Ordinary optical fiber is not just a glass thread for the internet. In quantum networks, it could become a channel for transmitting the elusive connection between particles known as entanglement. But to create it, you need a source of photon pairs born in a single event and matched in polarization. The authors of the new work placed a crystalline film of molybdenum disulfide (3R-MoS₂) directly between two standard fiber-optic connectors. When a laser pump beam is sent through this structure, spontaneous parametric down-conversion occurs in the film: one pump photon splits into two entangled photons of lower energy, which immediately travel into the fiber.

The key trick lies in the crystal's symmetry. 3R-MoS₂ has preferred directions along which pairs with specific polarization are born. By changing the pump laser's polarization, one can switch between different quantum states of the pair. This allows obtaining two of the four maximally entangled Bell states, as well as fully co-polarized pairs. It resembles tuning a radio: instead of turning a frequency knob, the experimenter rotates the pump polarization, and the desired "channel" of entanglement appears at the output.

Previously, such tuning required bulky setups with additional lenses, crystals, and precise alignment. Here, the entire source fits within a standard fiber-optic connector. This is an important step toward making quantum communication as compact and convenient as conventional laser modules.