Direct Detection of the Axion Scalar Field Using Trapped Ion Quantum Sensor

The atomic valence electron of a trapped ion can couple to the axion field a = a₀ cos ωt via a derivative fermionic interaction equivalent to an oscillating, monochromatic scalar field Beff ∼ ∇a. The axion field oscillates at its Compton frequency ω proportional to the mass of the axion particle m_a, and the strength of the effective magnetic field is proportional to the tangential velocity v, of the Sun’s orbit about the galactic centre. Through this axion-electron spin coupling, the atom would experience a time-modulate phase oscillating at the Compton frequency. In this paper, we describe a sensing protocol that leverages techniques in quantum sensing to enhance the phase measurement sensitivity for a single trapped ion qubit to time-dependent phases. We experimentally demonstrate that our protocol improves the quantum Fischer information scaling with measurement time almost quadratically, F_ω ∝ T^1.75(3). Lastly, we present our plans to improve the protocol using entangled qubits and further enhance our sensitivity to time-dependent phases.