We present a new measurement of the positive muon magnetic anomaly, $a_{\mu }\equiv (g_{\mu }-2)/2$, from the Fermilab Muon $g-2$ Experiment using data collected in 2019 and 2020. We have analyzed more than 4 times the number of positrons from muon decay than in our previous result from 2018 data. The systematic error is reduced by more than a factor of 2 due to better running conditions, a more stable beam, and improved knowledge of the magnetic field weighted by the muon distribution, ${\tilde{\omega }}_p^{\prime }$, and of the anomalous precession frequency corrected for beam dynamics effects, ${\omega }_a$. From the ratio ${\omega }_a/{\tilde{\omega }}_p^{\prime }$, together with precisely determined external parameters, we determine $a_{\mu }=116592057(25)\times {10}^{-11}$ (0.21 ppm). Combining this result with our previous result from the 2018 data, we obtain $a_{\mu }(\mathrm{FNAL})=116592055(24)\times {10}^{-11}$ (0.20 ppm). The new experimental world average is $a_{\mu }(\exp )=116592059(22)\times {10}^{-11}$ (0.19 ppm), which represents a factor of 2 improvement in precision.

A new measurement of the magnetic anomaly a μ of the positive muon is presented based on data taken from 2020 to 2023 by the Muon g − 2 Experiment at Fermi National Accelerator Laboratory (FNAL). This dataset contains over 2.5 times the total statistics of our previous results. From the ratio of the precession frequencies for muons and protons in our storage ring magnetic field, together with precisely known ratios of fundamental constants, we determine a μ = 1 165 920 710 ( 162 ) × 10 − 12 (139 ppb) for the new datasets, and a μ = 1 165 920 705 ( 148 ) × 10 − 12 (127 ppb) when combined with our previous results. The new experimental world average, dominated by the measurements at FNAL, is a μ ( exp ) = 1 165 920 715 ( 145 ) × 10 − 12 (124 ppb). The measurements at FNAL have improved the precision on the world average by over a factor of 4.

The applications of spin-based quantum sensors to measurements probing fundamental physics are surveyed. Experimental methods and technologies developed for spin-based quantum information science have rapidly advanced in recent years and these tools enable increasingly precise control and measurement of spin dynamics. Theories of beyond-the-standard-model physics predict, for example, discrete-symmetry-violating electromagnetic moments correlated with particle spins, exotic spin-dependent forces, and coupling of spins to ultralight bosonic dark-matter fields. Spin-based quantum sensors can be used to search for these myriad phenomena and offer a methodology for tests of fundamental physics that is complementary to particle colliders and large-scale particle detectors. Areas of technological development that can significantly enhance the sensitivity of spin-based quantum sensors to new physics are highlighted.

We present details on a new measurement of the muon magnetic anomaly, $a_{\mu }=(g_{\mu }-2)/2$. The result is based on positive muon data taken at Fermilab’s Muon Campus during the 2019 and 2020 accelerator runs. The measurement uses $3.1\mathrm{GeV}/c$ polarized muons stored in a 7.1-m-radius storage ring with a 1.45 T uniform magnetic field. The value of $a_{\mu }$ is determined from the measured difference between the muon spin precession frequency and its cyclotron frequency. This difference is normalized to the strength of the magnetic field, measured using nuclear magnetic resonance. The ratio is then corrected for small contributions from beam motion, beam dispersion, and transient magnetic fields. We measure $a_{\mu }=116592057(25)\times {10}^{-11}$ (0.21 ppm). This is the world’s most precise measurement of this quantity and represents a factor of 2.2 improvement over our previous result based on the 2018 dataset. In combination, the two datasets yield $a_{\mu }(\mathrm{FNAL})=116592055(24)\times {10}^{-11}$ (0.20 ppm). Combining this with the measurements from Brookhaven National Laboratory for both positive and negative muons, the new world average is $a_{\mu }(\exp )=116592059(22)\times {10}^{-11}$ (0.19 ppm).

There is a large body of evidence that atomic nuclei can undergo octupole distortion and assume the shape of a pear. This phenomenon is important for measurements of electric-dipole moments of atoms, which would indicate CP violation and hence probe physics beyond the Standard Model of particle physics. Isotopes of both radon and radium have been identified as candidates for such measurements. Here, we observed the low-lying quantum states in ²²⁴Rn and ²²⁶Rn by accelerating beams of these radioactive nuclei. We show that radon isotopes undergo octupole vibrations but do not possess static pear-shapes in their ground states. We conclude that radon atoms provide less favourable conditions for the enhancement of a measurable atomic electric-dipole moment.

We report development of a highly accurate (parts per billion) absolute magnetometer based on ${}^3\mathrm{He}$ NMR. Optical pumping polarizes the spins, long coherence times provide high sensitivity, and the ${}^3\mathrm{He}$ electron shell effectively isolates the nuclear spin providing accuracy limited only by corrections including materials, sample shape, and magnetization. Our magnetometer was used to confirm calibration, to 32 ppb, of the magnetic-field sensors used in recent measurements of the muon magnetic moment anomaly $(g_{\mu }-2)$, which differs from the standard model by 2.4 ppm. With independent determination of the magnetic moment of ${}^3\mathrm{He}$, this work will lead the way to a new absolute magnetometry standard.