Narrowband photonic quantum entanglement with counterpropagating domain engineering

Narrowband photonic entanglement is a crucial resource for long-distance quantum communication and quantum information processing, including quantum memories. We demonstrate the first polarization entanglement with 7.1 GHz inherent bandwidth by counterpropagating domain engineering, which is also confirmed by Hong–Ou–Mandel interference with 155-ps base-to-base dip width and ( 97.1 ± 0.59 ) % high visibility. The entanglement is harnessed with 18.5-standard-deviations Bell inequality violation, and further characterized with state tomography of ( 95.71 ± 0.61 ) % fidelity. Such narrowband entanglement sets a cornerstone for practical quantum information applications.