Researchers at the University of Pennsylvania have made significant progress in quantum communication by demonstrating a method to preserve a specific property of quantum light even after the connections—known as entanglement—between photons are severed. This advancement could enhance the capabilities of quantum computers by enabling them to share information via networks of light particles.
Quantum entanglement is sensitive to disturbances, and even minor disruptions can disrupt communication between quantum devices. In their study, the researchers utilized a mathematical concept called topology to maintain a distinctive pattern among photons despite the loss of their entangled state. “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 the university.
The experiment involved a structure known as a quantum skyrmion, characterized not by its physical shape but by a pattern combining two light properties: spin and orbital angular momentum. “The light oscillates in different directions depending on where you inspect it, similar to how regions of hair on your head grow in different directions,” explained Haoqi Zhao, the study’s lead author.
This technique created a topological pattern that covers all polarization states multiple times. Importantly, disturbances to parts of the pattern did not change its topological number, akin to displacing strands of hair without altering its overall style. To validate the resilience of this mathematical property, the team developed an integrated photonic chip composed of silicon nitride, which is commonly utilized in photonic devices.
The chip facilitated the generation of entangled photons through a process called spontaneous four-wave mixing, allowing the linking of quantum states even as the photons were physically separated. An array of miniature interferometers then directed the photons, with specific microring resonators imprinting the skyrmion pattern onto one of the photons. The researchers conducted a Bell-state measurement to transfer the encoded pattern into a shared quantum state.
Quantum state tomography was used to verify the resulting states, which showed a high fidelity rate of 89 percent. The initial single-photon skyrmion bore a measured topological number of 1.981, close to the expected value of 2. The team spent nearly five years refining this technique, creating prototypes at Penn’s Singh Center for Nanotechnology.
After establishing the system, the researchers intentionally introduced noise to assess how well the transferred pattern held up. They observed that while disturbances gradually deteriorated the quantum state and the entanglement disappeared, the skyrmion’s topological number largely remained intact. Zhao noted that previous research indicated these structures relied heavily on entanglement, making their findings unexpected.
The persistence of correlations measured as quantum discord suggests that the relationship between topology and quantum interactions may be more intricate than previously understood. This discovery may eventually contribute to the safeguarding of quantum information against noise in future quantum networks. However, the study does not yet demonstrate a practical communication system independent of entanglement.
The findings are detailed in a study published in the journal Nature Physics.
