We propose a unified physical principle for entanglement harvesting: the entanglement that two localized detectors can extract from a quantum field is determined solely by how localized the field's effective spectral density is. We demonstrate this in an analytically solvable model of two qubits coupled to a leaky single-mode cavity, which in turn couples to a continuous electromagnetic bath, and derive the maximum harvestable concurrence in closed form, Cmax(Q)=2e−π/(2Q)(1+e−π/(2Q))/(1+3e−π/Q)
Nearby in the stack
, where
Q≡∣Δ∣/κ
is the ratio of the qubit-cavity detuning
Δ
to the cavity linewidth
κ
. In the high-
Q
limit,
Cmax≃1−π2/(16Q2)
, so the entanglement is robust against cavity loss; in the low-
Q
limit it decays exponentially to zero, consistent with the irreversible-reservoir character of a continuous field, where maximal entanglement is unattainable. Since
Q
is proportional to the inverse participation ratio (IPR) of the effective spectral density, it is the single dimensionless parameter governing the crossover from deterministic gate-based entanglement (
Q→∞
) to vacuum harvesting (
Q→0
). Our framework operationalizes the Reeh-Schlieder theorem by quantifying the fraction of vacuum correlations accessible to localized detectors. It also reveals a formal correspondence of the maximal concurrence with the IPR, analogous to the conductivity-participation-ratio relation in Anderson localization. The predicted
Cmax(Q)
curve is, in principle, directly observable in superconducting circuit QED experiments.