A 20-meter fiber keeps photons entangled for complex operations

Researchers have demonstrated programmable quantum operations using a 20-meter multimode fiber, a distance previously limiting for maintaining quantum coherence. The work overcomes challenges posed by modal dispersion by harnessing ultra-long coherence from a warm 87Rb atomic ensemble to generate heralded single photons. The experimental setup relies on three key stages, beginning with the generation of heralded single photons via spontaneous four-wave mixing (SFWM) within the 87Rb atomic vapor cell, a process where pump and coupling lasers excite atoms to produce correlated photon pairs. These photons are then spatially filtered and coupled into single-mode fibers, forming the foundation for quantum information processing. The coincidence peak width, a measure of the photons’ temporal correlation, remains stable even after traveling through the 20-meter fiber, indicating minimal broadening and confirming the preservation of temporal coherence. This observation directly addresses a major obstacle in long-distance quantum communication: maintaining the delicate quantum state of photons over significant distances. Digital optical phase conjugation (DOPC)…