Zs. Sörlei

Population transfer in three-level Λ atoms with Doppler-broadened transition lines by a single frequency-chirped short laser pulse

We examine interaction of a single frequency-chirped laser pulse with three-level atoms that have a Λ configuration of levels. We show that it is possible to produce complete fast and robust population transfer of all atoms of the ensemble with Doppler-broadened transition lines from one ground state into the other ground state with negligibly small temporary population of the excited state by controlling the intensity of the laser pulse and the direction and speed of the frequency chirp.

Creation of a coherent superposition of quantum states by a single frequency-chirped short laser pulse

Traditional schemes for coherent population transfer or generation of coherent superposition states in multilevel atoms or molecules usually utilize two or more laser beams with radiation bandwidth smaller than the frequency interval between the working levels. We show the possibility of creation of the coherent superposition of three metastable states of a four-level atom with tripodlike level structure using a single short frequency-chirped laser pulse. The bandwidth of the pulse envelope (without chirp) must be comparable to or exceed the frequency distance between the two metastable levels. No appreciable excitation of the atom takes place during the creation of the coherent superposition state, thus diminishing significantly the effect of decoherence due to the spontaneous decay of the excited state. The proposed method of creation of superposition states is robust against variations in the laser pulse parameters. Since this method does not require maintaining steady resonance with the atomic transitions (owing to the frequency chirp of the laser pulse), it is effective both in homogeneously and inhomogeneously broadened media.

Theory of the adiabatic passage in two-level quantum systems with superpositional initial states

Abstract An analytic theory of the adiabatic passage (AP) regime of interaction of short frequency chirped laser pulses with a two-level quantum system (QS) being initially in the superpositional quantum state is presented. We show that the initial value of the non-diagonal elements of the density matrix can influence the dynamics of the population transfer during the action of the laser pulse, but do not influence essentially the final populations of the states of the QS obtained at the end of the interaction near the AP regime. A novel solution to the Bloch equations is obtained in the form of a converging power series generalizing the ordinary AP solution.

Manipulation of two-level quantum systems with narrow transition lines by short linearly polarized frequency-chirped laser pulses

We propose and investigate theoretically a novel scheme for transient slowing and cooling of two-level quantum systems with narrow transition linewidths by a sequence of counterpropagating, short, linearly polarized laser pulses with special frequency chirping. Both internal degrees of freedom and the motion of the center of mass of quantum systems are considered quantum mechanically. Interaction with a large number of laser pulses during the decay time permits a drastic decrease in the cooling time of such systems.

Nonlinear propagation effects in FIR optically pumped amplifiers

Abstract The nonuniform pump intensity distribution perpendicular to the propagation direction causes an index of refraction and gain inhomogeneity of the medium with signal frequency dispersion, for instance, in the amplifier type optically pumped lasers. The resulting waveguiding and gain focusing caused gain line distortion, which is investigated experimentally and theoretically in a methanol model material with a comparatively low pumping power level. The narrowing of the spectral region of amplification is observed with a decreasing pump beam size. This observation is interpreted as the result of propagation of the signal FIR beam in a waveguide induced by the pumping beam. This waveguiding is predicted to be a dominant mechanism in the amplifier type FIR lasers pumped by focused, high intensity short pulses.

Optical phase information writing and storage in populations of metastable quantum states

We propose a scheme for robust writing and storage of optical phase information in populations of metastable states of the atoms with a tripod structure of levels by using frequency-chirped laser pulses. The method provides much longer storage times compared with the schemes based on the collective atomic spin coherences. A negligible excitation of the atom provides immunity to decoherence induced by decay of the excited states. The method is robust against small-to-medium variations in the laser pulse intensity and speed of the chirp and, being insensitive to resonance conditions, it is effective both in homogeneously and inhomogeneously broadened media.

Stabilization and time resolved measurement of the frequency evolution of a modulated diode laser for chirped pulse generation

We have developed experimental methods for the generation of chirped laser pulses of controlled frequency evolution in the nanosecond pulse length range for coherent atomic interaction studies. The pulses are sliced from the radiation of a cw external cavity diode laser while its drive current, and consequently its frequency, are sinusoidally modulated. By the proper choice of the modulation parameters, as well as of the timing of pulse slicing, we can produce a wide variety of frequency sweep ranges during the pulse. In order to obtain the required frequency chirp, we need to stabilize the center frequency of the modulated laser and to measure the resulting frequency evolution with appropriate temporal resolution. These tasks have been solved by creating a beat signal with a reference laser locked to an atomic transition frequency. The beat signal is then analyzed, as well as its spectral sideband peaks are fed back to the electronics of the frequency stabilization of the modulated laser. This method is simple and it has the possibility for high speed frequency sweep with narrow bandwidth that is appropriate, for example, for selective manipulation of atomic states in a magneto-optical trap.

Creation of coherent superpositions between metastable atomic states in Doppler-broadened media

We propose and analyze a scheme for creation of coherent superposition of meta-stable states in a tripod-structured atom using frequency-chirped laser pulses with negligible excitation of the atoms. The underlying physics of the scheme is explained using the formalism of adiabatic states. We show that the proposed scheme may be equally efficient in homogeneously and Doppler-broadened media. By numerically solving master equation for the density matrix operator, we analyze the influence of relaxation processes on the efficiency of the creation of superposition states. We show that the proposed scheme is robust against small-to-medium variations of the laser fields parameters.

Coherent interaction of frequency-modulated laser pulses with Rb atoms

We investigate the behavior of Rb85 atoms in the field of a sequence of frequency-chirped short laser pulses for coherent manipulation of the Rb atomic beam. The analysis is based on numerical solution of equations for the density matrix elements of the hyperfine levels of the 5S1/2-5P3/2 transition in Rb85 atom. We analyze two different regimes of interaction including relatively short laser pulses (when the width of the pulse envelope spectrum is of order or exceeds frequency interval between the hyperfine levels resulting in effective mixing of them) and relatively long ones (when the ground hyperfine levels are resolved but the excited ones are not resolved). We show that in all regimes considered, the interaction of a frequency-chirped laser pulse with the multilevel Rb85 system is similar to the interaction with an effective two-level atom at sufficiently large peak intensities of the pulses. It allows us to perform efficient excitation of the multilevel atom by transferring populations of two hyperfine ground states to the excited ones and back to the ground states using a pair of frequency-chirped laser pulses. We present experimental results of utilizing this scheme of population transfer for coherent manipulation of an ensemble of Rb85 atoms in a magneto-optical trap.