The latest news shows that the virtually unhackable quantum internet service is approaching reality, as research teams from China, the United States, and the Netherlands independently made breakthroughs almost simultaneously.
The peer-reviewed journal Nature published two of the studies on Wednesday, saying the experiments in three real-world cities were the most advanced demonstrations of quantum internet technology to date. Each team used tens of kilometers of fiber optics to build a network in an urban environment based on entangled quantum phenomena that allow a pair of separated photons to remain closely connected in time and space.
The ability to harness entanglement is seen as an important step towards the quantum internet, which allows any two users to establish nearly unbreakable encryption keys to protect sensitive information.
Chinese researchers, led by China's "Father of Quantum" Pan Jianwei, described the achievement as a "key milestone" in the transition to larger-scale experiments. Hansen, a physicist at Delft University of Technology in the Netherlands and first author of the paper, said: "This step has now really moved out of the lab and into practical applications."
The Harvard research team, led by physicist Lukin, said the "key challenge" to achieving practical long-distance quantum communications was "involving strong entanglement between quantum storage nodes connected via fiber optic infrastructure".
American researchers say that each node contains qubits (the quantum version of a computer bit), a basic unit of information represented by 0 or 1, and can also exist in a third combined state — they communicate through "photonic channels."
While each team demonstrates quantum entanglement by using fiber optic cables to establish a secure connection between receiving node devices, the approach is different.
U.S. researchers used a 35-kilometer-long fiber-optic loop to connect two nodes placed side-by-side at Harvard University labs in Cambridge, Massachusetts, which extends to Boston.
The Chinese team set up three nodes – Alice, Bob and Charlie – in a triangular network around Hefei – with a central server lab in between, which are about 10 kilometers apart.
In the Netherlands, a total of 25 kilometers of fibre were laid from Delft to The Hague, connecting the two nodes with intermediate servers.
According to the paper, Pan and his researchers implemented a single-photon scheme that uses qubits encoded in a collection of rubidium atoms to send one photon from each node to a server for entanglement. Chinese researchers say that if two photons reach the server precisely at the same time, an entangled state is reached.
Pan Jianwei said his team hopes to build entanglement on 1,000 kilometers of fiber by 2020 with about 10 nodes.
Instead of relying on a collection of atoms, the American team used a diamond device that replaced carbon atoms with silicon atoms. "We're kind of entangled in two small quantum computers," Lukin said. ”
In the U.S. experiment, a single photon was sent to the first node, where it became entangled with a silicon atom, and since the light was already entangled with the first node, it could transfer this entanglement to the second node.
Dutch researchers used a similar approach to their American counterparts to embed nitrogen atoms into diamond crystals.
The Chinese and Dutch methods, which rely on the extremely precise time of the photons arriving at the central server, require some degree of fine-tuning, while the methods of the American researchers do not, according to the journal Nature.
But a report by the University of Science and Technology of China said that the Chinese researchers' methods were "two orders of magnitude" more efficient than their American counterparts.
Showing the entanglement between city nodes is a major achievement. Northup, a physicist at the University of Innsbruck in Austria, said: "These experiments are the most advanced demonstrations to date and demonstrate the technology needed to develop a quantum internet. ”