RESEARCH TRIANGLE PARK, N.C. — Two U.S. Army research projects advance quantum networking, which will likely play a key role in future battlefield operations.

Quantum networks will potentially deliver multiple novel capabilities not achievable with classical networks, one of which is secure quantum communication. In quantum communication protocols, information is typically sent through entangled photon particles. It is nearly impossible to eavesdrop on quantum communication, and those who try leave evidence of their tampering; however, sending quantum information via photons over traditional channels, such as fiber-optic lines, is difficult – the photons carrying the information are often corrupted or lost, making the signals weak or incoherent.

In the first project, the University of Chicago research team, funded and managed by the U.S. Army’s Combat Capability Development’s Army Research Laboratory’s Center for Distributed Quantum Information, demonstrated a new quantum communication technique that bypasses those traditional channels. The research linked two communication nodes with a channel and sent information quantum-mechanically between the nodes—without ever occupying the linking channel.

“This result is particularly exciting not only because of the high transfer efficiency the team achieved, but also because the system they developed will enable further exploration of quantum protocols in the presence of variable signal loss,” said Dr. Sara Gamble, program manager at the lab’s Army Research Office and co-manager of the Center for Distributed Quantum Information. “Overcoming loss is a key obstacle in realizing robust quantum communication and quantum networks.”

The research, published in the journal Physical Review Letters, developed a system that entangled two communication nodes using microwave photons—the same photons used in cell phones—through a microwave cable. For this experiment, they used a microwave cable about a meter in length. By turning the system on and off in a controlled manner, they were able to quantum-entangle the two nodes and send information between them—without ever having to send photons through the cable.

“We transferred information over a one-meter cable without sending any photons to do this, a pretty unusual achievement,” said Dr. Andrew Cleland, the John A. MacLean Sr. Professor of Molecular Engineering at Pritzker Molecular Engineering at University of Chicago and a senior scientist at Argonne National Laboratory. “In principle, this would also work over a much longer distance. It would be much faster and more efficient than systems that send photons through fiber-optic channels.”

Though the system has limitations, it must be kept very cold, at temperatures a few degrees above absolute zero, the researchers said it could also potentially work at room temperature with atoms instead of photons.

The team is now conducting experiments that would entangle several photons together in a more complicated state, which could ultimately enable enhanced quantum communication protocols and capabilities.

Entangled particles aren’t just limited to photons or atoms, however. In a second paper published June 12 in the peer-reviewed journal Physical Review X, the same Chicago team entangled two phonons—the quantum particle of sound—for the first time.

Using a system built to communicate with phonons, similar to the photon quantum communication system, the team entangled two microwave phonons, which have roughly a million times higher pitch than can be heard with the human ear.

Once the phonons were entangled, the team used one of the phonons as a herald, which was used to affect how their quantum system used the other phonon. The herald allowed the team to perform a so-called quantum eraser experiment, in which information is erased from a measurement, even after the measurement has been completed.

“Phonons give you a much bigger time window to do things and relieve some of the challenges in doing a quantum eraser experiment,” Cleland said.

Though phonons have a lot of disadvantages over photons—for example, they tend to be shorter-lived—they interact strongly with a number of solid-state quantum systems that may not interact strongly with photons. As a result, phonons could provide a better way to couple to these systems.

This coupling is a critical capability for many quantum networking applications, and may also benefit other quantum information science applications such as quantum computing. Additionally, the wavelengths of phonons are shorter than those of photons for the same frequency, potentially enabling smaller quantum circuits.

“Together, these experiments provide multiple avenues for future research into how we construct quantum networks that function in non-ideal environments, and reliably transfer quantum information between systems,” said Dr. Fredrik Fatemi, researcher at the laboratory and co-manager of the Center for Distributed Quantum Information. “Both are critically important for developing future quantum technologies.”

By U.S. Army CCDC Army Research Laboratory Public Affairs

RESEARCH TRIANGLE PARK, N.C. — A new U.S. Army-funded discovery could improve energy efficiency of technologies such as solar panels and fuel cells.

The research, funded by the Army Research Office, an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory, developed a novel technique to directly measure for the first time, energy distributions of hot electrons.

Hot electrons feature much larger energies than normal that can be generated in nanostructures.

Published in the journal Science, the research, conducted by a team of scientists from the University of Michigan and Purdue University, was part of the Department of Defense’s Multidisciplinary University Research Initiatives Program, supported by ARO.

“This multidisciplinary basic research effort sheds light on a unique way to measure the energy of charge carriers,” said Dr. Chakrapani Varanasi, an ARO program manager, who supported this study. “These results are expected to play a crucial role in developing future applications in energy conversion, photocatalysis and photodetectors, for instance, that are of great interest to the Department of Defense.”

The team demonstrated how a technique using a scanning tunneling microscope integrated with lasers and other optical components reveals the energy distribution of hot electrons.

“For example, if you wanted to employ light to split water into hydrogen and oxygen, you can use hot charge carriers because electrons that are more energetic can more readily participate in the reaction and drive the reaction faster,” said Dr. Edgar Meyhofer, a professor of mechanical engineering at University of Michigan, who co-led the research along with professors Pramod Sangi-Reddy and Vladimir Shalaev.

It’s one possible use for hot carriers in energy conversion and storage applications, Meyhofer said.

Hot electrons are typically generated by shining a certain frequency of light on a carefully engineered nanostructure made of metals such as gold or silver, exciting so-called surface plasmons. These plasmons are believed to eventually lose some of their energy to electrons, making them hot. While hot electrons can have temperatures as high as 2,000 degrees Fahrenheit, it’s their high energy – rather than the material temperature – that makes them useful for energy technologies.

“Measuring energy distribution means quantifying how many electrons are available at a certain amount of energy,” said Harsha Reddy, a doctoral candidate in Purdue’s School of Electrical and Computer Engineering and co-lead author on this paper. “That crucial piece of information was lacking for expanding the use of hot electrons.”

The team created the hot electrons by shining laser light onto a gold film just 13 nanometers thick, or hundred or so gold atoms thick, with tiny ridges spaced so that they would resonate with the incident laser light and generate the surface plasmon waves. Then they measured the energies of the charge carriers by employing carefully chosen molecules.

These molecules, some of which were synthesized by collaborators at the University of Liverpool, allow only charge carriers with certain energies to pass. By filtering the charge carriers siphoned off the nanostructure, the researchers figured out the energy distribution of the charge carriers.

“Electrons can be thought of as cars running at different speeds on a highway,” said Dr. Kun Wang, a postdoctoral fellow at the University of Michigan. “The molecule acts like an operator—it only allows cars travelling at a certain speed to pass through.”

Wang spent more than 18 months working with Harsha Reddy on how to make this idea work. Once they had developed a successful method, Wang and Reddy repeated the experiments with a second gold structure, this one about 6 nanometers thick. The results showed that light generates hot charge carriers more efficiently on a thinner metal film, confirming theoretical predictions.

“These advances were achieved by insightfully combining nanotechnology tools that were developed at the University of Michigan and nanophotonic expertise from our collaborators at Purdue University, which was only possible due to a Multi[disciplinary] University Research Initiative effort supported by ARO,” Sangi-Reddy said.

With the method now demonstrated, the team believes that others can also use it to explore and optimize nanostructures.

The Department of Energy and the Office of Naval Research provided additional funding for the research. Seed funding from the U-M Department of Mechanical Engineering supported complementary calculations.

By U.S. Army CCDC Army Research Laboratory Public Affairs