A state-insensitive optical trap could prove promising for the development of quantum networking, researchers in Georgia have found. Using clouds of ultracold atoms and a pair of lasers operating at optical wavelengths, Georgia Institute of Technology investigators entangled light with an optical atomic coherence composed of interacting rubidium atoms in two states — ground and highly excited (Rydberg). The large size of the Rydberg atoms — which have a radius of about 1 µm compared with typical subnanometer-sized ones — gives them exaggerated electromagnetic properties, enabling them to interact strongly with one another. A single Rydberg atom can block the formation of additional Rydberg atoms within an ensemble of atoms, enabling the creation of single photons on demand. The state-insensitive trap allowed the researchers to increase the rate at which they could generate photons from a few photons per second with earlier approaches to as many as 5000 per second. This increase by a factor of 100 compared with their previous work could help in the demonstration of quantum gates, which the researchers hope to achieve as they continue to optimize the technique. The experimental setup used by scientists at Georgia Tech to measure the entanglement between light and an optical atomic excitation in the laboratory of Alex Kuzmich. Images courtesy of Kuzmich Physics Lab. “We want to allow photons to propagate to distant locations so we can develop scalable protocols to entangle more and more nodes,” said Alex Kuzmich, a professor at Georgia Tech’s School of Physics. “If you can have coherence between the ground and Rydberg atoms, they can interact strongly while emitting light in a cooperative fashion. The combination of strong atomic interactions and collective light emissions results in entanglement between atoms and light. We think that this approach is quite promising for quantum networking.” Previous work from laboratories around the world to generate, distribute and control entanglement across quantum networks has resulted in ground states of single atoms or atomic ensembles being entangled with spontaneously emitted light, but the production of those photons has been through a probabilistic approach, which generated photons infrequently. Such a spontaneous emission process requires a relatively long time to create entanglement and limits the potential quantum network to just two nodes. To expand the potential for functional, multimode networks, scientists have explored other approaches, including entanglement between light fields and atoms in quantum superpositions of the ground and highly excited Rydberg electronic states. The latter approach enabled the deterministic generation of photons producing entanglement at a much higher rate. Until now, however, Rydberg atoms could not be excited to that state while confined to optical traps, so the traps had to be turned off for that step, allowing the confined atoms to escape and preventing the realization of atom-light entanglement. The state-insensitive trap — developed by the Georgia Tech team based on a suggestion from colleagues at the University of Wisconsin — overcame this problem by confining both ground-state and Rydberg atoms coherently. A key to the improved system is its operation at 1004- and 1012-nm wavelengths — so-called “magic” wavelengths tuned to both the Rydberg atoms and the ground-state atoms, said Lin Li, a graduate student in the Kuzmich laboratory. Georgia Tech graduate student Lin Li adjusts the optics on equipment being used to measure the entanglement between light and an optical atomic excitation in the laboratory of Alex Kuzmich. “We have experimentally demonstrated that in such a trap, the quantum coherence can be well preserved for a few microseconds and that we can confine atoms for as long as 80 milliseconds,” Li said. “There are ways that we can improve this, but with the help of this state-insensitive trap, we have achieved entanglement between light and the Rydberg excitation.” The current atomic confinement time, however, would be enough to operate complex protocols that might be part of a quantum network, the researchers say. “The system we have realized is closer to being a node in a quantum network than what we have been able to do before,” Kuzmich said. “It is certainly a promising improvement.” Three other approaches for creating entangled quantum memories to facilitate long-distance transmission of secure information are being worked on at Harvard University and the universities of Wisconsin and Michigan under a five-year program through the Multidisciplinary University Research Initiative (MURI) of the Air Force Office of Scientific Research. The findings were reported in an early edition of Nature (doi: 10.1038/nature12227). For more information, visit: www.gatech.edu