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arXiv:2101.09102 · Physical Review A

Quantum lock-in detection of a vector light shift

Kosuke Shibata, Naota Sekiguchi, Takuya Hirano

Published Apr 30, 2021• 9 citations
arXivHTMLPDF

Abstract

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 $^{87}\mathrm{Rb}$ 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 $F=2$ 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.

DOI
10.1103/physreva.103.043335

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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.
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.
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.
Abstract The detection of weak time-dependent alternating signals in a strongly noisy background is an important problem in physics and a critical task in metrology. Quantum lock-in amplifier can extract alternating signals within extreme noises by using suitable quantum resources, which has been widely used for magnetic field sensing, vector light shift detection, and force detection. In particular, entanglement-enhanced quantum lock-in amplifier can be realized via many-body quantum interferometry. The many-body lock-in measurement provides a feasible way to achieve high-precision detection of alternating signals, even in noisy environments. In this article, we review general protocol, experiment progresses and potential applications of quantum lock-in measurements.
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.
The high precision measurement of magnetic field vector based on quantum coherent superposition states due to the interaction of the atomic system with linearly polarized light and magnetic field is described. The coordinate systems are physically orthogonal, which defined by the directions of light polarization and propagation. Because the quantum beat signals resulted by the coherent superposition states are no longer affected by nonlinear Zeeman shift, the measurement has smaller systematic error. The method is suitable for the measurements of vector fields such as the Earth's magnetic field.
We develop a generalized principle of EIT vector magnetometry based on high-contrast EIT-resonances and the symmetry of atom-light interaction in the linearly polarized bichromatic fields. Operation of such vector magnetometer on the D1 line of 87Rb has been demonstrated. The proposed compass-magnetometer has an increased immunity to shifts produced by quadratic Zeeman and ac-Stark effects, as well as by atom-buffer gas and atom-atom collisions. In our proof-of-principle experiment the detected sensitivity to magnetic field orientation is 10^{-3} deg/Hz^{1/2}, which is limited by laser intensity fluctuations, light polarization quality, and the magnitude of the magnetic field.