Search for Millicharged Dark Matter with Trapped-Ion Quantum Processor*

Fractionally charged particles, including millicharged dark matter, are well-motivated extensions to the Standard Model. We present a search for heavy millicharged relics using the observed motion of a chain of trapped ions as a detector. Our method surpasses the alternative performance of room temperature single-ion detectors by several orders of magnitude by mapping the motion of multiple ions to individual spins using quantum information protocols to reduce systematic effects. Trapped ions offer a unique advantage with their low energy threshold, allowing detection of extremely cold millicharged particles that have thermalized with the Earth and accumulated over billions of years. This enables exploration of regions in the mass-charge parameter space that would otherwise be inaccessible.