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arXiv:1607.06898 · Physical Review A

Measurement and extinction of vector light shifts using interferometry of spinor condensates

A. A. Wood, Lincoln D. Turner, R. P. Anderson

Published Nov 2, 2016• 11 citations
arXivHTMLPDF

Abstract

We use differential Ramsey interferometry of ultracold atoms to characterize the vector light shift (VLS) from a far-off-resonance optical dipole trap at $\ensuremath{\lambda}=1064$ nm. The VLS manifests as a ``fictitious'' magnetic field, which we perceive as a change in the Larmor frequency of two spinor condensates exposed to different intensities of elliptically polarized light. We use our measurement scheme to diagnose the light-induced magnetic field and suppress it to $2.1(8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ of its maximum value by making the trapping light linearly polarized with a quarter-wave plate in each beam. Our sensitive measurement of the VLS-induced field demonstrates high-precision, in vacuo interferometric polarimetry of dipole-trapping light and can be adapted to measure vector shifts from other lasers, advancing the application of optically trapped atoms to precision metrology.

DOI
10.1103/physreva.94.052503

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We use differential Ramsey interferometry of ultracold atoms to characterize the vector light shift (VLS) from a far-off-resonance optical dipole trap at \ensuremathλ=1064\ensuremath{\lambda}=1064\ensuremathλ=1064 nm. The VLS manifests as a ``fictitious'' magnetic field, which we perceive as a change in the Larmor frequency of two spinor condensates exposed to different intensities of elliptically polarized light. We use our measurement scheme to diagnose the light-induced magnetic field and suppress it to 2.1(8)\ifmmode×\else\texttimes\fi10\ensuremath−42.1(8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}2.1(8)\ifmmode×\else\texttimes\fi10\ensuremath−4 of its maximum value by making the trapping light linearly polarized with a quarter-wave plate in each beam. Our sensitive measurement of the VLS-induced field demonstrates high-precision, in vacuo interferometric polarimetry of dipole-trapping light and can be adapted to measure vector shifts from other lasers, advancing the application of optically trapped atoms to precision metrology.
We demonstrate detection of a vector light shift (VLS) using the quantum lock-in method. The method offers precise VLS measurement with little affect from real magnetic field fluctuation. We detect a VLS on a Bose-Einstein condensate (BEC) of 87Rb^{87}\mathrm{Rb}87Rb atoms caused by an optical trap beam with a resolution less than 1 Hz. We demonstrate the elimination of a VLS by controlling the beam polarization to realize a long coherence time of an F=2F=2F=2 BEC in the stretched state. Quantum lock-in VLS detection should find wide application, including the study of spinor BECs, electric dipole moment searches, and precise magnetometry.
The mapping of the potential landscape with high spatial resolution is crucial for quantum technologies based on ultracold atoms. However, the imaging of optical dipole traps is challenging because purely optical methods, commonly used to profile laser beams in free space, are not applicable in a vacuum. In this work, we demonstrate precise in situ imaging of optical dipole traps by probing a hyperfine transition with Ramsey interferometry. Thereby, we obtain an absolute map of the potential landscape with micrometer resolution and shot-noise-limited spectral precision. The idea of the technique is to control the polarization ellipticity of the trap laser beam to induce a differential light shift proportional to the trap potential. By studying the response to polarization ellipticity, we uncover a small but significant nonlinearity in addition to a dominant linear behavior, which is explained by the geometric distribution of the atomic ensemble. Our technique for imaging of optical traps can find wide application in quantum technologies based on ultracold atoms, as it applies to multiple atomic species and is not limited to a particular wavelength or trap geometry.
We describe a dispersive Faraday optical probe of atomic spin which performs a weak measurement of spin projection of a quantum gas continuously for more than one second. To date, focusing bright far-off-resonance probes onto quantum gases has proved invasive due to strong scalar and vector light shifts exerting dipole and Stern-Gerlach forces. We show that tuning the probe near the magic-zero wavelength at 790 nm between the fine-structure doublet of 87Rb^{87}\mathrm{Rb}87Rb cancels the scalar light shift, and careful control of polarization eliminates the vector light shift. Faraday rotations due to each fine-structure line reinforce at this wavelength, enhancing the signal-to-noise ratio for a fixed rate of probe-induced decoherence. Using this minimally invasive spin probe, we perform microscale atomic magnetometry at high temporal resolution. Spectrogram analysis of the Larmor precession signal of a single spinor Bose-Einstein condensate measures a time-varying magnetic field strength with 1 $\ensuremath{\mu}\mathrm{G}$ accuracy every 5 ms; or, equivalently, makes more than 200 successive measurements each at 10pT/Hz10\phantom{\rule{0.16em}{0ex}}\mathrm{pT}/\sqrt{\mathrm{Hz}}10pT/Hz​ sensitivity.
A precise, accurate, and relatively straightforward in situ method to measure and control the ellipticity of light resonantly interacting with an atomic vapor is described. The technique can be used to minimize vector light shifts. The method involves measurement of ellipticity-induced resonances in the magnetic-field dependence of nonlinear magneto-optical rotation of frequency-modulated light. The light propagation direction is orthogonal to the applied magnetic field B\mathbf{B}B and the major axis of the light polarization ellipse is along B\mathbf{B}B. When the light modulation frequency matches the Larmor frequency, elliptically polarized light produces a precessing atomic spin orientation transverse to B\mathbf{B}B via synchronous optical pumping. The precessing spin orientation causes optical rotation oscillating at the Larmor frequency by modulating the atomic vapor's circular birefringence. Based on this technique's precision, in situ nature (which avoids systematic errors arising from optical interfaces) and independent control of the most important systematic errors, it is shown that the accuracy of light ellipticity measurements achievable with this technique can exceed that of existing methods by orders of magnitude.
We demonstrate a suppression of the light shift in a Coherent-Population-Trapping (CPT) atomic magnetometer by using linearly polarized light and a differential measurement between magnetic resonances. The radio frequency that creates the optical sidebands for CPT quickly switches between two magnetic sensitive transitions, and the magnetic field is extrapolated from the difference of the center frequencies of the magnetic resonances. Light shifts and common drifts like collisional shifts can be suppressed through careful choice of measured resonances, and we show the light shift reduction by more than a factor of 20 compared to excitation with circularly-polarized light. Various limitations of the method are discussed.
We have measured the vector light shift due to a cavity built-up optical lattice by using a variation of the Hanle effect with trapped Cs atoms, where the time-evolving population of all magnetic sublevels is measured in situ. The measurement is linearly sensitive to the electric field of the nonlinearly polarized light, which allows unprecedented sensitivity to absolute linear polarization quality, to the level of 10(-10) in fractional intensity. Our approach to measuring and improving linear polarization can be applied to electron electric dipole moment searches, optical lattice clocks, magnetometery, and quantum computing.