Integrated electrical and photonic circuits (PIC) operating at cryogenic
temperatures are fundamental building blocks required to achieve scalable
quantum computing, and cryogenic computing technologies. Optical interconnects
offer better performance and thermal insulation than electrical wires and are
imperative for true quantum communication. 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. While
detectors and lasers perform better at low temperature, cryogenic optical
switching remains an unsolved challenge. 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. We report the nonlinear
optical (NLO) properties of BaTiO3 in a temperature range which has previously
not been explored, showing an effective Pockels coefficient of 200 pm/V at 4 K.
We demonstrate the largest EO bandwidth (30 GHz) of any cryogenic switch to
date, 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. It removes major roadblocks for the
realisation of novel cryogenic-compatible systems in the field of quantum
computing and supercomputing, and for interfacing those systems with the real
world at room-temperature
