Self-compensating alkali–noble-gas comagnetometers are the most sensitive laboratory probes of exotic spin couplings, but their quantum-noise limit, including the backaction of the optical readout, has not been established. We derive it from an input-output model of the Faraday readout coupled to the linearized alkali–noble-gas dynamics, which reduces to a closed two-quadrature response and resolves the exotic-field spectral density into spin-projection, imprecision, and backaction terms. The central result is that the slow-mode cancellation which rejects magnetic fields also shields the measured alkali quadrature from the probe backaction. The backaction enters the unmeasured quadrature and reaches the readout only through the effective alkali precession, which the compensation condition sets to zero. Backaction therefore limits the device only between a knee of a few hertz and the compensation resonance, while in the subhertz dark-matter band the standard quantum limit lies within a few percent of the projection-noise floor. There, squeezed probe light helps only optically thin cells and a prepared spin-squeezed state is a transient resource; lowering the floor requires a continuously squeezed alkali reservoir, to which the compensated readout is well suited.

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