A mathematical property gives quantum skyrmions an unexpected survival advantage

Quantum computers could become far more powerful if scientists could connect them through networks, allowing them to share information using particles of light. However, there is a big catch.

The quantum connections that make this possible are very sensitive. Even small disturbances can destroy the entanglement between photons, and bring communication between quantum devices to a halt.

Now, in a new study, researchers at the University of Pennsylvania have demonstrated an intriguing way to preserve a particular property of quantum light even after this connection breaks down.

Using a mathematical concept called topology, they transferred a distinctive pattern between photons and found that the pattern remained intact even after the photons lost their quantum connection (entanglement).

“It’s as if one dancer’s unique movement suddenly became part of a duet, and persisted even when the dancers were separated,” Liang Feng, senior study author professor in materials science and engineering at the University of Pennsylvania, said.

Giving photons a pattern that refuses to disappear

The secret behind the experiment lies in a structure called a quantum skyrmion. Unlike a conventional object, its defining feature is not its physical shape but a pattern involving two properties of light: its spin, related to polarization, and its orbital angular momentum, which describes how its wavefront twists.

“The light oscillates in different directions depending on where you inspect it. It’s sort of like how regions of hair on your head grow in different directions,” Haoqi Zhao, first study author and a postdoctoral fellow at Yale, said

Together, these variations create a pattern that covers all possible polarization states a fixed number of times. Mathematicians describe this property using a topological number. The interesting part is that disturbing individual portions of the pattern does not necessarily change this number.

“Think of a swirl in someone’s hair. You can push individual strands out of place without changing the overall pattern,” Zhao added.

To test whether this mathematical resilience could help quantum information survive disturbances, the team first needed a way to create and transfer these patterns using photons.

Building a tiny factory for quantum light

The researchers developed an integrated photonic chip made from silicon nitride, a material commonly used in photonic devices. Its components were designed to generate entangled photons, direct them along different paths and imprint a skyrmion pattern onto one of them.

The process began with a femtosecond laser pulse directed into two spiral waveguides on the chip. Through a process called spontaneous four-wave mixing, the light generated pairs of photons that shared quantum entanglement. In simple terms, their quantum states became linked, even when the photons were subsequently separated.

An array of miniature interferometers then directed the photons through different parts of the chip. One photon entered specially designed microring resonators, where carefully arranged structures on their inner and outer edges produced different polarization and angular momentum components. Combining these components created the desired skyrmion pattern.

The researchers then performed a measurement on the first photon’s polarization and momentum, known as a Bell-state measurement. This allowed the topological pattern encoded in that photon to be transferred into a quantum state shared by both photons.

The team verified the resulting states using quantum state tomography, a technique that reconstructs a quantum system from measurements.

The original single-photon skyrmion had a measured topological number of 1.981, very close to the expected value of 2. The transferred two-photon states showed similarly high values, with an average quantum-state fidelity of 89 percent, indicating how closely the experimentally produced state matched the intended one.

Nearly half a decade of refining techniques went into developing the chip, with the team fabricating prototypes in Penn’s Singh Center for Nanotechnology.

“It’s a bit like building an entire factory in miniature. One part of the chip generates entangled photons, another directs them where they need to go and still another creates the skyrmion pattern. The challenge was getting all the parts to work together,” said Yichi Zhang, one of the study authors and a researcher at the University of Pennsylvania.

The surprising part came when entanglement disappeared

With the system working, the researchers deliberately introduced noise into the photons, including unwanted background light, to see how well the transferred pattern could withstand interference.

As the disturbances increased, the quantum state gradually deteriorated and the entanglement between the photons eventually disappeared. Yet the skyrmion’s topological number remained largely unchanged. The pattern only broke down when the researchers introduced still stronger disturbances.

This was particularly surprising because “previous work suggested that these structures depended on entanglement. Our findings show that the relationship is more complicated than we thought,” Zhao said.

The researchers also found evidence of these remaining correlations through a measure called quantum discord. Unlike entanglement, quantum discord can capture certain quantum correlations that remain even when two particles are no longer entangled.

Its persistence suggests that topology may be connected to a broader range of quantum relationships than scientists previously thought.

The discovery could eventually help protect quantum information against noise in future quantum networks. However, the experiment does not yet demonstrate a practical communication system that works without entanglement.

The study is published in the journal Nature Physics.