University of Vienna Team Builds Native Entanglement Source for Quantum Photonics

Utilising ultrathin semiconductors, researchers have demonstrated a new technique for efficiently generating entangled photons. Benjamin Braun and colleagues at University of Vienna, in collaboration with University of L’Aquila and Columbia University, utilise periodically-poled transition metal dichalcogenides to overcome limitations imposed by material coherence length. The research introduces quasi-phase matching by mechanically altering the material’s nonlinearity, successfully scaling pair-production rates while maintaining high-fidelity, symmetry-generated polarization entanglement exceeding 99 percent. It offers a new approach to engineering quantum light within nanophotonic systems and clarifies the relationship between crystal symmetry and propagation in thin nonlinear media.

High-fidelity entanglement via quasi-phase matching in transition metal dichalcogenides

Entanglement now surpasses 99 percent, representing a significant advance over earlier methods limited by material coherence length, which historically restricted both efficiency and photon pair production scaling. Traditionally, achieving high-fidelity entanglement necessitated complex optical arrangements and bulky nonlinear materials to satisfy phase-matching conditions for spontaneous parametric down-conversion (SPDC). SPDC is a nonlinear optical process where a pump photon spontaneously splits into two lower-energy photons, known as the signal and idler, while conserving energy and momentum. However, periodically-poled transition metal dichalcogenides offer a solution by enabling quasi-phase matching. This new approach mechanically alters the material’s nonlinearity at precise intervals, a process known as quasi-phase matching, to enhance entangled photon pair creation without reducing entanglement quality. The coherence length, a critical parameter in nonlinear optics, dictates the effective interaction length for SPDC; beyond this length, the phase difference between the generated photons accumulates, diminishing entanglement. Quasi-phase matching effectively extends this interaction length by periodically reversing the nonlinear susceptibility of the material, compensating for the phase mismatch and allowing for efficient down-conversion even in materials with short coherence lengths.

Concurrence, a measure quantifying entanglement strength ranging from 0 to 1, was mapped across the Poincaré sphere of pump polarization, consistently revealing high values for linear polarizations and confirming the entanglement’s robust character. Pump polarization was varied between horizontal, vertical, left- and right-circular settings, maintaining entanglement along the sphere’s equator, with concurrence values approaching unity. This indicates a high degree of polarization correlation between the generated photon pairs. Simulated heatmaps corroborated these experimental findings, demonstrating a clear connection between crystal symmetry and entanglement preservation; the material’s structure natively generates polarization-entangled photon pairs. Efficient generation of these states, however, remains limited by the material’s coherence length, and scaling to larger areas or more complex quantum circuits will necessitate improvements in material quality and fabrication techniques. Further research is needed to optimise the poling period and material thickness to maximise entanglement efficiency and minimise unwanted background noise.

Mechanical limitations hinder scalable periodic poling of transition metal dichalcogenides

Polarization entanglement fidelities exceeding 99 percent were achieved through efficient generation of entangled photon pairs using periodically-poled transition metal dichalcogenides. Quasi-phase matching was successfully implemented by mechanically altering the material’s nonlinearity at intervals corresponding to its coherence length. The coherence length is determined by the material’s refractive index and the wavelength of the generated photons, typically on the order of micrometres for these materials. Maintaining or automating this precise alignment for larger structures presents a key limitation to the scalability of this mechanical ‘flipping’ process, as does fabrication with consistently accurate periodic poling over extended areas. The mechanical process currently relies on manual manipulation, making it challenging to create large-scale, integrated quantum photonic circuits. Developing automated poling techniques, such as focused ion beam milling or laser-induced forward transfer, could address this limitation.

Recent investigations into these materials’ nonlinear optical effects at the nanoscale underpin this work. Earlier studies established that these materials could generate polarization-entangled photons due to their unique crystal symmetry, specifically their lack of inversion symmetry, but conversion efficiency was restricted by the material’s coherence length. The current work overcomes this restriction by effectively extending the interaction length beyond this inherent limit. These findings clarify the relationship between crystal symmetry and light propagation within these thin materials, providing a new pathway for designing quantum light sources within nanophotonic systems. Transition metal dichalcogenides, such as molybdenum disulphide (MoS₂) and tungsten diselenide (WSe₂), are two-dimensional materials with strong light-matter interactions, making them ideal candidates for nonlinear optical applications.

Enhanced photon pair generation via periodic poling of transition metal dichalcogenides

A method to scale the production of entangled photons was developed using periodically-poled transition metal dichalcogenides, a class of ultrathin materials crucial for technologies like quantum computing and secure communication. Traditionally, efficient generation of these pairs required bulky optical components to achieve phase matching. This approach bypassed this limitation by utilising these materials, which exhibit strong nonlinear optical properties even in extremely thin layers, allowing them to circumvent conventional phase-matching techniques and rely on the material’s inherent crystal symmetry to generate polarization-entangled photon pairs. The strong nonlinearity arises from the unique electronic band structure of these materials, enabling efficient photon pair generation even with low pump power.

This research clarifies the relationship between crystal structure and light propagation, building upon knowledge from crystals such as beta-barium borate and periodically-poled lithium niobate used in spontaneous parametric down-conversion. These established materials have been widely used for SPDC due to their relatively high nonlinear coefficients and ability to be phase-matched. The technique boosted the rate of entangled photon pair production while maintaining high fidelity, exceeding 99% in polarization entanglement. This understanding provides a new pathway for designing quantum light sources within nanophotonic systems. Fabricating ultrathin semiconductors with a repeating structural pattern enables quasi-phase matching, effectively boosting the efficiency of entangled photon pair creation and circumventing limitations previously imposed by the coherence length. The ability to integrate these quantum light sources onto compact nanophotonic chips could pave the way for developing practical quantum technologies, including quantum key distribution and quantum sensors.

Researchers successfully scaled up the production of entangled photons by utilising periodically-poled transition metal dichalcogenides. This method overcomes limitations of traditional techniques which require bulky optics for phase matching, instead relying on the material’s inherent symmetry to generate entanglement. By mechanically altering the material’s nonlinearity at intervals matching the coherence length, the pair-production rate was increased while maintaining over 99% fidelity in polarization entanglement. The work clarifies how crystal symmetry and light propagation interact in thin materials, offering a new route for engineering quantum light in nanophotonic systems.