Advance opens door for lightning-fast quantum applications without specialized infrastructure
Northwestern University engineers are the first to successfully demonstrate quantum teleportation over a fiberoptic cable already carrying Internet traffic.
The discovery introduces the new possibility of combining quantum communication with existing Internet cables -- greatly simplifying the infrastructure required for distributed quantum sensing or computing applications.
The study will be published on Friday (Dec. 20) in the journal Optica.
"This is incredibly exciting because nobody thought it was possible," said Northwestern's Prem Kumar, who led the study. "Our work shows a path towards next-generation quantum and classical networks sharing a unified fiberoptic infrastructure. Basically, it opens the door to pushing quantum communications to the next level."
An expert in quantum communication, Kumar is a professor of electrical and computer engineering at Northwestern's McCormick School of Engineering, where he directs the Center for Photonic Communication and Computing.
Only limited by the speed of light, quantum teleportation could make communications nearly instantaneous. The process works by harnessing quantum entanglement, a technique in which two particles are linked, regardless of the distance between them. Instead of particles physically traveling to deliver information, entangled particles exchange information over great distances -- without physically carrying it.
"In optical communications, all signals are converted to light," Kumar explained. "While conventional signals for classical communications typically comprise millions of particles of light, quantum information uses single photons."
Before Kumar's new study, conventional wisdom suggested that individual photons would drown in cables filled with the millions of light particles carrying classical communications. It would be like a flimsy bicycle trying to navigate through a crowded tunnel of speeding heavy-duty trucks.
Kumar and his team, however, found a way to help the delicate photons steer clear of the busy traffic. After conducting in-depth studies of how light scatters within fiberoptic cables, the researchers found a less crowded wavelength of light to place their photons. Then, they added special filters to reduce noise from regular Internet traffic.
"We carefully studied how light is scattered and placed our photons at a judicial point where that scattering mechanism is minimized," Kumar said. "We found we could perform quantum communication without interference from the classical channels that are simultaneously present."
To test the new method, Kumar and his team set up a 30 kilometer-long fiberoptic cable with a photon at either end. Then, they simultaneously sent quantum information and regular Internet traffic through it. Finally, they measured the quality of the quantum information at the receiving end while executing the teleportation protocol by making quantum measurements at the mid-point. The researchers found the quantum information was successfully transmitted -- even with busy Internet traffic whizzing by.
Next, Kumar plans to extend the experiments over longer distances. He also plans to use two pairs of entangled photons -- rather than one pair -- to demonstrate entanglement swapping, another important milestone leading to distributed quantum applications. Finally, his team is exploring the possibility of carrying out experiments over real-world inground optical cables rather than on spools in the lab. But, even with more work to do, Kumar is optimistic.
"Quantum teleportation has the ability to provide quantum connectivity securely between geographically distant nodes," Kumar said. "But many people have long assumed that nobody would build specialized infrastructure to send particles of light. If we choose the wavelengths properly, we won't have to build new infrastructure. Classical communications and quantum communications can coexist."
The study, "Quantum teleportation coexisting with classical communications in optical fiber," was supported by the U.S. Department of Energy (grant number DE-AC02-07CH11359).