Session U4: Cooling and Trapping

Magnetic trapping of copper and silver using buffer gas loading

Atomic silver and coppper are magnetically trapped using buffer gas loading. Copper (Cu) is trapped in the $4s\;^2{\rm S}_{1/2}$, $m_j=1/2$ state with lifetimes as long as 8s. Silver (Ag) is trapped in the $5s\;^2{\rm S}_{1/2}$, $m_j=1/2$ state with lifetimes as long as 2.3 s. Lifetimes are limited by collisions with background $^3$He. Inelastic Zeeman state-changing collisions are observed between Ag and $^3$He. The ratio of transport to inelastic cross-sections for Ag-$^3$He is found to be $2.9\pm 0.2\times 10^6$ at 410 mK in a 4.0 T anti-Helmholtz trapping field. The spin relaxation cross-section is observed to vary with temperature as $T^{5.8\pm 0.4}$ between 300 mK and 630 mK and vary with trap magnetic field detph as $B^{-0.9\pm 0.2}$ between 2 T and 4 T. The transport to inelastic cross-section ratio for Cu-$^3$He collisions is found to be $8.0\pm 0.3\times 10^{6}$ at 400 mK. Comparison is made to alkali-noble gas theory, showing that additional considerations are necessary beyond the typical treatment restricted to the $s$ valence electron.   

Magnetic Trapping of Atomic Nitrogen and Cotrapping of NH

We observe magnetic trapping of atomic nitrogen ($^{14}$N) and cotrapping of $^{14}$NH ($X^3\Sigma^-$). We use buffer gas cooling to load the magnetic trap directly from a room temperature molecular beam generated by a radio-frequency plasma source. We trap approximately $1 \times 10^{11}$ $^{14}$N atoms at a peak density of $5 \times 10^{11}$ cm$^{-3}$ at a temperature of $\approx$ 550 mK. The $1/e$ lifetime of nitrogen in the trap of 12 +5/$-3$ s is limited by collisions with the helium buffer gas. This lifetime sets a limit on the inelastic rate constant for $^{14}$N-$^3$He collisions of $\Gamma_{in} < 2.2 \times 10^{-16}$ cm$^3$\thinspace s$^{-1}$. $^{14}$N and $^{14}$NH are cotrapped, with $4 \times 10^{10}$ $^{14}$N atoms and $1\times 10^8$ $^{14}$NH molecules at peak densities of $n_{N} \approx 1 \times 10^{11}$ cm$^{-3}$ and $n_{NH} \approx 1 \times 10^{8}$ at a temperature of $\approx 550$ mK.   

AC electric trapping of neutral atoms

We have demonstrated trapping of ultracold ground-state $^{87}$Rb atoms in a macroscopic ac electric trap [1]. Trapping by ac electric fields has been previously achieved for polar molecules [2], as well as Sr atoms on a chip [3], and recently for Rb atoms in a three-phase electric trap [4]. Similar to trapping of ions in a Paul trap, three-dimensional confinement in an ac electric trap is obtained by switching between two saddle-point configurations of the electric field. For the first time, this dynamic confinement is directly visualized with absorption images taken at different phases of the ac switching cycle. Stable electric trapping is observed in a narrow range of switching frequencies around 60 Hz, in agreement with trajectory calculations. In a typical experiment, about 3 x 10$^{5}$ Rb atoms are trapped with lifetimes on the order of 9 s and trap depths of about 10 $\mu $K. Additionally, we show that the atoms can be used to sensitively probe the electric fields in the trap by imaging the cloud while the fields are still on. \textbf{References}: 1. S. Schlunk et al., PRL \textbf{98}, 223002 (2007) 2. H. L. Bethlem et al., PRA \textbf{74}, 063403 (2006) 3. T. Kishimoto et al., PRL \textbf{96}, 123001 (2006) 4. T. Rieger et al., PRL \textbf{99, }063001 (2007)   

Progress towards a buffer gas cooled BEC of metastable He

We report recent progress towards a BEC of metastable helium ($^4$He$^*$) using buffer gas cooling. $10^{11}$ $^4 $He$^*$ atoms are produced via RF-discharge and magnetically trapped at an initial temperature of 400 mK in an anit-helmholtz quadrupole field. These atoms are evaporatively cooled into the ultracold regime and transferred to a superconducting QUIC trap with trap frequencies $\omega_{axial} = 2\pi \times 20$ Hz and $\omega_{radial} = 2\pi \times 150$ Hz, resulting in a cloud of $\sim 10^9$ atoms at a temperature of 1 mK. Trap lifetimes are limited only by collisions with residual background gas. Further cooling is achieved via RF evaporation, and the cloud is detected via absorption imaging at 1083 nm or 390 nm.   

Narrow-Line Laser Cooling and Trapping of Strongly Magnetic Atoms

Narrow optical transitions can be used for Doppler laser cooling to the sub-microkelvin temperature regime in multi- electron atoms such as the alkaline earths. This technique should also be useful for reaching ultracold temperatures in erbium, a strongly magnetic rare-earth element with a narrow (8~kHz) transition at 841~nm. However, a conventional magneto-optical trap (MOT) on this transition is destabilized by competition between optical and magnetic forces. To overcome this difficulty, we developed an unusual narrow-line MOT using cooling light tuned to the blue side of the atomic transition. In the resulting stable trap, we find that atoms are spin polarized and reach temperatures as low as 1~$\mu$K. These methods should be applicable to the other rare-earth elements and may enable narrow-line cooling of metastable alkaline earths on transitions that are too weak to compensate gravity.   

Collisional cooling of ultra-cold atom ensembles

We study a new cooling mechanism, collisional cooling, which capitalizes on the energy selectivity of inelastic scattering in a Feshbach resonance. We study this mechanism by simulating an ensemble of fermionic atoms in the quantum degenerate regime in the presence of such a resonance via a quantum kinetic approach. We find that by tuning the resonance energy appropriately, the temperature of the system can be lowered to a temperature comparable to the width of the resonance, and that thermalization is achieved through elastic scattering, which is also strongly enhanced due to the presence of the resonance.   

Trapping cold Cs atoms in an optical bottle beam

We demonstrate a blue detuned bottle beam optical trap for confinement of cold Cs atoms. The use of blue detuned, dark traps is of interest for reducing inhomogeneous light shifts in atomic ensembles, and for Rydberg atom trapping. The bottle beam is created using a Mach-Zehnder interferometer with unequal magnification in the two arms. The resulting optical field has an intensity zero surrounded by bright regions in all directions. The bottle beam is loaded by spatially overlapping it with a cloud of cold atoms in a MOT, giving a localized sample of several thousand atoms.   

State-insensitive two-color optical trapping

We propose a scheme for state-insensitive trapping of neutral atoms by using two-color light at convenient wavelengths. In this scheme, a combination of trapping and control lasers is used to minimize the variance of the potential experienced by Rb atom in ground and excited $5p_{3/2}$ states. We predict the values of trap and control wavelengths for which the $5s$ and $5p_{3/2}$ levels have same ac Stark shifts in the presence of two laser fields. The calculations are based on the relativistic all-order method where all single and double excitations of the Dirac-Hartree-Fock wave function are included to all orders of perturbation theory.   

Spin Relaxation in Dark Optical Traps

Blue-detuned, dark, optical traps confine atoms to regions of low intensity. This enables measurements to be done in near field-free conditions and provides for longer state purity in ensembles of atoms. We explore spontaneous-scattering-induced spin relaxation in blue-detuned traps and investigate the effects of trap tightness and wavelengths on state lifetime. Because hotter atoms in blue detuned traps are exposed to more intense light, they tend to scatter more photons. Hence, most atoms that undergo a state-changing photon exchange are from the outlying regions of the trap. By quickly removing these atoms, state purity is maintained in the remaining ensemble. Simulations show that this method of retaining state purity can be combined with evaporative cooling to obtain a very cold and very pure-state sample of atoms.   

Evaporative Cooling Of a Photon Fluid to Quantum Degeneracy

We demonstrate the conditions necessary for the condensation of photons in a Fabry-Perot cavity and the rise of coherent properties using evaporative cooling mechanisms traditionally associated with ultracold atomic condensates. The photons are able to collide through an atom-mediated interaction and become a photon fluid. The photons enter into a Poissonian number distribution and possess a narrow spectral linewidth. This state expresses both coherent and superfluid properties that should be able to be accessed experimentally.   

Optomechanical cooling and trapping in a three-mirror cavity

We present a theoretical analysis of optomechanical cooling and trapping of a moving mirror located inside a cavity with two fixed end mirrors, substantiating recent experiment and theory [1]. This three-mirror configuration turns out to have technological as well as physical advantages over the usual two-mirror set-up. We consider fully as well as partially reflective middle mirrors [2,3]. In the latter case we find two regimes, one dissipative and the other dispersive, depending on the placement of the middle mirror. This allows us to propose a two-color cooling and trapping scheme that improves on current configurations. \newline \newline [1] J. D. Thompson et. al, arXiv:0707.1724v2[quant-ph](2007). \newline [2] M. Bhattacharya and P. Meystre, Phys. Rev. Lett. \textbf{99},073601 (2007). \newline [3] M. Bhattacharya, H. Uys and P. Meystre, arXiv:0708.4078v1 [quant-ph] (2007).   

Using a Laguerre-Gaussian beam to cool and trap the rotational motion of a mirror

Cavity-enabled optomechanical cooling and trapping of mirrors has lately become experimentally feasible, indicating the possibility of reaching the quantum mechanical ground state of a macroscopic mirror. All theory and experiment so far has been based on the exchange of \textit{linear} momentum between light beams and vibrating mirrors. We consider Laguerre-Gaussian beams carrying \textit{angular} momentum in a cavity made of two highly reflective spiral phase elements. We show that this configuration should enable the optical trapping and cooling of the rotational motion of the movable cavity end-mirror down to its quantum mechanical ground state [1], as well as robust entanglement of its ro-vibrational modes [2]. \newline \newline [1] M. Bhattacharya and P. Meystre, Phys. Rev. Lett. \textbf{99}, 153603-1 (2007). \newline [2] M. Bhattacharya, P.-L. Giscard and P. Meystre, Phys. Rev. Lett. submitted.