In optical fibers, chips and other light-guiding devices, minor defects can scatter photons and disrupt their path. A new study demonstrates a way to reveal properties which, in certain systems, help light wave behavior withstand such flaws. The method could help speed up the search for new kinds of electronics and optical technologies, and contribute to more reliable quantum devices.

Reporting in an upcoming edition of the journal Physical Review Letters, researchers at KTH Royal Institute of Technology demonstrated a programmable chip that can detect and measure “topological properties,” or features of wave behavior that don’t change even when a system is altered. (An open access version of the research article is available to view on arxiv.org)

KTH Associate Professor Ali Elshaari says understanding these properties can help devices keep working despite imperfections.

Topological properties can make materials unusually stable, Elshaari says. “When I explain topology to students, I start with a knot in a rope,” he says. “You can stretch it, twist it or shake it, but the knot stays until you cut the rope.

“Physicists have found that some materials and devices carry similar knots in how waves move through them.”

KTH doctoral student Andrea Cataldo, the study’s lead author, says that kind of stability is attractive for technologies that must work despite noise and defects, such as future electronics and quantum devices.

“In the real world, every system loses some energy,” Cataldo says. “So we need ways to study these properties under realistic conditions.”

The light-based chip can imitate energy-losing systems and directly reveal their hidden stable properties, he says.

The chip reliably distinguished the "knotted" (topological) cases from ordinary ones, Cataldo says. It also detected a special point where energy-losing systems behave in unusual ways, called an exceptional point.

“We use a mathematical trick to run the lossy system on the chip without actually losing the signal,” he says. The researchers then compare the light against reference beams—an approach they compare to tuning an instrument against a reference note.

“This lets us read the hidden information even when energy loss would normally hide it,” he says.

Because the chip can be reprogrammed, many different scenarios can be tested on one device.