  We present a scheme to implement a passive and deterministic
controlled-variable phase gate on photonic qubits encoded in the frequency
basis. Our gate employs a cascade system with the ground to first excited state
interacting with the control photon of a given polarization, and the first to
second excited state transition interacting with the target photon of the
orthogonal polarization. By controlling the relative detuning between the
target photon and the frequency of the transition between the first and second
excited states of the cascade emitter, we enable any controlled-phase operation
from 0 to $\pi$. This gate does not utilize any active control and needs only a
single cascade emitter, enabling low-footprint and more efficient decomposition
of quantum circuits, especially those rooted in the quantum Fourier transform.
