Quantum Differential Decoherence Spectroscopy: Quantitative Limits on Systematic Suppression in Solid-State Quantum Sensors

The characterization of systematic decoherence backgrounds in solid-state quantum sensors constitutes a fundamental metrological

challenge: model uncertainties on conventional decoherence sources (δΓ_model ∼ 2–5 Hz) exceed by orders of magnitude the

precision required for emerging applications in quantum sensing, including proposals for ultralight dark matter detection where signal

rates ΓDM ≲ 10⁻⁶ Hz demand fractional precisions of 10⁻⁶ to 10⁻¹¹. This work introduces Quantum Differential Decoherence

Spectroscopy (QDIFF), a measurement methodology employing paired nitrogen-vacancy (NV) center diamond sensors that differ only

in a controlled variable—primarily the crystallographic orientation of the NV axis—while sharing an identical cryogenic environment

(T < 20 mK). By subtracting simultaneous decoherence rate measurements, QDIFF cancels common-mode systematics without

requiring explicit background modeling. A rigorous quantitative analysis of residual noise channels—including thermal fluctuations,

surface two-level systems (TLS), electromagnetic interference, measurement back-action, cross-correlations, and the systematic

uncertainty of the additive Lindblad decomposition itself (α ≈ 0.15–0.30)—demonstrates improvement factors of 3–12× over absolute

single-sensor measurements, depending on the dominant noise channel and fabrication scenario. The resulting residual floor

(δΓ_residual ≈ 0.42–7.6 Hz) establishes QDIFF as a metrological tool for quantifying the irreducible systematic uncertainty floor in

differential quantum sensors, while honestly acknowledging that direct competition with dedicated ultralight dark matter detectors

(CASPEr, ABRACADABRA) remains out of reach. The explicit propagation of model-framework uncertainty constitutes a

methodological standard for future proposals in differential quantum sensing.