On-chip modulator-free optical transmitter for quantum and classical communications

The practical combination of quantum cryptography and classical communications will require convergence of their technologies. In this pivotal time where both fields are transitioning towards photonic integrated architectures, it is essential to develop devices that fully leverage their hardware compatibilities, while still addressing the key issues of cost reduction, miniaturization and infrastructure energetic footprint, essential for future high- bandwidth, low-latency networks. Here, we address these issues by developing an on-chip transmitter consisting of just 3 building blocks but capable of transmitting both quantum encrypted photons and classical multi-level modulation signals. By combining optical injection locking and direct phase modulation we are able to encode pulse trains with multiple levels of differential phase, without the need of high-speed electro-optic modulators and their associated power footprint. We generate return-to-zero differential phase shift keying signals with up to 16 distinct levels. Moreover, we demonstrate multi-protocol quantum key distribution delivering state-of-the-art secure key rates. Our on-chip transmitter will facilitate the flexible combination of quantum and classical communications within a single, power-efficient device that can readily be integrated in existing high connectivity networks.