Metro-scale QKD using multimode fiber

We report a proof-of-principle realization of a decoy-state BB84 quantum key distribution (QKD) protocol with phase encoding over a record-breaking 17 km of multimode fiber (MMF) at a rate of 193 kbits/s, as well as over 1 Mbit/s at a distance of 1 km. These results suggest that QKD can be deployed over MMF in metropolitan-scale telecommunication connections. Such MMF metropolitan networks are ubiquitous—thus, this advance could pave the way to wide scale metropolitan deployment. We also assess the advantages of adapting the OM3 channel using mode-matching photonic lanterns on the quantum bit error rate, signal gain, and key rate and compare different encoding techniques in light of MMF propagation effects. This work confirms the suitability of current QKD technology for use in existing MMF links, unlocking new opportunities for quantum applications using legacy fiber.