Session M03: Atomic Comagnetometry for Tests of Fundamental Physics

Two approaches for nuclear spin co-magnetometry

I will discuss two methods commonly used for nuclear spin co-magnetometry in fundamental physics measurements. One technique uses a comparison between magnetic fields measured by nuclear and alkali-metal spins, another approach compares the precession frequencies of two nuclear spin species. Alkali-nuclear spin comparison achieves the highest energy resolution at short time scales, but does not have high absolute stability. We recently investigated a pulsed pump laser approach to understand the limitations of absolute stability. A comparison of two nuclear spin precession frequencies provides better long-term stability and has a potential for further improvement by many orders of magnitude in sensitivity before reaching fundamental quantum limits. We recently focused on comparison between $^{3}$He and $^{21}$Ne spins that have hour-long coherence times and weak spin-interaction effects.   

Search for new physics using a 129Xe-131Xe-Rb comagnetometer

In a compact 129Xe-131Xe-Rb comagnetometer, polarized Rb atoms are used both to hyperpolarize Xe atoms and to serve as an in-situ magnetometer to sense the nuclear spin signals of Xe atoms. The two Xe isotopes are chosen for their long nuclear spin coherence times, and for their nearly equal collisional shifts in the Rb vapor. To suppress the effect due to the nuclear quadrupole moment of 131Xe, we amplify and resolve the quadrupole splittings by deliberately employing an elongated cell geometry. A modulated Rb polarization scheme is used to suppress systematic effects by two orders of magnitude. We have used this comagnetometer to search for monopole-dipole interactions at the sub-millimeter range and spin-dependent gravitational interactions at the Earth range. In both searches, we measure the ratio of nuclear spin-precession frequencies between 129Xe and 131Xe, and search for a correlated change of this ratio with either the movement of an external mass or the sensor orientation in the Earth gravitational field. The null results of both experiments set new upper limits on the coupling strength of aforementioned exotic interactions.   

Polarization dynamics in a nuclear spin gyroscope

Self-compensated comagnetometers, employing overlapping samples of spin-polarized alkali and noble gases (for example K-³He) are promising sensors for exotic beyond-the-standard-model fields and high-precision metrology such as rotation sensing. When the comagnetometer operates in the so-called self-compensated regime, the effective field, originating from contact interactions between the alkali valence electrons and the noble-gas nuclei, is compensated with an applied magnetic field. When the comagnetometer begins operation in a given magnetic field, spin-exchange optical pumping establishes equilibrium between the alkali electron-spin polarization and the nuclear-spin polarization. Subsequently, when the magnetic field is tuned to the compensation point, the spin polarization is brought out of the equilibrium conditions. This causes a practical issue for long measurement times. We report on a novel method for closed-loop control of the compensation field. This method allows optimization of the operating parameters, especially magnetic field gradients, in spite of the inherently slow (hours to days) dynamics of the system. With the optimization, higher stable nuclear polarization, longer relaxation times and stronger electron-nuclear coupling are achieved which is useful for nuclear-spin-based quantum memory, spin amplifiers and gyroscopes.

Furthermore I will discuss the results of a search for gradient coupled axion-like dark matter conducted with such a machine.   

Optical Atomic Comagnetometry for Tests of Fundamental Physics

Optical atomic magnetometry is widely used to search for physics beyond the Standard Model. However, uncontrollable magnetic fields can be a significant source of noise, reducing sensitivity and leading to systematic errors and false-positive signals. To mitigate these issues, comagnetometric methods have been implemented to limit the influence of magnetic fields and improve the sensitivity to nonmagnetic spin couplings.

The Global Network of Optical Magnetometers for Exotic physics searches (GNOME) is an international collaboration aiming to detect correlations in noise of magnetically-shielded optical spin sensors (e.g., optical magnetometers). The network particularly focuses on the detection of transient and oscillatory signals induced by ultralight bosonic dark matter. Although over the last years several measurement campaigns of the network were performed with optical atomic magnetometers, it has been speculated that further development of GNOME requires the construction and optimization of a sensor for the detection of transient and oscillatory non-magnetic signals.

We will report on our efforts in the development and characterization of a self-compensating atomic comagnetometer, a sensor first proposed by Michael Romalis and collaborators, which utilizes a mixture of alkali-metal vapors and a buffer gas. Implementation of these gases allows for efficient hybrid pumping of atoms and buffer gas, which, when combined with appropriate operating conditions, such as sample temperature, static magnetic field, and laser tuning, significantly enhances the sensitivity to non-exotic couplings by several orders of magnitude compared to traditional atomic magnetometers. Furthermore, we will present an analytical model that enables the calibration of the sensor for nonmagnetic couplings, based on studies of its magnetic response. Lastly, we will briefly discuss the operation of the sensor in the latest GNOME campaign.