Coherent collective response in many-qubit systems for dark matter detection

We propose an array of Ramsey-type interferometers using N superposition states, ( 0 + 1)⊗ N, as a sensor to detect wave-like dark matter. After exposure to the dark matter wave, which induces coherent qubit transitions, the signal is the imbalance between the numbers of 0 and 1 outcomes. The signal-to-noise ratio in this scheme is proportional to N α², where $α$ is the coupling of dark matter to the qubits, and thus the sensitivity to the coupling scales as δα~ 1 / √N. For comparison, in the detection scheme based on the Rabi-type transition, 0 → 1, this scaling is achieved only when N highly entangled qubits are used. Since the Ramsey-type measurement does not require entangled states, one can consider much larger N by simply placing a large number of qubits within the de Broglie wavelength of the dark matter. We demonstrate that, using trapped-ion qubits in linear Paul traps as the sensor, the projected sensitivity to the coupling matches or surpasses existing laboratory, astrophysical, and cosmological bounds for N 10⁶-10⁸. We also evaluate its sensitivity to high-frequency gravitational waves. Our general framework should, in principle, be useful for other quantum sensing platforms.