Single-photon skyrmion topology transferred between entangled photons

Researchers have transferred the topology of a single-photon quantum skyrmion to an entangled bi-photon state on a chip. The topological invariant remained intact after entanglement vanished, offering a noise-resistant method for quantum information transfer. A team from the University of Pennsylvania, the University of the Witwatersrand, and Nanyang Technological University generated a single-photon skyrmion state, theoretically described as (|0,L> + |2,R>)/√2 with a skyrmion number N_sk = 2. Quantum state tomography retrieved an experimental single-photon skyrmion number of N_sk = 1.981 ± 0.002 with a fidelity of 86% ± 2%. Haoqi Zhao, Liang Feng, and colleagues authored the paper, with contributions from Andrew Forbes’s group in South Africa and Yijie Shen’s team in Singapore. The findings are published in Nature Physics.

Device and Method

The experiment used a silicon-nitride-on-silica photonic integrated circuit. The chip featured spiral waveguides and microring resonators to generate and manipulate photon pairs via spontaneous four-wave mixing.

The platform uses a 310-nanometre-thick Si3N4 layer on SiO2. Each of the two on-chip spiral waveguides is 11.5 millimetres long and 650 nanometres wide, optimized for momentum matching. Microring resonators are 600 nanometres wide to enhance spin purity and spin-orbit locking, with an azimuthal quantum number of 414 for clockwise whispering gallery modes. A femtosecond pump pulse drove spontaneous four-wave mixing in the spirals, generating a bi-photon path entanglement state: (|P1>_A|P1>_B + |P2>_A|P2>_B)/√2. Thermo-optic Mach-Zehnder interferometer arrays routed photons. Inner gratings on the resonators scattered photons as spin-up, left-circularly polarized with zero orbital angular momentum. Outer gratings produced spin-down, right-circularly polarized photons with an orbital angular momentum charge of two. The final chip footprint was smaller than a fingernail.

Topology Transfer and Resilience

The skyrmion topology was coherently transferred to entangled photons and remained intact under noise, sustained by quantum discord even after entanglement vanished.

A thermo-optic Mach-Zehnder interferometer array routes photons to different on-chip skyrmion generators for reconfigurability. Performing a spin-momentum Bell-state measurement on photon A and applying a conditioned Pauli operation on photon B's polarization transferred the single-photon topology to a shared entangled skyrmion topology. The four terms in the superposition are Bell-resolved components of a single coherent state. Each experimental run heralds one entangled skyrmion state per Bell-state measurement outcome. After entanglement vanishes, the transferred topological invariant remains intact while non-zero quantum discord persists.

Quantum state tomography showed average fidelities of 89 percent, with retrieved entangled skyrmion numbers averaging approximately 1.98. Under isotropic noise, the skyrmion number remained unchanged even as state purity fell below 0.34. Concurrence, a measure of entanglement, dropped to zero at that purity, but quantum discord remained non-zero. The skyrmion number collapsed to zero only when purity approached roughly 0.25. Tests with anisotropic noise showed skyrmion topology preserved up to a noise ratio of 0.5, at which point quantum discord vanished in phase-flip and bit-phase-flip channels. The skyrmion number acts as a topological witness of non-classical correlations beyond entanglement. The authors note that no operational advantage for quantum state transfer has yet been demonstrated.

Experimental Details

Specific measurements confirmed high-fidelity skyrmion number retrieval and precise control over photon properties.

The bi-photon momentum-polarization entanglement was characterized with a quantum fidelity of 91% ± 2%. Photon A was coupled into a clockwise whispering gallery mode. The inner angular grating generates a spin-up polarized mode with zero orbital angular momentum charge. The outer grating delivers a spin-down component with an orbital angular momentum charge. In the fabricated device, some outer sidewall scatterers were omitted locally in the bus-waveguide coupling region due to limited space, but the grating periodicity and emitted orbital angular momentum order were unchanged. Photon B was directed through an interferometer array into a grating coupler converting paths to horizontal and vertical polarizations. Photon A was routed via another array into two identical microring resonators to encode the single-photon topological state. Extraction from the resonators gives photon A two different momenta, producing a tensor product state that intertwines the skyrmion with entanglement. Spin-momentum Bell basis states were defined for the measurement.

The approach enables robust quantum interconnects and advances distributed quantum information technology mediated by topology. It addresses challenges in quantum networks where entanglement is fragile and error correction is resource-intensive. The research provides a foundation for exploring topological observables in mixed-state quantum information processing.