In an article recently published in the journal Nature Communications, researchers performed the very first search for dark photons. They utilized a sophisticated long-baseline quantum sensor network comprising 15 atomic magnetometers functioning as a dark matter haloscope.
a Distribution of the signal-to-noise ratio of the network-averaged cross-correlation spectrum. The 95% confidence level (C.L.) is determined with Monte Carlo simulation. b Limits on kinetic mixing ϵ of dark photon dark matter in the mass range from 4.1 feV to 2.1 peV. The blue-shaded region is excluded from our network measurement at 95% C.L. The black line shows our future projection of an upgraded magnetometer network. The light-blue shaded regions show other constraints derived from terrestrial or extra-terrestrial experiments, including observing the magnetic fields of Earth and Jupiter, measurements with a network of magnetometers in quiet magnetic environments (SNIPE), and Cavendish-Coulomb experiments. The dashed lines show the limits from cosmological and astrophysical observations, including cosmic microwave background (CMB) photon’s transition to dark photon, and DPDM heating the plasma. Image Credit: https://www.nature.com/articles/s41467-024-47566-0
Background
Despite decades of astrophysical evidence supporting the existence of dark matter, directly detecting its non-gravitational interactions with standard model (SM) fields and particles has proven challenging. Numerous theories have emerged in attempts to solve this mystery, with many proposing the existence of new fundamental particles beyond the SM that could account for dark matter.
Among these proposed particles, ultralight bosons such as dark photons and axions have emerged as particularly promising candidates for dark matter. These bosons were initially predicted within theoretical frameworks, positing the existence of extra dimensions. Over the years, several experiments have been conducted in pursuit of detecting ultralight bosonic dark matter.
Previous Axion and Dark Photon Searches
Previous searches have mostly focused on axion-like particles and axions using inverse Primakoff effects, where these particles turn into photons in the presence of a powerful magnetic field. However, kinetically mixed dark photon dark matter (DPDM) searches do not rely on electromagnetic fields as DPDM generates magnetic fields/resonant cavity modes via effective oscillating currents in an electromagnetically shielded room.
Recently, different strategies, including radio telescopes, atomic spectroscopy, geomagnetic fields, and resonant LC circuits, have been employed to search for DPDM. Additionally, several experiments, like QUALIPHIDE, SuperMAG, and FUNK, have already established experimental constraints on DPDM.
Yet, mass constraints on DPDM below neV primarily depend on cosmological and astrophysical observations, like cosmic microwave background (CMB) distortion and heating up of plasma anomalously, which are reliant on astrophysical modeling.
The Study
In this study, researchers performed the first search for dark photons/correlated dark-photon signals by employing a long-baseline quantum sensor network of 15 atomic magnetometers/synchronized atomic magnetometer arrays in large shields (AMAILS) consisting of 15 atomic magnetometers. These magnetometers were situated in two separate meter-scale electromagnetically shielded rooms in Suzhou and Harbin, China, with a distance of approximately 1700 km between the locations, and were synchronized using the Global Positioning System (GPS).
A standard halo model was assumed for the momentum distribution of DPDM that forms after virialization. Both rooms were built using five-layer mu-metal, with their innermost layer being cuboid-shaped. The dimensions of the innermost layer were 2 × 2 × 2 m³. Meter-scale electromagnetic shields that can convert magnetic fields within their confines were used to detect the effective current-generated electromagnetic signal.
Atomic magnetometers, which were recognized as a quantum sensor type that exploits the spin-exchange relaxation-free effect to improve measurement sensitivity and atomic coherence time, were employed as DPDM detectors. Although one atomic magnetometer can theoretically detect the radio signal from DPDM, distinguishing the DPDM signal from several noise sources under realistic experimental conditions can be challenging.
Thus, researchers monitored several magnetometers simultaneously to address this problem, with two magnetometers installed in the Harbin station and 13 in the Suzhou station for extracting potential events from their correlated signals. They calculated the cross-correlation spectra for each magnetometer pair. Specifically, the cross-correlations between magnetometers in separate shielded rooms or the same shielded room were calculated.
Previous search experiments/searches on wave-like dark matter were primarily limited to searches for local signals with one detector, where confidently distinguishing dark matter signals from local technical noises was challenging. Recently, networked quantum sensors, including networks of optical and atomic clocks and a global network of optical magnetometers (GNOME), have been used for dark matter searches.
These networks consider situations where dark-matter particles interact among themselves and produce topological defects instead of wave-like dark matter. This study also performed dark matter investigations using specialized quantum magnetometers. These instruments offered enhanced potential for advancements and the integration of innovative features in a distinct mass range.
Significance of the Study
By correlating the separated magnetometers' readouts in the network, researchers successfully demonstrated the first long-baseline quantum sensor network searching for DPDM-correlated signals over a distance of 1700 km. The shields' large size and the network's multiple sensors significantly enhanced the expected dark-photon electromagnetic signals, while the long-baseline measurements effectively reduced several local noise sources.
Furthermore, the magnetometers exhibited exceptional sensitivity at the femtotesla level, enabling the detection of the magnetic field radiated by the DPDM in close proximity to the shield room's walls.
Through this network, researchers were able to narrow down the parameter space describing the kinetic mixing of dark photons, or the kinetic mixing coefficient of DPDM, across a mass range spanning from 4.1 feV to 2.1 peV.
These constraints surpassed those achieved by current terrestrial DPDM searches and stand as the most stringent limits obtained from any terrestrial experiment within this mass range. In essence, this study presents promising avenues for exploring wave-like dark matter using long-baseline quantum sensors.
Journal Reference
Jiang, M., Hong, T., Hu, D., Chen, Y., Yang, F., Hu, T., Yang, X., Shu, J., Zhao, Y., Peng, X., Du, J. (2024). Long-baseline quantum sensor network as dark matter haloscope. Nature Communications, 15(1), 1-7. https://doi.org/10.1038/s41467-024-47566-0, https://www.nature.com/articles/s41467-024-47566-0
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