Modulator-free frequency-bin entanglement certification over multimode channels

We demonstrate a novel, passive method for processing frequency-bin encoded quantum information using linear interferometry and time-resolved detection, eliminating the need for electro-optic modulation or nonlinear optics. This approach is compatible with multimode propagation, a crucial step for free-space quantum communication, and favorable for size, weight, and power (SWaP) requirements. Using a high-brightness multi-resonator source, we generate frequency-bin entangled photons and show the ability to perform arbitrary projective measurements in both single- and multi-mode propagation scenarios. For the first time, we measure the joint temporal amplitude of the generated state, directly linking its entangled nature to the temporal oscillation of non-local correlations. We leverage this to certify entanglement with a Bell curve visibility exceeding 95%. Our technique enables full quantum state tomography, yielding a state fidelity of up to 91% for a maximally entangled state. This work paves the way for robust entanglement deployment over free-space and satellite-based links.