Integrated on-chip quantum light sources on a van der Waals platform

Scalable photonic quantum information technologies require a platform combining quantum light sources, waveguides, and detectors on a single chip. Here, we introduce a van der Waals platform comprising strain-engineered bilayer WSe$_2$ quantum emitters, integrated on multimode WS$_2$ waveguides with optimized grating couplers, enabling efficient on-chip quantum light sources. The emitters exhibit bright, highly polarized emission that couples efficiently into WS$2$ waveguides. Under resonant p-shell excitation, we observe high-purity, waveguide-coupled single-photon emission, measured using both an off-chip Hanbury Brown-Twiss configuration ($g^{(2)}(0) = 0.003^{+0.030}{-0.003}$) and an on-chip configuration ($g^{(2)}(0) = 0.076\pm0.023$). For a single output, the out-coupled single-photon count rate at the first lens reaches approximately 320 kHz under continuous-wave p-shell excitation, corresponding to an estimated waveguide-coupled rate of 1.7 MHz. 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.

💡 Research Summary

**
The authors present a fully integrated quantum photonic platform based entirely on van der Waals (vdW) materials, demonstrating an on‑chip single‑photon source with high purity and efficiency. Strain engineering is used to create localized quantum emitters in bilayer WSe₂, which exhibit bright, linearly polarized emission in the 700–800 nm range. These emitters are transferred onto multimode WS₂ slab waveguides (500 nm × 150 nm cross‑section) that have been patterned by electron‑beam lithography and reactive‑ion etching to achieve smooth sidewalls and low surface roughness. The waveguide geometry is deliberately chosen to maximize the overlap between the dipole field of the emitters and the guided modes; finite‑difference time‑domain (FDTD) simulations and scattering‑type scanning near‑field optical microscopy (sSNOM) confirm the existence of several TE and TM modes with an effective index of ≈2.9 at wavelengths around 800 nm.

A key component of the system is a compact grating coupler designed for microscope‑grade operation. Inspired by silicon‑nitride half‑ring gratings, the authors adapt the geometry to the higher refractive index of WS₂, arriving at a four‑half‑ring design with a 780 nm pitch and 55 % duty cycle. FDTD analysis predicts an out‑coupling efficiency of 18.8 % for a collection objective with NA = 0.81, which is experimentally validated by measuring the far‑field radiation pattern.

The quantum emitters are resonantly excited via p‑shell excitation at ≈802 nm. In free space, the second‑order correlation function yields g²(0) = 0.043 ± 0.027, confirming high‑purity single‑photon emission. When the emitters are coupled to the WS₂ waveguide, the authors perform two types of Hanbury Brown‑Twiss (HBT) measurements: (i) an off‑chip configuration that collects light from a grating coupler, giving g²(0) = 0.003⁺⁰·³⁰₋₀·₀₀₃, and (ii) an on‑chip configuration that uses the waveguide itself as a 50/50 beam splitter, collecting photons from both grating outputs simultaneously, yielding g²(0) = 0.076 ± 0.023. These values demonstrate that the waveguide coupling does not significantly degrade photon statistics.

Photon count rates are also reported. From the first grating coupler, an out‑coupled rate of ≈320 kHz is measured at the first collection lens; the second coupler provides ≈240 kHz. Assuming the simulated 18.8 % extraction efficiency, the internal waveguide‑coupled photon flux is estimated at 1.7 MHz (first output) and 1.28 MHz (second output). This represents one of the highest on‑chip single‑photon generation rates reported for a fully 2D‑material‑based system.

Beyond the source, the paper outlines a broader vision: WS₂ waveguides and gratings can be extended to form interferometers, filters, and resonators; superconducting NbSe₂ nanowires can be patterned as on‑chip superconducting nanowire single‑photon detectors (SNSPDs); and the entire stack can be assembled without complex deposition or high‑temperature steps, thanks to the dangling‑bond‑free nature of vdW crystals. The authors argue that this monolithic vdW platform circumvents the material incompatibilities and fabrication bottlenecks of hybrid silicon/III‑V approaches, offering a path toward scalable quantum photonic circuits that integrate generation, routing, and detection on a single chip.

In summary, the work demonstrates:

1. Deterministic creation of strain‑induced quantum emitters in bilayer WSe₂.
2. Efficient coupling of these emitters to multimode WS₂ waveguides with well‑characterized modal properties.
3. Optimized grating couplers that extract guided photons with ≈19 % efficiency.
4. Preservation of single‑photon purity in both off‑chip and on‑chip HBT measurements.
5. High internal photon flux (≈1.7 MHz) and a clear roadmap toward fully integrated vdW quantum photonic circuits.

These results constitute a significant step toward practical, scalable quantum information processing using only two‑dimensional materials.