Researchers at the Department of Energy's Oak Ridge National Laboratory joined forces with EPB of Chattanooga and the University of Tennessee at Chattanooga to demonstrate the first transmission of an entangled quantum signal using multiple wavelength channels and automatic polarization stabilization over a commercial network with no downtime.
The successful trial of this innovation marks another step toward the eventual creation of a quantum internet that could prove to be more capable and secure than existing networks.
The demonstration used automatic polarization compensation, or APC, to stabilize the polarization, or direction of the electric field oscillation in a light wave, of a signal sent over the EPB's fiber-optic commercial quantum network. The approach used reference signals generated by lasers to continuously check the transmitted polarization, detected with an ultrasensitive method known as heterodyne detection.
APCs reduce data interference caused by outside forces like wind and temperature changes that can affect the fiber optic cables used to transmit quantum signals.
"One of our goals all along has been to develop quantum communications systems that operate seamlessly for users," said Joseph Chapman, an ORNL quantum research scientist who led the study . "This is the first demonstration of this method, which enabled relatively fast stabilization while preserving the quantum signals, all with 100% uptime - meaning the people at either end of this transmission won't notice any interruption in the signal and don't need to coordinate scheduled downtime."
The method enabled continuous transmission of the signals with no interruptions for more than 30 hours between the node on the University of Tennessee Chattanooga campus and two other EPB quantum network nodes, each about half a mile away. The UTC node held an entangled-photon source developed by Muneer Alshowkan, an ORNL quantum research scientist.
Quantum computing relies on quantum bits, or qubits, to store information. Qubits, unlike the binary bits used in classical computing, can exist in more than one state simultaneously via quantum superposition, which allows combinations of physical values to be encoded on a single object.
The ORNL study used light particles, or photons, as qubits and transmitted the polarization-entangled qubits on photon pairs via quantum entanglement distribution. Entangled qubits are so intertwined that one can't be described independently of the other. That entanglement allows the information encoded in qubits to be transmitted from one place to another via quantum teleportation without physical travel through space. Entanglement distribution and quantum teleportation form the bedrock of more advanced quantum networks.
Photons can be encoded as qubits via polarization, along with other properties of light, and can be transmitted over existing fiber-optic cable systems. But wind, moisture, changes in temperature and other stresses on the cable can disrupt the photons' polarization and interfere with the signal. Chapman and the ORNL team wanted to find a way to stabilize the polarization and reduce interference while keeping the network running at maximum bandwidth.
"Most previous solutions didn't necessarily work for all types of polarizations and required trade-offs like periodically resetting the network," Chapman said. "People using the network need it up and running. Our approach controls for any type of polarization and doesn't require the network to periodically shut down."
Chapman and Alshowkan tested the compensation method by generating test signals from entangled photons using entanglement-assisted quantum process tomography, which estimates the properties of a quantum channel - such as the in-ground fiber with APC - to measure for changes. The transmissions remained relatively stable with minimal added noise when APC was enabled.
"An experienced musician with a good ear can tell the difference when two instruments are out of tune," Chapman said. "In our APC, we're using a laser to do the same thing with our reference signals."
Chapman has applied for a patent on the method . Next steps include adjusting the approach to increase bandwidth and compensation range to enable high-performance operation under a wider variety of conditions.
"Working with organizations like ORNL provides valuable feedback for how we can continue to enhance EPB Quantum Network as a resource for researchers, startups and academic customers," said David Wade, EPB's CEO. "Since launching a commercially viable quantum network, we've begun working to prepare our community to benefit from the advancements in the quantum future and establish Chattanooga as a destination for developers and investment."
UTC officials pledged to continue their support.
"We're excited about being part of this successful teamwork," said Reinhold Mann, vice chancellor for research at UTC. "This partnership advances quantum information science and technology and adds to our special experiential learning offering for our students."
Support for this research came from the ORNL Laboratory Directed Research and Development program, from the DOE Office of Science's Advanced Scientific Computing Research program, and from the UTC Quantum Initiative.
In celebration of the International Year of Quantum Science and Technology in 2025, ORNL continues to empower the pursuit of quantum innovation, advancing world-leading scientific discovery to enable a quantum revolution that promises to transform a vast range of technologies critical to American competitiveness. Click here to learn more about Quantum Science at ORNL.
About EPB
EPB is a customer-focused technology company that delivers innovative power and telecommunications solutions to the Chattanooga area in pursuit of its mission to enhance the quality of life for the community it serves. In 2010, EPB completed a 100% fiber optic network accessible to all its customers as the basis for launching America's first community-wide Gig speed internet. The company still operates the world's fastest community-wide internet service today at speeds up to 25 Gig. EPB also utilizes Chattanooga's fiber optic network as the communications backbone for the most advanced and highly automated power distribution system in the United States. In 2022, EPB continued its commitment to keeping Chattanooga on the cutting edge by establishing the nation's first commercially available quantum network-EPB Quantum Network℠. This effort aligns local job creation efforts with the national priority to accelerate the commercialization of quantum technology.
Since switching the lights on for its first electric customer in 1939, EPB has grown to serve nearly half a million people across a 600-square-mile service area with cutting-edge infrastructure that integrates power distribution and telecommunications. At the same time, EPB keeps customer benefit at the center of all its efforts, earning recognition from J.D. Power as the #1 Mid-Sized Utility in the South for the last nine consecutive years based on customer satisfaction ratings. Learn more at epb.com.
UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. DOE's Office of Science is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science . - Matt Lakin
