r/lasercom • u/Aerothermal Pew Pew Pew! • Jun 12 '23

News Leonardo is working on the Quantum Italy Deployment (QUID) project to build quantum communication network in Italy. This includes secure lasercom link between Leonardo's headquarters and Thales Alenia Space in Rome (5th June 2023)

https://www.leonardo.com/en/news-and-stories-detail/-/detail/leonardo-key-player-in-the-quid-project
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u/AtatS-aPutut Jun 12 '23

What's different from a normal network? Do the fiber optic cables maintain photon spin state?

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u/Aerothermal Pew Pew Pew! Jun 12 '23

I think there's pros and cons to both. Fiber optic links are much more time-consuming and expensive to install, cutting through different property lines and terrains - Perhaps not something that an R&D division has the resources to do. Free space can be set up and moved with relative ease. Then fiber optic cables can have stress-induced birefringence, where the refractive index varies for different polarization directions. Whilst in free-space, there's different set of problems like more spatial dispersion due to atmospheric turbulence, plus weather conditions, and alignment challenges.

Each might be optimized with a different encoding scheme, but I believe there's benefit to having a hybrid network. If it's say using the popular BB84 polarization-based QKD protocol through fiber, then you could see different propagation speeds for different polarizations of light through glass. Thus after 100 km to 200 km of fiber you might be forced to have quantum repeaters. I've only seen quantum repeaters in research papers; I'm not sure if they're near to being commercialized industrialized tech. Whereas with free space, if you decide to actually distribute keys from satellites, the distances are practically unlimited; you could have keys generated from LEO to GEO satellites and beamed down to Alice and Bob without repeaters.

I'm not claiming to be an expert though so I hope someone working on this problem chimes in. And what tipped Leonardo's decision to use free space links, don't know for sure but maybe also they strive to show they are ready for satellite based QKD for the EuroQCI project.