Quantum computing is one of the great scientific challenges of the 21st century. Small-scale systems today promise to surpass classical computers in the coming years and to enable the solution of classically intractable computational tasks in the fields of quantum chemistry, optimization, cryptography and more. In contrast to classical computers, quantum computers based on superconducting quantum bits (qubits) can to date not be linked over long distance in a network to improve their computing capacity, since devices, which preserve the quantumstate when it is transferred from one machine to another, are not available. Several approaches are being pursued to realize such a component, one of themost promising to date makes use of an intermediary, micromechanical element that enables quantum coherent conversion between the information present in the quantum computer and an optical fiber, without compromising the quantum nature of the information, via optomechanical interaction. This approach could allow fiber-optic quantum networks between separate quantum computers based on superconducting qubits in the future. In this work a platformfor such a microwave-to-optic link was developed based on the piezoelectric material gallium phosphide. This III-V semiconductor offers not only a piezoelectric coupling between the electric field of a microwave circuit and a mechanicalmode, but also a wide optical bandgap E_g = 2.26eV which reduces nonlinear optical absorption in the device and a large refractive index n(1550nm) = 3.01 which allows strong optical confinement at near-infrared wavelengths. Importantly and in contrast to other approaches with gallium phosphide, an epitaxially grown, single crystal thin film of the material is integrated directly on a silicon wafer with pre-structured niobium electrodes by direct wafer-bonding. This opens up the possibility of integrating the device design presented here directly with superconducting qubits fabricated with this material system. A microwave-to-optical transducer design was simulated and fabricated in the galliumphosphideon- silicon platform. The device was found to exhibit large vacuum optomechanical coupling rates g0/2 pi ~ 290kHz and a high intrinsic optical quality factor Q >10^5 while at the same time permitting electromechanical coupling to a microwave electrode. Coherent microwave-tooptical transductionwas shown at room temperature for this device and the electromechanical coupling rate could be extracted from a model derived by input-output theory. The electromechanical coupling between the electro-optomechanical device and a superconducting qubit was estimated to be g/2 pi = O(200kHz) which indicates that strong coupling between the here presented device and a superconducting transmon qubit is achievable. In addition, superconducting microwave cavities with high quality factor at single photon energy Q ~ 5x10^5 were fabricated and measured to verify that fabrication process of the microwave-to-optical transducer is compatible with high-quality superconducting microwave circuits.
Quantum computing is one of the great scientific challenges of the 21st century. Small-scale systems today promise to surpass classical computers in the coming years and to enable the solution of classically intractable computational tasks in the fields of quantum chemistry, optimization, cryptography and more. In contrast to classical computers, quantum computers based on superconducting quantum bits (qubits) can to date not be linked over long distance in a network to improve their computing capacity, since devices, which preserve the quantumstate when it is transferred from one machine to another, are not available. Several approaches are being pursued to realize such a component, one of themost promising to date makes use of an intermediary, micromechanical element that enables quantum coherent conversion between the information present in the quantum computer and an optical fiber, without compromising the quantum nature of the information, via optomechanical interaction. This approach could allow fiber-optic quantum networks between separate quantum computers based on superconducting qubits in the future. In this work a platformfor such a microwave-to-optic link was developed based on the piezoelectric material gallium phosphide. This III-V semiconductor offers not only a piezoelectric coupling between the electric field of a microwave circuit and a mechanicalmode, but also a wide optical bandgap E_g = 2.26eV which reduces nonlinear optical absorption in the device and a large refractive index n(1550nm) = 3.01 which allows strong optical confinement at near-infrared wavelengths. Importantly and in contrast to other approaches with gallium phosphide, an epitaxially grown, single crystal thin film of the material is integrated directly on a silicon wafer with pre-structured niobium electrodes by direct wafer-bonding. This opens up the possibility of integrating the device design presented here directly with superconducting qubits fabricated with this material system. A microwave-to-optical transducer design was simulated and fabricated in the galliumphosphideon- silicon platform. The device was found to exhibit large vacuum optomechanical coupling rates g0/2 pi ~ 290kHz and a high intrinsic optical quality factor Q >10^5 while at the same time permitting electromechanical coupling to a microwave electrode. Coherent microwave-tooptical transductionwas shown at room temperature for this device and the electromechanical coupling rate could be extracted from a model derived by input-output theory. The electromechanical coupling between the electro-optomechanical device and a superconducting qubit was estimated to be g/2 pi = O(200kHz) which indicates that strong coupling between the here presented device and a superconducting transmon qubit is achievable. In addition, superconducting microwave cavities with high quality factor at single photon energy Q ~ 5x10^5 were fabricated and measured to verify that fabrication process of the microwave-to-optical transducer is compatible with high-quality superconducting microwave circuits.
Abstract Electrically actuated optomechanical resonators provide a route to quantum-coherent, bidirectional conversion of microwave and optical photons. Such devices could enable optical interconnection of quantum computers based on qubits operating at microwave frequencies. Here we present a platform for microwave-to-optical conversion comprising a photonic crystal cavity made of single-crystal, piezoelectric gallium phosphide integrated on pre-fabricated niobium circuits on an intrinsic silicon substrate. The devices exploit spatially extended, sideband-resolved mechanical breathing modes at ~3.2 GHz, with vacuum optomechanical coupling rates of up to g0/2π ≈ 300 kHz. The mechanical modes are driven by integrated microwave electrodes via the inverse piezoelectric effect. We estimate that the system could achieve an electromechanical coupling rate to a superconducting transmon qubit of ~200 kHz. Our work represents a decisive step towards integration of piezoelectro-optomechanical interfaces with superconducting quantum processors.
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.
Electro-optic modulation is central to classical optical communications and emerging quantum technologies. High-confinement optomechanical crystal modulators enable microwave-optical transduction through strong optomechanical interactions and offer a promising interface between superconducting qubits and optical fibers. However, their performance is limited by thermal noise from optical absorption. Release-free optomechanical crystals provide improved thermal anchoring but have not yet been integrated into a microwave-optical transducer. Here, we demonstrate a release-free electro-optomechanical transducer combining strong optomechanical interactions in silicon with the efficient piezoelectricity of lithium niobate via micro-transfer printing. We observe electro- and optomechanical coupling rates compatible with quantum-level operation when co-integrated with a superconducting microwave circuit. This advance moves release-free electro-optomechanical devices toward practical microwave-optical interfaces.
Coherent transduction of quantum states from the microwave to the optical domain can play a key role in quantum networking and distributed quantum computing. We present the design of a piezo-optomechanical device formed in a hybrid lithium niobate on silicon platform, that is suitable for microwave-to-optical quantum transduction. Our design is based on acoustic hybridization of an ultra-low mode volume piezoacoustic cavity with an optomechanical crystal cavity. The strong piezoelectric nature of lithium niobate allows us to mediate transduction via an acoustic mode which only minimally interacts with the lithium niobate, and is predominantly silicon-like, with very low electrical and acoustic loss. We estimate that this transducer can realize an intrinsic conversion efficiency of up to 35% with <0.5 added noise quanta when resonantly coupled to a superconducting transmon qubit and operated in pulsed mode at 10 kHz repetition rate. The performance improvement gained in such hybrid lithium niobate-silicon transducers make them suitable for heralded entanglement of qubits between superconducting quantum processors connected by optical fiber links.
Optical quantum networks provide a natural route for connecting distant superconducting quantum processors, enabling distributed quantum computation, sensing, and communication. Piezo-optomechanical transducers are among the leading candidates for scalable microwave-to-optical quantum interfaces. However, prior one-dimensional piezo-optomechanical transducers remain limited by optical-absorption-induced heating and the resulting thermal noise. Two-dimensional optomechanical crystals offer substantially improved thermalization from better thermal anchoring, but their structural complexity has so far hindered the realization of a fully integrated two-dimensional transducer. Here, we overcome this challenge with a new design strategy based on band structure engineering. We fabricate the devices and experimentally characterize the response, measuring an electromechanical damping rate of 4.6 kHz at room temperature and electromechanical coupling rate of 0.17 MHz by wire-bonding to a multi-mode microwave resonator at 10 mK. Bidirectional transduction is performed with a calibrated internal efficiency of 0.85\% in the continuous-wave operation, alongside pulsed photon-phonon pair generation near its quantum ground state. Our results represent a significant step toward high-efficiency and low-noise transducers for entangling remote superconducting qubits.
Quantum transduction between microwave and optical photons offers the potential to merge the long-range connectivity of optical photons with the deterministic quantum operations of superconducting microwave qubits. A promising approach to achieving this uses an intermediary mechanical mode along with piezo-optomechanical interactions. Traditionally, these transducers are suspended to confine mechanical fields, but this complicates manufacturing and comes with the major challenge of poor thermal anchoring and a trade-off between noise and efficiency. To overcome these issues, we introduce the—to our knowledge—first design of a release-free electro-optomechanical quantum transducer. Our release-free, i.e., non-suspended, design leverages a silicon-on-sapphire platform. It combines release-free lithium niobate electromechanical crystals with silicon optomechanical crystals on a sapphire substrate, optimizing thermal anchoring and microwave and mechanical coherence. Despite departing from the traditional suspended transducer paradigm, our release-free design achieves coupling rates sufficient for quantum-level interactions between microwave photons, phonons, and optical photons. Unconventionally, it utilizes high-wavevector mechanical modes tightly confined to the chip surface. Beyond quantum science and engineering, this platform and its design principles could also propel low-power acousto-optic systems in integrated photonics.