John Hannegan, a graduate student and research scientist at the University of Maryland, adjusts photon collection optics in preparation for linking the trapped ion lab with another quantum lab.

ADELPHI, Md. -- Army researchers, in collaboration with researchers at the University of Maryland, built a precursor to a hybrid quantum network that shows promise for the future.

The researchers demonstrated two-photon interference between photons originating from a trapped ion system and a neutral atom system for the first time. Two-photon interference, also known as the Hong-Ou-Mandel effect, occurs when two identical photons enter a beam splitter from different input ports but exit together out of the same output port.

Trapped ion systems and neutral atomic ensemble systems represent two of the leading approaches in quantum information science because of their versatility in use and robust operation. Any future quantum system that employs both techniques will be able to use each of their complementary strengths to overcome individual weaknesses, researchers said.

“Trapped ions have the highest fidelity quantum operations reported and neutral atoms are excellent at manipulating photons that carry quantum information,” said Dr. Qudsia Sara Quraishi, a physicist at the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. “As with classical networks, practical implementation of quantum networks will require connecting hybrid components. Interference of the photons emitted by each source is needed to establish a connecterized network. This work makes possible a joint platform consisting of trapped ions and neutral atoms.”

This milestone came into fruition when the lab and the University of Maryland brought together researchers who specialize in trapped ion systems and researchers who study neutral atom quantum systems.

A map depicts two quantum systems linked via three optical fibers: one quantum channel and two classical channels.

The full team set up an experiment on the Maryland campus, where they connected a trapped ion system in one building and a neutral atom system in another building with commercially available fiber lines. The researchers installed three fiber links between the two labs, one of which transmitted the single photons while the other two synchronized the two experiments.

The researchers identified the state of each quantum node based on how the node’s photon behaved at the beam splitter. To get the photons to interact, they sent photons from one lab to the other.

“One lab has a beam splitter that allows the photon two paths to exit; there’s a 50 percent chance of it coming out of one port and a 50 percent chance of it coming out the second port,” said John Hannegan, a fourth-year graduate student in Quraishi’s lab. “If you send two individual photons into the beam splitter, you’d expect that each photon to select an output port independently; however, due to quantum interference, photons with identical properties that arrive at the beam splitter bunch together, meaning they always leave the same port together, not individually.”

Bunching the photons means that the nodes themselves can now exhibit correlations or entanglement even without direct interactions. Though technical challenging, this procedure is normally straightforward; however, the task becomes essentially impossible with different colored photons.

In this experiment, the specific breakthrough that the team accomplished arose from the researchers’ ability to make the photons of two different quantum systems identical enough for quantum interference to even occur.

Army researchers used quantum frequency conversion to convert a blue-colored photon emitted by a trapped ion system in one building into a near-infra-red photon identical to the one emitted by a neutral atom system in the other building. Once the colors matched, the researchers synchronized the timing of the photons so that they would arrive at the beam splitter at the same time and measured the resulting interference signal.

“Photons from a trapped ion system have a different temporal shape and frequency than photons from a neutral atom system,” Quraishi said. “No prior experiment has shown interference between photons produced by these two important quantum systems.”

Quraishi said that the results of this experiment acted as a huge step forward in an area of quantum information research that has seen very little work until the development of frequency conversion, which bridged fundamentally different operation wavelengths.

According to Quraishi, this work extends the baseline between quantum networking nodes and allows for user-defined photonic channels rather than those restricted to the node’s default operation wavelength. This research may even enable future Army overmatch in areas such as secure information distribution and increase network capacity through quantum entanglement.

The team published their research in a paper, Quantum Interference between Photons from an Atomic Ensemble and a Remote Atomic Ion, in the peer-reviewed journal Physical Review Letters.

CCDC Army Research Laboratory is an element of the U.S. Army Combat Capabilities Development Command. As the Army’s corporate research laboratory, ARL discovers, innovates and transitions science and technology to ensure dominant strategic land power. Through collaboration across the command’s core technical competencies, CCDC leads in the discovery, development and delivery of the technology-based capabilities required to make Soldiers more lethal to win the nation’s wars and come home safely. CCDC is a major subordinate command of the U.S. Army Futures Command.