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arXiv:2209.13086 · Physical Review A

Magnetic-field-independent spin-exchange relaxation-free magnetometer

Mark Dikopoltsev, Uriel Levy, Or Katz

Published Mar 3, 2025• 7 citations
arXivHTMLPDF

Abstract

Spin-exchange relaxation-free magnetometers based on dense ensembles of alkali-metal spins are precision quantum sensors that hold the record of measured and projected sensitivity to magnetic fields, in the microgauss to milligauss range. At geomagnetic fields however, these sensors quickly lose their magnetic sensitivity due to spin decoherence by random spin-exchange collisions. Here we discover that atoms with nuclear spin $I=\frac{1}{2}$ can operate in the spin-exchange relaxation-free regime even at high magnetic field. We counterintuitively show that frequent collisions between a dense and optically inaccessible $(I=\frac{1}{2})$ gas with another optically accessible spin gas ($I>\frac{1}{2}$) improve the fundamental magnetic sensitivity of the latter. We analyze the performance of a dual-species potassium and atomic hydrogen magnetometer and project a fundamental sensitivity of about $10\phantom{\rule{0.28em}{0ex}}\mathrm{aT}\phantom{\rule{0.16em}{0ex}}{({\mathrm{cm}}^{3}\text{/}\mathrm{Hz})}^{1/2}$ at geomagnetic fields for feasible experimental conditions.

DOI
10.1103/physreva.111.032602

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Spin-exchange relaxation-free magnetometers based on dense ensembles of alkali-metal spins are precision quantum sensors that hold the record of measured and projected sensitivity to magnetic fields, in the microgauss to milligauss range. At geomagnetic fields however, these sensors quickly lose their magnetic sensitivity due to spin decoherence by random spin-exchange collisions. Here we discover that atoms with nuclear spin I=12I=\frac{1}{2}I=21​ can operate in the spin-exchange relaxation-free regime even at high magnetic field. We counterintuitively show that frequent collisions between a dense and optically inaccessible (I=12)(I=\frac{1}{2})(I=21​) gas with another optically accessible spin gas (I>12I>\frac{1}{2}I>21​) improve the fundamental magnetic sensitivity of the latter. We analyze the performance of a dual-species potassium and atomic hydrogen magnetometer and project a fundamental sensitivity of about 10aT(cm3/Hz)1/210\phantom{\rule{0.28em}{0ex}}\mathrm{aT}\phantom{\rule{0.16em}{0ex}}{({\mathrm{cm}}^{3}\text{/}\mathrm{Hz})}^{1/2}10aT(cm3/Hz)1/2 at geomagnetic fields for feasible experimental conditions.
In this paper, a new type of high-sensitivity magnetometer that can surpass SQUID magnetometers operating in liquid-helium in sensitivity and spatial resolution is discussed. The sensitivity of atomic magnetometers is determined by the electron spin relaxation time and the number of alkali-metal atoms. The relaxation time is usually limited by spin-exchange collisions between atoms and drops at higher alkali-metal density. Spin-exchange collisions preserve the total spin of the atoms, but can change their hyperfine state, which reverses the direction of spin precession in the magnetic field. Thus, spin-exchange collisions usually cause spin decoherence. However, this relaxation process can be suppressed if the spin precession frequency is much smaller than the rate of spin-exchange collisions. These conditions lead to 'motional narrowing', where the spin precession angle between the collisions is so small that the atoms do not lose coherence. A magnetometer operating near zero magnetic field can simultaneously achieve high alkali-metal density and long spin-relaxation time, resulting in very high sensitivity.
An optically pumped atomic magnetometer using a hybrid cell of potassium and rubidium atoms was demonstrated to yield high sensitivity to magnetic fields. We operated the magnetometer with the four possible combinations of optically pumped and optically probed atoms and found that the combination of optically pumped potassium and optically probed rubidium showed the highest sensitivity among the four combinations because the rubidium atoms were denser than those of potassium. Furthermore, we investigated the dependence of the sensitivity on the power densities of the pump and probe beams and the wavelength of the probe beam. The magnetometer using the hybrid cell required higher pump-beam power and had narrower magnetic linewidth than those of the single alkali-metal cell. However, the magnetic linewidth was larger than the theoretical value, ignoring the spin relaxation caused by the spin-exchange collisions. By adjusting the laser conditions, the highest sensitivity approached 30 fTrms/Hz1/2.
At the most fundamental level, the performance of atomic sensors is limited by quantum decoherence. The problem of decoherence has been addressed at low magnetic fields with atomic samples, where the limiting factor of the coherence lifetime arises from spin-exchange collisions. In this paper, we demonstrate the complex role of the collisions in the relaxation of quantum states of alkali-metal atoms. The detailed understanding of the collision role allows us to reduce the ground-state relaxation in stronger magnetic fields (tens $\ensuremath{\mu}\mathrm{T}$). Reduction of the relaxation rate enables improvement of the performance of atomic sensors. In particular, enhancement of the sensitivity of optical magnetometers in the detection of stronger magnetic fields may be obtained. Reduced transverse relaxation also enables increasing quantum-information storage time in atomic vapor.
Alkali-metal magnetometers compete with SQUID detectors as the most sensitive magnetic field sensors. Their sensitivity is limited by relaxation due to spin-exchange collisions. We demonstrate a K magnetometer in which spin-exchange relaxation is completely eliminated by operating at high K density and low magnetic field. Direct measurements of the signal-to-noise ratio give a magnetometer sensitivity of $10\text{ }\mathrm{f}\mathrm{T}\text{ }\mathrm{H}{\mathrm{z}}^{\mathrm{\ensuremath{-}}\mathrm{1}\mathrm{/}\mathrm{2}}$, limited by magnetic noise produced by Johnson currents in the magnetic shields. We extend a previous theoretical analysis of spin exchange in low magnetic fields to arbitrary spin polarizations and estimate the shot-noise limit of the magnetometer to be $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}\text{ }\mathrm{T}\text{ }\mathrm{H}{\mathrm{z}}^{\mathrm{\ensuremath{-}}\mathrm{1}\mathrm{/}\mathrm{2}}$.
High precision measurement of magnetic fields has been of great importance for both fundamental physics and practical applications. Magnetometers based on atomic spin effect provide a new technique for high precision measurement and have been enabled as the most sensitive magnetic sensors at present. However, the atomic spins used as probes to sense the magnetic fields are prone to the decoherence effects. For the atomic spin-exchange relaxation free (SERF) magnetometer which operates with high alkali atoms density in very low magnetic fields, the main relaxation mechanism is the spin-destruction collisions between atoms. In this paper, it is demonstrated by numerical simulations that the precision of the atomic SERF magnetometer is significantly affected by this spin-destruction collisions.
Ultra sensitive detection and high precision measurement of magnetic fields have been of great importance for both fundamental physics and practical applications. Recent developments in the technology of atomic magnetometers have enabled the spin-exchange relaxation free (SERF) atomic magnetometers as the most sensitive devices for detecting and measuring static or quasi-static magnetic fields. In this paper, we analyze the effect of the most prominent relaxation mechanism (i.e., the spin-destruction collisions) on the precision of SERF magnetometers. For static magnetic fields, it is explicitly demonstrated how the spin-destruction collisions degrade the precision of the SERF magnetometers from the Heisenberg limit to the standard limit even with quantum resources being employed.