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Comment by binarymax | Hacker News Reader

Tens of thousands of kilometers sounds good. But millions of kilometers will be a huge challenge. The advantage of the laser is its pinpoint precision. But when you get to the distance where the speed of light is not fast enough, and the receiving end has moved, you have an interesting problem. You will need to point the laser to where the receiver will be. Also, as you start thinking about even further distances and gravity 'refracting' light it gets complicated very quickly.

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It isn't that pinpointed, calculating and tracking the position of something in our solar system relative to earth to a few (hundred) thousand square kilometers precision should be little problem.

"For example, a typical Ka-band signal from Mars spreads so wide that when it reaches Earth the diameter of the energy is many times larger than the diameter of the Earth. A typical optical signal, however, will spread only over the equivalent of a small portion of the United States, requiring less energy use and waste."

From http://www.nasa.gov/pdf/742122main_LCRDFactSheet3.pdf

Interesting. That would increase the need for more terrestrial transceivers or satellite transceivers to maintain active communication as objects spin.

They're already working on quantum communications between Earth and satellites, too, but it's obviously still very early days. But it will probably be what we'll use for very large distances in the future.

http://www.extremetech.com/extreme/152906-european-scientist...

http://spectrum.ieee.org/tech-talk/aerospace/satellites/chin...

As I understand it, this is about "secure" communications and not some magical new way of information transmission. These "quantum communications" would still require a standard channel to communicate over.... which would likely be laser/radio as per normal.

Do you need that for quantum entanglement communication, too?

From various discussions I've had with physicists on the matter I found the answer is yes.

My thoughts were summed up by having a device here on earth that is entangled with a device just above the surface of the sun. Now imagine the sun vanishes and the devices communicate this information over the entanglement, thus (to my mind) breaking the "light barrier" as we receive an 8 minute warning that the lights have gone out. I don't fully remember (or understand!) the answer I got, but it most definitely had a reliance upon a relativistic communication channel.

[also: what H3g3m0n said - entanglement is not about the transfer of information]

'Quantum communications' isn't a real thing outside of scifi (and generally the crappier kind).

It's sometimes (inaccurately) used to describe quantum entanglement (or quantum encryption) but it doesn't actually allow for the transfer of information.

It can be used for quantum encryption but the actual information transfer has to take place over a traditional medium. When you hear those stories about quantum teleportation and such that is generally what they are talking about.

Compensating for the movement in the way you mention it shouldn't be too hard I guess.

I'm curious if they will need to compensate for the speed differential between the source and the target. Can anyone explain if the following will cause any problems for this technology? If the source and target move away from eachother quickly, the target will observe an increased wavelength, and vice versa. Apparently this is called Redshift [http://en.wikipedia.org/wiki/Redshift] and technically there is some similarity to the Doppler effect.

Would that matter if the information weren't encoded using the wavelength? Eg, imagine that they toggle the light very quickly, Morse-code style, to encode messages. If the frequency changes slightly, no big deal.

EDIT: I suppose this effect could make the speed of the toggling appear faster or slower, but that could be adjusted for.

The Doppler effect will be present, but will not cause any problems that aren't already present. It exists in current radio communications - light is no different in that regard as it's all just electromagnetic waves.

The Doppler effect (of which redshift is a special case) already has to be taken into account with radio communications (which is EM radiation just like light). In fact, all data from the Huygens Titan lander was almost lost because the software onboard the Cassini probe, on which Huygens piggybacked, couldn't have locked onto the lander's carrier wave due to too steep a Doppler shift. This was fixed by altering the Titan approach trajectory so that the Huygens' relative velocity didn't grow too high.

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