  Quantum photonic technologies, such as quantum key distribution, are already
benefiting greatly from the rise of integrated photonics. However, the
flexibility in design of these systems is often restricted by the properties of
the integration material platforms. Here, we overcome this choice by using
hybrid integration of ultra-low-loss silicon nitride waveguides with indium
phosphide electro-optic modulators to produce high-performance quantum key
distribution transceiver chips. Access to the best properties of both materials
allows us to achieve active encoding and decoding of photonic qubits on-chip at
GHz speeds and with sub-1% quantum bit error rates over long fibre distances.
We demonstrate bidirectional secure bit rates of 1.82 Mbps over 10 dB channel
attenuation and positive secure key rates out to 250 km of fibre. The results
support the imminent utility of hybrid integration for quantum photonic
circuits and the wider field of photonics.
