One of the challenges that physicists face in creating a quantum Internet is to distribute entangled photons around the planet. The idea is that a user in Tokyo could use this entanglement to send a perfectly secure message to somebody in Moscow or Johannesburg or New York.
The problem is that entangled photons are difficult to send over these distances because optical fibers absorb then. This process of absorption limits the distance that physicists can distribute entanglement to about 100 kilometers.
One solution is to place quantum repeaters along a fiber that pass on the entanglement without destroying it. Physicists are currently developing these kinds of devices and expect to have them operating in the next few years.
However, quantum repeaters will operate at temperatures close to absolute zero and require their own power and cooling infrastructure. That is all possible on land but is much harder to make work for transoceanic cables. Which is why physicists are looking for alternative ways to distribute entanglement over long distances.
Today, Kristine Boone at the University of Calgary in Canada and a few pals outline a plan to distribute entanglement around the planet from satellites orbiting a couple of hundred kilometers above the Earth. “Our proposed scheme relies on realistic advances in quantum memories and quantum non-demolition measurements and only requires a moderate number of satellites equipped with a tangled photon pair sources,” they say.
One feature of quantum technology is that it is rapidly changing as advances are made in laboratories all over the planet. But any technology aboard a satellite cannot be changed once it is launched. So a potential danger with a satellite-based network is that it would be unable to take advantage of important advances.
Boone and co get around this by keeping much of the most advanced technology on the ground. Their proposed satellites will be little more than vehicles for producing entangled photons, a process that is relatively well understood and straightforward to achieve.
Each satellite will generate a constant stream of entangled pairs. Each member of the pair will be sent to separate stations on the ground, where it will be stored in quantum memories. In this way, the satellites will entangle quantum memories across the globe.
The ground stations will consist of relatively small one-meter telescopes, aimed at the satellites as they pass overhead. These will collect photons and direct them towards quantum memories. It is the quantum memories that are likely to advance rapidly in the coming years.
Once the entanglement is stored on the ground, it can then be used as needed to send secure messages, or even sent locally across the quantum Internet using short optical fibers.
Simon and co perform various calculations to show that their proposal is well founded. “We have argued that quantum repeaters based on LEO satellite links are a viable approach to global quantum communication,” they say.
An interesting question is whether the system they propose would be better than the one we discussed last week in which entanglement is transported around the world in quantum memories on containerships. At first glance, that seems to have the potential to be cheaper given that the transport infrastructure is already in place and known to be cost-effective. By contrast, rocket launches, and the satellites they carry, are hugely expensive.
One thing is clear. Entanglement is set to become a valuable resource that is likely to be bought and sold, much like oil and gas today. Just how the incipient market for entanglement emerges will be interesting to watch.
Ref: http://arxiv.org/abs/1410.5384 : Entanglement Over Global Distances via Quantum Repeaters with Satellite Links
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