Physicists at the
University of Vienna and the Austrian Academy of Sciences have achieved quantum
teleportation over a record distance of 143 km. The experiment is a major step
towards satellite-based quantum communication. The results have now been
published in Nature.

An
international team led by the Austrian physicist Anton Zeilinger has
successfully transmitted quantum states between the two Canary Islands of La Palma
and Tenerife, over a distance of 143 km. The previous record, set by
researchers in China just a few months ago, was 97 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.

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 fibers 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). Ma also said “An important
step for our successful teleportation was a method known as ‘active
feed-forward’, which we have used for the first time in a long-distance
experiment. It helped us to double the transfer rate”. In an active
feed-forward protocol, conventional data is sent alongside the quantum information,
enabling the recipient to decipher the transferred signal with a higher
efficiency.

“Our
experiment shows how mature ‘quantum technologies’ are today, and how useful
they can be for practical applications,” says Anton Zeilinger. “The
next step is satellite-based quantum teleportation, which should enable quantum
communication on a global scale. We have now taken a major step in this
direction and will use our know-how in an international cooperation, which
involves our colleagues at the Chinese Academy of Sciences. The goal is to
launch a ‘quantum satellite mission’.”

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.”

Source: University of Vienna