Researchers at the Technical University of Denmark achieved a g(2)(0) value of 0.003 in single-photon emission from a new device, demonstrating a significant step toward reliable quantum communication. The circuit integrates strain-engineered bilayer WSe2 quantum emitters with WS2 waveguides, forming a “van der Waals platform” for scalable quantum computing.
Measuring approximately 357 kHz for out-coupled single photons, the device’s estimated on-chip generation rate reaches 1.68 MHz, a surprisingly high speed for an integrated quantum system. These results demonstrate an efficient, integrated single-photon source and establish a pathway toward scalable photonic quantum information processing centered around nanoengineered van der Waals materials.
Van der Waals Platform for Integrated Quantum Photonics
Achieving a second-order correlation function of g(2)(0) = 0.003+0.030 -0.003, a measurement indicating exceptionally high purity in the emitted single photons and a critical step toward reliable quantum communication networks. This level of purity, determined through analysis of both single-port and cross-correlated photon streams, demonstrates the potential for generating consistent, well-defined quantum states essential for complex quantum operations.
Finite-difference time-domain models revealed that the observed increase in single-photon rates stems from the use of a multimode waveguide configuration, optimizing light propagation and collection. By employing discrete-state excitation, the team achieved both higher single-photon purity and brighter photoluminescence, minimizing spectral background noise that can interfere with quantum signals.
Strain-Engineered Bilayer WSe2 as Quantum Emitters
The integrated device demonstrated a g(2)(0) value of 0.076, achieved through multi-port photon stream analysis. The use of hexagonal boron nitride as a spacer during transfer proved beneficial, limiting fracturing of the WSe2 bilayer. This careful material selection and device architecture allowed researchers to observe bright, highly polarised emission that couples efficiently into the waveguides, a critical requirement for practical quantum photonic circuits. The resulting system supports MHz-level single-photon rates, indicating efficient coupling between the emitters and the waveguides.
This work builds toward a future implementation of a fully integrated quantum photonic circuit, where distinct TMD materials each serve a specialized function: WSe2 as single-photon emitters, WS2 as low-loss photonic circuits with couplers and beam splitters, and niobium diselenide nanowires as fast single-photon detectors. “Notably, we observe a maximum around 800 nm and consistently higher transmittance between 750-850 nm with the grated structure,” the researchers note, highlighting the optimized performance of the fabricated components.
Multimode WS2 Waveguides Enhance Emitter Coupling
Multimode WS2 waveguides demonstrate efficient coupling of light from bilayer WSe2 quantum emitters. Exploitation of bidirectional coupling within the WS2 waveguide facilitated a two-port second-order correlation experiment, confirming the robustness of the integrated system. This all-van der Waals platform offers a promising route toward bright, integrated sources for future quantum photonic circuits.
g(2)(0) = 0.003: High-Purity Single-Photon Emission
A measured g(2)(0) value of 0.003 demonstrates the high purity of single-photon emission from this new device, a result achieved by integrating strain-engineered bilayer WSe2 quantum emitters with multimode WS2 waveguides. Confocal measurements registered a 0.043±0.027 g(2)(0) value, demonstrating the inherent single-photon purity of the quantum emitter itself before waveguide coupling. Removing the beam splitter in the Hanbury Brown and Twiss setup and implementing a second collection spot aligned with another grating coupler allowed for two-port cross-correlation, further refining the analysis of single-photon emission characteristics.
Waveguide-Coupled Rate Exceeds 1.68 MHz
An on-chip quantum light source achieved a waveguide-coupled single-photon rate of at least 1.68 MHz, a figure representing a substantial advance in integrated quantum photonics. This rate, determined from an out-coupled count of approximately 357 kHz, establishes a lower bound for photon generation within the device itself. Measurements yielded g(2)(0) = 0.003+0.030 -0.003 and g(2)(0) = 0.076 ± 0.023. The device’s architecture addresses a key challenge in scaling quantum technologies: the need for monolithic integration of diverse material systems.
Current implementations often combine silicon or silicon nitride photonics with III-V semiconductors for emitters and superconducting materials for detectors, requiring complex fabrication processes. These results demonstrate an efficient, integrated single-photon source and establish a pathway toward scalable photonic quantum information processing centred around nanoengineered van der Waals materials.
The team’s approach uses the versatility of transition metal dichalcogenides, allowing for the combination of distinct TMDs to engineer specific optical and electronic functionalities. The researchers note that fabrication imperfections and multimode coupling likely contribute to this upper bound, suggesting the actual generation rate could be even higher.
The ability to achieve these rates with a single material system represents a step toward realizing scalable photonic quantum information processing, potentially enabling the creation of complex quantum photonic circuits on a single chip. The work establishes a foundation for future research focused on refining the device’s performance and exploring the integration of additional quantum photonic components within the van der Waals platform.
This result validates the potential of the architecture for complex quantum photonic circuits, moving beyond simple single-emitter characterization. The device, built from bilayer WSe2 quantum emitters integrated with multimode WS2 waveguides, demonstrated efficient on-chip coupling of polarized single photons.
hBN Spacer Improves WSe2 Transfer and Brightness
Employing hexagonal boron nitride (hBN) as a spacer during the transfer process demonstrably improves both the integrity and performance of bilayer WSe2 quantum emitters, a critical step in fabricating robust on-chip quantum light sources. The material limits fracturing during transfer, a common issue when working with two-dimensional materials, and importantly, increases the overall brightness of the emitters themselves.
This enhancement is particularly valuable as it simplifies the experimental setup required for characterizing these delicate quantum systems. Excitation via discrete states, rather than above-band excitation, further suppresses background emission and amplifies the signal from the quantum emitter, a technique that directly contributes to improved measurement clarity.
TMD Integration Roadmap for Quantum Circuits
An estimated minimum waveguide-coupled single-photon rate of 1.68 MHz demonstrates the efficiency of a newly integrated device. This on-chip source achieves this rate by directly integrating quantum emitters with a waveguide, a departure from current implementations that rely on disparate material systems like silicon, silicon nitride and III-V semiconductors. Combining these materials requires complex fabrication processes that present a significant bottleneck for scaling photonic quantum technologies.
The device’s architecture aims to address this challenge by utilizing transition metal dichalcogenides (TMDs) as a unified material platform, using their versatile van der Waals assembly. The team transferred a thin hexagonal boron nitride layer onto a waveguide, followed by the WSe2 bilayer, maintaining proximity between the two materials to facilitate coupling. The researchers state, emphasizing the replicability of the results, the bidirectional coupling of the WS2 waveguide was exploited to establish a two-port second-order correlation experiment, indicating potential for advanced quantum networking.
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