Ultralow voltage, high-speed, and energy-efficient cryogenic electro-optic modulator

Abstract

Photonic integrated circuits (PICs) at cryogenic temperatures enable a wide range of applications in scalable classical and quantum systems for computing and sensing. A promising application of cryogenic PICs is to provide optical interconnects by upconverting signals from the electrical to the optical domain, allowing a massive data transfer from 4 K superconducting (SC) electronics to the room temperature environment. Such a solution can overcome a major bottleneck in the scalability of cryogenic systems that currently rely on bulky coaxial cables that suffer from limited bandwidth, a large heat load, and poor scalability. A key element to realize a cryogenic-to-room temperature optical interconnect is a high-speed, electro-optic (EO) modulator operating at 4 K with a modulation voltage at the mV scale, compatible with SC electronics. Although several cryogenic EO modulators have been demonstrated, their driving voltages are substantially large (several hundred mV to a few V) compared to the mV scale voltage provided by SC circuits. Here, we demonstrate a cryogenic modulator with ∼10 mV peak-to-peak driving voltage and Gb/s data rate, with an ultralow electric energy consumption of ∼10.4 aJ=bit and an optical energy consumption of ∼213 fJ=bit.We achieve this record performance by designing and fabricating a compact optical ring resonator modulator in a heterogeneous InP-on-Si platform, where we optimize a multi-quantum-well layer of InAIGaAs to achieve a strong EO effect at 4 K. Unlike other semiconductors such as silicon, our platform benefits from the high-carrier mobility and minimal freecarrier freezing of III-V compounds at low temperatures, with a moderate doping level and a correspondingly low loss (intrinsic resonator Q ∼272;000). These modulators can pave the path for complex cryogenic photonic functionalities and massive data transmission between cryogenic and room-temperature electronics.