Quantum Skyrmions Take a Ride on a Photonic Chip

Physicists have long sought ways to protect fragile quantum information from the noise that relentlessly degrades it. Now, a team working across the University of Pennsylvania, the University of the Witwatersrand and Nanyang Technological University has demonstrated something remarkable: quantum skyrmions, exotic topological textures of light, can be created, manipulated and transferred between single photons and entangled photon pairs entirely on a silicon nitride chip. Writing in Nature Physics, Haoqi Zhao, Liang Feng and colleagues, together with Andrew Forbes’ group in South Africa and Yijie Shen’s team in Singapore, report a programmable nanophotonic platform that not only generates entangled skyrmions on chip for the first time but also reveals a form of resilience that survives even after the entanglement linking the photons has completely vanished.

To appreciate why this matters, it helps to understand what a skyrmion is. Originally conceived by physicist Tony Skyrme to model atomic nuclei, skyrmions are particle-like topological objects in which a field wraps around a sphere an integer number of times. In optics, a photonic skyrmion is built by sculpting the polarization of light so that, across the beam profile, the spin vector sweeps through every possible polarization state on the Poincaré sphere a whole number of times. That integer winding count, the skyrmion number, is a topological invariant: it cannot be changed by smooth deformations. Local distortions, scattering, turbulence and other perturbations may scramble individual polarization values, but the global winding structure resists such assaults, much like a knot in a rope cannot be untied by gentle shaking.

Previous work had already produced quantum skyrmions in two separate settings. Single-photon skyrmions were generated on chip using nanocavity- or metasurface-coupled quantum emitters, while entangled skyrmions, in which a skyrmionic spin-orbit texture is distributed across two spatially separated photons, were created in bulk crystals using spontaneous parametric down-conversion. What remained missing was a way to bridge the two regimes: to generate entangled skyrmions on an integrated chip, to reconfigure them, and to transfer skyrmion topology coherently from a local single-photon state to a non-local, distributed entangled state. The new experiment closes that gap with a complete integrated architecture.

The heart of the platform is a pair of spin-orbit-locked microring resonators fabricated on a silicon-nitride-on-silica chip with a 310-nanometre-thick device layer. Each resonator supports a clockwise whispering gallery mode with an azimuthal quantum number of 414, and its inner and outer sidewalls carry independently designed angular gratings. When a photon circulates in the resonator, the inner grating scatters it out as a spin-up, left-circularly polarized component carrying zero orbital angular momentum, while the outer grating produces a spin-down, right-circularly polarized component carrying an orbital angular momentum charge of two. The superposition of these two components forms a single-photon skyrmion state with a skyrmion number of two, encoded in the non-separable coupling between the photon’s spin and its spatial degree of freedom.

Generating the entangled partner photon relies on spontaneous four-wave mixing in two on-chip spiral waveguides, each 11.5 millimetres long and tailored to a width of 650 nanometres to maximize the correlated photon-pair generation efficiency. A femtosecond pump pulse coherently drives both nonlinear processes, producing a path-entangled photon pair. Thermo-optic Mach-Zehnder interferometer arrays then route photon A into the microring skyrmion generators and photon B into a two-dimensional grating coupler that converts its paths into horizontal and vertical polarizations. The result is a tensor-product state in which a local single-photon skyrmion coexists with momentum-polarization entanglement between the two photons, all prepared and controlled on a single chip smaller than a fingernail.

The topology transfer itself works much like entanglement swapping. The chip-prepared state can be rewritten in the spin-momentum Bell basis of photon A, revealing four Bell-resolved components, each containing an entangled skyrmion shared between photons A and B. Performing a spin-momentum Bell-state measurement on photon A heralds which component has been produced, and applying the appropriate Pauli correction to photon B completes the mapping: the skyrmion topology originally encoded locally on photon A is now distributed non-locally across the orbital angular momentum of photon A and the polarization of photon B. Quantum state tomography on the resulting states yielded fidelities averaging 89 percent, with retrieved entangled skyrmion numbers of approximately 1.98, in excellent agreement with the theoretical value of two.

The most striking result, however, concerns what happens when noise is deliberately injected. The researchers subjected the transferred entangled skyrmion state to controlled, nearly isotropic noise approximating an effective depolarizing channel, using background and incoherent white-light noise of adjustable intensity. As expected, the state fidelity and purity declined steadily. But the skyrmion number remained essentially unchanged even as the purity fell below 0.34, the point at which the concurrence, the standard measure of entanglement, dropped to zero. In that regime, the state still exhibited non-zero quantum discord, indicating that subtler non-classical correlations persist beyond entanglement and continue to support the topological winding structure. Only when the purity approached roughly 0.25, where isotropic noise overwhelms the original state entirely, did the skyrmion number finally collapse to zero.

The team then tested anisotropic noise channels implemented directly on the chip. By manipulating the on-chip phase of photon B, they realized a phase-flip quantum channel, which in the Poincaré-sphere picture contracts the polarization sphere into an ellipsoid elongated along the S-z direction. Despite continuous degradation of the quantum correlations as the noise ratio increased, the topological number of the entangled skyrmion remained intact up to a critical threshold of 0.5, precisely where the quantum discord vanished. A bit-phase-flip channel, realized by swapping the two paths of photon B through the Mach-Zehnder interferometer, produced the same result: topology preserved until the discord disappeared. These findings establish the skyrmion number as a genuine topological witness of non-classical correlations that extends beyond entanglement, a quantity that survives in a mixed-state regime where conventional quantum state transfer protocols would already have failed.

The implications reach toward the practical challenges of building quantum networks. Distributed quantum computing and quantum communication depend on optical interconnects to move quantum states between nodes, yet entanglement links are notoriously fragile, and error correction or entanglement purification demand substantial overhead in resources and control precision. A topological observable that tolerates noise and even the complete loss of entanglement offers a fundamentally different protection strategy, one baked into the structure of the quantum state itself rather than bolted on through active correction. The authors are careful to note that their experiment does not yet demonstrate an operational advantage for quantum state transfer based on the skyrmion number, but it provides an experimentally accessible foundation for exploring whether topological observables can be harnessed in mixed-state quantum information processing.

Equally significant is the platform itself. The precise, scalable control of photon entanglement, together with flexible thermo-optic routing and switching among different on-chip skyrmion generators, positions integrated photonics as an ideal laboratory for complex topological protocols. The same architecture can, in principle, implement arbitrary local unitary transformations, opening the door to more general topological operations beyond the identity transformations used in this demonstration. As researchers worldwide pursue robust quantum interconnects and distributed quantum technologies, this chip-mediated toolbox for creating, transferring and protecting quantum skyrmions suggests that the future of noise-resistant quantum information may be written not in error-correcting codes alone, but in the topological geometry of light itself.

Subject of Research: On-chip generation and topology transfer of entangled photonic quantum skyrmions

Article Title: Chip-mediated manipulation of photonic quantum skyrmions

Article References: Chip-mediated manipulation of photonic quantum skyrmions. (n.d.). https://doi.org/10.1038/s41567-026-03487-x

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03487-x

Keywords: quantum skyrmions, integrated photonics, silicon nitride, quantum entanglement, topological protection, quantum discord, orbital angular momentum, microring resonator, spontaneous four-wave mixing, quantum state tomography, quantum interconnects, Nature Physics