Metasurfaces Generate Maximally Entangled Bell States via Hamiltonian-Driven Spin-Entanglement

Entanglement, a fundamental phenomenon in quantum mechanics, underpins emerging technologies such as quantum computing and secure communication, but creating and maintaining entanglement remains a significant challenge. Ramaseshan R, Prateek P. Kulkarni, and Sharanya Madhusudhan, all from PES University, alongside Kaustav Bhowmick, present a theoretical framework demonstrating how optical metasurfaces, ultrathin engineered materials, offer a potentially revolutionary approach to generating strong quantum correlations. Their work reveals that these compact surfaces can efficiently transform initially unconnected quantum states into highly entangled Bell states, achieving a concurrence of approximately 99.5%. This research establishes metasurfaces not simply as miniaturised optical components but as a viable platform for building scalable and robust photonic systems capable of sustaining quantum correlations for up to 29 microseconds, paving the way for more practical and efficient quantum technologies.

Entanglement Sources and Conventional Limitations

Entanglement represents a key phenomenon in quantum mechanics, enabling advances in quantum computing, cryptography, and teleportation. Traditionally, physical systems such as optical beam splitters, nonlinear crystals, trapped ions, superconducting qubits, and quantum dots have served as the primary means of generating and manipulating entanglement.

Optical beam splitters and parametric down-conversion in nonlinear media have proven instrumental in producing entangled states. However, these conventional approaches face limitations, motivating the exploration of alternative methods for generating and controlling entangled states.

Metasurface Fabrication for Quantum Photon Control

Conventional methods often require bulky optical setups, precise alignment, and phase stability, making large-scale integration challenging. Recent advancements in structured photonic systems, particularly metasurfaces, have opened new possibilities for entanglement generation and quantum information processing.

Metasurfaces, artificially engineered two-dimensional arrays of sub-wavelength resonators, offer powerful tools for controlling light at the nanoscale. Unlike traditional optical elements, metasurfaces allow for tailored wavefront shaping, spin-orbit interactions, and polarization control on an ultrathin platform.

Experimental work demonstrates the viability of engineered metasurfaces for single-photon control, proving their consistency for quantum applications. A key feature of metasurfaces is their ability to mediate spin-dependent interactions, enabling photon-photon coupling via structured optical responses.

This work demonstrates that a metasurface can generate Bell states through spin-photon interactions governed by a Hamiltonian approach. Unlike conventional entanglement mechanisms, this approach utilizes a linearly arranged periodic nano-ring metasurface, applying its properties to a Hamiltonian that best describes its behaviour to obtain spin entanglement between incident photons.

The evolution of the system under this Hamiltonian shows that an initially separable spin state evolves into a maximally entangled Bell state, confirming metasurfaces as viable quantum entanglement generators. Furthermore, the nature of quantum correlations in this system has been analyzed by distinguishing classical correlations from quantum correlations and calculating quantum discord to quantify non-classical correlations that persist even under environmental decoherence.

This establishes metasurfaces as a better platform to realize quantum photonics, offering a compact and tunable alternative to traditional optical components in quantum information processing. The key novelties of this work lie in formulating a spin-interaction Hamiltonian tailored to a realistic metasurface geometry, introducing a spatially varying interaction strength derived experimentally from power distributions, and demonstrating that metasurfaces sustain quantum discord under realistic decoherence, confirming their suitability for quantum technologies.

Metasurfaces Preserve and Generate Photon Entanglement

This study presents a comprehensive theoretical investigation of metasurface-based quantum photonic systems as an efficient platform for generating and preserving quantum correlations. Beginning with a Hamiltonian framework, the research derives the evolution of photon spin entanglement, demonstrating that a maximally entangled Bell state can be achieved through metasurface-mediated spin-spin coupling.

The spatially varying interaction strength, estimated from measured power distributions, reveals that linear dielectric metasurfaces, such as silicon, exhibit the highest peak coupling and most localized interaction region. This contrasts with nonlinear materials, which show lower peak coupling and broader spatial profiles due to their dispersive behaviour.

The fidelity of entanglement was quantified using the concurrence measure, where the metasurface achieved an average concurrence of 0.921, outperforming spontaneous parametric down-conversion (SPDC) and proving comparable to spontaneous four-wave mixing (SFWM). To evaluate the coherence retention capabilities of each platform, the coherence times of SPDC, SFWM, and the metasurface were analysed.

SPDC is limited by its broad spectral bandwidth and group velocity mismatch, resulting in sub-picosecond coherence times, while SFWM coherence is constrained by atomic spin-wave dephasing mechanisms, yielding lifetimes on the order of tens of nanoseconds. In contrast, the metasurface architecture maintains spin coherence up to 26 μs, limited primarily by Rayleigh scattering in air.

The evolution of quantum discord, modelled using a Werner-state decoherence framework, shows that discord persists in the metasurface configuration up to approximately 29.6 μs, orders of magnitude longer than in SPDC or SFWM implementations. These results confirm that metasurfaces not only enable the generation of Bell states but also provide superior longevity in preserving quantum correlations.

As such, metasurfaces are validated as a promising and scalable platform for quantum information processing and discord-driven quantum technologies.