Quantum Network Milestone: Scientists Entangle Light with Optical Atomic Coherence

Scientists may have just reached a quantum network milestone. Using clouds of ultra-cold atoms and a pair of lasers operating at optical wavelengths, they've entangled light with an optical atomic coherence composed of interacting atoms in two different states. This new research could potentially pave the way for functional, multi-node quantum networks.

Currently, researchers are working on generating, distributing and controlling entanglement across quantum networks around the world. While in earlier work ground states of single atoms or atomic ensembles have been entangled with spontaneously-emitted light, the production of these photons has been through a probabilistic approach, which generated photons infrequently. Instead, scientists wanted to expand the potential for multi-node networks.

In order to accomplish this feat, the researchers used a new type of optical trap that simultaneously confined both ground-state and highly-excited (Rydberg) atoms of the element rubidium. The large size of the Rydberg atoms, which have a radius of about one micron instead of a usual sub-nanometer size, gives them exaggerated electromagnetic properties and allows them to interact strongly with one another.

The scientists used the optical trap to increase the rate at which they could generate photons by a factor of 100 compared to their previous work.  More specifically, they first confined an ultra-cold gas of rubidium atoms in a one-dimensional optical lattice using lasers operating at specific wavelengths. These atoms were then driven from the collective ground state into a single excited state. The researchers then created an entangled state by applying a laser field. This retrieved field was then mixed with the coherent field by using polarizing beam-splitters, followed by measurement at single-photon detectors.

"If you can have coherence between the ground and Rydberg atoms, they can interact strongly while emitting light in a cooperative fashion," said Alex Kuzmich, one of the researchers, in a news release. "The combination of strong atomic interactions and collective light emissions results in entanglement between atoms and light. We think this approach is quite promising for quantum networking."

The researchers were actually able to preserve quantum coherence for a few microseconds and could even confine atoms for as long as 80 milliseconds. In addition, the scientists are quick to note that there are ways to improve this.

"The system we have realized is closer to being a node in a quantum network than what we have been able to do before," said Kuzmich. "It is certainly a promising improvement."

The findings are published in the journal Nature.