I wrote:
Optical repeaters have to pass your signal through an intermediate electronic stage anyway, since we have no purely optical valve/transistor equivalents (bosons don't interact with each other at all).
Dave Emery responds:
This is not true. There is now a whole technology of optical amplifiers for fiber communications systems that used Ettrium doped fibers pumped with strong light from a laser at a slightly shorter wavelength. These fiber optical amplifiers have gains in the order of 10-12 db in a section of special doped fiber only about 10 feet long.
You're right, I do remember reading about these somewhere... didn't realize they were already in use. Even so, I still don't think such a repeater would pass quantum-crypto signals, excepting any photons that happened to just "leak" directly through. Your useful quantum state information resides in the individual photons originally sent, and any even the optical repeaters you describe achieve gain by by gating in *more* photons under the incoming signal's control. In so doing it will collapse the wavefunctions of these incoming photons. Not to say repeaters on the line aren't possible, but they'll have to decode your data using a copy of the "secret" key, then re-encode for transmission... so this will be a potential break-in point and need good physical security.
The current generation of undersea cables from the US to Europe use these amplifiers instead of the more traditional regenerating repeaters that convert the light to electronic signals, reclock the data stream and convert it back to light with another laser diode. There is no conversion from light to digital electronic signals all the way from Rhode Island to England - the same light pulses that go into the fiber on one side of the Atlantic come out on the other end without ever having been converted to electronic form in between.
You said power for the amps comes from a high-intensity, shorter-wavelength beam... can this be superimposed on the original signal at the point of origin, as with in-line coax-cable amplifiers?
These amplfiers have enourmous bandwidth, and can be used to amplify several slightly different wavelengths of light allowing wavelength division multiplexing of multiple streams of light flashes of slightly different "colors" (all the current technology works at around 1500 nm which is well into the infrared). This can expand the capacity of a single fiber to four to six times the 5 Gb/sec that is the current state of the art.
Nice... :) - alex