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Quantum technologies

Dieter Meschede's research group
Home Group members Seokchan Yoon
Group members
Dr. Seokchan Yoon
Last position
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Field of research
in our group:
Cavity QED

Publications(up to 2015)

  • R. Reimann, W. Alt, T. Kampschulte, T. Macha, L. Ratschbacher, N. Thau, S. Yoon and D. Meschede
    Cavity-Modified Collective Rayleigh Scattering of Two Atoms, Phys. Rev. Lett. 114, 023601 (2015)arXivBibTeXPDF

    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.

  • T. Kampschulte, W. Alt, S. Manz, M. Martinez-Dorantes, R. Reimann, S. Yoon, D. Meschede, M. Bienert and G. Morigi
    Electromagnetically-induced-transparency control of single-atom motion in an optical cavity, Phys. Rev. A 89, 033404 (2014)arXivBibTeXPDF

    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 electromagnetically induced transparency (EIT)–like interference in an atomic lambda-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 among atomic, photonic, and mechanical degrees of freedom.

  • R. Reimann, W. Alt, T. Macha, D. Meschede, N. Thau, S. Yoon and L. Ratschbacher
    Carrier-free Raman manipulation of trapped neutral atoms, New J. Phys. 16, 113042 (2014)arXivBibTeXPDF

    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.

  • S. Brakhane, W. Alt, T. Kampschulte, M. Martinez-Dorantes, R. Reimann, S. Yoon, A. Widera and D. Meschede
    Bayesian Feedback Control of a Two-Atom Spin-State in an Atom-Cavity System, Phys. Rev. Lett. 109, 173601 (2012)arXivBibTeXPDF

    We experimentally demonstrate real-time feedback control of the joint spin-state of two neutral Caesium 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 loops achieves near optimal conditions by steering the system to the target state marginally exceeding the time to retrieve information about its state.

  • S. Yoon, Y. Choi, S. Park, W. Ji, J. Lee and K. An
    Characteristics of single-atom trapping in a magneto-optical with a high magnetic-field gradient, J. of Phys.: Conf. Ser. 80, 012046 (2007)BibTeX
    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.
  • Y. Choi, S. Yoon, S. Kang, W. Kim, J. Lee and K. An
    Direct measurement of loading and loss rates in a magneto-optical trap with atom-number feedback, Phys. Rev. A 76, 013402 (2007)BibTeX
    We have measured the loading and loss rates in a magneto-optical trap only with a few atoms by directly counting atom-number changing events. Unambiguous formulas are presented for the calculation of those rates from a step-wise time sequence of the fluorescence of the trapped atoms. With a recently developed atom-number feedback technique we could efficiently measure the loading rate as a function of the magnetic field gradient for the initial number of trapped atoms of zero. We could also measure the one- and two-atom loss rates as functions of the trap laser intensity for a precisely prepared initial number of trapped atoms. Each of these rates has been measured independently by directly counting the corresponding atom-number-changing events, without any influence from or inference to the other rates.
  • H. Chon, G. Park, S. Lee, S. Yoon, J. Kim, J. Lee and K. An
    The dependence of transverse and longitudinal resolutions on incident Gaussian beam widths in the illumination part of optical scanning microscopy, J. Opt. Soc. Am. A 24, 60 (2007)BibTeX
    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 0.5μm 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.
  • S. Kang, S. Yoon, Y. Choi, J. Lee and K. An
    Dependence of fluorescence-level statistics on bin-time size in a few-atom magneto-optical trap, Phys. Rev. A 74, 013409 (2006)BibTeX
    We have analyzed the statistical distribution of the fluorescence signal levels in a magneto-optical trap containing a few atoms and observed that it strongly depends on the relative size of the bin time with respect to the trap decay time. We derived analytic expressions for the signal distributions in two limiting cases, long and short bin time limits, and found good agreement with numerical simulations performed regardless of the size of the bin time. We found an optimal size of the bin time for minimizing the probability of indeterminate atom numbers while providing accurate information on the instantaneous number of atoms in the trap. These theoretical results are compared with actual experimental data. We observed super-Poisson counting statistics for the fluorescence from trapped atoms, which might be attributed to uncorrelated motion of trapped atoms in the inhomogeneous magnetic field in the trap.
  • S. Yoon, Y. Choi, S. Park, J. Kim, J. Lee and K. An
    Definitive number of atoms on demand: controlling the number of atoms in a-few-atom magneto-optical trap, Appl. Phys. Lett. 88, 211104 (2006)BibTeX
    A few math 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.
  • Y. Lim, S. Yoon, K. Yee, J. Lee, D. Kim and D. Lee
    Coherent phonon oscillations excited by second-order Raman scattering in CdZnTe multiple quantum wells and cubic ZnTe, Phys. Rev. B 68, 153308 (2003)BibTeX
    We report observation of strong phonon-pair oscillations in CdZnTe multiple quantum wells and cubic ZnTe(001). Using femtosecond laser pulses with photon energies far below the bandgap, the coherent phonon-pair oscillations are generated via the impulsive stimulated Raman scattering of the second order. Polarization- and phase-sensitive measurements allow us to resolve both relative Raman polarizabilities of various symmetry modes and their initial phases, simultaneously.
  • Y. Lim, S. Yoon, K. Yee, Y. Ahn, E. Oh and J. Lee
    Coherent optical phonon oscillations in cubic ZnSe, Appl. Phys. Lett. 82, 2446 (2003)BibTeX
    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.