Ultrasensitive Atomic Comagnetometer with Enhanced Nuclear Spin Coherence

Kai Wei, Tian Zhao, Xiujie Fang, Zitong Xu, Chang Liu, Qian Cao, Arne Wickenbrock, Yanhui Hu, Wei Ji, Jiancheng Fang, and Dmitry Budker

Phys. Rev. Lett. 130, 063201 – Published 10 February 2023

Abstract

Achieving high energy resolution in spin systems is important for fundamental physics research and precision measurements, with alkali-noble-gas comagnetometers being among the best available sensors. We found a new relaxation mechanism in such devices, the gradient of the Fermi-contact-interaction field that dominates the relaxation of hyperpolarized nuclear spins. We report on precise control over spin distribution, demonstrating a tenfold increase of nuclear spin hyperpolarization and transverse coherence time with optimal hybrid optical pumping. Operating in the self-compensation regime, our ${}^{21}{Ne} − Rb − K$ comagnetometer achieves an ultrahigh inertial rotation sensitivity of $3 \times 10^{- 8} rad / s / {Hz}^{1 / 2}$ in the frequency range from 0.2 to 1.0 Hz, which is equivalent to the energy resolution of $3.1 \times 10^{- 23} {eV / Hz}^{1 / 2}$ . We propose to use this comagnetometer to search for exotic spin-dependent interactions involving proton and neutron spins. The projected sensitivity surpasses the previous experimental and astrophysical limits by more than 4 orders of magnitude.

Received 22 October 2022
Accepted 9 January 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.063201

Physics Subject Headings (PhySH)

Coherent control Dark matter Light-matter interaction Optical pumping Quantum measurements

Atomic ensemble Atoms Laser systems Optical sources & detectors

Atomic, Molecular & Optical

Authors & Affiliations

Kai Wei^1,2, Tian Zhao^1,2, Xiujie Fang^2,3, Zitong Xu^1,2, Chang Liu^1,2, Qian Cao^1,2, Arne Wickenbrock ^4,5, Yanhui Hu ^6,*, Wei Ji ^4,5,†, Jiancheng Fang^1,2, and Dmitry Budker ^4,5,7

¹School of Instrumentation Science and Opto-electronics Engineering, Beihang University, Beijing, 100191, China
²Hangzhou Extremely Weak Magnetic Field Major Science and Technology Infrastructure Research Institute, Hangzhou, 310051, China
³School of Physics, Beihang University, Beijing 100191, China
⁴Helmholtz-Institut, GSI Helmholtzzentrum fur Schwerionenforschung, Mainz 55128, Germany
⁵Johannes Gutenberg University, Mainz 55128, Germany
⁶Department of Physics, King’s College London, Strand, London WC2R 2LS, United Kingdom
⁷Department of Physics, University of California, Berkeley, California 94720-7300, USA

^*yanhui.hu@kcl.ac.uk
^†wei.ji.physics@gmail.com

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Vol. 130, Iss. 6 — 10 February 2023

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Images

Figure 1
(a) The pump and probe configuration of the ${}^{21}{Ne} − Rb − K$ comagnetometer. Hybrid Rb-K atoms are applied to transfer the spin momentum of pumping-light photons to ${}^{21}{Ne}$ nuclear spins. The precession of ${}^{21}{Ne}$ spins due to exotic fields or inertial rotation is transferred to the alkali spins, which are read out by probe light based on optical rotation. (b) Suppression factor $S F_{x}$ as a function of the noble-gas nuclear spin polarization $P_{z}^{n}$ (solid curves with lower axis) and the noble-gas-spin transverse relaxation rate $R_{2}^{n}$ (dashed curves with upper axis). The $S F_{x}^{low}$ and $S F_{x}^{mid}$ decrease with $P_{z}^{n}$ , and the $S F_{x}^{low}$ increases with $R_{2}^{n}$ , consistent with the theoretical model.
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Figure 2
(a) The simulated spatial distribution of electron polarization $P_{z}^{e}$ for density ratio $ξ$ of 10 and 400, respectively, in the $Y − Z$ plane of the cell center at $190 ° C$ . (b) The calculated homogeneity factor $η^{e}$ of $P_{z}^{e}$ and the volume-averaged nuclear polarization ${\bar{P}}_{z}^{n}$ as functions of $ξ$ . $η^{n} \approx 1$ and ${\bar{P}}_{z}^{e} \approx 0.5$ are not plotted here. (c) The noble-gas spin polarization $P_{z}^{n}$ and transverse relaxation rate $R_{2}^{n}$ as a function of $ξ$ at $200 ° C$ . By optimizing the $ξ$ , the $P_{z}^{n}$ and the $T_{2}^{n} = 1 / R_{2}^{n}$ are improved by nearly 1 order of magnitude, respectively. (d) The averaged suppression factors $S F_{x}^{low}$ and $S F_{x}^{mid}$ as a function of the $ξ$ at $200 ° C$ .
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Figure 3
The noise spectrum of the SC comagnetometer. In the frequency range from 0.2 to 1.0 Hz, the averaged noise is $3 \times 10^{- 8} rad / s / {Hz}^{1 / 2}$ . Below 0.2 Hz, the noise spectrum is dominated by the $1 / f$ noise. The peak in the noise spectrum is due to the vibration, which is confirmed with a precision seismometer. The probe noise is measured with unpolarized spin ensembles by blocking the pump laser.
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Figure 4
Existing limits and projected sensitivity on the coupling-constant product $g_{S} g_{P}$ of the spin-dependent force between the neutron (proton) spin and unpolarized nucleon. The black dashed and red dashed lines are the sensitivity in this proposal using the ${}^{21}{Ne}$ neutron spin and proton spin, respectively. The blue solid line “Venema 1992 nN” [39] and black solid line “Lee 2018 nN” [7] represent the direct experimental limits on the coupling to neutron spins, while the red solid line “Kimball 2017 pN” [18] represents the direct experimental limits on the coupling to proton spins. The green solid line “Astro 2020 NN” [43] represents the astrophysical limits on the coupling between nucleons (not distinguishing between protons and neutrons).
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