5,356 research outputs found

    Creating long-lived neutral-atom traps in a cryogenic environment

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    We describe techniques for creating long-lived magneto-optical and magnetostatic traps for neutral atoms. These traps exist in a sealed cryogenic environment with a temperature near 4 K, where the background gas pressure can be extremely low. To date we have observed cesium magneto-optical traps with background-limited lifetimes in excess of 1 h, and magnetostatic traps with lifetimes of nearly 10 min. From these observations we use the known He-Cs van der Waals collision cross section to infer typical background gas pressures in our apparatus below 4×10^(-12) Torr. With hardware improvements we expect this pressure can be made much lower, extending the magnetostatic-trap lifetimes one to two orders of magnitude. Furthermore, with a cryogenic system one can use superconducting magnets and SQUID detectors to trap and to nondestructively sense spin-polarized atoms. With superconducting microstructures one can achieve very large magnetic-field gradients and curvatures, as high as ∼10^6 G/cm and ∼10^9 G/cm^2, respectively, for use in magnetic and magneto-optical traps

    Bit-Interleaved Coded Modulation Revisited: A Mismatched Decoding Perspective

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    We revisit the information-theoretic analysis of bit-interleaved coded modulation (BICM) by modeling the BICM decoder as a mismatched decoder. The mismatched decoding model is well-defined for finite, yet arbitrary, block lengths, and naturally captures the channel memory among the bits belonging to the same symbol. We give two independent proofs of the achievability of the BICM capacity calculated by Caire et al. where BICM was modeled as a set of independent parallel binary-input channels whose output is the bitwise log-likelihood ratio. Our first achievability proof uses typical sequences, and shows that due to the random coding construction, the interleaver is not required. The second proof is based on the random coding error exponents with mismatched decoding, where the largest achievable rate is the generalized mutual information. We show that the generalized mutual information of the mismatched decoder coincides with the infinite-interleaver BICM capacity. We also show that the error exponent -and hence the cutoff rate- of the BICM mismatched decoder is upper bounded by that of coded modulation and may thus be lower than in the infinite-interleaved model. We also consider the mutual information appearing in the analysis of iterative decoding of BICM with EXIT charts. We show that the corresponding symbol metric has knowledge of the transmitted symbol and the EXIT mutual information admits a representation as a pseudo-generalized mutual information, which is in general not achievable. A different symbol decoding metric, for which the extrinsic side information refers to the hypothesized symbol, induces a generalized mutual information lower than the coded modulation capacity.Comment: submitted to the IEEE Transactions on Information Theory. Conference version in 2008 IEEE International Symposium on Information Theory, Toronto, Canada, July 200

    Stability of Magneto-optical Traps with Large Field Gradients: Limits on the Tight Confinement of Single Atoms

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    We report measurements of the stability of magneto-optical traps (MOTs) for neutral atoms in the limit of tight confinement of a single atom. For quadrupole magnetic field gradients at the trap center greater than ∼1 kG/cm, we find that stochastic diffusion of atoms out of the trapping volume becomes the dominant particle loss mechanism, ultimately limiting the MOT size to greater than ∼5 μm. We measured and modeled the diffusive loss rate as a function of laser power, detuning, and field gradient for trapped cesium atoms. In addition, for as few as two atoms, the collisional loss rates become very high for tightly confined traps, allowing the direct observation of isolated two-body atomic collisions in a MOT

    The canonical controller and its regularity

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    This paper deals with properties of canonical controllers. We first specify the behavior that they implement. It follows that a canonical controller implements the desired controlled behavior if and only if the desired behavior is implementable. We subsequently investigate the regularity of the controlled behavior. We prove that a canonical controller is regular if and only if every controller is regular. In other words, canonical controllers are maximally irregular

    Faster universal modeling for two source classes

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    The Universal Modeling algorithms proposed in [2] for two general classes of finite-context sources are reviewed. The above methods were constructed by viewing a model structure as a partition of the context space and realizing that a partition can be reached through successive splits. Here we start by constructing recursive counting algorithms to count all models belonging to the two classes and use the algorithms to perform the Bayesian Mixture. The resulting methods lead to computationally more efficient Universal Modeling algorithms