Ultrafast laser-driven Rabi oscillations of a trapped atomic vapor

Abstract

We investigate Rabi oscillation of an atom ensemble in Gaussian spatial distribution. By using the ultrafast laser interaction with the cold atomic rubidium vapor spatially confined in a magnetooptical trap, the oscillatory behavior of the atom excitation is probed as a function of the laser pulse power. Theoretical model calculation predicts that the oscillation peaks of the ensemble-atom Rabi flopping fall on the simple Rabi oscillation curve of a single atom and the experimental result shows good agreement with the prediction. We also test the the three-pulse composite interaction Rx(π/2)Ry(π)Rx(π/2) to develop a robust method to achieve a higher fidelity population inversion of the atom ensemble.PACS numbers: 32.80. Qk, 32.80.Wr, 42.65.Re Rabi oscillation is a fundamental concept in physics with a significant pedigree first discovered in the context of nuclear magnetic resonance (NMR) [1][2][3] and later extended to atomic physics and quantum optics [4,5]. In the presence of an oscillatory driving field E(t) = A(t) cos(ωt), a two-state quantum system undergoes a cyclic change of Bloch vector ρ manifested by the precessionabout an effective torque Ω = (−µA(t)/2 , 0, δ), where µ is the transition dipole moment between the two energy states, A(t) is the field envelope, and δ is the frequency detuning under the slowly-varying envelope approximation [4]. This generic feature of Rabi oscillation is universally found in a vast variety of material systems ranging from simple atoms and molecules [6][7][8][9][10] When a two-state atom interacts with a resonant (δ = 0) laser pulse, the dynamics of the excited state probability, which we may refer to as single-atom Rabi oscillation (SARO), is represented bywhere Θ o is the pulse area defined by Θ o = µA(t)dt/ . Since the pulse area is subject to both the pulse duration and the electric-field envelope, Rabi oscillations of an ultra-short time scale can be implemented by ultrafast optical interaction at a strong-enough laser intensity regime. However, the spatial extent of the laser beam over the laser-atom interaction region inevitably causes spatial average effect that often leads to vanishing of the oscillatory behavior. To overcome this problem, homogenizing the spatial profile of laser beams [22,23] and * Electronic address: jwahn@kaist.ac.kr adapting chirped laser interaction [24] have been considered. This paper aims quantitative analysis of spatially averaged Rabi oscillation. For this, we use the atom ensemble localized in a magneto-optical trap (MOT) [25] interacted with ultrafast laser pulses. As a theoretical model to investigate the spatially inhomogeneous interaction, we consider a Gaussian laser beam propagating along z direction. The pulse area in Eq. (2) is then represented in the polar coordinate system aswhere r = x 2 + y 2 , w(z) is the beam waist at z, w o = w(0) is the minimal beam waist, Θ o is the maximal pulse area, and Θ z = w o Θ o /w(z). When we assume the atom density profile in the MOT is also a Gaussian, i.e., ρ(r, z) = ρ o e −(r 2 +z 2 ...

Rabi oscillations of Morris-Shore–transformedN-state systems by elliptically polarized ultrafast laser pulses

“…(16) and (17) are confirmed. The theoretical calculations are respectively plotted with dashed lines, of which the deviation from the experimental result is due to the spatially inhomogeneous laser interaction with the atom ensemble [30]. When the spatial averaging effect is taken into account (by assuming the Gaussian laser beam and atom ensemble size ratio is 2.5), the new calculations, shown with solid lines, agree well with the experimental results.…”

“…The signal linearity was assured by operating the experiment in the one-photon perturbation regime. The error due to three-photon ionizations by the pump pulse was estimated below 5% at a pulse-area of 3π [30], which was negligible in the considered experimental con-dition. The whole experiment (four steps: MOT-turn off, laser-control, ionization, MOT-turn on) was repeated at 2 Hz by cyclically turning on and off three mechanical shutters.…”

Subpicosecond X rotations of atomic clock states

“…Experimental investigation of the subpicosecond X rotation of the atomic clock states was performed in a setup described in our early works [25][26][27]. The setup composed of a magneto-optical trap (MOT) [28] for cold rubidium atoms ( 87 Rb) and a femtosecond laser amplifier [29].…”

Quantum dynamics of a two-state system induced by a chirped zero-area pulse

“…By adjusting the diameters of the cooling and repumping laser beams, an atomic cloud of about 200 µm diameter and 6 × 10 9 cm 3 atom density was prepared. The atoms were tightly confined in particular to achieve a uniform laseratom interaction [27]. When the laser pulses were focused on to the atomic vapor, the laser beam diameter of 600 µm was about 3 times bigger than the diameter of the atom cloud.…”