Nat. Phys. (2019)

Long-distance fibre-based quantum communications systems tend to operate in the telecommunications wavelength band (1.5 μm) in the near-infrared (NIR). In contrast, quantum systems for performing local operations, such as storage and computation, make use of optical transitions in the visible or shorter NIR wavelengths from trapped ions or atoms, defect centres, quantum dots and spins in a crystal. To link these two systems together entangled photon pair sources that bridge the telecoms and visible bands are desired. Using a high-Q silicon nitride microresonator platform, Xiyuan Lu and co-workers from the USA have now demonstrated a system that generates narrow-band visible–telecom photon pairs with a coincidence-to-accidental ratio of up to 3,780 ± 140 at an on-chip pair flux of 1,200 ± 300 pairs per second, at sub-milliwatt pump power. Unlike existing visible–telecom pair sources, the system combines an on-chip solution with phase- and frequency-matching of narrow-band modes over a spectral separation of >250 THz. The team report time–energy entanglement of the pairs with a visibility of 82.7 ± 0.2%. When the entanglement was distributed over >20 km in optical fibre, a visibility of 58 ± 1% was measured.

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