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arXiv:physics/0408028 · Physical Review A

High-density mesoscopic atom clouds in a holographic atom trap

Jennifer Sebby-Strabley, Raymond Newell, J. O. Day, Erik Brekke, Thad G Walker

Published Feb 9, 2005• 31 citations
arXivHTMLPDF

Abstract

We demonstrate the production of micron-sized high-density atom clouds of interest for mesoscopic quantum information processing. We evaporate atoms from $60\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$, $3\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{atoms}∕{\mathrm{cm}}^{3}$ samples contained in a highly anisotropic optical lattice formed by interfering diffracted beams from a holographic phase plate. After evaporating to $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ by lowering the confining potential, in less than a second the atom density reduces to $8\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at a phase space density approaching unity. Adiabatic recompression of the atoms then increases the density to levels in excess of $1\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The resulting clouds are typically $8\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in the longest dimension. Such samples are small enough to enable mesoscopic quantum manipulation using the Rydberg blockade and have the high densities required to investigate collision phenomena.

DOI
10.1103/physreva.71.021401

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We demonstrate the production of micron-sized high-density atom clouds of interest for mesoscopic quantum information processing. We evaporate atoms from 60\ensuremathμK60\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}60\ensuremathμK, 3\ifmmode×\else\texttimes\fi1014atoms∕cm33\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{atoms}∕{\mathrm{cm}}^{3}3\ifmmode×\else\texttimes\fi1014atoms∕cm3 samples contained in a highly anisotropic optical lattice formed by interfering diffracted beams from a holographic phase plate. After evaporating to 1\ensuremathμK1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}1\ensuremathμK by lowering the confining potential, in less than a second the atom density reduces to 8\ifmmode×\else\texttimes\fi1013cm\ensuremath−38\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}8\ifmmode×\else\texttimes\fi1013cm\ensuremath−3 at a phase space density approaching unity. Adiabatic recompression of the atoms then increases the density to levels in excess of 1\ifmmode×\else\texttimes\fi1015cm\ensuremath−31\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}1\ifmmode×\else\texttimes\fi1015cm\ensuremath−3. The resulting clouds are typically 8\ensuremathμm8\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}8\ensuremathμm in the longest dimension. Such samples are small enough to enable mesoscopic quantum manipulation using the Rydberg blockade and have the high densities required to investigate collision phenomena.
We demonstrate the production of high-density cold 87Rb samples (2 x 10(14) atoms/cm3) in a simple optical lattice formed with YAG light that is diffracted from a holographic phase plate. A loading protocol is described that results in 10,000 atoms per 10 microm x 10 microm x 100 microm unit cell of the lattice site. Rapid free evaporation leads to a temperature of 50 microK and phase space densities of 1/150 within 50 ms. The resulting small, high-density atomic clouds are very attractive for a number of experiments, including ultracold Rydberg atom physics.
We describe the operation of a very small optical dipole trap that is designed to store and manipulate individual atoms. Due to the very small dipole trap volume, a “collisional blockade ” mechanism locks the average number of trapped atoms on the value 0.5 over a large range of loading rates. We demonstrate experimentally the existence of this regime, and we describe also the “weakloading ” and “strong loading ” regimes outside the blockade range. In addition, we describe methods to measure the oscillation frequencies of a single atom in the trap with a high accuracy, and a “release and recapture ” method designed for temperature measurements at the single atom level. The measured “single atom temperature ” is 35 µK, that is in the sub-Doppler regime. Finally, by exploiting the extremely high density observed in the strong loading regime, we present preliminary results for evaporative cooling, where typically 30 atoms at 200 µK are forced to evaporate into about 10 atoms at 15 µK. 1
Summary form only given. Interactions between neutral atoms are generally weak, but exciting these atoms in a Rydberg state considerably enhances their interaction through a long-range dipole-dipole potential. An interesting application of this dipole-dipole interaction is the dipole blockade in Rydberg excitation. This effect offers exciting possibilities for manipulating quantum bits stored in a single collective excitations in mesoscopic ensembles, or for realizing scalable quantum logic gate. The dipole blockade effect has been demonstrated in atomic ensembles by different groups, and recently also with two independently trapped atoms, which shows the possibility of creating entangle states and quantum logic gates. To extend and continue these results with more atoms, a promising method is to use atoms trapped in an optical lattice in the Mott insulator phase. To excite atoms into a Rydberg state, and to control the interactions and the dipole blockade, a high resolution laser scheme is needed. We use a two-photon excitation scheme 5s1/2rarr 6p3/2rarr ns or nd. The first step, at 420 nm, is realized by doubling a MOPA laser, with the second step at ~ 1000 nm. Both lasers are locked to a Fabry-Perot cavity, and the total excitation bandwidth is less than 3 MHz. After the excitation into the Rydberg state, we apply a pulsed electric field to ionize and detect the Rydberg atoms with an ion detector. To characterize and improve the efficiency of our ion detection, we have explored the direct ionization of ultracold atoms in a magneto-optical trap, or in a Bose-Einstein condensate. This calibration will allows us to characterize precisely the Rydberg excitation and to monitor its time evolution. First results on the Rydberg excitation of ultracold atoms in dipole traps and optical lattices will be presented. The prospect for creating Rydberg excitations inside a Mott insulator will also be discussed.
The phase space density of dense, cylindrical clouds of atoms in a 2D magneto-optic trap is investigated. For a large number of trapped atoms (>10(8)), the density of a spherical cloud is limited by photon reabsorption. However, as the atom cloud is deformed to reduce the radial optical density, the temperature of the atoms decreases due to the suppression of multiple scattering leading to an increase in the phase space density. A density of 2 x 10(-4) has been achieved in a magneto-optic trap containing 2 x 10(8) atoms.
Sodium atoms have been loaded from a slowed beam into a dark magneto-optical trap which confines atoms mostly in the non-absorbing F=1 ground state. This method has allowed us to achieve samples of more than 10{sup 10} atoms with optical densities in excess of 150. Temperatures around 1.2 mK and densities around 10{sup 12} cm{sup -3} have been measured using weak probe absorption. At this density an elastic collisions rate is already approximately 100 Hz. Our goal is to reach the regime where the rate is high enough to allow evaporative cooling of the trapped sample enroute to Bose-Einstein Condensation, for which yet higher densities and lower temperatures are required. We report the current status of experiments underway to further increase the phase space density of the trapped atoms by cooling with polarization-gradient molasses and reloading them into a magnetic trap with a spherical quandrupole field.