The image shows the two clouds of caesium atoms. The atoms have been entangled using laser light. The atoms spontaneously emit photons in all directions. By designing the experiment in a very precise way the NBI team succeeded in maintaining the entanglement for up to an hour. Credit: (Credit: Christine Muschik) |
Quantum
communication could be an option for the absolutely secure transfer of
data. The key component in quantum communication over long distances is
the special phenomenon called entanglement between two atomic systems.
Entanglement between two atomic systems is very fragile and up until now
researchers have only been able to maintain the entanglement for a
fraction of a second. But in new experiments at the Niels Bohr Institute
researchers have succeeded in setting new records and maintaining the
entanglement for up to an hour. The results are published in the
scientific journal Physical Review Letters.
Entanglement
is a curious phenomenon in quantum mechanics which Albert Einstein
called “spukhafte Fernwirkung” (spooky action at a distance). Two
separate entangled systems have a ghostlike connection even when they
are placed at a large distance without being directly connected to each
other. It is said that their states are correlated. This means that if
you read out the one system, the other system will ‘know’ about it. In
the experiments at the Niels Bohr Institute, the spins of two gas clouds
of caesium atoms are entangled.
Control of a spontaneous process
To
create the entangled state of the two atomic clouds the researchers use
light. Light consists of photons, which are the smallest parts (a
quantum) of a light pulse. When you shine a laser beam on atoms the
photons are absorbed and subsequently re-emitted spontaneously. This
process has been an impediment to the experiments because it is
uncontrolled.
“Now
we have managed to control this ‘spontaneous’ process and use it,” says
Eugene Polzik, Professor and Director of the Danish National Research
Foundation Center, Quantop at the Niels Bohr Institute at the University
of Copenhagen.
Maintaining entanglement
In
the Quantop laboratories the research group conducted experiments with
entanglement using two clouds of caesium atoms placed in separate glass
containers. By illuminating both clouds of atoms with laser light, the
collective spins of the atoms are manipulated. The two atomic clouds
become entangled, which means that some of their properties are
correlated.
But
the atoms emit photons in all directions and this causes the
entanglement to disappear. This usually happens in a fraction of a
second.
“What
we have done is that we have developed a technique where we renew the
entanglement as fast as it disappears. In this way we have been able to
maintain the entanglement between the two atomic clouds as long as the
experiment lasted, that is to say up to an hour,” says Hanna Krauter,
who is a quantum physicist and researcher at Quantop at the Niels Bohr
Institute.
From theory to reality
The
research has been conducted in collaboration with the Max Planck
Institute of Quantum Optics in Germany, where they have been working
with the theoretical models. Theoretical physicists have suggested
similar techniques for about five years, but it is only now that the NBI
team has succeeded in conducting the physical experiments based on
these methods and getting them to work.
“The
breakthrough has great potential and provides, among other things, a
new approach to quantum communication. It is a step towards getting
quantum communication to function in practice–not just in the
laboratory, but also in the real world of networking á la the Internet.
In addition, it means an improvement of ultra-precise measurements of
miniscule magnetic fields with atomic magnetometers. Sensitive
magnetometers could be used to measure electrical activity in the human
brain and heart,” says Professor Eugene Polzik.
