We present an introductory review of the latest advancements cold Rydberg atom research. First, we briefly summarize the exaggerated properties of Rydberg atoms, and we discuss the new perspectives of Rydberg atom research that has been enabled by laser cooling and trapping technique. We then highlight the latest developments and achievements in the newly emerged research fields for Rydberg molecules and cold neutral plasmas. Various applications of the Rydberg blockade effect for quantum optics and quantum information science are also reviewed.
We investigate coherent control of the two-photon transition pathways of a four-level atomic system in a diamond configuration. When an ultrashort laser pulse interacts with this system in the ground state 5S 1/2 of rubidium, the two-photon transition probability amplitude of 5D 3/2 is obtained by a summation of all possible resonant and nonresonant two-photon transition probability amplitudes via 5P 1/2 and 5P 3/2. Second-order perturbation theory predicts that the maximal constructive interference of the transition probability amplitudes occurs when the phases of eight different spectrum blocks satisfy four different phase relations. Experiments carried out with spectrally phase-coded laser pulses show good agreement with the theoretical prediction.
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 ...
We present an experimental investigation of ultrafast-laser driven Rabi oscillations of atomic rubidium. Since the broadband spectrum of an ultrafast laser pulse simultaneously couples all the electronic hyperfine transitions between the excited and ground states, the complex excitation linkages involved with the D1 or D2 transition are energy degenerate. Here, by applying the MorrisShore transformation, it is shown that this multi-state system is reduced to a set of independent two-state systems and dark states. In experiments performed by ultrafast laser interactions of atomic rubidium in the strong interaction regime, we demonstrate that the ultrafast dynamics of the considered multi-state system is governed by a sum of at most two decoupled Rabi oscillations when this system interacts with ultrafast laser pulses of any polarization state. We further show the implication of this result to possible controls of photo-electron polarizations.
Quantum computing is based on unitary operations in a two-level quantum system, a qubit, as the fundamental building block, and the ability to perform qubit operations in an amount of time that is considerably shorter than the coherence time is an essential requirement for quantum computation. Here, we present an experimental demonstration of arbitrary single-qubit SU(2) quantum gate operations achieved at a terahertz clock speed. Implemented by coherent control methods of tailored ultrafast laser interaction with cold rubidium atomic qubits, Bloch vector manipulation about all three rotational axes was successfully demonstrated. The dynamic evolution of the qubits was successfully measured by devised femtosecond Ramsey interferometry. We anticipate this demonstration to be a starting point to process quantum algorithm in a simplified manner by a programmed sequence of femtosecond laser pulses.
The chirped-pulse interaction in the adiabatic coupling regime induces cyclic permutations of the energy states of a three-level system in the V -type configuration, which process is known as the three-level chirped rapid adiabatic passage. Here we show that a spectral hole in a chirped pulse can turn on and off one of the two adiabatic crossing points of this process, reducing the system to an effective two-level system. The given hybrid adiabatic-nonadiabatic transition results in selective excitation of the three-level system, controlled by the laser intensity and spectral position of the hole as well as the sign of the chirp parameter. Experiments are performed with shaped femtosecond laser pulses and the three lowest energy-levels (5S 1/2 , 5P 1/2 , and 5P 3/2 ) of atomic rubidium ( 85 Rb), of which the result shows good agreement with the theoretically analyzed dynamics. The result indicates that our method, being combined with the ordinary chirped-RAP, implements an adiabatic transitions between the two excited states. Furthermore the laser intensity-dependent control may have applications including selective excitations of atoms or ions arranged in space when being used in conjunction with laser beam profile programming.
We present a method that harnesses coherent control capability to two-dimensional Fourier-transform optical spectroscopy. For this, three ultrashort laser pulses are individually shaped to prepare and control the quantum interference involved in two-photon interexcited-state transitions of a V-type quantum system. In experiments performed with atomic rubidium, quantum control for the enhancement and reduction of the 5P 1/2 → 5P 3/2 transition was successfully tested in which the engineered transitions were distinguishably extracted in the presence of dominant one-photon transitions.
We show theoretically and experimentally that two-photon coherent control in a V -shape three-level system projects a one-photon coherent transient in a simple two-level system. The second-and third-order spectral phase terms of a shaped laser pulse play the roles of time and quadratic spectral phase, respectively, in conventional coherent transients. In a three-pulse coherent control experiment of atomic rubidium, the phase and amplitude of controlled transition probability is retrieved from a two-dimensional Fourier-transform spectral peak. It is hoped that this control scheme may harness coherent control capability on multidimensional Fourier-transform spectroscopy.
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