Dynamics of a stored Zeeman coherence grating in an external magnetic field

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.

Power-law entanglement growth from typical product states

Generic quantum many-body systems typically show a linear growth of the entanglement entropy after a quench from a product state. While entanglement is a property of the wave function, it is generated by the unitary time evolution operator and is therefore reflected in its increasing complexity as quantified by the operator entanglement entropy. Using numerical simulations of a static and a periodically driven quantum spin chain, we show that there is a robust correspondence between the entanglement entropy growth of typical product states with the operator entanglement entropy of the unitary evolution operator, while special product states, e.g. $\sigma_z$ basis states, can exhibit faster entanglement production. In the presence of a disordered magnetic field in our spin chains, we show that both the wave function and operator entanglement entropies exhibit a power-law growth with the same disorder-dependent exponent, and clarify the apparent discrepancy in previous results. These systems, in the absence of conserved densities, provide further evidence for slow information spreading on the ergodic side of the many-body localization transition.Comment: 11 pages, 6 figures, Journal versio

Inhibition of electromagnetically induced absorption due to excited state decoherence in Rb vapor

The explanation presented in [Taichenachev et al, Phys. Rev. A {\bf 61}, 011802 (2000)] according to which the electromagnetically induced absorption (EIA) resonances observed in degenerate two level systems are due to coherence transfer from the excited to the ground state is experimentally tested in a Hanle type experiment observing the parametric resonance on the $$ line of $^{87}$Rb. While EIA occurs in the $F=1\to F^{\prime}=2$ transition in a cell containing only $Rb$ vapor, collisions with a buffer gas ($30 torr$ of $Ne$) cause the sign reversal of this resonance as a consequence of collisional decoherence of the excited state. A theoretical model in good qualitative agreement with the experimental results is presented.Comment: 8 pages, 7 figures, submitted to Physical Review

Polarization dependence of four-wave mixing in a degenerate two-level system

Nearly degenerate four-wave mixing (NDFWM) within a closed degenerate two-level atomic transition is theoretically and experimentally examined. Using the model presented by A. Lezama et al [Phys. Rev. A 61, 013801 (2000)] the NDFWM spectra corresponding to different pump and probe polarization cases are calculated and discussed. The calculated spectra are compared to the observation of NDFWM within the $6S_{1/2}(F=4)\to 6P_{3/2}(F=5)$ transition of cesium in a phase conjugation experiment using magneto optically cooled atomsComment: 10 pages, 13 figures; submitted to Phys. Rev.

Spectroscopic observation of the rotational Doppler effect

We report on the first spectroscopic observation of the rotational Doppler shift associated with light beams carrying orbital angular momentum. The effect is evidenced as the broadening of a Hanle/EIT coherence resonance on Rb vapor when the two incident Laguerre-Gaussian laser beams have opposite topological charges. The observations closely agree with theoretical predictions.Comment: Submited to Physical Review Lette

Probe spectroscopy in an operating magneto-optical trap: the role of Raman transitions between discrete and continuum atomic states

We report on cw measurements of probe beam absorption and four-wave-mixing spectra in a $^{85}$Rb magneto-optical trap taken while the trap is in operation. The trapping beams are used as pump light. We concentrate on the central feature of the spectra at small pump-probe detuning and attribute its narrow resonant structures to the superposition of Raman transitions between light-shifted sublevels of the ground atomic state and to atomic recoil processes. These two contributions have different dependencies on trap parameters and we show that the former is inhomogeneously broadened. The strong dependence of the spectra on the probe-beam polarization indicates the existence of large optical anisotropy of the cold-atom sample, which is attributed to the recoil effects. We point out that the recoil-induced resonances can be isolated from other contributions, making pump-probe spectroscopy a highly sensitive diagnostic tool for atoms in a working MOT.Comment: 9 pages, 8 figure

Safety-Aware Apprenticeship Learning

Apprenticeship learning (AL) is a kind of Learning from Demonstration techniques where the reward function of a Markov Decision Process (MDP) is unknown to the learning agent and the agent has to derive a good policy by observing an expert's demonstrations. In this paper, we study the problem of how to make AL algorithms inherently safe while still meeting its learning objective. We consider a setting where the unknown reward function is assumed to be a linear combination of a set of state features, and the safety property is specified in Probabilistic Computation Tree Logic (PCTL). By embedding probabilistic model checking inside AL, we propose a novel counterexample-guided approach that can ensure safety while retaining performance of the learnt policy. We demonstrate the effectiveness of our approach on several challenging AL scenarios where safety is essential.Comment: Accepted by International Conference on Computer Aided Verification (CAV) 201

200 años de la política exterior peruana

El libro 200 años de la política exterior peruana reúne las reflexiones y disertaciones expuestas en el foro del mismo nombre que organizó el programa de Relaciones Internacionales de la Universidad Peruana de Ciencias Aplicadas (UPC). Este ejemplar, dividido en dos partes, analiza el desarrollo del país en el escenario internacional desde las perspectivas histórica (1821-2000) y contemporánea (2001-2021).El libro 200 años de la política exterior peruana reúne las reflexiones y disertaciones expuestas en el foro del mismo nombre que organizó el programa de Relaciones Internacionales de la Universidad Peruana de Ciencias Aplicadas (UPC). Este ejemplar, dividido en dos partes, analiza el desarrollo del país en el escenario internacional desde las perspectivas histórica (1821-2000) y contemporánea (2001-2021). De esta manera, los artículos recopilados en el libro, elaborados por protagonistas de la historia y de las relaciones exteriores del país, hacen de esta publicación un documento indispensable para comprender la política exterior peruana, a través de las voces emblemáticas del servicio diplomático peruano de Carlos Buller, Romy Tincopa, Gustavo Meza-Cuadra, Marcela López, Luis Enrique Chávez y Allan Wagner, uno de los diplomáticos más representativos del país