  The advancement of Rydberg atoms in quantum sensing is driving a paradigm
shift from classical receivers to atomic receivers. Capitalizing on the extreme
sensitivity of Rydberg atoms to external disturbance, atomic receivers can
measure radio-waves more precisely than classical receivers to support
high-performance wireless communication and sensing. Although the atomic
receiver is developing rapidly in quantum-sensing domain, its integration with
wireless communications is still at a nascent stage. Particularly, systematic
methods to enhance communication performance through this integration are
largely uncharted. Motivated by this observation, we propose to incorporate
atomic receivers into multiple-input-multiple-output (MIMO) communication to
implement atomic-MIMO receivers. Specifically, we establish the framework of
atomic-MIMO receivers exploiting the principle of quantum sensing, and reveal
that its signal detection is intrinsically a non-linear biased phase-retrieval
(PR) problem, as opposed to the linear model in classical MIMO systems. To this
end, we modify the Gerchberg-Saxton (GS) algorithm, a typical PR solver, with a
biased GS algorithm to solve the discovered biased PR problem. Moreover, we
propose an Expectation-Maximization-GS (EM-GS) algorithm by introducing a
high-pass filter constructed by Bessel functions into the iteration of GS,
which improves the detection accuracy efficiently. Finally, the effectiveness
of atomic MIMO receivers is demonstrated by theoretical analysis and numerical
simulation.
