Penn Engineers Make Quantum 'Skyrmions' That Outlast the Entanglement Behind Them

Quantum networks could one day link quantum computers into more powerful systems and enable new forms of secure communication. A basic obstacle stands in the way: quantum information is extremely fragile, and the entanglement between particles of light that such networks depend on is easily disrupted.

Engineers at the University of Pennsylvania now report a possible workaround based on topology. In a paper in Nature Physics, they describe a chip that creates a pair of entangled photons, then gives one of them a distinctive pattern known as a skyrmion by polarizing its light in a particular way. Topology is the branch of mathematics that treats a doughnut and a coffee mug as equivalent because each has a single hole that persists when the shape is distorted.

The team found it could transfer the skyrmion pattern engraved on a single photon, locally, to the quantum state shared by the two photons, remotely. More surprisingly, when the researchers introduced enough disruption to cut the photons' entanglement, the topological property stayed intact.

"It's as if one dancer's unique movement suddenly became part of a duet, and persisted even when the dancers were separated," said Liang Feng, a professor of materials science and engineering at Penn and the paper's senior author. "If we can learn to encode information in such resilient patterns, that could lay the groundwork for much more reliable quantum networks."

A quantum skyrmion is defined not by a physical shape but by a pattern in how light is polarized. "The light oscillates in different directions depending on where you inspect it," said first author Haoqi Zhao, a postdoctoral fellow at Yale and an alumnus of Feng's lab. In a skyrmion, those variations cover all possible polarization states a fixed number of times, and that number stays the same even if parts of the pattern are disturbed. "Think of a swirl in someone's hair," Zhao said. "You can push individual strands out of place without changing the overall pattern."

Other researchers have made skyrmions with individual photons and skyrmions shared between entangled photons. The Penn team says no one else has shown how to transfer a skyrmion's topological properties from one photon to a state shared by two, much less on a small chip of the kind future quantum networks would need.

The chip took nearly five years of refinement and was fabricated at Penn's Singh Center for Nanotechnology. "It's a bit like building an entire factory in miniature," said Yichi Zhang, a doctoral student and co-author. One section produces entangled photons, another steers them, and a third creates the skyrmion pattern. "The challenge was getting all the parts to work together."

To test resilience, the researchers added increasingly strong disturbances, including unwanted background light. The quantum state degraded and the entanglement eventually vanished, but the skyrmion's topological property held, breaking down only under still stronger interference. The work is a lab demonstration, not a working network, and the team's next steps would be to see whether information can be reliably encoded in such patterns over longer distances.