Ultralow voltage, High-speed, and Energy-efficient Cryogenic Electro-Optic Modulator

: 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 up-converting signals from electrical to optical domain, allowing massive data-transfer from 4 K superconducting (SC) electronics to room temperature environment. Such a solution is central to overcome the major bottleneck in the scalability of cryogenic systems, which currently rely on bulky copper cables that suffer from limited bandwidth, large heat load, and do not show any scalability path. A key element for realizing a cryogenic-to-room temperature optical interconnect is a high-speed electro-optic (EO) modulator operating at 4 K with operation voltage at mV scale, compatible with SC electronics. Although several cryogenic EO modulators have been demonstrated, their driving voltages are significantly large (several hundred mV to few V) compared to the mV scale voltage required for SC circuits. Here, we demonstrate a cryogenic modulator with ~10 mV peak-to-peak driving voltage and gigabits/sec data rate, with ultra-low electric and optical energy consumptions of ~10.4 atto-joules/bit and ~213 femto-joules/bit, respectively. We achieve this record performance by designing a compact optical ring resonator modulator in a heterogeneous InP-on-Silicon 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 free carrier freezing of III-V compounds at low temperatures, with moderate doping level and 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 as low as 10.4 aJ/bit. Those record results have been reached by combining the large EO functionality of QWs in the InP layer with the photonic integration capability of silicon. The minimal free-carrier freezeout of InP and its alloys is another central enabler to perform efficient modulation in our device with moderate doping, low loss, and wide modulation bandwidth at 4 K. With this active semiconductor platform, we achieve intrinsic Qs as high as 272,000 at 4 K, with much higher Q for larger radius resonators. In this work we demonstrate a single modulator device, but integrating an array of modulators with wavelength multiplexers can increase the aggregate interconnect data bandwidth. Our CRISP modulator can be integrated with CMOS and SC electronics, and it can strongly benefit emerging cryogenic classical and quantum information systems by paving the way for energy-efficient and high-bandwidth optical data-links between 4 K and room temperature, as well as for quantum sensing applications at cryogenic temperatures.