Physicists at the University of Vienna and the Austrian Academy of Sciences have successfully transmitted quantum states between La Palma (from the Jacobus Kapteyn Telescope) and Tenerife,
over a record distance of 143 km.
Breaking the distance record wasn't the scientists' primary goal though. This experiment provides
the basis for a worldwide information network, in which quantum mechanical effects enable the exchange of messages with greater security,
and allow certain calculations to be
performed more efficiently than with conventional technologies. In such a future 'quantum internet', quantum teleportation will be a key protocol for the transmission of information between quantum computers.
In a quantum teleportation experiment, quantum states — but not matter — are exchanged between two parties over distances that can be, in principle, arbitrarily long. The process works even if the location
of the recipient is not known. Such an exchange can be used either for the transmission of messages, or as an operation in future quantum computers. In these applications the photons that encode the quantum
states have to be transported reliably over long distances without compromising the fragile quantum state. The experiment of the Austrian physicists, in which they have now set up a quantum connection suitable
for quantum teleportation over distances of more than 100 km, opens up new horizons.
ESA's Optical Ground Station (OGS) is 2400 m above sea level, at the Teide Observatory on the neighbouring island of Tenerife.
Visible green laser beams are used for stabilising the sending and receiving telescopes on the two islands.
The invisible infrared single photons used for quantum teleportation are sent from La Palma and received by the 1-m
Telescope located under the dome of the OGS. Credits: IQOQI Vienna, Austrian Academy of Sciences [ JPG
Xiao-song Ma, one of the scientists involved in the experiment, says: "The realization of quantum teleportation over a distance of 143 km has been a huge technological challenge."
The photons had to be sent directly through the turbulent atmosphere between the two islands. The use of optical fibres is not suitable for teleportation experiments over such great distances,
as signal loss would be too severe. To reach their goal, the scientists had to implement a series of technical innovations. Support came from a theory group at the Max Planck Institute for Quantum
Optics in Garching (Germany) and an experimental group at the University of Waterloo (Canada).
Rupert Ursin, who has been working with Zeilinger on long-distance experiments since 2002, adds: "Our latest results are very encouraging with a view to future experiments in which we either exchange signals between
Earth and satellites or send messages from one satellite to another." Satellites in ‘low-Earth orbit' fly between 200 and 1200 km above the surface of the Earth. (The International Space Station, for example,
orbits at an altitude of about 400 km.) "On the way through the atmosphere from La Palma to Tenerife, our signals have been attenuated by a factor of roughly one thousand. Nevertheless,
we managed to perform a quantum teleportation experiment. In satellite-based experiments, the distances to be travelled are longer, but the signal will have to pass through less atmosphere.
We have now created a sound basis for such experiments."
Stabilising laser being launched from the roof of the JKT building.
Initial experiments with entangled photons were performed in 2007, but teleportation of quantum states could only be achieved in
2012 by improving the performance of the set-up. Credits: IQOQI Vienna, Austrian Academy of Sciences
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