Off the top of my head, I foresee two problems with this:
1. It’s a big (10+ cm diameter) evacuated, inflexible tube. Not fun to install over long distances.
2. It’s big — many many wavelengths wide. It’s surely very very far from single-mode. For classical fiber, AIUI this mostly matters because the modes propagate at slightly different speed, thus spreading a signal in time and reducing useful bandwidth. For quantum communication, if your input modes get scrambled on the way through, you need a way to unscramble them — you can’t just add up the power in each mode.
3. Again with modes (and I’m not an optics expert at all — I could be wrong here), the paper very briefly mentions the fundamental Gaussian mode and gives the tiniest bit of attention to diffraction loss. But I would expect some loss due to the fundamental mode losing some light into other modes when it gets to the next mirror, and that light getting further diffracted. ISTM this should depend on aberration, and there was no discussion of lens design to minimize this.
1. It’s a big (10+ cm diameter) evacuated, inflexible tube. Not fun to install over long distances.
2. It’s big — many many wavelengths wide. It’s surely very very far from single-mode. For classical fiber, AIUI this mostly matters because the modes propagate at slightly different speed, thus spreading a signal in time and reducing useful bandwidth. For quantum communication, if your input modes get scrambled on the way through, you need a way to unscramble them — you can’t just add up the power in each mode.
3. Again with modes (and I’m not an optics expert at all — I could be wrong here), the paper very briefly mentions the fundamental Gaussian mode and gives the tiniest bit of attention to diffraction loss. But I would expect some loss due to the fundamental mode losing some light into other modes when it gets to the next mirror, and that light getting further diffracted. ISTM this should depend on aberration, and there was no discussion of lens design to minimize this.