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arXiv:quant-ph/0605163 · Fortschritte der Physik

Quantum information processing in optical lattices and magnetic microtraps

P. Treutlein, T. Steinmetz, Yves Colombe, Benjamin Lev, Peter Hommelhoff, Jakob Reichel, Markus Greiner, O. Mandel, A. Widera, T. Rom, I. Bloch, TheodorW. Hänsch

Published Aug 1, 2006• 95 citations
arXivHTMLPDF

Abstract

Abstract We review our experiments on quantum information processing with neutral atoms in optical lattices and magnetic microtraps. Atoms in an optical lattice in the Mott insulator regime serve as a large qubit register. A spin‐dependent lattice is used to split and delocalize the atomic wave functions in a controlled and coherent way over a defined number of lattice sites. This is used to experimentally demonstrate a massively parallel quantum gate array, which allows the creation of a highly entangled many‐body cluster state through coherent collisions between atoms on neighbouring lattice sites. In magnetic microtraps on an atom chip, we demonstrate coherent manipulation of atomic qubit states and measure coherence lifetimes exceeding one second at micron‐distance from the chip surface. We show that microwave near‐fields on the chip can be used to create state‐dependent potentials for the implementation of a quantum controlled phase gate with these robust qubit states. For single atom detection and preparation, we have developed high finesse fiber Fabry‐Perot cavities and integrated them on the ato m chip. We present an experiment in which we detected a very small number of cold atoms magnetically trapped in the cavity using the atom chip.

DOI
10.1002/prop.200610325

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Abstract We review our experiments on quantum information processing with neutral atoms in optical lattices and magnetic microtraps. Atoms in an optical lattice in the Mott insulator regime serve as a large qubit register. A spin‐dependent lattice is used to split and delocalize the atomic wave functions in a controlled and coherent way over a defined number of lattice sites. This is used to experimentally demonstrate a massively parallel quantum gate array, which allows the creation of a highly entangled many‐body cluster state through coherent collisions between atoms on neighbouring lattice sites. In magnetic microtraps on an atom chip, we demonstrate coherent manipulation of atomic qubit states and measure coherence lifetimes exceeding one second at micron‐distance from the chip surface. We show that microwave near‐fields on the chip can be used to create state‐dependent potentials for the implementation of a quantum controlled phase gate with these robust qubit states. For single atom detection and preparation, we have developed high finesse fiber Fabry‐Perot cavities and integrated them on the ato m chip. We present an experiment in which we detected a very small number of cold atoms magnetically trapped in the cavity using the atom chip.
We review our experiments on quantum information processing with neutral atoms in optical lattices and magnetic microtraps. Atoms in an optical lattice in the Mott insulator regime serve as a large qubit register. A spin-dependent lattice is used to split and delocalize the atomic wave functions in a controlled and coherent way over a defined number of lattice sites. This is used to experimentally demonstrate a massively parallel quantum gate array, which allows the creation of a highly entangled many-body cluster state through coherent collisions between atoms on neighbouring lattice sites. In magnetic microtraps on an atom chip, we demonstrate coherent manipulation of atomic qubit states and measure coherence lifetimes exceeding one second at micron-distance from the chip surface. We show that microwave near-fields on the chip can be used to create state-dependent potentials for the implementation of a quantum controlled phase gate with these robust qubit states. For single atom detection and preparation, we have developed high finesse fiber Fabry-Perot cavities and integrated them on the atom chip. We present an experiment in which we detected a very small number of cold atoms magnetically trapped in the cavity using the atom chip.
The system comprised of an atom strongly coupled to photons, known as cavity quantum electrodynamics (QED), provides a rich experimental setting for quantum information processing, both in the implementation of quantum logic gates and in the development of quantum networks. Moreover, studies of cavity QED will help elucidate the dynamics of continuously observed open quantum systems with quantum-limited feedback. To achieve these goals in cavity QED, a neutral atom must be tightly confined inside a high-finesse cavity with small mode volume for long periods of time. Microfabricated wires on a substrate---known as an atom chip---can create a sufficiently high-curvature magnetic potential to trap atoms in the Lamb-Dicke regime. We have recently integrated an optical fiber Fabry-Perot cavity with such a device. The microwires allow the on-chip collection and laser cooling of neutral atoms, and allow the magnetic waveguiding of these atoms to an Ioffe trap inside the cavity mode. Magnetically trapped intracavity atoms have been detected with this cavity QED system. A similar experiment employing microdisks and photonic bandgap cavities is nearing completion. With these more exotic cavities, a robust and scalable atom-cavity chip system will deeply probe the strong coupling regime of cavity QED with magnetically trapped atoms. Atom chips have found great success in producing and manipulating Bose-Einstein condensates and in creating novel atom optical elements. An on-chip BEC has been attained in a miniaturized system incorporating an atom chip designed for atom interferometry and for studies of Josephson effects of a BEC in a double-well potential. Using similar microfabrication techniques, we created and demonstrated a specular magnetic atom mirror formed from a standard computer hard drive. This device, in conjunction with micron-sized charged circular pads, can produce a 1-D ring trap which may prove useful for studying Tonks gases in a ring geometry and for creating devices such as a SQUID-like system for neutral atoms. This thesis describes the fabrication and employment of these atoms chips in experiments at both Caltech and Munich, the latter in collaboration with Professors Theodore Haensch and Jakob Reichel at the Max Planck Institute for Quantum Optics.
We review novel methods for the investigation, control and manipulation of neutral atoms in optical lattices. These setups allow unprecedented quantum control over large numbers of atoms and thus are very promising for applications in quantum information processing. After introducing optical lattices we discuss the superfluid (SF) and Mott insulating (MI) states of neutral atoms trapped in such lattices and investigate the SF-MI transition as observed experimentally recently. In the second part of the paper we give an overview of proposals for quantum information processing and show different ways to entangle the trapped atoms, in particular the usage of cold collisions and Rydberg atoms. Finally, we discuss briefly the implementation of quantum simulators, entanglement enhanced atom interferometers, and ideas for robust quantum memory in optical lattices.
Neutral atoms offer a promising platform for single- and many-body quantum control, as required for quantum information processing. This includes excellent isolation from the decohering influence of the environment, and the existence of well developed techniques for atom trapping and coherent manipulation. We present a review of our work to implement quantum control and measurement for ultra-cold atoms in far-off-resonance optical lattice traps. In recent experiments we have demonstrated coherent behavior of mesoscopic atomic spinor wavepackets in optical double-well potentials, and carried out quantum state tomography to reconstruct the full density matrix for the atomic spin degrees of freedom. This model system shares a number of important features with proposals to implement quantum logic and quantum computing in optical lattices. We present a theoretical analysis of a protocol for universal quantum logic via single qubit operations and an entangling gate based on electric dipole-dipole interactions. Detailed calculations including the full atomic hyperfine structure suggests that high-fidelity quantum gates are possible under realistic experimental conditions.
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.