Chinese satellite is one giant step for the quantum
internet
Craft due to launch in August is first in a wave of planned
quantum space experiments.
27
July 2016
China is poised to launch the world’s first satellite
designed to do quantum experiments. A fleet of quantum-enabled craft is likely
to follow.
First up could be more Chinese satellites, which will
together create a super-secure communications network, potentially linking
people anywhere in the world. But groups from Canada, Japan, Italy and
Singapore also have plans for quantum space experiments.
“Definitely, I think there will be a race,” says Chaoyang Lu, a physicist at the -University of
Science and Technology of China in Hefei, who works with the team behind the
Chinese satellite. The 600-kilogram craft, the latest in a string of Chinese space-science satellites,
will launch from Jiuquan Satellite Launch Center in August. The Chinese Academy
of Sciences and the Austrian Academy of Sciences are collaborators on the
US$100-million mission.
Quantum communications are secure because any tinkering with
them is detectable. Two parties can communicate secretly — by sharing a
encryption key encoded in the polarization of a string of photons, say
— safe in the knowledge that any eavesdropping would leave its mark.
So far, scientists have managed to demonstrate quantum communication up to about 300
kilometres. Photons travelling through optical fibres and the air
get scattered or absorbed, and amplifying a signal while preserving a photon’s
fragile quantum state is extremely difficult. The Chinese researchers hope that
transmitting photons through space, where they travel more smoothly, will allow
them to communicate over greater distances.
At the heart of their satellite is a crystal that produces
pairs of entangled photons, whose properties remain entwined however far apart
they are separated. The craft’s first task will be to fire the partners in
these pairs to ground -stations in Beijing and Vienna, and use them to generate
a secret key.
During the two-year mission, the team also plans to perform
a statistical measurement known as a Bell test to prove that entanglement can
exist between particles separated by a distance of 1,200 kilometres. Although
quantum theory predicts that entanglement persists at any distance, a Bell test would prove it.
The team will also attempt to ‘teleport’ quantum states,
using an entangled pair of photons alongside information transmitted by more
conventional means to reconstruct the quantum state of a photon in a new
location.
“If the first satellite goes well, China will definitely
launch more,” says Lu. About 20 satellites would be required to enable secure
communications throughout the world, he adds.
The teams from outside China are taking a different tack. A
collaboration between the National University of Singapore (NUS) and the
University of Strathclyde, UK, is using cheap 5-kilogram satellites known as cubesats to
do quantum experiments. Last year, the team launched a cubesat that created and
measured pairs of ‘correlated’ photons in orbit; next year, it hopes to launch
a device that produces fully entangled pairs.
Costing just $100,000 each, cubesats make space-based
quantum communications accessible, says NUS physicist Alexander Ling, who is
leading the project.
A Canadian team proposes to generate pairs of entangled
photons on the ground, and then fire some of them to a microsatellite that
weighs less than 30 kilograms. This would be cheaper than generating the
photons in space, says Brendon Higgins, a physicist at the University of
Waterloo, who is part of the Canadian Quantum Encryption and Science Satellite
(QEYSSat) team. But delivering the photons to the moving satellite would be a
challenge. The team plans to test the system using a photon receiver on an
aeroplane first.
An even simpler approach to quantum space science, pioneered
by a team at the University of Padua in Italy led by Paolo Villoresi, involves
adding reflectors and other simple equipment to regular satellites. Last year,
the team showed that photons bounced back to Earth off an existing satellite
maintained their quantum states and were received with low enough error rates
for quantum cryptography (G. Vallone et al. Phys. Rev. Lett. 115, 040502; 2015).
In principle, the researchers say, the method could be used to generate secret
keys, albeit at a slower rate than in more-complex set-ups.
Researchers have also proposed a quantum experiment aboard
the International Space Station (ISS) that would simultaneously -entangle the
states of two separate properties of a photon — a technique known as
hyperentanglement — to make teleportation more reliable and efficient.
As well as making communications much more secure, these
satellite systems would mark a major step towards a ‘quantum internet’ made up
of quantum computers around the world, or a quantum computing cloud, says Paul
Kwiat, a physicist at the University of Illinois at Urbana–Champaign who is
working with NASA on the ISS project.
The quantum internet is likely to involve a combination of
satellite- and ground-based links, says Anton Zeilinger, a physicist at the
Austrian Academy of Sciences in Vienna, who argued unsuccessfully for a
European quantum satellite before joining forces with the Chinese team. And
some challenges remain. Physicists will, for instance, need to find ways for
satellites to communicate with each other directly; to perfect the art of
entangling photons that come from different sources; and to boost the rate of
data transmission using single photons from megabits to gigabits per second.
If the Chinese team is successful, other groups should find
it easier to get funding for quantum satellites, says Zeilinger. The United
States has a relatively low profile when it comes to this particular space
race, but Zeilinger suggests that it could be doing more work on the topic that
is classified.
Eventually, quantum teleportation in space could even allow
researchers to combine photons from satellites to make a distributed telescope
with an effective aperture the size of Earth — and enormous resolution. “You
could not just see planets,” says Kwiat, “but in principle read licence plates
on Jupiter’s moons.”
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