Nanophotonics Reconstructs 6×6 Qudit States

Researchers at the Technion, Israel Institute of Technology have achieved an advance in quantum computing by transforming between 4×4 and 6×6 density matrices for qudit states using a nanophotonic platform. The team, led by Professor Guy Bartal from the Helen Diller Quantum Center and the Viterbi Department of Electrical & Computer Engineering, reconstructed and calculated a Stokes representation of a photonic anti-skyrmion state, a complex topological structure originally observed in particle physics and condensed matter systems. This breakthrough addresses limitations of existing quantum systems, which often struggle with scalability and deterministic generation of high-dimensional quantum states; the new platform allows for joint control of near-field quantum states and their mapping into a large free-space Hilbert space. Skyrmions have emerged as a robust medium for information processing, fundamentally reshaping our approach to nanoscale data, suggesting potential for secure information processing and advanced computation.

Nanophotonic TAM Control Enables Single-Photon Entanglement

Unlike earlier approaches that required intricate setups or relied on fragile multi-photon entanglement, this system leverages a miniature optical chip to robustly transform single photons into high-dimensional entangled states. The core of the innovation lies in a surface plasmon platform, a meticulously patterned gold-air interface, designed to couple light into surface plasmon polariton (SPP) modes carrying a well-defined TAM. This design allows the system to function as both a dissipative and entangling quantum circuit, according to the research team; the angular momentum of the vector SPP mode is solely characterized by its TAM, effectively making the system a dissipative and entangling non-unitary quantum circuit. The platform achieves three key functions simultaneously: coupling photons into near-field SPP modes defined by TAM, scattering these modes into free-space to create SAM-OAM entanglement, and performing quantum state tomography to map the evolution of the TAM state.

The researchers reconstructed and calculated a Stokes representation of a photonic anti-skyrmion, with a controlled topological invariant of -1.911 ± 0.236, eliminating the need for post-selection, a significant hurdle in previous experiments. The authors emphasize that this single-photon skyrmion is naturally generated by the nanophotonic platform directly from the TAM states and does not require dual-beam implementations to synthesize the vector field or multi-photon entanglement. This inherent resilience, stemming from the topology of the skyrmions, promises a more stable and scalable approach to quantum information processing, potentially enabling on-chip sources for qudits and qudit-based quantum key distribution without compromising efficiency.

Optical skyrmions emerged in 2018, but translating these benefits to single photons, creating quantum skyrmions, presented significant challenges, often requiring complex setups or fragile entanglement. This advancement differs from previous methods by reconstructing and calculating a Stokes representation of an anti-skyrmion, bypassing the need for dual-beam implementations or multi-photon entanglement. The team envisions this technology enabling scalable, high-dimensional quantum information processing, on-chip qudit sources, and secure quantum key distribution, all within the same device geometry without compromising efficiency.

Free-Space Qudit Mapping & Topological Invariant of -2

Building on earlier work demonstrating near-field photon entanglement in total angular momentum (TAM), Professor Guy Bartal’s team has created a system capable of transforming single photons into complex, entangled states without the need for post-selection or complex beam configurations. Unlike existing techniques that often rely on fragile multi-photon entanglement, this method naturally generates high-dimensional quantum states, enabling a seamless transition from near-field manipulation to free-space propagation. The team demonstrated a transformation between free-space qudit density matrices, mapping a 4×4 matrix representing TAM eigenstates into a 6×6 matrix within a linear-polarization and Bessel-mode Hilbert space, and similarly transforming a superposition state. The researchers successfully reconstructed and calculated a Stokes representation of an anti-skyrmion with a measured topological invariant of -1.911 ± 0.236, confirming the platform’s ability to characterize these complex quantum states. The scientists forecast that this technique can be used to generate novel quantum states of light with SAM and OAM by leveraging nonlinear interactions, potentially paving the way for scalable, on-chip sources for qudits and secure quantum communication protocols.