  Entangled photon-pair sources are at the core of quantum applications like
quantum key distribution, sensing, and imaging. Operation in space-limited and
adverse environments such as in satellite-based and mobile communication
requires robust entanglement sources with minimal size and weight requirements.
Here, we meet this challenge by realizing a cubic micrometer scale entangled
photon-pair source in a 3R-stacked transition metal dichalcogenide crystal. Its
crystal symmetry enables the generation of polarization-entangled Bell states
without additional components and provides tunability by simple control of the
pump polarization. Remarkably, generation rate and state tuning are decoupled,
leading to equal generation efficiency and no loss of entanglement. Combining
transition metal dichalcogenides with monolithic cavities and integrated
photonic circuitry or using quasi-phasematching opens the gate towards
ultrasmall and scalable quantum devices.
