Hybrid entangled states composed of multiple degrees of freedom may improve the transmission and conversion of quantum information for communications and computing. In this paper, Francesconi et al. demonstrate how to directly produce polarization-frequency entangled-photon pairs at room temperature and at telecommunications wavelengths. By carefully crafting an AlGaAs integrated photonic waveguide with a small modal birefringence—the refractive index is engineered to be slightly different for the transverse electric and magnetic modes—the researchers produce a unique biphoton state through type-II spontaneous parametric down conversion. In this hybrid state, the frequency of each photon is always correlated with the photon’s polarization, thus comprising composite polarization-frequency encoding. An advantage of this kind of entanglement is its versatility, as it allows for the biphoton wavefunction to be adapted to specific experiments with different encodings using standard optical elements, such as polarizers, optical filters, or modulators. Photon pairs are produced at a rate of 10 million pairs per second at the output of the chip using 30 mW of pump power. The fidelity to the ideal biphoton state is 0.85, which may be further improved by reducing the optical loss at the chip facets and correcting imperfections in the pump spatial mode. These results are encouraging for a variety of quantum information tasks, especially for improving the data transmission rates and resilience to noise for quantum communications.

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