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arXiv:quant-ph/0311197 · Physical Review Letters

Coherence in Microchip Traps

Philipp Treutlein, Peter Hommelhoff, T. Steinmetz, Theodor W. Hänsch, Jakob Reichel

Published May 21, 2004• 253 citations
arXivHTMLPDF

Abstract

We report the coherent manipulation of internal states of neutral atoms in a magnetic microchip trap. Coherence lifetimes exceeding 1 s are observed with atoms at distances of 5-130 microm from the microchip surface. The coherence lifetime in the chip trap is independent of atom-surface distance within our measurement accuracy and agrees well with the results of similar measurements in macroscopic magnetic traps. Because of the absence of surface-induced decoherence, a miniaturized atomic clock with a relative stability in the 10(-13) range can be realized. For applications in quantum information processing, we propose to use microwave near fields in the proximity of chip wires to create potentials that depend on the internal state of the atoms.

DOI
10.1103/physrevlett.92.203005

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We report the coherent manipulation of internal states of neutral atoms in a magnetic microchip trap. Coherence lifetimes exceeding 1 s are observed with atoms at distances of 5-130 microm from the microchip surface. The coherence lifetime in the chip trap is independent of atom-surface distance within our measurement accuracy and agrees well with the results of similar measurements in macroscopic magnetic traps. Because of the absence of surface-induced decoherence, a miniaturized atomic clock with a relative stability in the 10(-13) range can be realized. For applications in quantum information processing, we propose to use microwave near fields in the proximity of chip wires to create potentials that depend on the internal state of the atoms.
Microchip traps provide a promising approach to quantum information processing and communication (QIPC) with neutral atoms: strong and complex potentials can be produced for acting on the qubit atoms, and the potentials can be scaled to higher qubit numbers by virtue of the microfabrication process. We describe experimental results that are relevant to use in QIPC, such as the transport of Bose-Einstein-condensed atomic ensembles along the chip surface with the help of a magnetic conveyor belt. The second part of the paper is devoted to single-atom detection on the 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 spectacular progress in the field of ultracold quantum gases is intimately connected with the availability of sophisticated techniques for quantum-level control of internal states, motional states, and collisional interactions. Atom chips provide such control in compact, robust, and scalable setups, which makes them attractive for both applications and fundamental studies. In this thesis I report on experiments that use a new method for coherent manipulation of ultracold atoms. The method is based on microwave near-fields, provided by a waveguide structure that is fully integrated on an atom chip. We generate microwave near-field potentials that combine the versatility of optical traps with the robustness and tailorability of static magnetic microtraps. These potentials depend on the internal atomic state, and we use them for state-selective splitting of Rb Bose-Einstein condensates. We show for the first time combined coherent manipulation of internal and motional states on an atom chip, realizing a trapped-atom interferometer with internal state labeling of the interferometer paths. Moreover, we use microwave near-field potentials for the preparation of spin-squeezed states for quantum-enhanced metrology through controlling state-dependent collisional interactions. In addition, very promising proposals exist for the implementation of a quantum phase gate using these potentials. Measuring microwave fields is important for engineering of microwave devices as well as in science, e.g. to characterize the field homogeneity in the interaction regions of an atomic clock. We develop a novel technique that uses clouds of uncondensed ultracold atoms as sensitive, tunable and non-invasive probes for microwave field imaging with micrometer spatial resolution. The microwave magnetic field components drive Rabi oscillations on atomic hyperfine transitions whose frequency can be tuned with a static magnetic field. Readout is accomplished using state-selective absorption imaging. Quantitative data extraction is simple and it is possible to reconstruct the amplitudes and phases of the different microwave magnetic field components. While we demonstrate 2D imaging, an extension to 3D imaging is straightforward. We use the method to determine the microwave near-field distribution around the on-chip waveguide and reconstruct the corresponding current distribution. For our experimental parameters, the method provides a microwave magnetic field sensitivity of 0.2 mG, which can even be improved further with variants discussed. The experiments presented in this thesis open the path for the realization of portable quantum-enhanced interferometer devices, the implementation of a quantum phase gate as well as for a new generation of microwave field sensors.
We report on the loading and trapping of ultracold atoms in a one-dimensional permanent magnetic lattice of period 10 µm produced on an atom chip. The grooved structure which generates the magnetic lattice potential is fabricated on a silicon substrate and coated with a perpendicularly magnetized multilayered TbGdFeCo/Cr film of effective thickness 960 nm. Ultracold atoms are evaporatively cooled in a Z-wire magnetic trap and then adiabatically transferred to the magnetic lattice potential by applying an appropriate bias field. Under our experimental conditions trap frequencies of up to 90 kHz in the magnetic lattice are measured and the atoms are trapped at a distance of less than 5 µm from the surface with a measured lifetime of about 450 ms. These results are important in the context of studies of quantum coherence of neutral atoms in periodic magnetic potentials on an atom chip.
Microwave near-fields are a key ingredient for quantum information processing with atom chips. Our goal is to realize a quantum gate with the following features: the qubit is encoded in the hyperfine states |1rang equiv |F=1, mF=-1rang and |2rang equiv |F=2, mF=+1rang of87Rb, which are both magnetically trappable and allow for very long coherence lifetimes. Microwave near-fields guided on the atom chip are used to drive single-qubit rotations and provide state-selectivity to the magnetic trapping potential. The quantum phase gate is implemented by state-selective collisions of two qubit atoms in this potential. Besides applications in quantum information processing, microwave near-fieIds on atom chips can also be used for atom interferometry and chip-based atomic clocks.
Abstract The limitations for the coherent manipulation of neutral atoms with fabricated solid state devices, so-called ‘atom chips’, are addressed. Specifically, we examine the dominant decoherence mechanism, which is due to the magnetic noise originating from the surface of the atom chip. It is shown that the contribution of fluctuations in the chip wires at the shot noise level is not negligible. We estimate the coherence times and discuss ways to increase them. Our main conclusion is that future advances should allow for coherence times as long as 1 second, a few µm away from the surface. PACS: 03.75.-b Matter waves – 32.80.Lg Mechanical effects of light on atoms and ions – 03.67.Lx Quantum computation – 05.40.-a Fluctuation phenomena and noise In the quest for physical implementations of quantum information processing, “atom chips ” are currently of great interest.