Ultracompact and tunable Bell state source using transition metal dichalcogenides

Entangled photon-pairs are crucial for applications like quantum key distribution, sensing and imaging. For prospective use in real world devices, the challenge for Entangled Photon-Pair Sources (EPS) is to simultaneously meet high requirements regarding state fidelity, tunability etc. while maintaining a small footprint and high robustness. In this work, we develop an EPS that meets these demands. Using a sub-micron thick Transition Metal Dichalcogenide (TMD) crystal, we show tunable generation of polarization entangled Bell states via Spontaneous Parametric Down-Conversion (SPDC). To the best of our knowledge, this is the first realization of SPDC in a TMD. In particular, we employ the TMD 3R-phase molybdenum disulfide (3R-MoS2), which due to its crystal symmetry intrinsically creates entanglement without needing external optical components. We experimentally demonstrate tuning between different maximally entangled states with constant generation efficiency and show pathways towards highly efficient and tunable TMD-based EPS using quasi-phasematching or cavity integration.