Session J3: New Techniques for Laser Cooling and Trapping

Optimization of collisional Feshbach cooling of an ultracold nondegenerate gas

We optimize a collision-induced cooling process for ultracold atoms in the nondegenerate regime. It makes use of a Feshbach resonance, instead of rf radiation in evaporative cooling, to selectively expel hot atoms from a trap. Using functional minimization we analytically show that for the optimal cooling process the resonance energy must be tuned such that it linearly follows the temperature. Here, optimal cooling is defined as maximizing the phase-space density after a fixed cooling duration. The analytical results are confirmed by numerical Monte-Carlo simulations. In order to simulate more realistic experimental conditions, we show that background losses do not change our conclusions, while additional non-resonant two-body losses make a lower initial resonance energy with non-linear dependence on temperature preferable.   

Towards slowing and trapping of a buffer-gas beam of CaF

Cryogenic buffer-gas beam (CBGB) sources [1] are now routinely used to create slow atomic and molecular beams. While atoms from a CBGB can be loaded directly into a magneto-optical trap (MOT) without any Zeeman slower [2], additional slowing stages are required to load molecules due to their lower capture velocity ($<$ 10 m/s). Here, we report on our progress to slow a beam of CaF molecules using laser light at 606 nm in combination with two vibrational repumpers at 548 nm and 628 nm. The slowing lasers are broadened to cover the source's velocity spread and to remain resonant while molecules are being slowed. Our approach for slowing is similar to those for SrF and YO [3-5]. Since CaF requires remixing of the magnetic sub-states to keep it in the optical cycle, we additionally implement an alternating-current (AC) MOT for trapping. \\[4pt] [1] N.~R.~Hutzler, et al., Chem. Rev. 112, 4803 (2012)\\[0pt] [2] B.~Hemmerling, et al., New J. Phys. 16, 063070 (2014)\\[0pt] [3] E.~F.~Shuman, et al., Nature 467, 820 (2010)\\[0pt] [4] M.~T.~Hummon, et al., Phys. Rev. Lett. 110, 143001 (2013)\\[0pt] [5] M.~Yeo, et al., arXiv:1501.04683 (2015)   

Core-Shell Magneto-Optical Trap for Alkaline-Earth-Metal-Like Atoms

We propose and demonstrate a new type of magneto-optical trap (MOT) for alkaline-earth-metal-like (AEML) atoms where the narrow intercombination $^{1}S_{0}\rightarrow{}^{3}P_{1}$ transition and the broad $^{1}S_{0}\rightarrow{}^{1}P_{1}$ transition are spatially arranged into a core-shell configuration. Our scheme resolves the main limitations of previously adopted MOT schemes, leading to significant increases in both the loading rate and the steady state atom number. We apply this scheme to $^{174}$Yb atoms and compare it with the conventional intercombination MOT, where we observe more than two orders of magnitude improvement in the loading rate and ten-fold improvement in the steady state atom number. The increase in loading rate and trapped atomic number can lead to enhancement of the statistical sensitivity in many different types of precision experiments using cold AEML atoms, such as lattice clock experiments and the electric dipole moment experiments.   

Experimental Demonstration of Synthetic Lorentz Force on Cold Atoms by Using Radiation Pressure

The quest for synthetic magnetism in quantum degenerate atomic gases is motivated by producing controllable quantum emulators, which could mimic complex quantum systems such as interacting electrons in magnetic fields [1, 2]. Experiments on synthetic magnetic fields for neutral atoms have enabled realization of the Hall effect, Harper and Haldane Hamiltonians, and other intriguing topological effects. Here we present the first demonstration of a synthetic Lorentz force, based on the radiation pressure and the Doppler effect, in cold atomic gases captured in a Magneto-Optical Trap (MOT). Synthetic Lorentz force on cold atomic cloud is measured by recording the cloud trajectory. The observed force is perpendicular to the cloud velocity, and it is zero for the atomic cloud at rest. The proposed concept is straightforward to implement in a large volume and different geometries, it is applicable for a broad range of velocities, and it can be realized for different atomic species. The experiment is based on the theoretical proposal introduced in [3]. [1] I. Bloch, J. Dalibard, and S. Nascimbene, Nat. Phys. 8, 267 (2012). [2] J. Dalibard, F. Gerbier, G. Juzeliunas, and P. Ohberg, Rev. Mod. Phys. 83, 1523 (2011). [3] T. Dubcek, N. Santic, D. Jukic, D. Aumiler, T. Ban, and H. Buljan, Phys. Rev. A 89, 063415 (2014).   

Laser cooling and trapping with optical frequency combs

A large number of atoms and molecules are difficult to control with continuous wave lasers because generating sufficient power at all of the necessary wavelengths is technologically challenging. Mode-locked lasers, through their enhanced efficiency of~nonlinear frequency conversion, provide some of these hard to access wavelengths. As a step towards control of exotic atoms and molecules we report on laser cooling and trapping of atoms using an optical frequency comb~in two different regimes. Using a~single~comb,~we have created a simultaneous dual-species (isotopes) MOT, demonstrating that multiple comb teeth~can be used in parallel~to cool and confine species requiring many cw lasers. Separately, we demonstrate comb-based laser cooling on a two-photon transition, which efficiently uses the full time-averaged optical power of the entire comb [1].~Our progress toward~extending this to include trapping by making a MOT using this two-photon transition is presented. This work is supported by the National Science Foundation. \\[4pt] [1] D. Kielpinski, Phys. Rev. A 73, 063407 (2006)   

Laser Cooling by Stimulated Emission

We present a laser cooling schemes based on the stimulated emission of the two level atoms, by the bichromatic field. The improved efficiency of the scheme is suggested by the Carnot-like thermal cycle. The controllability of the stimulated and the cooling of the internal degrees of freedom of the atom are the strong candidates for enabling us to expand the scheme to more complex atoms as well as the molecules.   

Frequency-comb-induced radiative force on cold rubidium atoms

Excitation of atoms by trains of ultrashort laser pulses yields atomic coherence effects, such as accumulation of excited-state population and coherence [1]. These effects become more pronounced in atomic systems at (ultra)low temperatures (no dephasing due to collisions). Somewhat surprisingly, experiments with (ultra)cold atomic gases involving trains of ultrashort pulses are scarce in the literature. The same also applies to optomechanical effects on atoms induced by resonant frequency comb (FC) excitation. We will present results of the radiative force measurements in cold rubidium atoms induced by the coherent pulse train (i.e. FC) excitation [2]. Various experimental geometries will be studied, including single pulse train excitation, two in-phase and out-of-phase counter-propagating pulse trains, and various time delays between the pulse trains. The force measurements will be supported by theoretical modeling using optical Bloch equations, and supplemented by laser-induced fluorescence measurements. \\[4pt] [1] D. Aumiler, T. Ban, and G. Pichler, Phy. Rev. A 79, 063403 (2009).\\[0pt] [2] G. Kregar, N. \v{S}anti\'{c}, D. Aumiler, H. Buljan, and T. Ban, Phys. Rev. A 89, 053421 (2014).   

Line generated 2D grid for neutral atom trapping

A phase-insensitive light field is desirable for stably trapping neutral atoms for quantum computing. Techniques are presented for creating a rectilinear array of atom trapping sites using holographically shaped beams with a gaussian profile in one transverse dimension, and a top-hat line profile in the other transverse dimension. This line generated grid creates an inherently 2D trap array at the focal plane, so that a low-crosstalk addressable qubit register may be implemented. The light field is projected from only one direction, and is insensitive to phase fluctuations and to misalignment in 2 degrees of freedom. This technique creates traps with depth twice that of previous singly-projected designs. Preliminary results for an 81-site cesium qubit register are presented.   

Grating chips for quantum technologies

Laser cooled atomic samples have resulted in profound advances in frequency metrology, however the technology is typically complex and bulky. In the cover story of the May 2013 issue of Nature Nanotechnology [1] we describe a micro-fabricated optical element that greatly facilitates miniaturisation of ultra-cold atom technology. Portable devices should be feasible with accuracy vastly exceeding that of equivalent room-temperature technology, with a minimal footprint. Laser cooled samples will be ideal for measurement devices e.g. portable atomic clocks and magnetometers and, moreover, they hold great potential for longer-term breakthroughs exploiting e.g. optical lattices for all-optical clocks and Bose-Einstein condensates for atom interferometry. Here we will discuss next generation diffractive optical elements (DOE) and demonstrate quantum based measurements on samples of ultra-cold atoms created using our miniaturised optical setup.\\[4pt] [1] C. C. Nshii et al., {\it A surface-patterned chip as a strong source of ultra-cold atoms for quantum technologies,} Nature Nanotech. {\bf 8}, 321 (2013).   

Cooling optically levitated dielectric nanoparticles via parametric feedback

The inability to leverage resonant scattering processes involving internal degrees of freedom differentiates optical cooling experiments performed with levitated dielectric nanoparticles, from similar atomic and molecular traps. Trapping in optical cavities or the application of active feedback techniques have proven to be effective ways to circumvent this limitation. We present our nanoparticle optical cooling apparatus, which is based on parametric feedback modulation of a single-beam gradient force optical trap. This scheme allows us to achieve effective center-of-mass temperatures well below 1 kelvin for our $\sim 1 \times 10^{-18}$ kg particles, at modest vacuum pressures. The method provides a versatile platform, with parameter tunability not found in conventional tethered nanomechanical systems. Potential applications include investigations of nonequilibrium nanoscale thermodynamics, ultra-sensitive force metrology, and mesoscale quantum mechanics and hybrid systems.