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Two days ago, China launched the world's first large scale quantum communication satellite Micius, ready to test quantum teleportation(QT) and key distribution(QKD) between space and ground, separated by over 400 km.

We know that QT and QKD have been tested on earth for similar length scale.

For example, see a paper in 2015 by Austrian and American physicists that tests QKD over 307 km of optical fiber: http://www.nature.com/nphoton/journal/v9/n3/full/nphoton.2014.327.html#affil-auth

And another paper in 2012 that tests QT over 143 km in free space: http://www.nature.com/nature/journal/v489/n7415/full/nature11472.html#affil-auth

So my question is: what changes when we do quantum communication from earth to space? Can fellow users of physics SE generate a list of possible problems that might disrupt the implementation of this new technology?

Zhengyan Shi
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    Related to http://physics.stackexchange.com/questions/274751/how-is-chinas-quantum-satellite-going-to-send-information-through-entangled-p? But it also seems to be a speculative big-list type of questions, and in my view probably off-topic. – dmckee --- ex-moderator kitten Aug 18 '16 at 02:02
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    For me it's rather an engineering oriented project. The key point here is not if the KD/QT can be achieved, but to make them reliable/robust in an application oriented scenario. For example, the accurate aiming between sender/receiver, robustness against different weather conditions, etc. – XXDD Aug 18 '16 at 05:24

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The short theoretical physics answer is: Nothing much changes, it actually gets easier. The short experimentalist answer is probably: you can't manipulate the experiment as much as you might like to after the launch, but all of this is more of an engineering problem.

For the physics: Communication through space is easier than through air on earth, because space is mostly empty. This means there is much less interference from the environment enhancing decoherence times. Since the atmosphere gets thin really quickly compared to the scales we are talking about, it is easier to do long-distance space-based communication than long-distance commmunication through air.

From the engineering side, there are probably many potential problems, most of which I won't know as a theorist. Probably the biggest change is of course that one half of the aparatus is not that easy to manipulate. In most of the earth-based exeriments, both sending and receiving parties were some labs where physicists could easily access all parts of the experiment and take them apart if needed. Obviously, the manipulation of the satellite is more limited, so you have to do a better engineering job in advance. This is a purely engineering problem.

The satellite will not be in a geostationary orbit, hence it's a moving target. However, QKD with moving receivers has already been demonstrated in other experiments (see for instance http://arxiv.org/abs/1505.00292), so that's not a new thing.

Martin
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