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CRC Press eBooks

Trapping Cold Atoms with Laser Light

Glen D. Gillen, Katharina Gillen, Shekhar Guha

Published Oct 30, 2013• 0 citations
Publisher

Abstract

In this chapter we present an interesting application of the propagation of laser light through linear optical media: the ability to spatially confine (trap) cold atoms within localized light intensity patterns. We first present some of the fundamentals for understanding how light can trap an atom. Depending upon the relationship between the properties of the atom and the properties of the laser light, atoms can be trapped in either localized dark spots, or localized bright spots. We will describe several light patterns for trapping atoms: The tightly localized bright and dark regions in the near-field diffraction patterns presented in Chapters 7 and 8 , The near-field patterns projected to a location far away from the diffracting aperture, and Polarization-dependent movable light field traps based on the near-field diffraction patterns.

DOI
10.1201/b15993-11

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•Jan 1, 2019•0 citations
In this chapter we present an interesting application of the propagation of laser light through linear optical media: the ability to spatially confine (trap) cold atoms within localized light intensity patterns. We first present some of the fundamentals for understanding how light can trap an atom. Depending upon the relationship between the properties of the atom and the properties of the laser light, atoms can be trapped in either localized dark spots, or localized bright spots. We will describe several light patterns for trapping atoms: The tightly localized bright and dark regions in the near-field diffraction patterns presented in Chapters 7 and 8 , The near-field patterns projected to a location far away from the diffracting aperture, and Polarization-dependent movable light field traps based on the near-field diffraction patterns.
We theoretically investigate the feasibility of using intensity distributions of light of a single laser beam diffracted by a circular aperture as optical dipole traps for cold neutral atoms. Localized and cylindrically symmetric traps on the central axis of the circular aperture exist for both blue- and red-detuned laser light. Experimental mapping of the spots of interest using CO2{\mathrm{CO}}_{2}CO2​ laser light demonstrates the existence of these light distributions for laboratory conditions and their agreement with theoretical predictions.
Scalar diffraction theory is combined with beam-propagation techniques to investigate the projection of near-field diffraction patterns to spatial locations away from the aperture for use in optically trapping cold neutral alkali-metal atoms. Calculations show that intensity distributions with localized bright and dark spots usually found within a millimeter of the diffracting aperture can be projected to a region free from optical components such as a cloud of cold atoms within a vacuum chamber. Calculations also predict that the critical properties of the optical dipole atom traps are not only maintained for the projected intensity patterns but also can be manipulated and improved by adjustment of the optical components outside the vacuum chamber.
We propose and present a quantitative analysis of neutral atom microtraps based on optical near fields produced by the diffraction of a laser wave on small apertures in a thin screen. We show that near-field atom microtraps are capable of storing atoms in micron-sized regions, with estimated trap lifetimes of about 1s1\phantom{\rule{0.3em}{0ex}}\mathrm{s}1s, when using a moderate laser intensity of about 10W∕cm210\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}10W∕cm2. The depth of the proposed Fresnel atom microtraps is about 0.1mK0.1\phantom{\rule{0.3em}{0ex}}\mathrm{mK}0.1mK. An array of such atom microtraps could have applications to site-selective manipulation of cold atoms.
In recent years great progress was made in the spatial control of light with dynamic phase and amplitude modulators such as spatial light modulators and digital micromirror devices. In this work we describe the theory and practice of light shaping with such devices, detailing the spatial control of amplitude, phase and polarisation of coherent laser beams. We use our expertise in generating and measuring light fields with spatially dependent polarisation structures to characterise the correlations between spatial modes and polarisation in such light fields. We do this by adapting concurrence, a quantum measure of entanglement, to these classical correlations. One of the most promising application of coherent laser light is in the control of atomic media via atom-light interactions. In this work we describe the construction of simple external cavity diode lasers designed for the generation of resonant light for atomic physics applications. We exploit these lasers and spatial light modulators to create and measure three-dimensional atomic population structures in a warm rubidium vapour. We also implement a magneto-optic and a dynamic dark spontaneous-force optical trap for rubidium. These traps produce dense (~ 10^11 cm^-3) and cold (~100 uK) clouds of rubidium atoms. We develop the theory of spatially dependent electromagnetically induced transparency in such traps using rate equations. We find that the absorption of linearly polarised light depends on the relative direction of a magnetic field and the light polarisation. We use the cold atom clouds to measure the direction of magnetic fields by using this dependence and laser beams with structured polarisation.
Configurable trapping potentials are of great interest in cold atom physics, as they enable production of dynamical highly flexible fields that exhibit unprecedented stability and diverse geometries. Direct imaging can be used to create large area trapping potentials but is often overlooked due to its inability to correct for wavefront aberrations of the optical system [1]. This need not be a major disadvantage for a well-corrected optical system and brings advantages including the simplicity and speed of direct imaging. This is in contrast to the Fourier plane method which requires complex calculations to generate proper holograms and suffers from phase defects and speckle. For applications in cold atom trapping, these effects are especially detrimental as the atoms are sensitive to perturbations at the ~1% level of the optical potential. Our approach uses off-the-shelf lenses and microscope objectives and is able to achieve 630(10) nm full width half maximum (FWHM) patterning resolution using a 0.45 NA objective, within 5% of the diffraction limit of the system, while imaging through 1.25 mm of glass. The light field patterning is done using a digital micromirror device (DMD) which allows for dynamic trapping potentials due to its ability to store 13,889 frames and its 22 kHz full frame refresh rate. We use this method to pattern planar potentials for the purpose of cold atom experiments and have found that for atoms, which tend to respond relatively slowly to perturbations, it is possible to combine half-toning and time averaging to produce grey scale patterns, additionally allowing for pattern correction [2]. [1] P. C. Mogensen and J. Glückstad, Optics Communications 175, 75–81 (2000). [2] G. Gauthier, I. Lenton, N. McKay Parry, M. Baker, M. J. Davis, H. Rubinsztein-Dunlop, and T. W. Neely, arXiv preprint arXiv:1605.04928 (2016).
Many types of quantum systems are being explored for use in quantum computers. One type of quantum system that shows promise for quantum computing is trapped neutral atoms. They have long coherence times, since they have multiple stable ground states and have minimal coupling with other atoms and their environment, and they can be trapped in arrays, making them individu- ally addressable. Once trapped, they can be initialized and operated on using laser pulses. This experiment utilizes a pinhole diffraction pattern, which can trap atoms in both bright and dark areas. To maximize trap strength, an injection-locked laser amplification system is used to provide a high-power, precisely tunable light source for the diffraction pattern. This amplification system depends on the coupling between a “seed” laser and a “receiving” laser. In previous analyses of the coupling between these two laser diodes, the effects on injection locking when varying temperature and current of both lasers have been shown to be highly nonlinear, and few general trends have been identified. In this project, in order to optimize and better characterize the system, automation of the data-taking and varying parameters was implemented. High efficiency lock zones of laser currents and temperatures were identified, and these locking zones will allow us to create strong atom traps for our experiment.