Physicists at Ludwig Maximilian University (LMU), with colleagues at Saarland University, have demonstrated the transport of an entangled state between an atom and a photon via an optical fiber over a distance of up to 20 km. According to the researchers, this is a new record for distance traveled by an atom and a photonic channel in an entangled state — the previous record was 700 meters. “The experiment represents a milestone, insofar as the distance covered confirms that quantum information can be distributed on a large scale with little loss,” professor Harald Weinfurter said. The researchers entangled a rubidium (Rb) atom with a photon. Following targeted excitation, the Rb atom emitted a photon with a wavelength of 780 nm, in the near-infrared (NIR) region of the spectrum. “In an optic fiber made of glass, light at this wavelength is rapidly absorbed,” Weinfurter said. Conventional telecomm networks use wavelengths around 1550 nm to reduce losses in transit. Picture of the single atom trap. In the ultrahigh vacuum glass cell a single Rubidium atom is captured, which later will be entangled with a photon. Courtesy of C. Olesinski/LMU. To improve the team’s chances of successfully transporting an atom-photon entanglement through an optical fiber, Saarland University researcher Matthias Bock built a quantum frequency converter that increased the wavelength of the emitted photons from 780 to 1520 nm. Bock faced several challenges: He needed to make sure that conversion took place between single photons only — from one photon to one other photon — and that no other properties of the entangled state were altered during the conversion process. Otherwise, the entangled state would be lost. “Thanks to the use of this highly efficient converter, we were able to maintain the entangled state over a much longer range at telecommunications wavelengths, and therefore to transport the quantum information that it carries over long distances,” Weinfurter said. Next, the researchers plan to convert the frequency of the light emitted by a second atom, which should enable them to generate entanglement between the two atoms over long telecommunications fibers. The properties of glass-fiber cables vary depending on factors such as the temperature and strain to which they are exposed. For this reason, the team intends to first carry out its next experiment under controlled conditions in the laboratory. If the laboratory experiment is a success, the researchers will undertake field experiments and add new nodes to their network. After all, they said, even long journeys can be successfully completed by taking one step at a time. The research was published in Physical Review Letters (www.doi.org/10.1103/PhysRevLett.124.010510).