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Laser Spectroscopy

ARRAY OF MESOSCOPIC ENSEMBLES ON A MAGNETIC ATOM CHIP

Atreju Tauschinsky, Caspar F. Ockeloen-Korppi, Rutger M. T. Thijssen, S. Whitlock, R. J. C. Spreeuw

Published Feb 1, 2010• 0 citations
Publisher

Abstract

We report on the storage and manipulation of hundreds of mesoscopic ensembles of ultracold 87Rb atoms in a vast two-dimensional array of magnetic microtraps, defined lithographically in a permanently magnetized film. The atom numbers typically range from tens to hundreds of atoms per site. The traps are optically resolved using absorption imaging and individually addressed using a focused probe laser. We shift the entire array, without heating, along the surface by rotating an external bias field. We evaporatively cool the atoms to the critical temperature for quantum degeneracy. At the lowest temperatures, density dependent loss allows small and well defined numbers of atoms to be prepared in each microtrap. This microtrap array is a promising novel platform for quantum information processing, where hyperfine ground states act as qubit states, and Rydberg excitation may orchestrate interaction between neighbouring sites.

DOI
10.1142/9789814282345_0007

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We report on the storage and manipulation of hundreds of mesoscopic ensembles of ultracold 87Rb atoms in a vast two-dimensional array of magnetic microtraps, defined lithographically in a permanently magnetized film. The atom numbers typically range from tens to hundreds of atoms per site. The traps are optically resolved using absorption imaging and individually addressed using a focused probe laser. We shift the entire array, without heating, along the surface by rotating an external bias field. We evaporatively cool the atoms to the critical temperature for quantum degeneracy. At the lowest temperatures, density dependent loss allows small and well defined numbers of atoms to be prepared in each microtrap. This microtrap array is a promising novel platform for quantum information processing, where hyperfine ground states act as qubit states, and Rydberg excitation may orchestrate interaction between neighbouring sites.
Arrays of trapped atoms are the ideal starting points for developing registers comprising large numbers of physical qubits for storing and processing quantum information. One very promising approach involves neutral atom traps produced on microfabricated devices known as atom chips, as almost arbitrary trap configurations can be realized in a robust and compact package. Until now, however, atom chip experiments have focused on small systems incorporating single or only a few individual traps. Here, we report experiments on a two-dimensional array of trapped ultracold atom clouds prepared using a simple magnetic-film atom chip. We are able to load atoms into hundreds of tightly confining and optically resolved array sites. We then cool the individual atom clouds in parallel to the critical temperature required for quantum degeneracy. Atoms are shuttled across the chip surface utilizing the atom chip as an atomic shift register and local manipulation of atoms is implemented using a focused laser to rapidly empty individual traps.
We describe the fabrication and construction of a setup for creating lattices of magnetic microtraps for ultracold atoms on an atom chip. The lattice is defined by lithographic patterning of a permanent magnetic film. Patterned magnetic-film atom chips enable a large variety of trapping geometries over a wide range of length scales. We demonstrate an atom chip with a lattice constant of 10 μm, suitable for experiments in quantum information science employing the interaction between atoms in highly excited Rydberg energy levels. The active trapping region contains lattice regions with square and hexagonal symmetry, with the two regions joined at an interface. A structure of macroscopic wires, cutout of a silver foil, was mounted under the atom chip in order to load ultracold (87)Rb atoms into the microtraps. We demonstrate loading of atoms into the square and hexagonal lattice sections simultaneously and show resolved imaging of individual lattice sites. Magnetic-film lattices on atom chips provide a versatile platform for experiments with ultracold atoms, in particular for quantum information science and quantum simulation.
We present two different strategies for developing a quantum information science platform, based on our experimental results with magnetic microtrap arrays on a magnetic-film atom chip. The first strategy aims for mesoscopic ensemble qubits in a lattice of ~5 μm period, so that qubits can be individually addressed and interactions can be mediated by Rydberg excitations. The second strategy aims for direct quantum simulators using sub-optical lattices of ~100 nm period. These would allow the realization of condensed matter inspired quantum many-body systems, such as Hubbard models in new parameter regimes. The two approaches raise quite different issues, some of which are identified and discussed.
We propose the use of periodic arrays of permanent magnetic films for producing magnetic lattices of microtraps for confining, manipulating and controlling small clouds of ultracold atoms and quantum degenerate gases. Using analytical expressions and numerical calculations we show that periodic arrays of magnetic films can produce one-dimensional (1D) and two-dimensional (2D) magnetic lattices with non-zero potential minima, allowing ultracold atoms to be trapped without losses due to spin flips. In particular, we show that two crossed layers of periodic arrays of parallel rectangular magnets plus bias fields, or a single layer of periodic arrays of square-shaped magnets with three different thicknesses plus bias fields, can produce 2D magnetic lattices of microtraps having non-zero potential minima and controllable trap depth. For arrays with micron-scale periodicity, the magnetic microtraps can have very large trap depths ($\sim$0.5 mK for the realistic parameters chosen for the 2D lattice) and very tight confinement.
A novel kind of magnetic microtrap is demonstrated for ultracold neutral atoms. It consists of two concentric loops of radii r1r_1r1​ and r2r_2r2​ having oppositely oriented currents. A magnetic field minimum is generated in three dimensions that can be used to trap the atoms with a trap depth of about 1 mK using a current of 2.6 A. The condition r2/r1r_2/r_1r2​/r1​ = 2.2 maximizes the restoring force on the atoms toward the trap center. Unlike conventional magnetic microtraps, an external bias field is not required. Moreover, a one dimensional array of double-loop microtraps can be created by daisy chaining single microtrap circuits. \nA linear array of three microtraps having r1r_1r1​ = 300 mu\\mumum was fabricated as part of an atom chip. The following three techniques were developed to load the microtrap array: 1) atoms initially contained in a magneto-optical trap (MOT) were transported to the atom chip by a conventional magnetic trap, 2) atoms were first loaded into a mirror MOT and 3) atoms were initially loaded in a far off resonance optical dipole trap (FORT). Each technique loaded greater than 10510^5105 87^{87}87Rb atoms into the microtrap array. The lowest temperature of 30 mu\\mumuK for the microtrapped atom cloud was achieved using FORT loading. The strength and the position of the microtrap could be precisely adjusted over a range from 300 to 50 mu\\mumum above the atom chip surface by applying an external bias magnetic field. The lifetime of the atoms trapped in the microtrap array was measured to be 350 ms which was limited by the collisions with residual background gas.
A linear array of 11 closely spaced microtraps was demonstrated. Each microtrap was created using the magnetic field generated by two concentric circular wire loops having radii of 60 and 132 μ m carrying oppositely oriented currents as well as a bias magnetic field oriented perpendicular to the atom chip surface containing the microwire loops. 87 Rb atoms were initially cooled using either a surface magneto-optical trap (MOT) or a far off resonance optical dipole trap (FORT) created using an infrared laser directed along the microtrap array axis. The microtrap current was turned on and over 12 000 atoms were loaded into each microtrap. The advantage of FORT loading was that the 11 microtraps were about equally populated and the microtrapped atom temperature of 11 μ K was one quarter of that obtained using the MOT.