Integrated photonic circuits (PICs) operating at cryogenic temperatures are fundamental building blocks required to achieve scalable quantum computing, and cryogenic computing technologies 1,2. Silicon PICs have matured for room temperature applications, but their cryogenic performance is limited by the absence of efficient low temperature electro-optic (EO) modulation. Here we demonstrate EO switching and modulation from room temperature down to 4 K by using the Pockels effect in integrated barium titanate (BaTiO3)based devices 3. We investigate the temperature-dependence of the nonlinear optical (NLO) properties of BaTiO3, showing an effective Pockels coefficient of 200 pm/V at 4 K. The fabricated devices exhibit an EO bandwidth of 30 GHz, ultra-low-power tuning which is 10 9 times more efficient than thermal tuning, and high-speed data modulation at 20 Gbps. Our results demonstrate a missing component for cryogenic PICs. Our results remove major roadblocks for the realisation of cryogenic-compatible systems in the field of quantum computing, supercomputing and sensing, and for interfacing those systems with instrumentation at room-temperature. Cryogenic technologies are becoming essential for future computing systems, a trend fuelled by the worldwide quest to develop quantum computing systems and future generations of highperformance classical computing systems 4,5. While most computing architectures rely solely on electronic circuits, photonic components are becoming increasingly important (Supplementary Note, SN 1). First, PICs can be used for quantum computing approaches where the quantum nature of photons is exploited as qubits 1,2. Second, optical interconnects can overcome limitations in

Photonic integrated circuits have facilitated a drastic increase in the complexity of quantum information processing, from which near-term quantum networks can benefit. Here, we report monolithically fabricated indium phosphide transmitters capable of performing measurement-deviceindependent quantum key distribution. We demonstrate an estimated 1 kbps key rate at 100 km, with predicted distances over 350 km possible. The scheme removes detector vulnerabilities through a centralised and untrusted resource, enabling quantum-secured communication with cost-effective, mass-manufacturable devices.

We present a miniaturized quantum key distribution system, designed to augment the more mature quantum key distribution systems currently commercially available. Our device is designed for the consumer market, and so size, weight and power are more important than raw performance. To achieve our form factor, the transmitter is handheld and the receiver is a larger fixed terminal. We envisage users would bring their transmitters to centrally located receivers and exchange keys which they could use at a later point. Transmitting qubits at 80 MHz, the peak key rate is in excess of 20 kbps. The transmitter device fits within an envelope of <150 ml, weighs 65 g and consumes 3.15 W of power.

We demonstrate the first electro-optic switch operating at cryogenic temperatures of 4 K with a high electrooptic bandwidth of >18 GHz. Our novel technology exploits the Pockels effect in barium titanate thin films co-integrated with silicon photonics and offers low losses, pure phase modulation, and sub-pW electro-optic tuning.