Hybrid integrated photonics for coherent microwave-optical transduction

Abstract

Superconducting circuits are important technological tools that have found various applications, notably hypothetical future superconducting quantum computers. Scaling such systems further is, however, proving challenging because of thermal heat load and space constraints in cryostats. One way of improving the scalability of such systems would be to resort to optical fibers for controlling the circuits in cryostats, as well as mediating long-distance interaction between them while preserving quantum coherence. Such a strategy relies on building a coherent interface between GHz frequencies and optical frequencies. This interface, referred to as "microwave-optical transducer", would need to exhibit high photon conversion efficiencies and very low added noise, ideally less than a quantum, in a cryogenic environment. Since the prospects of weaving networks of optically connected superconducting quantum processors have now attracted the attention of the scientific community for more than a decade, very efficient transducers have already been proposed and realized. However, despite their impressive performances against selected metrics, often equivalent to optimizing the on-chip conversion efficiency alone, none of the approaches pursued so far really solved the qubit interconnection problems in ways that could be applied to large-scale systems in the near future. The aim of this thesis was to develop transduction approaches that would ultimately enable optical driving of superconducting circuits and quantum state transfer between microwave and optical frequencies, while at the same time ensuring good scalability, both in terms of wafer manufacturability and integration in cryogenic environments. Integrated photonic circuits based on silicon nitride appeared as a promising platform, as silicon nitride is a CMOS-certified material, has a transparency window compatible with optical fibers and lacks two photon absorption. Realizing the coupling between microwave and optical frequencies in a way similar to what is done in optomechanics is desirable, because the field of optomechanics has seen the establishement of techniques that enable control of the low frequency degree of freedom at the quantum mechanical level, including ground state cooling, control of the susceptibility by dynamical backaction, quantum state transfer and entanglement. The heterogeneous integration of nonlinear crystalline thin films, such as lithium niobate and lithium tantalate, onto silicon nitride photonic circuitry endows it with an electro-optic activity that emulates the optomechanical interaction, while offering an inherent access to the microwave degree of freedom. However, material nonlinearities, including photorefraction, charge screening, and hysteresis, introduce complications that motivate exploring alternative approaches. Here, alongside an electro-optic implementation, a transducer that leverages the interaction of bulk acoustic resonances excited by micro-electromechanical systems with whispering-gallery-like modes of microresonators is presented. Coherent transduction between microwave and optical fields is demonstrated, and optomechanical dynamical backaction is observed. The transducers fast bandwidth, compatibility with multiplexing and ability to down-convert microwaves suggest interesting prospects, particularly upon further improvements of the optomechanical coupling.