Date of Award
Spring 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Electrical Engineering (ENAS)
First Advisor
Tang, Hong
Abstract
A quantum network combining superconducting processors and low-loss optical fibers has thepotential to revolutionize the world of information technology. However, the large frequency gap between superconducting qubits (∼ 7 GHz) and telecommunication fibers (∼ 200 THz) present a significant challenge that requires the development of microwave-to-optical (M2O) frequency transducers. For a M2O transducer to successfully transmit quantum signals, two stringent criteria need to be met: near-unity efficiency and near-ground state added noise. Despite encouraging progress in the past decade, quantum signal transduction remains elusive with many challenges to be addressed. In this thesis, we focus on M2O transducers based on cavity electro-optics leveraging Pock- els effect that allows direct interaction between microwave and optical fields. A triple-resonant scheme is adopted to enhance conversion efficiency, which requires frequency matching between optical resonant frequency differential and microwave resonant frequency. To address this chal- lenge, we developed a frequency-tuning technology for high-quality factor superconducting res- onators with a novel grid design on the high-kinetic inductance wire. Frequency tuning is realized by applying an external magnetic field which induced screening current around the holes. The screening current then modulates the kinetic inductance and therefore changes the resonant fre- quency. With this method, we achieved frequency tuning over 3% and quality factor over 80k for a superconducting resonator made from thin-film NbN. We also explored a flip-chip approach to fabricate an electro-optical M2O transducer. This approach allows independent optimization of both superconducting and photonic components and largely improves fabrication yield. Through 2 careful calibration of the transducer operating in a millikelvin environment, we achieve an internal efficiency of 0.15% and a total efficiency of 2.4 ∗ 10^−5. Although higher transduction efficiency has been achieved at elevated temperatures, our demonstration is the first to show the transduction of integrated electro-optical transducer in the quantum ground state. The rest of the thesis focuses on studying the added noise of a M2O transducer and noise mitigation strategies. We mainly investigate the microwave fluctuations of the superconducting mode as the main source of transducer added noise. To characterize the microwave noise from a superconducting resonator, we developed a protocol to precisely calibrate the microwave noise on the sub-single-photon level using quantum-limited superconducting parametric amplifiers. We then investigate the origin of the optically-induced microwave fluctuations in a EO transducer by studying the noise dynamics as well as dependence on the optical drives. Our results reveal differ- ent noise generation mechanisms and we discuss corresponding strategies for noise suppression. Since superconducting parametric amplifiers are integral to weak signal readout, we developed a novel nanobridge kinetic-inductance parametric amplifier (NKPA) that can be integrated to a wide range of experiments requiring magnetic fields. By exploiting thin-film NbN nanobridge as the source of nonlinearity, we achieved ultra-low noise amplification of NKPA that is limited by quantum mechanics. Moreover, we show that NKPA maintains the excellent performance in magnetic fields up to 427 mT which is limited by testing instrument. We also investigated an experimental scheme to effectively balance high power dissipation and ground state cooling required by efficient low-noise M2O transduction. The solution is to leverage radiative cooling of the superconducting resonator to effectively suppress the microwave thermal excitation while allowing the device material to be thermalized to an elevated temperature. This effect is first demonstrated in our experiments where a 10.5 GHz superconducting resonator is mounted on the 1 K plate while the resonance is cooled to near-ground-state by radiative dissipa- tion to a cold load at milli-Kelvin temperature. This cooling scheme would allow microwave-to- optical transducers to access higher cooling power for pump dissipation while maintaining ground 3 state operation. We conclude the thesis with pathways towards M2O transduction of microwave quantum sig- nals, where we discuss a new cavity electro-optical transducer design that could reach desired performance and an experimental proposal to demonstrate the quantum transduction of such a M2O transducer.
Recommended Citation
Xu, Mingrui, "Towards Microwave-to-Optical Transduction of Quantum Microwave Signals" (2023). Yale Graduate School of Arts and Sciences Dissertations. 1064.
https://elischolar.library.yale.edu/gsas_dissertations/1064
