Quantum entanglement could mean completely secure data transfer

Quantum communication should be an option for the absolutely secure transfer of knowledge. The main component in quantum communication over long distances is the special phenomenon called entanglement between two atomic systems. Entanglement between two atomic systems is awfully fragile and up previously researchers have only been in a position to maintain the entanglement for a fragment of a second. But in new experiments on the Niels Bohr Institute researchers have succeeded in setting new records and maintaining the entanglement for as much as an hour. The effects are published inside 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 if they’re placed at a big distance without being directly connected to one another. It’s said that their states are correlated. Which means that in case you read out the only system, the opposite system will ‘know’ about it. Within the experiments on the Niels Bohr Institute, the spins of 2 gas clouds of caesium atoms are entangled.

The picture shows the 2 clouds of caesium atoms. The atoms had been entangled using laser light. The atoms spontaneously emit photons in all directions. By designing the experiment in a completely precise way the NBI team succeeded in maintaining the entanglement for as much as an hour. (Credit: Christine Muschik)

Control of a spontaneous process

To create the entangled state of both atomic clouds the researchers use light. Light comprises photons, that are the smallest parts (a quantum) of a mild pulse. Should you shine a laser beam on atoms the photons are absorbed and subsequently re-emitted spontaneously. This process have been an impediment to the experiments since it is uncontrolled.

“We’ve managed to govern this ‘spontaneous’ process and use it”, explains Eugene Polzik, Professor and Director of the Danish National Research Foundation Center, Quantop on the Niels Bohr Institute on the University of Copenhagen.
Maintaining entanglement

Within 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 2 atomic clouds become entangled, because of this a number of their properties are correlated.
However the atoms emit photons in all directions and this causes the entanglement to vanish. This usually happens in a fragment of a second.

“What now we have done is that we’ve got developed one way where we renew the entanglement as fast because it disappears. During this way we now have been in a position to maintain the entanglement between the 2 atomic clouds so long as the experiment lasted, that’s to mention as much as an hour”, explains Hanna Krauter, who’s a quantum physicist and researcher at Quantop on the Niels Bohr Institute.

From theory to reality

The research was conducted in collaboration with the Max Planck Institute of Quantum Optics in Germany, where they’ve been working with the theoretical models. Theoretical physicists have suggested similar techniques for roughly five years, nevertheless it is purely now that the NBI team has succeeded in conducting the physical experiments in accordance with these methods and getting them to work.

“The breakthrough has great potential and gives, among other things, a brand new way to quantum communication. This can be a step towards getting quantum communication to operate in practice – not only inside the laboratory, but in addition inside the real world of networking á la the web. Additionally, it means an improvement of ultra-precise measurements of miniscule magnetic fields with atomic magnetometers. Sensitive magnetometers can be used to measure electrical activity within the human brain and heart”, explains Professor Eugene Polzik.