Quantum interference and correlation control of frequency-bin qubits

We implement distinct quantum gates in parallel on two entangled frequency-bin qubits for the first time. Our basic quantum operation controls the spectral overlap between adjacent frequency bins, allowing us to observe frequency-bin Hong-Ou-Mandel interference with a visibility of 0.971±0.007, a record for two photons of different colors. By integrating this tunability with frequency parallelization, we synthesize independent gates on entangled qubits in the same optical fiber and flip their spectral correlations. The ultralow noise of our gates preserves entanglement purity, as evidenced by a 9.8σ violation of an entropic separability bound. Our results constitute the first closed, user-defined gates on frequency-bin qubits in parallel, with application to the development of frequency-based quantum information processing.