Quantum teleportation starts out with an EPR pair being distributed among Alice and Bob i.e. one qubit goes to Alice and the other goes to Bob. The problem that I see in this scenario is the state of the EPR pair being maintained over long distances over an indefinite period of time i.e. the quantum system consisting of the pair of qubits continues to remain in the same state( the initial Bell state ).

My question is

Is there technology currently available to be able to do this? If so, what is it and how does it work? If no, any contenders for this job?


I'm not an expert (mathematical physicist here), but as far as I know, the answer is no over really long distances and yes over shorter distances.

There are inherently two problems that your question calls attention to:

  • entangling particles over long distance
  • maintaining this entanglement over longer time

You only address the second one, but the first is actually as much of a problem, so let me also address it!

The first question is addressed with the help of so-called quantum repeaters. To this extend, especially because you ask about technology (not just proof-of-principle experiments), you should check out the work of Nicolas Gisin. He also cofounded a company for quantum communication technology, IDquantique. Probably, other people are doing similar work, but I only know his work from a talk he gave a few months ago.

In this talk, Gisin said that entanglement generation to almost arbitrary fidelity is possible and has been demonstrated up to about 400km - for some (decoherence?) reason, there seems to be a barrier at about that point for the moment. Current technology seems to exist for quantum key-distribution through particular fibre networks for up to 80km (see this page - obviously details about the implementation are not given, but it should be based on quantum teleportation). Here is an older paper demonstrating teleportation over current telecommunication technology: Quantum teleportation over the Swisscom telecommunication network.

How to overcome the 400km distance barrier? This leads us to the question of how to store entanglement. This is what people call a quantum memory. Currently, there are no good quantum memories available, as far as I know. You can find an overview about current approaches for photonic memories here (open access as far as I know). I cannot summarize all the different approaches here, also, because I don't understand most of the words in the abstract. One very promising way seem to be rare-earth quantum memories. An overview can be found in this doctoral thesis Rare-earth quantum memories for single photons and entanglement from Imam Usmani of the group of Nicolas Gisin. The idea seems to be to use carefully doped rare-earth systems that are cooled down. The system is then sometimes called a "frozen gas" and because the doped atoms are trapped and interact only weakly with the environment, they are useful for storage. I imagine that one will need quantum error correction to assist the memory. Here is one paper using this approach for a quantum network (again, work by Gisin et al.).

Once again, many other ideas seem to be floating around. The fact that there are so many ideas indicates that no "best contender" with advanced technology is available as of now.

Finally, let me come back to the question of establishing long-distance quantum teleportation, because I wanted to link to this paper, which I found really funny: High-spped quantum networking by ship


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