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arXiv:1802.02190 · Physical Review A

Velocimetry of cold atoms by matter-wave interferometry

Max Carey, Jack Saywell, David Elcock, Mohammad Belal, Tim Freegarde

Published Feb 28, 2019• 9 citations
arXivHTMLPDF

Abstract

We present an elegant application of matter-wave interferometry to the velocimetry of cold atoms whereby, in analogy to Fourier transform spectroscopy, the one-dimensional velocity distribution is manifest in the frequency domain of the interferometer output. By using stimulated Raman transitions between hyperfine ground states to perform a three-pulse interferometer sequence, we have measured the velocity distributions of clouds of freely expanding $^{85}\mathrm{Rb}$ atoms with temperatures of 34 and $18\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{K}$. Quadrature measurement of the interferometer output as a function of the temporal asymmetry yields velocity distributions with excellent fidelity. Our technique, which is particularly suited to ultracold samples, compares favorably with conventional Doppler and time-of-flight techniques, and it reveals artefacts in standard Raman Doppler methods. The technique is related to, and provides a conceptual foundation of, interferometric matter-wave accelerometry, gravimetry, and rotation sensing.

DOI
10.1103/physreva.99.023631

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We present an elegant application of matter-wave interferometry to the velocimetry of cold atoms whereby, in analogy to Fourier transform spectroscopy, the one-dimensional velocity distribution is manifest in the frequency domain of the interferometer output. By using stimulated Raman transitions between hyperfine ground states to perform a three-pulse interferometer sequence, we have measured the velocity distributions of clouds of freely expanding 85Rb^{85}\mathrm{Rb}85Rb atoms with temperatures of 34 and 18\ensuremathμK18\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{K}18\ensuremathμK. Quadrature measurement of the interferometer output as a function of the temporal asymmetry yields velocity distributions with excellent fidelity. Our technique, which is particularly suited to ultracold samples, compares favorably with conventional Doppler and time-of-flight techniques, and it reveals artefacts in standard Raman Doppler methods. The technique is related to, and provides a conceptual foundation of, interferometric matter-wave accelerometry, gravimetry, and rotation sensing.
We consider the matterwave interferometric measurement of atomic velocities, which forms a building block for all matterwave inertial measurements. A theoretical analysis, addressing both the laboratory and atomic frames and accounting for residual Doppler sensitivity in the beamsplitter and recombiner pulses, is followed by an experimental demonstration, with measurements of the velocity distribution within a 20 μK cloud of rubidium atoms. Our experiments use Raman transitions between the long-lived ground hyperfine states, and allow quadrature measurements that yield the full complex interferometer signal and hence discriminate between positive and negative velocities. The technique is most suitable for measurement of colder samples.
We report the 1D cooling of ^{85}Rb atoms using a velocity-dependent optical force based upon Ramsey matter-wave interferometry. Using stimulated Raman transitions between ground hyperfine states, 12 cycles of the interferometer sequence cool a freely moving atom cloud from 21 to 3 μK. This pulsed analog of continuous-wave Doppler cooling is effective at temperatures down to the recoil limit; with augmentation pulses to increase the interferometer area, it should cool more quickly than conventional methods and be more suitable for species that lack a closed radiative transition.
Cold atom interferometer is sensitive to both gravity and rotation.In order to measure the two parameters respectively,by exerting several horizontal Raman pulses in multiple time-domains on cold atom clouds,we realize the cold atom splitting was realized,and the difference of atom velocity was increased.Then by extending vertical sequences of Raman pulses to cold atoms,two sub-interferometers with different mean velocity of atoms were achieved.Based on the principle of interferometry phase shift that is related to rotation but not acceleration,two measure methods were presented.One makes the mean horizontal velocity of some atoms zero,the other makes the direction of mean horizontal velocity of two parts opposite.Theory analysis and calculation show that it can measure gravity and rotation respectively based on experiment data.
Ultracold samples of laser-cooled atoms are quantum systems over which modern atomic physicists can exert exquisite control. Largely decoupled from their environment, they can act as near-ideal test masses for inertial sensors based on atom interferometry and are well suited to experiments in coherent control of quantum systems that probe the fundamental nature of quantum mechanics and pave the way for practical quantum simulation and computation. This thesis details a series of experimental results that arise from the coherent control of rubidium atoms with laser light, focusing on the interplay between these interactions and atomic velocities; the laser frequency an atom ‘sees’ is Doppler shifted according to its velocity, while conservation of momentum dictates that, in exchanging photons with a laser, an atom’s velocity is altered. Coherent light–atom interactions can thus be tailored either to measure or to narrow the spread of velocities in an ultracold atomic gas. Alternatively, it can be desirable to design interactions that are homogeneous across a large spread of atomic velocities. All of these aspects are explored in this thesis. The velocity-sensitive interactions that lie at the heart of atom-interferometric inertial sensors are reexamined in a manner that yields considerable insight into the underlying processes and culminates in a novel, precise and elegant technique for measuring the velocity of ultracold atoms that is used to reveal the Gaussian nature of the velocity spread in a cloud with an effective temperature of 18.7(6) µK, undistorted by artefacts that plague other methods. Furthermore, optimal control techniques are applied to the problem of coherently and uniformly manipulating the quantum states of atoms in an ensemble with a large spread of velocities, even subject to variations in laser intensity. A broadband inversion pulse is demonstrated to change the internal state of 99.8(3) % of atoms in a ∼35 µK ensemble and — for the first time — this technique is used to optimise an entire atom interferometry sequence, yielding a threefold enhancement in the measurement contrast. Finally, a version of grey molasses cooling — in which atoms accumulate in velocity-dependent ‘dark’ states, narrowing their momentum spread and increasing their phase space density — is demonstrated with phase-coherent cooling beams; dark states that exist in this system prove to be particularly resilient to the spatially varying light shifts that are present in an optical dipole trap, and this is used both to enhance the number of atoms loaded into such a trap — by a factor of 7 compared to loading from a conventional optical molasses — and to further cool them once they are loaded in a technique that has promising prospects for the rapid production of ultracold, trapped, atoms.
The use of matter-wave interferometry to detect inertial forces is already a powerful tool in precision measurement, but extending it beyond the lab to navigation applications requires optimization for compact setups. Here researchers characterize point-source atom interferometry (PSI) in a centimeter-scale cell, using an expanding cloud of cold atoms and an optical Raman interaction for simultaneous measurement of the system's acceleration along the Raman laser beams plus its rotation vector projected into the plane perpendicular to the beams. The sensitivity measurements presented here inform the further development of quantum inertial sensing that balances size and performance.