Quantum skyrmions and high dimensional entanglement mediated by nanophotonics

Figure 1 | Nanophotonic generation, tomography, and reconstruction of a photonic anti-skyrmion state. a. Decomposition of the vector field components into the component perpendicular to the plane (out of plane) and the in-plane components and SEM micrograph of the nanophotonic system - a nanopatterned gold layer evaporated on a glass substrate. The circular input coupler (A), which couple's photons of a given polarization to plasmons with a well-defined TAM, is milled through the entire gold layer. The annular out-coupler ring (B) is milled only through half of it, scattering the SPPs into photonic modes propagating towards the camera. b. Quantum state tomography of photonic free-space photons created from near-field qubits. The panels show the intensity of the projection of a heralded single photon on the four different polarizations, as recorded by an EMCCD camera. This action is performed for four different polarizations of the incident photon. The left column lists the 4 possibilities of the polarization of photons incident upon the nanophotonic system, while the top row lists the measured polarization.  Left panel: Experimental results. Right panel: Fitting results (via mean squared error) for each intensity image to the state of the photon projected to ideal Bessel and Bessel-Hermite bases. c. Transformation between free space qudit density matrices. The 4X4 density matrix orienting from the TAM eigenstate J₁ transformed into a 6X6 density matrix in the linear-polarization and HB modes Hilbert space, and The 4X4 density matrix orienting from the TAM superposition state J_- ​, transformed into a 6X6 density matrix circular-polarization and Bessel-modes Hilbert space. d. Reconstructed and calculated Stokes representation c. Vector representation of the unit vector of the electric field (color coded for the value of its axial component red =1 and blue =-1), showing generation of anti-skyrmion with topological invariant of -2. Left: Reconstruction, and Right: Simulation. We obtain N=-1.911±0.236. @Guy Bartal et al.

Skyrmions - intricate, topologically protected vector field textures - have emerged as a profoundly robust medium for information processing, fundamentally reshaping our approach to nanoscale data. These topological structures were originally conceptualized in particle physics and widely utilized in condensed matter. Skyrmions were brought into the photonic realm only in 2018, through the pioneering discovery of optical skyrmions by the Technion team. Now, as we enter the quantum era, the demand to translate these topological benefits to the single-photon level has driven the pursuit of quantum skyrmions for high-dimensional quantum computing and secure information processing. However, conventional quantum systems are typically bulky and can hardly be deployed in scalable on-chip settings. Existing methods often require complex dual-beam implementations to synthesize the necessary vector fields, rely on fragile multi-photon entanglement, or necessitate post-selecting a skyrmionic component from a broader quantum state. As such, they lack both the capacity for device miniaturization and the natural, deterministic generation of high-dimensional quantum states.

In a new paper, an experimental team led by Professor Guy Bartal from the Helen Diller Quantum Center and the Viterbi Department of Electrical & Computer Engineering, Technion – Israel Institute of Technology ,have developed an advanced nanophotonic platform for jointly controlling quantum states in the near-field and mapping them into a large free-space Hilbert space. This work builds directly upon the group's foundational experimental demonstration of near-field photon entanglement in total angular momentum (TAM), published last year in Nature. When photons are tightly confined on the sub-wavelength scale, their spin angular momentum (SAM) and orbital angular momentum (OAM) become inseparable, leaving TAM as the good quantum number. Based on this principle, the researchers designed a miniature optical chip that utilizes TAM to realize the robust transformation of heralded single photons into complex, entangled states. Remarkably, the researchers showed that these TAM states naturally form quantum optical Stokes skyrmion with controlled topological invariant of ±2, completely eliminating the need for post-selection. As such, their platform allows us to robustly morph quantum information and map non-trivial topology seamlessly into free-space.

The experimental quantum system is centered around a surface plasmon platform - a gold-air interface patterned with a circular grating meticulously designed to couple light into surface plasmon polariton (SPP) modes that carry this well-defined TAM. “Since the angular momentum of the vector SPP mode is solely characterized by its TAM, the system effectively acts as both a dissipative and entangling non-unitary quantum circuit” say the researchers.

The circularly-symmetric nanophotonic platform combines three purposes in one: (1) to couple incident polarized photons into near-field SPP modes defined purely by their TAM, (2) scatter these propagating TAM modes out to free-space , thereby inherently transform the TAM state into SAM-OAM entanglement; and (3) to perform precise quantum state tomography (QST) using heralded single photons, definitively mapping how the near-field TAM state evolves into a high-dimensional free-space entangled Bell like state."

“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 nor multi-photon entanglement” added the authors. “this provides a robust generation of quantum skyrmions, on top of their inherent resilience to perturbations, emanating from their topology”.

"The presented technique can be used to generate novel quantum states of light with SAM and OAM by leveraging nonlinear interactions between a strong free-space pump and quantum surface-confined states. This breakthrough could open a new venue for scalable, high-dimensional quantum information processing, on-chip sources for qudits, and qudit-based quantum key distribution relying on exactly the same device geometry without causing efficiency issues," the scientists forecas.

Reference

Quantum skyrmions and high dimensional entanglement mediated by nanophotonics

Amit Kam, Shai Tsesses, Lior Fridman, Yigal Ilin, Amir Sivan, Guy Sayer, Stav Lotan, Kobi Cohen, Amit Shaham, Liat Nemirovsky-Levy, Larisa Popilevsky, Aviv Karnieli, Meir Orenstein, Mordechai Segev & Guy Bartal

https://link.springer.com/article/10.1186/s43593-026-00124-1

Light Publishing Center, Changchun Institute of Optics, Fine Mechanics And Physics, CAS