Ennio Arimondo

Coherent population trapping in laser spectroscopy

Publisher Summary This chapter describes the coherent population trapping in laser spectroscopy. Coherent population trapping may be also described as the pumping of the atomic system in a particular state, the coherent superposition of the atomic states, which is a nonabsorbing state. The exciting radiation creates an atomic coherence, such that the atoms evolution is prepared exactly out of phase with the incoming radiation and no absorption takes place. The chapter discusses the basic properties of an atomic system prepared with the coherent population-trapping superposition of states and outlines experimental observations concerned with the establishment of coherent trapping in different discrete systems. The chapter also discusses the theoretical and experimental aspects of trapping that involve states of the continuum and reviews the theoretical and experimental features associated with coherent population trapping in laser cooling, adiabatic transfer, lasing without inversion, pulse matching, and photon statistics. The theoretical aspect of coherent population trapping created by spontaneous emission is also discussed in the chapter.

Bloch Oscillations and Mean-Field Effects of Bose-Einstein Condensates in 1D Optical Lattices

We have loaded Bose-Einstein condensates into one-dimensional, off-resonant optical lattices and accelerated them by chirping the frequency difference between the two lattice beams. For small values of the lattice well depth, Bloch oscillations were observed. Reducing the potential depth further, Landau-Zener tunneling out of the lowest lattice band, leading to a breakdown of the oscillations, was also studied and used as a probe for the effective potential resulting from mean-field interactions as predicted by Choi and Niu [Phys. Rev. Lett. 82, 2022 (1999)]. The effective potential was measured for various condensate densities and trap geometries, yielding good qualitative agreement with theoretical calculations.

Dynamical control of matter-wave tunneling in periodic potentials.

We report on measurements of dynamical suppression of interwell tunneling of a Bose-Einstein condensate (BEC) in a strongly driven optical lattice. The strong driving is a sinusoidal shaking of the lattice corresponding to a time-varying linear potential, and the tunneling is measured by letting the BEC freely expand in the lattice. The measured tunneling rate is reduced and, for certain values of the shaking parameter, completely suppressed. Our results are in excellent agreement with theoretical predictions. Furthermore, we have verified that, in general, the strong shaking does not destroy the phase coherence of the BEC, opening up the possibility of realizing quantum phase transitions by using the shaking strength as the control parameter.

Strong relative intensity squeezing by four-wave mixing in rubidium vapor

We have measured ¿6.3 dB of relative intensity squeezing at 795 nm, generated by stimulated, nondegenerate four-wave mixing in a hot rubidium vapor. This scheme is of interest for experiments involving cold atoms or atomic ensembles.

High-fidelity quantum driving

Transforming a quantum system with high fidelity is usually a trade-off between an increase in speed—thereby minimizing decoherence—and robustness against fluctuating control parameters. Protocols at these two extreme limits are now demonstrated and compared using Bose–Einstein condensates in optical traps.

Measurements of trapping efficiency and stiffness in optical tweezers

Abstract We report an experimental study concerning the radial forces of an optical tweezers acting on spherical polystyrene particles diluted in water solution. The radius of the trapped beads varied between 0.5 and 7.5 μm, i.e., in an intermediate range between Rayleigh and geometric optics regime. As a force calibration method we used the viscous drag exerted by a fluid flow. A parametric study of the transverse trapping forces was made as a function of bead radius and laser power for two objective lenses having numerical apertures of 0.65 (40×) and 1.25 (100×). Measured forces were compared with numerical estimates based on generalized Lorenz–Mie theory reported in literature. Optical tweezers were also characterized in terms of the optical potential well by measuring the displacement of trapped particles experiencing a viscous drag at a fluid flow below the critical velocity. Following this procedure, trap stiffness was determined for several bead radii and at different numerical apertures of the objective lenses.

Repetitive passive Q-switching and bistability in lasers with saturable absorbers

We report detailed experimental data on the passive Q-switching operation in a CO2 laser with CH3I saturable absorber, and on the transient behaviour in the near-Q-switching situation. Under suitable operating conditions, we found bistability in the output power. In some cases, we observed the simultaneous presence of bistability and passive Q-switching. The theoretical part of the paper starts from the four-level model of laser with saturable absorber, as formulated by other authors. By adiabatically eliminating the variables of the resonant levels, we reduce the problem to a set of three differential equations, from which we derive explicit analytical conditions for the rise of passive Q-switching. These conditions turn out to be in good qualitative and partially quantitative agreement with our experimental findings as well as with other experimental data previously obtained by other authors. Finally we classify the possible combinations of passive Q-switching and bistability that one can find in this type of experiments.

Ultraslow propagation of matched pulses by four-wave mixing in an atomic vapor

We have observed the ultraslow propagation of matched pulses in nondegenerate four-wave mixing in a hot atomic vapor. Probe pulses as short as 70 ns can be delayed by a tunable time of up to 40 ns with little broadening or distortion. During the propagation, a probe pulse is amplified and generates a conjugate pulse which is faster and separates from the probe pulse before getting locked to it at a fixed delay. The precise timing of this process allows us to determine the key coefficients of the susceptibility tensor. The fact that the same configuration has been shown to generate quantum correlations makes this system very promising in the context of quantum information processing.

Observation of photon-assisted tunneling in optical lattices.

We have observed tunneling suppression and photon-assisted tunneling of Bose-Einstein condensates in an optical lattice subjected to a constant force plus a sinusoidal shaking. For a sufficiently large constant force, the ground energy levels of the lattice are shifted out of resonance and tunneling is suppressed; when the shaking is switched on, the levels are coupled by low-frequency photons and tunneling resumes. Our results agree well with theoretical predictions and demonstrate the usefulness of optical lattices for studying solid-state phenomena.

Coherent control of dressed matter waves.

We demonstrate experimentally that matter waves can be coherently and adiabatically loaded and controlled in one-, two-, and three-dimensional strongly driven optical lattices. This coherent control is then used in order to reversibly induce the superfluid-Mott insulator phase transition by changing the strength of the driving. Our findings pave the way for studies of driven quantum systems and new methods for controlling matter waves.