A record for maintaining the entanglement of the spins of two gas clouds of cesium atoms for up to an hour has been set by researchers from Niels Bohr Institute (NBI) at the University of Copenhagen. Entanglement, a key component in quantum communication over long distances, is very fragile and, until now, researchers had been able to maintain entanglement for only a fraction of a second. According to Albert Einstein’s “spukhafte Fernwirkung” (spooky action at a distance) phenomenon in quantum mechanics, two separate entangled systems have a ghostlike connection even when placed at a large distance without being directly connected to each other. It is said that their states are correlated, which means that if you read out the one system, the other system will know about it. Two clouds of cesium atoms that 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: (Image: Christine Muschik) In the Quantop laboratories at NBI, the researchers conducted experiments with entanglement using two clouds of cesium 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, causing 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,” said Hanna Krauter, researcher at NBI. Conducted in collaboration with Max Planck Institute of Quantum Optics in Germany, the researchers 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 a la the Internet. In addition, it means an improvement of ultraprecise measurements of minuscule magnetic fields with atomic magnetometers. Sensitive magnetometers could be used to measure electrical activity in the human brain and heart,” said Eugene Polzik, a professor at NBI. The results are published in Physical Review Letters. For more information, visit: www.nbi.dk