Seokchan Yoon

Cavity-Modified Collective Rayleigh Scattering of Two Atoms

We report on the observation of cooperative radiation of exactly two neutral atoms strongly coupled to the single mode field of an optical cavity, which is close to the lossless-cavity limit. Monitoring the cavity output power, we observe constructive and destructive interference of collective Rayleigh scattering for certain relative distances between the two atoms. Because of cavity backaction onto the atoms, the cavity output power for the constructive two-atom case (N=2) is almost equal to the single-emitter case (N=1), which is in contrast to free-space where one would expect an N^{2} scaling of the power. These effects are quantitatively explained by a classical model as well as by a quantum mechanical model based on Dicke states. We extract information on the relative phases of the light fields at the atom positions and employ advanced cooling to reduce the jump rate between the constructive and destructive atom configurations. Thereby we improve the control over the system to a level where the implementation of two-atom entanglement schemes involving optical cavities becomes realistic.

Bayesian feedback control of a two-atom spin-state in an atom-cavity system.

We experimentally demonstrate real-time feedback control of the joint spin-state of two neutral cesium atoms inside a high finesse optical cavity. The quantum states are discriminated by their different cavity transmission levels. A Bayesian update formalism is used to estimate state occupation probabilities as well as transition rates. We stabilize the balanced two-atom mixed state, which is deterministically inaccessible, via feedback control and find very good agreement with Monte Carlo simulations. On average, the feedback loop achieves near optimal conditions by steering the system to the target state marginally exceeding the time to retrieve information about its state.

Coherent optical phonon oscillations in cubic ZnSe

We report the observation of coherent optical phonon oscillations in cubic bulk ZnSe(001). With a photon energy far below the band gap, the generation mechanism of the coherent longitudinal optical phonon mode is revealed to be the impulsive stimulated Raman scattering. Dephasing of the coherent longitudinal optical phonon modes by electron-phonon interaction and anharmonic processes is studied by investigating excitation intensity and temperature dependence of the dephasing rates.

Electromagnetically-induced-transparency control of single-atom motion in an optical cavity

We demonstrate cooling of the motion of a single neutral atom confined by a dipole trap inside a high-finesse optical resonator. Cooling of the vibrational motion results from EIT-like interference in an atomic Λ-type configuration, where one transition is strongly coupled to the cavity mode and the other is driven by an external control laser. Good qualitative agreement with the theoretical predictions is found for the explored parameter ranges. Further we demonstrate EIT-cooling of atoms in the dipole trap in free space, reaching the ground state of axial motion. By means of a direct comparison with the cooling inside the resonator, the role of the cavity becomes evident by an additional cooling resonance. These results pave the way towards a controlled interaction between atomic, photonic and mechanical degrees of freedom.

Definitive number of atoms on demand: Controlling the number of atoms in a few-atom magneto-optical trap

A few Rb85 atoms were trapped in a micron-size magneto-optical trap with a high quadrupole magnetic-field gradient and the number of atoms was precisely controlled by suppressing stochastic loading and loss events via real-time feedback on the magnetic-field gradient. The measured occupation probability of a single atom was as high as 99%. Atoms up to five were also trapped with high occupation probabilities. The present technique could be used to make a deterministic atom source.

Dependence of transverse and longitudinal resolutions on incident Gaussian beam widths in the illumination part of optical scanning microscopy

We studied both theoretically and experimentally the intensity distribution of a Gaussian laser beam when it was focused by an objective lens with its numerical aperture up to 0.95. Approximate formulas for full width at half-maximum (FWHM) of the intensity distribution at focus were derived for very large and very small initial beam waists with respect to the entrance pupil radius of the objective lens. In experiments, the energy flux through a 0.5 microm pinhole was measured for various pinhole positions. It was found in theoretical analysis and confirmed in experiments that the FWHMs at focus in the transverse and longitudinal directions do not increase much from the ultimate FWHMs until the input beam waist is reduced below half of the entrance pupil radius.

Carrier-free Raman manipulation of trapped neutral atoms

We experimentally realize an enhanced Raman control scheme for neutral atoms that features an intrinsic suppression of the two-photon carrier transition, but retains the sidebands which couple to the external degrees of freedom of the trapped atoms. This is achieved by trapping the atom at the node of a blue detuned standing wave dipole trap, that acts as one field for the two-photon Raman coupling. The improved ratio between cooling and heating processes in this configuration enables a five times lower fundamental temperature limit for resolved sideband cooling. We apply this method to perform Raman cooling to the two-dimensional vibrational ground state and to coherently manipulate the atomic motion. The presented scheme requires minimal additional resources and can be applied to experiments with challenging optical access, as we demonstrate by our implementation for atoms strongly coupled to an optical cavity.

Characteristics of single-atom trapping in a magneto-optical trap with a high magnetic-field gradient

A quantitative study on characteristics of a magneto-optical trap with a single or a few atoms is presented. A very small number of 85Rb atoms were trapped in a micron-size magneto-optical trap with a high magnetic-field gradient. In order to find the optimum condition for a single-atom trap, we have investigated how the number of atoms and the size of atomic cloud change as various experimental parameters, such as a magnetic-field gradient and the trapping laser intensity and detuning. The averaged number of atoms was measured very accurately with a calibration procedure based on the single-atom saturation curve of resonance fluorescence. In addition, the number of atoms in a trap could be controlled by suppressing stochastic loading events by means of a real-time active feedback on the magnetic-field gradient.

Observation of a non-equilibrium steady state of cold atoms in a moving optical lattice

Non-equilibrium dynamics expands our understanding on physical processes based on the conventional equilibrium physics. In particular, non-equilibrium steady states with continuous flow among them have drawn much interest related to various biochemical processes, biomolecular motors, and high-temperature quantum entanglement as well as Bose–Einstein condensates. Here we report observation of a non-equilibrium steady states of atoms achieved in a hybrid of a moving optical lattice and a surrounding cold atom cloud in a phase-stabilized magneto-optical trap. A part of atoms are localized and transported in the moving optical lattice and the rest are not localized in the lattice while trapped as a cold cloud of atoms. These motional states coexist with continuous transition between them. Our model calculations well reproduce the key features of the experimental observations including stepwise transitions, confirming the existence of a non-equilibrium steady state with characteristics of asymmetric simple exclusion process in the cold atom system.Non-equilibrium dynamics of cold atoms have recently attracted attention revealing unconventional phenomena. The authors report here the experimental observation of a non-equilibrium steady state in a hybrid trap composed of a magneto-optical trap and a moving optical lattice.

Measurement of loading and loss rates in a magneto-optical trap by direct event counting with atom number feedback

By using direct event-counting with atom-number feedback we investigated the loading rate of a magneto-optical trap as a function of the magnetic field gradient. We also studied the one- and two-atom loss rates as functions of trap laser intensity. All measurements were performed with a precisely prepared initial number of atoms.