali-metal atoms, which are commonly used, 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
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 due to the presence of multiple excited levels.
Given this model, we also show that a well-designed optical pumping enables to
significantly recover the transparency.
01/2011;
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ABSTRACT: 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 Λ configuration.
Optics Letters 12/2010; 35(23):3937-9. · 3.40 Impact Factor
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ABSTRACT: 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. Comment: 9 pages, 6 figures
08/2010;
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ABSTRACT: 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
05/2010;
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ABSTRACT: We report on a detailed investigation of the coupling between a femtosecond-laser frequency comb and a cw diode laser interacting with an atomic medium of variable density. The comb is printed on a Doppler-broadened atomic transition and the frequency-dependent transmission of the cw laser is monitored as it is scanned over the inhomogenously broadened absorption profile. The printing process and its probing are analysed, experimentally and theoretically, as a function of both laser intensities and the atomic density. The results reveal the importance of optical pumping and power broadening by both lasers, allowing us to determine various regimes of competition between them.
Journal of Physics B Atomic Molecular and Optical Physics 02/2010; 43:55001. · 1.88 Impact Factor
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ABSTRACT: 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.
Quantum Electronics and Laser Science Conference, Washington, USA.; 01/2010
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ABSTRACT: The paper reports on the storage of superposition of orbital angular momentum (OAM) states, as well as its manipulation through an applied transverse magnetic field. In the experiment , Zeeman sublevels of the degenerate two-level system is used. In another series of experiments,an external dc magnetic field is applied, nearly orthogonal to the plane defined by the incident beams. In this case,a series of collapses and revivals of the stored Zeeman coherence is observed. These observations demonstrate that superposition of OAM states can be stored into the atomic ensemble, and also the reversible transfer of OAM between light and atoms. Moreover, it also shows that the stored OAM can be manipulated with an external magnetic field.
Lasers and Electro-Optics 2009 and the European Quantum Electronics Conference. CLEO Europe - EQEC 2009. European Conference on; 07/2009
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ABSTRACT: 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 $\mu s$.
02/2009;
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ABSTRACT: 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. Comment: Submitted to Phys. Rev. A
04/2008;
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ABSTRACT: 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.
12/2006;
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ABSTRACT: 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.
10/2006;
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ABSTRACT: 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 micros, 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.
Physical Review Letters 10/2006; 97(11):113603. · 7.37 Impact Factor
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ABSTRACT: 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 (for storing the states) can enable scalable architectures for quantum computation, communication and metrology. Here we report observations of entanglement between two atomic ensembles located in distinct, spatially separated set-ups. 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 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 represent significant progress in the ability to distribute and store entangled quantum states.
Nature 01/2006; 438(7069):828-32. · 36.28 Impact Factor
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ABSTRACT: Pairs of pulses from an incoherent source are used to investigate the time-resolved four-wave mixing response of atomic rubidium when a two-photon resonance is involved in the nonlinear process. By varying the relative polarization of the pulse pairs, we are able to select the quantum pathways and clearly distinguish optical and quantum interferences.
Optics Letters 08/2005; 30(14):1876-8. · 3.40 Impact Factor
