Beyond QKD, which types of quantum entanglement experiments will QUESS likely facilitate?

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.

Answer

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.