Quantum Network Experiment Marks Major Milestone

The realization of a sophisticated quantum network took a giant leap forward when researchers at the Georgia Institute of Technology announced that, through the use of ultra-cold atoms and pair of lasers operating at optical wavelengths, they were able to entangle light with an optical atomic coherence composed of interacting atoms in two different states.

The discovery, according to the researchers, could pave the way for functional, multi-node quantum networks, considered the "holy grail" for security experts due to the fact that messages sent through them cannot be read without changing them.

To achieve this end, the scientists used a new type of optical trap that simultaneously confined both ground-state and highly-excited, or Rydberg, atoms of the element rubidium.

Because of the large size of the Rydberg atoms, which boast a radius of about one micron versus the usual sub-nanometer size, they have exaggerated electromagnetic properties and interact strongly with one another.

A single Rydberg atom, for example, can block the formation of additional Rydberg atoms within an ensemble of atoms, which in turn allowed the researchers to create single photons on demand.

Back in 2012, Georgia Tech's Alex Kumich and collaborators published a report on the Rydberg single-photon source demonstrating, for the first time, many-body Rabi oscillations of an atomic ensemble. A year later, the state-insensitive trap allowed the researches to increase the rate at which they could generate photons by a factor of 100 compared to their previous work.

"We want to allow photons to propagate to distant locations so we can develop scalable protocols to entangle more and more nodes," said Kuzmich, a professor in 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."

Generating, controlling and distributing entanglements across quantum networks are the primary goals of quantum information scientists around the world are currently pursuing. At this point, ground states of single atoms or atomic ensembles have been entangled with spontaneously-emitted light in previous studies. However, the production of those photons was through a probabilistic approach that generated photons infrequently and was limited to just two nodes.

In order to expand the potential for multi-node networks, researchers have investigated a number of possible solutions, such as entanglement between light fields and atoms in quantum superpositions of the ground and highly-excited Rydberg electronic states. The latter allows for the deterministic generation of photons that produces entanglement much more frequently than the spontaneous model.

However, previous to the most recent study, Rydgber atoms could not be excited to that state while confined to optical traps, meaning the traps had to be turned off for that step in order to allow the confined atoms to escape. This, however, prevented the realization of atom-light entanglement.

The team from Georgia Tech found a solution to this problem based on a suggestion from colleagues at the University of Wisconsin, which was a state-insensitive optical trap able to confine both ground-state and Rydberg atoms coherently.

Within this trap, atoms persist for as long as 80 milliseconds while being excited into the Rydberg state. Moreover, the researchers believe this can be extended with additional improvements though even the current atomic confinement time would be enough to operate complex protocols that a quantum network may require.

"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."

Key to the improved system is the operation of an optical trap at wavelengths of 1,004 and 1,012 nanometers, the so-called "magic" wavelengths tuned to both the Rydberg and ground-state atoms.

"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."

In all, the rate of generating entangled photons increased from a few photons per second with the earlier approaches to as many as 5,000 photons per second, allowing researchers to pursue future research goals, such as the demonstration of quantum gates.