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Physical Review Letters

Suppression of Dephasing due to a Trapping Potential and Atom-Atom Interactions in a Trapped-Condensate Interferometer

Munekazu Horikoshi, Ken’ichi Nakagawa

Published Oct 29, 2007• 41 citations
Publisher

Abstract

We propose and demonstrate a novel trapped-condensate interferometer using optical Bragg diffractions in a harmonic magnetic potential, which can realize a long coherence time with low dephasing. Dephasing of wave packets due to the magnetic potential is canceled by setting the interrogation time equal to the oscillation period of the harmonic potential. The harmonic potential also helps to suppress dephasing due to condensate atom-atom interactions. An interference signal with a fringe contrast of 30% is observed at an interrogation time of 58 ms. For a longer interrogation time about 100 ms, the spatial coherence of the condensate is still maintained with low dephasing, although the interference fringe is washed out by external vibrational noise.

DOI
10.1103/physrevlett.99.180401

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We propose and demonstrate a novel trapped-condensate interferometer using optical Bragg diffractions in a harmonic magnetic potential, which can realize a long coherence time with low dephasing. Dephasing of wave packets due to the magnetic potential is canceled by setting the interrogation time equal to the oscillation period of the harmonic potential. The harmonic potential also helps to suppress dephasing due to condensate atom-atom interactions. An interference signal with a fringe contrast of 30% is observed at an interrogation time of 58 ms. For a longer interrogation time about 100 ms, the spatial coherence of the condensate is still maintained with low dephasing, although the interference fringe is washed out by external vibrational noise.
We have demonstrated a novel atom interferometer which uses a free oscillation of 87Rb BEC atoms in a magnetic harmonic potential on an atom chip. This interferometer can effectively suppress the dephasing due to the trap potential by applying two Bragg laser pulses with an time interval T equal to the oscillation period of the axial mode of the magnetic trap. We observed a high contrast (~30 %) interference fringe signal for T = 58 ms. When we increased the time interval T to 97 ms, an interference fringe was washed out by the random phase fluctuation. We attributed this phase fluctuation to the external vibrational noise from the floor.
A Mach-Zehnder-type atom interferometer with a Bose-Einstein condensate has been investigated on an atom chip by using optical Bragg diffraction. A phase shift and a contrast degradation, which depend on the atomic density and the trapping frequency of the magnetic-guide potential, have been observed. Also, the output wave packets were found to exhibit a spatial interference pattern. The atom-atom interaction and the guide potential induce the spatially inhomogeneous phase evolution of the wave packets in each arm of the interferometer. The observed contrast degradation can be quantitatively explained as a dephasing due to this inhomogeneous phase evolution.
We report on a trapped atom interferometer based on Bragg diffraction and Bloch oscillations with alkaline-earth-metal atoms. We use a Ramsey--Bord\'e Bragg interferometer with 88Sr^{88}\mathrm{Sr}88Sr atoms combined with Bloch oscillations to extend the interferometer time. Thanks to a long coherence time for Bloch oscillations of 88Sr^{88}\mathrm{Sr}88Sr atoms, we observed interference up to 1 s evolution time in the lattice. A detailed study of decoherence sources during the Bloch phase is also presented. While still limited in sensitivity by lattice lifetime and beam inhomogeneity this result opens the way to high contrast trapped interferometers with extended interrogation time.
We observe the coherence of an interacting two-component Bose-Einstein condensate (BEC) surviving for seconds in a trapped Ramsey interferometer. Mean-field-driven collective oscillations of two components lead to periodic dephasing and rephasing of condensate wave functions with a slow decay of the interference fringe visibility. We apply spin echo synchronous with the self-rephasing of the condensate to reduce the influence of state-dependent atom losses, significantly enhancing the visibility up to 0.750.750.75 at the evolution time of 1.51.51.5 s. Mean-field theory consistently predicts higher visibility than experimentally observed values. We quantify the effects of classical and quantum noise and infer a coherence time of 2.82.82.8 s for a trapped condensate of $5.5\ifmmode\times\else\texttimes\fi{}{10}^{4}$ interacting atoms.
Atom interferometers are quantum mechanical devices sensitive to gravitational and inertial forces, with applications in fundamental physics and inertial sensing in the field. Their performance is currently limited by the interrogation time available to freely falling atoms in Earth’s gravitational field, as well as noise due to mechanical and acoustic vibrations. Our experiment probes gravitational potentials by holding, rather than dropping, atoms. We realize an interrogation time of 20 seconds by suspending the spatially separated atomic wave packets in an optical lattice. This record coherence is enabled by the smooth lattice wave fronts, which are mode-filtered by an optical cavity. This trapped geometry suppresses phase variance due to vibrations by three to four orders of magnitude, overcoming the dominant noise source in atom-interferometric gravimeters. The later part of the talk describes recent progress in characterizing and reducing dephasing of the interferometer. An upgraded optical lattice interferometer experiment is currently being commissioned, with the goal of increased sensitivity to gravity.
In our original paper (Altin et al 2011 New J. Phys. 13 065020), we presented the results from a Ramsey atom interferometer operating with an optically trapped sample of up to 10 6 Bose-condensed 87 Rb atoms in the m F = 0 clock states. We were unable to observe projection noise fluctuations on the interferometer output, which we attribute to the stability of our microwave oscillator and background magnetic field. Numerical simulations of the Gross–Pitaevskii equations for our system show that dephasing due to spatial dynamics driven by interparticle interactions accounts for much of the observed decay in fringe visibility at long interrogation times. The simulations show good agreement with the experimental data when additional technical decoherence is accounted for, and suggest that the clock states are indeed immiscible. With smaller samples of 5 × 10 4 atoms, we observe a coherence time of τ = 1.0 +0.5 −0.3 s.