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In the recent Xinhua press release China launches first-ever quantum communication satellite the last two sentences (which are two paragraphs) outline several distinct experiments planned for the Quantum Experiments at Space Scale (QUESS) satellite in conjunction with either one or two ground stations.

With the help of the new satellite, scientists will be able to test quantum key distribution between the satellite and ground stations, and conduct secure quantum communications between Beijing and Xinjiang's Urumqi.

QUESS, as planned, will also beam entangled photons to two earth stations, 1,200 kilometers apart, in a move to test quantum entanglement over a greater distance, as well as test quantum teleportation between a ground station in Ali, Tibet, and itself.

If I try to make an outline, I get the following.

  1. QKD between the satellite and ground stations
  2. secure quantum communications between Beijing and... Urumqi
  3. beam entangled photons to two earth stations (1,200km apart)
  4. test quantum teleportation between a ground station... and itself

Here is my current understanding of those:

  1. QUESS produces a pair of entangled photons, sends one photon to a ground station and simultaneously measures the other internally.
  2. Don't understand - QKD by #3, or something different? Uses second public path for the encrypted data? Since Beijing to the Xinjiang Observatory is over 2400km, I'm thinking Store and Repeat?
  3. QUESS produces a pair of entangled photons, sends both photons - each to different ground stations for simultaneous measurement.
  4. Not sure - is this more of a "science experiment" related to entanglement but not necessarily for secure information transfer?

I'm looking for an answer which helps me understand the most likely interpretation of the experiments mentioned in items 2 and 4 - a clarification of what the experiment is likely to be intended to do, not an explanation of the underlying quantum mechanics of entanglement.

Because of the mixture of publicly conducted science and government interests, the answer may need to contain a bit of speculation or 'most likely experimental scenario' from someone familliar with the field.

This helpful answer and the Nature article cited there has helpful information, and also goes on to mention some confusion on geographic sites in a different (New York Times) article. But I don't find an answer to my question there.

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  • $\begingroup$ #3 seems to distribute entangled pairs along a long distance for the verification of entanglement properties at a larger distance. Since with optic fibre or on-ground experiments, the entangled photons can only be distributed with a distance around hundreds km, here they can test 1200km. $\endgroup$ – XXDD Aug 26 '16 at 14:44
  • $\begingroup$ @X.Dong Thanks, I'm asking about 2 and 4. $\endgroup$ – uhoh Aug 26 '16 at 15:08
  • $\begingroup$ #2, I believe is essentially a QKD between the satellite and ground station. But I am not sure about how the QKD is achieved (for example by measuring distributed entangled pairs? but it seems this is not secure if somebody can capture/measure the photon before it reaches the ground station and then send a copy of the 'collapsed' state to the ground station). $\endgroup$ – XXDD Aug 28 '16 at 6:41
  • $\begingroup$ #4, it seems just a teleportation between the satellite and the ground station. I think on one hand, this is a scientific experiments to test the entanglement with a long distance. On the other hand, if it's stable and reliable, teleprotation can later be used to achieve secure communication(not QKD based), for example by teleporting 2 different quantum states to represent 0/1. Not sure if this is part of their plan. $\endgroup$ – XXDD Aug 28 '16 at 6:45
  • $\begingroup$ @X.Dong Since #1 is basically "QKD between the satellite and ground stations" I am guessing that #2 is not also " essentially a QKD between the satellite and ground station." Since it is already said to be between two ground stations. "secure quantum communications between Beijing and... Urumqi" $\endgroup$ – uhoh Aug 28 '16 at 10:31
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It's very hard to answer this question, because all details are missing - and the detailed description of the QKD protocol is necessary.

But here is my guess: The main goal of the satellite seems to be quantum key distribution between two ground stations. That's the obvious goal - that's where real world impact happens. Everything else is academic.

But how do you do that? Well, a first step is clearly establishing QKD between one ground station and the satellite. To do this, there are several possibilities, but all of them rely on entangled photons. Clearly, in order to do QKD between two stations, you'll therefore need:

beam entangled photons to two earth stations (1,200km apart)

We start with one station and the satellite. Now we want to do QKD. What this means is that you need to check whether the states are actually nearly maximally entangled. This can be done in several ways - one example would be a Bell test. If that doesn't work, you won't have QKD. In any case the second step after entangling photons is

QKD between the satellite and ground stations

is not precise enough to know what they attempt to do - we need the name of the protocol (I couldn't find it) or a description of what's going on.

Now we have QKD between the satellite and a ground station. If we want to extend this to QKD between two ground stations, there are also several possibilities. You could do QKD between ground station to satellite and then QKD between satellite and ground station again and do decoding and reencoding on the satellite. But that's not really good, because then the satellite actually has the decoded message, which you don't want, so what you really want is to use the satellite only as an intermediary to create entanglement between the two ground stations directly, so that they can generate the secret key directly.

In order to do this, the satellite will need to be a quantum repeater, which works by teleporting the qubits. So in order to implement a quantum repeater to implement QKD between two ground stations, we need to do teleportation. We can start by

test quantum teleportation between a ground station... and itself

Doing this with itself means that we can just work with one lab and not two for starters. But we'll need to extend it later.

So in the end, all of these are probably just toy experiments to implement certain steps of the real thing:

secure quantum communications between Beijing and... Urumqi

Saying "QKD between two ground stations via a satellite" comprises everything else discussed. The other "experiments" are just there to drop a few buzzwords and to show that the mission can still be a success if the main goal fails.

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  • $\begingroup$ This is a great help, thanks! Sorry for being redundant, but If the satellite generated entangled pairs and could somehow manage to simultaneously direct one to each of two ground stations, is that rudementary QKS between two ground stations ? (assuming the ground stations are suitably equipped) Could that by itself be called quantum teleportation ? $\endgroup$ – uhoh Aug 30 '16 at 11:39
  • $\begingroup$ No and no. Teleportation is not sending entangled pairs, it's sending another (unknown, that's the interesting part!) quantum state through an entangled state. What you need to do is to have an entangled state between the two labs on earth. If you want to send it through the satellite, this is a way to do it: Create an entangled pair between each lab and the satellite. Using the entagled pair between lab B and the satellite, teleport the state of the satellite from the pair between lab A and the satellite to lab B. $\endgroup$ – Martin Aug 30 '16 at 12:52
  • $\begingroup$ If you could to everything perfectly, this would mean that in the end the pair shared between A and the satellite is now shared between A and B and the satellite is out of the picture. Then you can start doing QKD. But that's exaclty where the problem is: There is not enough specification to see what's actually happening. $\endgroup$ – Martin Aug 30 '16 at 12:54
  • $\begingroup$ If you want to understand how any of the steps might work, I advise you to read about a) what entangled states are and how to create them, b) what teleportation is, c) what a quantum repeater is and d) what QKD is by reading an article about either the protocol BB84 or the protocol E91. You have to do it in that order to understand anything! $\endgroup$ – Martin Aug 30 '16 at 12:55
  • $\begingroup$ Thank you very much for the extra time to clarify! I think this is going to be a good answer for linking to as more questions appear. $\endgroup$ – uhoh Aug 30 '16 at 13:13

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