Book excerpt: Cryogenic technologies promise to overcome existing bottlenecks in a range of science and engineering fields including quantum computing, high performance computing and single-photon communication systems. As these technologies mature and systems scale, a readout solution for high speed, low power data transfer between the cryogenic environment and room temperature becomes essential. The use of optical links for such data transfer - in what is known as cryogenic optical readout - is appealing due to the low heat conduction of optical fiber and the possibility to exploit wavelength division multiplexing architectures. However, existing demonstrations suffer from large power dissipation associated with amplifying the millivolt signals generated by the cryogenic systems. This thesis deals with the development of silicon photonic modulators operating at cryogenic temperatures and capable of modulating an optical carrier with millivol-level driving signals. We show cryogenic operation of CMOS photonic resonant modulators in the forward bias regime with high modulation efficiency and reduced power dissipation, and demonstrate cryogenic optical readout of a superconducting single photon detector. We also present a new operation mode for optical modulators that leverages parasitic photocurrent to achieve electrical gain and reduce power dissipation. Modulation with signal levels down to 4 mVpp and electrical power dissipation in the zJ/bit range is demonstrated. This thesis sets the foundation for silicon photonics to realize scalable, low power, high throughput cryogenic readout, addressing one of the key remaining challenges for the wide adoption of cryogenic technologies.