Quantum interference between photons of different color in electro-optic driven coupled microresonators

Two-photon interference is at the heart of many quantum information protocols. However, frequency mismatch between different single photon sources degrades the fidelity of the interference. We consider two approaches to bridge this gap, based on coherent electro-optic control of a photonic two-level system formed by coupled thin-film lithium niobate microring resonators. With appropriate balancing of the resonator coupling rates to the waveguide and the Rabi-like electro-optic coupling between the two resonance levels, both frequency shifting and frequency beam-splitting can be achieved using the same device. First, we operate the device as a frequency shifter and validate the preservation of quantum coherence by interfering signal and idler photons produced by nondegenerate spontaneous parametric downconversion at a passive beam splitter, after frequency-shifting into alignment (visibility >85%). Second, we present progress toward true 50:50 frequency beam splitting at a single photon level. In this scheme, signal and idler interfere directly in the frequency domain and produce frequency bunching. We achieve low parasitic loss rates and stable experimental operation after optimized adiabatic microring tapering and optical packaging. The device-level improvements achieved here may broaden the application space of time-frequency encoded quantum information, such as frequency-domain entanglement swapping.