Sending entangled photons to opposite sides of the planet will require a small fleet of orbiting satellites, say physicists. |
As reported by MIT Technology Review: 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: arxiv.org/abs/1410.5384 : Entanglement Over Global Distances via Quantum Repeaters with Satellite Links
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