Einstein–Quantum Resonance Spacetime Extension: A Phenomenological Frequency-Dependent Modification of the Einstein Field Equations Description: This work introduces the Einstein–Quantum Resonance Spacetime Extension (EQRSE), a phenomenological, frequency-dependent modification of the Einstein field equations. The model incorporates a weak Lorentzian resonance factor that couples microscopic oscillatory modes to macroscopic spacetime curvature. A full covariant formulation is provided, together with resonance profiles, modified stress–energy coupling, and implications for gravitational-wave propagation. We present line-excluded PSD analyses using publicly available LIGO, Virgo, and KAGRA data. No statistically significant broadband excess is observed near , but the data remain fully consistent with a weak resonance whose amplitude lies below current detector sensitivity. The model is therefore not excluded; instead, this work provides the first observational upper bounds on the coupling strength. The manuscript includes theoretical development, observational constraints, and appendices with the variation of the action, field equations, and resonance functions. Current interferometric detectors (LIGO, Virgo, KAGRA) cannot conclusively test the predicted resonance: the relevant frequency band overlaps with instrument-induced mechanical modes. A reliable verification requires optical atomic clocks (Sr, Yb⁺, Al⁺), whose 10⁻¹⁸–10⁻¹⁹ stability can detect the predicted small frequency modulations. Keywords: General Relativity, Modified Gravity, Quantum Resonance, Spacetime Curvature, Gravitational Waves, Phenomenological Models, Frequency-Domain Gravity