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AIP conference proceedings

Engineering Multiparticle Entanglement with Neutral Atoms in Optical Lattices

Immanuel Bloch

Published Jan 1, 2005• 0 citations
Publisher

Abstract

The controlled creation of entanglement is one of the most challenging goals of modern quantum physics. Here we show how spin dependent optical lattice potentials together with controlled collisions between atoms in an optical lattice potential can be used to realize massively parallel operating quantum gates. In a second experiment, the generation of Bell states of atom pairs on single lattice sites is demonstrated by employing a newly found interstate Feshbach resonance in 87Rb. This Feshbach resonance has allowed us to efficiently modify the scattering length between two atoms in different spin states and is used to induce an entanglement dynamics in the system.

DOI
10.1063/1.1928866

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•Jan 1, 2007•4 citations
The controlled creation of entanglement is one of the most challenging goals of modern quantum physics. Here we show how spin dependent optical lattice potentials together with controlled collisions between atoms in an optical lattice potential can be used to realize massively parallel operating quantum gates. In a second experiment, the generation of Bell states of atom pairs on single lattice sites is demonstrated by employing a newly found interstate Feshbach resonance in 87Rb. This Feshbach resonance has allowed us to efficiently modify the scattering length between two atoms in different spin states and is used to induce an entanglement dynamics in the system.
Summary form only given. Controlled interactions between pairs of neutral atoms in optical lattices allow for the creation of highly entangled multiparticle states and the implementation of quantum logic gates. Such systems benefit from suppressed decoherence because neutrals couple weakly to the environment and from the possibility of processing with the massively parallel quantum registers in the atomic ensemble. The main source of decoherence is spontaneous emission which can be made negligible if the two atom interactions are performed fast compared to the scattering rate. We have shown the possibility of using induced electric dipole-dipole interactions between pairs of atoms to perform the two qubit gates /spl radic/(SWAP) and CPHASE with high fidelity.
Summary form only given. Optical lattices provide new opportunities to create entangled states of neutral atoms for quantum information processing. Such systems have some attractive features: decoherence is suppressed because neutrals couple weakly to the environment, and operations can be performed in parallel on a large ensemble of trapped atoms, thus offering avenues for scaling to many qubits. The main source of decoherence is spontaneous emission, but this can be negligible if all manipulations are performed rapidly compared to the photon scattering rate. We consider here the possibility of using resonantly induced electric dipole-dipole interactions to engineer two-qubit operations such as the swap gate and the controlled-phase which, together with single qubit operations, can perform arbitrary quantum computations.
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.
Summary form only given. Controlled coherent interactions between pairs of neutral atoms can be designed in tight micro-traps such as optical lattices which allow the creation of highly entangled multiparticle states. Such systems benefit from suppressed decoherence because neutrals couple weakly to the environment and the interactions are strictly pairwise. The main source of decoherence is spontaneous emission which can be made negligible if the two atom interactions are performed fast compared to the scattering rate.We have shown the possibility of using induced electric dipole-dipole interactions between pairs of atoms to perform quantum logic gates with high fidelity.
We report on a matter wave interferometer realized with entangled pairs of trapped 87Rb atoms. Each pair of atoms is confined at a single site of an optical lattice potential. The interferometer is realized by first creating a coherent spin superposition of the two atoms and then tuning the interstate scattering length via a Feshbach resonance. The selective change of the interstate scattering length leads to an entanglement dynamics of the two-particle state that can be detected in a Ramsey interference experiment. This entanglement dynamics is employed for a precision measurement of atomic interaction parameters. Furthermore, the interferometer allows us to separate lattice sites with one or two atoms in a nondestructive way.
This thesis reports on the experimental investigation of controlled spin dependent interactions in a sample of ultracold Rubidium atoms trapped in a periodic optical potential. In such a situation, the most basic interaction between only two atoms at one common potential well, forming a micro laboratory for this atom pair, can be investigated. Spin dependent interactions between the atoms can lead to an intriguing time evolution of the system. In this work, we present two examples of such spin interaction induced dynamics. First, we have been able to observe and control a coherent spin changing interaction. Second, we have \nachieved to examine and manipulate an interaction\ninduced time evolution of the relative phase of a spin 1/2-system, both in the case of particle pairs and in the more general case of N interacting particles. The first part of this thesis elucidates the spin-changing interaction mechanism underlying many fascinating effects resulting from interacting spins at ultracold temperatures. This process changes the spin states of two colliding particles, while preserving total magnetization. If initial and final states have almost equal energy, this process is resonant and leads to large amplitude\noscillations between different spin states. The measured coupling parameters of such a process allow to precisely infer atomic scattering length differences, that e.g. determine the nature of the magnetic ground state of the hyperfine states in Rubidium.\nMoreover, a method to tune the spin oscillations at will based on the AC-Zeeman effect has been implemented. This allowed us to use resonant spin changing \ncollisions as a quantitative and non-destructive particle pair probe in the optical lattice. This led to a series of experiments shedding light on the Bosonic superfluid to Mott insulator transition.\nIn a second series of experiments we have been able to coherently manipulate the interaction induced time evolution of the relative phase in an ensemble of spin 1/2-systems. For two particles, interactions can lead to an entanglement oscillation of the particle pair. For the general case of N interacting particles, the ideal time evolution leads to the creation of spin squeezed states and even Schrödinger cat states. In the experiment we have been able to control the underlying\ninteractions by a Feshbach resonance. For particle pairs we could directly observe the entanglement oscillations. For the many particle case we have been able to observe and reverse the interaction induced dispersion of the relative phase.\nThe presented results demonstrate how correlated spin states can be engineered through \ncontrol of atomic interactions. Moreover, the results point towards the possibility to simulate quantum\nmagnetism phenomena with ultracold atoms in optical traps, and to realize and analyze many novel quantum spin states which have not been experimentally realized so far.