Coherent backscattering of nonmonochromatic light by an ensemble of cold atoms

The coherent backscattering of light by an ensemble of cold atoms in a magneto-optical trap is studied theoretically. The effect of the width and shape of the incident light spectrum on the enhancement factor and total intensity of the backscattered light is analyzed. The calculations are performed for ⁸⁵Rb atoms with allowance for the entire hyperfine and Zeeman structure of the ground and excited states of the atoms, spatial inhomogeneity of the atomic cloud, and motion of the atoms.     

Destructive interference in coherent backscattering of light by an ensemble of cold atoms

The coherent backscattering of light by an ensemble of cold atoms located in a magneto-optical trap is investigated theoretically. The dependence of the gain coefficient on the probe frequency is analyzed in a wide spectral range covering the entire hyperfine structure of the excited state. The calculation is performed for ⁸⁵Rb atoms. It is found that destructive interference can be observed at certain frequencies, which results in gain coefficients smaller than unity. The angular distribution of scattered light is investigated for corresponding frequencies and the dependence of the shape of the cone of destructive interference on the size of the atomic cloud and its optical thickness is analyzed.     

Visible superfluorescence from optically pumped europium atoms

We have observed superfluorescence (SF) on five atomic transitions at visible wavelengths 633.58, 635.00, 640.09, 640.61 and 736.22 nm in Doppler broadened gas of europium (Eu) atom. The nanosecond SF pulses were observed by longitudinally pumping Eu vapor column with a pulsed dye laser to upper states 4f⁶5d6s², ⁸D_7/2at 346.79 nm and 4f⁷5d6p, ¹⁰F_5/2at 348.73 nm from the ground state 4f⁷6s², ⁸S_7/2. High optical conversion efficiency ≈10% was measured for these SF transitions. Our experiment deals with the large sample multilevel SF in the regime where the length of the excited column L is greater than the maximum value of the Arecchi-Courtens length (L_c). The observed variation of SF peak intensity (I_fl) and time delay for SF evolution (τ_D) with number of atoms in the excited state (N) resemble theoretically predicted SF scaling laws for transverse excitation, namely I_fl∝N and t_D μ 1/?N\tau_D \propto 1/{\sqrt N} although the experimental condition is similar to the swept excitation. This could be due to the Rabi frequency associated with the pump transition which is comparable to the SF delay time precluding the initiation of SF at different times along the sample and results in transverse (instantaneous) excitation. The experimental τ_D values were found to be in agreement with the quantum mechanical calculations describing SF.     

Echo responses of an ensemble of cold atoms in optical lattices

The dynamics of an ultracold dilute boson gas in an optical lattice is described in the framework of the Bose-Hubbard model in which the parameters of the system are controlled by light. Within this model, an ensemble of Bose-Einstein condensates trapped in cells of a double optical lattice generates an echo response in the quantum-mechanical probability current. The echo signal is excited by biharmonic radiation pulses under two-photon (Raman) resonance conditions. The time profile of the echo signal and the role played by the inhomogeneous broadening and the interaction of atoms in the formation of the boson echo are discussed.     

Gain and lasing of optically pumped metastable rare gas atoms

Optically pumped alkali vapor lasers are currently being developed in several laboratories. The objective is to construct high-powered lasers that also exhibit excellent beam quality. Considerable progress has been made, but there are technical challenges associated with the reactivity of the metal atoms. Rare gas atoms (Rg) excited to the np(5)(n+1)s (3)P(2) configuration are metastable and have spectral properties that are closely similar to those of the alkali metals. In principle, optically pumped lasers could be constructed using excitation of the np(5)(n+1)p←np(5)(n+1)s transitions. We have demonstrated this potential by observing gain and lasing for optically pumped Ar(*), Kr(*) and Xe(*). Three-level lasing schemes were used, with He or Ar as the collisional energy transfer agent that established the population inversion. These laser systems have the advantage of using inert reagents that are gases at room temperature.     

Polarization analysis of the light emitted by an optically pumped3He gas

Optical measurements of the nuclear polarizationM of an optically pumped pure³He gas have been performed. They consist in monitoring the circular polarization of various helium lines emitted by the discharge. We have demonstrated the possibility to use the orange line (587.5 nm) to measureM and we have calibrated this technique over the pressure range 0.4–6.5 Torr. The circular polarization of the light has been analyzed using a new device which includes a static polarimeter and photodiodes as light sensors. The intensity of the discharge was modulated and the detection was made at the frequency of modulation.Laboratoire associé au Centre National de la Recherche Scientifique et a l'Université P. M. Curie     

Saturation resonance of atoms optically pumped by a monomode laser beam

The existence of two types of phenomena (the population effect and the Zeeman coherence effect) in saturation resonances by magnetic mode crossing have been predicted theoretically. A direct experimental proof is given here, by the observation of the Π component of the fluorescence light emitted by neon atoms optically pumped by a monomode 6328 Å laser beam.     

Atomic coherence effects on optically pumped cesium atoms in static magnetic field

The population difference between ground states F = 4 and F = 3 is calculated for Cs atoms pumped on the D₂ line by a resonant laser beam. The pumping efficiency for Cs atoms in a static magnetic field on two hyperfine transitions (6S_1/2 F = 4 → 6P_3/2 F′ = 3 and F′ = 4) is calculated for various pump laser intensities. The population difference as a function of the static magnetic field exhibits a dip centered at the zero magnetic field, which corresponds well with the Zeeman coherence between sublevels of the F = 4 state. The full-width at half-maximum (FWHM) of the dip as a function of the pump laser intensity shows abnormal power broadening behavior that differs for different hyperfine transitions. We present experimental results that agree with the theoretical calculations.     

Special features of the dynamics of multipole moments of atoms at rest in weak light fields

The dynamics of radiative relaxation of multipole moments of atoms at rest in the ground state is considered for a low degree of dipole transition saturation. Using the field representation in the bipolar harmonic basis, the special features of the characteristic relaxation times are studied as functions of the j_g j_e transition type, ellipticity, and field frequency offset from resonance.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 57–64, December, 2004.     

Focusing ground-state atoms with an electrostatic field

A general formula for the trajectory of atoms in an arbitrary potential with axial symmetry is derived. We apply this formula to show that an axially symmetric electrostatic imaging lens for ground-state neutral atoms is possible. Because of its simple construction and capability of focusing atoms of any species, such a lens system can be used in a variety of applications. PACS 03.75.Be; 39.25.+k; 32.80.Pj     

Polarization spectrum of three-level atoms in weak polychromatic fields

Differential density matrix equations for a three-level atomic system driven by polychromatic fields are solved numerically. The polarization spectrum of the probe field is obtained. Ultra-narrow resonances in the polarization spectrum on the probe field frequency emerge at multiple harmonics and subharmonics.     

Observation of optically induced feshbach resonances in collisions of cold atoms

We have observed optically induced Feshbach resonances in a cold ( <1 mK) sodium vapor. The optical coupling of the ground and excited-state potentials changes the scattering properties of an ultracold gas in much the same way as recently observed magnetically induced Feshbach resonances, but allows for some experimental conveniences associated with using lasers. The scattering properties can be varied by changing either the intensity or the detuning of a laser tuned near a photoassociation transition to a molecular state in the dimer. In principle this method allows the scattering length of any atomic species to be altered. A simple model is used to fit the dispersive resonance line shapes.     

Wavelength modulation absorption spectrometry from optically pumped collision broadened atoms and molecules

A theoretical investigation of the influence of optical pumping on wavelength modulation absorption spectrometry (WMAS) signals from collision broadened atoms and molecules is presented. General expressions for the nf-WMAS signal from atomic and molecular systems, modeled as three-level systems that can accommodate both optical saturation and optical pumping, are derived by the use of a previously developed Fourier series-based formalism in combination with rate equations solved under steady-state conditions. The expressions are similar to those describing the nf-WMAS signal from two-level systems that can accommodate optical saturation [Schmidt FM, Foltynowicz A, Gustafsson J, Axner O, WMAS from optically saturated collision-broadened transitions. JQSRT 2005;94:225-54], the difference being the value of the saturation flux, wherefore the general parametric dependence of WMAS signals from optically pumped systems is the same as that from optically saturated systems. The additional effect of optical pumping on the WMAS signal is investigated for three typical cases: molecules or atoms in an ordinary atmosphere, atoms in an inert atmosphere, and atoms or molecules possessing metastable states. The possibility to describe any of these systems with a two-level model is investigated.     

Continuous weak measurement and nonlinear dynamics in a cold spin ensemble

A weak continuous quantum measurement of an atomic spin ensemble can be implemented via Faraday rotation of an off-resonance probe beam, and may be used to create and probe nonclassical spin states and dynamics. We show that the probe light shift leads to nonlinearity in the spin dynamics and limits the useful Faraday measurement window. Removing the nonlinearity allows a nonperturbing measurement on the much longer time scale set by decoherence. The nonlinear spin Hamiltonian is of interest for studies of quantum chaos and real-time quantum state estimation.     

Diffraction of light by weak ultrasonic fields

Assuming the validity ofRaman andNath's simplified theory for small widths of weak sound fields, an elegant and simple method is presented to derive an expression for the intensity of the first order in the case of weak sound fields. Expressions obtained are found to be identical with those ofDavid, andBhatia andNoble. Extending this treatment to the case of diffraction by stationary waves it is shown that the intensity of a first order in this case is half that of the corresponding order for progressive waves and this result has also been verified experimentally.     

Entangled light pulses from single cold atoms

The coherent interaction between a laser-driven single trapped atom and an optical high-finesse resonator allows one to produce entangled multiphoton light pulses on demand. The mechanism is based on the mechanical effect of light. The degree of entanglement can be controlled through the parameters of the laser excitation. Experimental realization of the scheme is within reach of current technology. A variation of the technique allows for controlled generation of entangled subsequent pulses, with the atomic motion serving as intermediate memory of the quantum state.     

Anisotropic gain in an optically pumped cw FIR laser

The observed asymmetry of tuning curves of a 70.6 μm CH₃OH laser is interpreted as a consequence of unidirectional amplification in the optically pumped gas, a quantum mechanical double resonance effect. The occurrence of this effect. was verified by FIR gain measurements.