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High-overtone bulk acoustic-wave resonators as a platform for microwave-to-optical quantum state transfer

Tom Schatteburg

Published Jan 1, 2025• 0 citations
Publisher

Abstract

As superconducting quantum computing devices grow more and more powerful [1–3], the quest to connect them over large-scale distances [4] becomes increasingly relevant. Microwave-to-optical transducers offer a pathway for such connections via a low-loss and thermal noise-free optical fiber network. However, achieving simultaneously high efficiency and low added noise is currently limited by a trade-off between both quantities due to pump absorption heating [5–9]. High-overtone bulk acoustic-wave resonators (HBARs) promise a solution to this problem due to their crystalline material and low surface-to-bulk ratio. In this thesis, we introduce an HBAR-based microwave-to-optical transducer to achieve quantum state transfer. As a first milestone, we build a Brillouin-optomechanical cavity that self-aligns when cooled down and is relatively insensitive to vibrations, which allows optomechanical coupling in an environment that is compatible with superconducting circuits. We measure the phonon mode occupation of the HBAR to be below 0.5, demonstrating the heaviest mechanical object measured in the quantum ground state to date. In addition, we show that the mode occupations are insensitive to laser pump power, indicating negligible absorption heating and confirming the unique advantage of this system. We then build a transducer device which combines Brillouin-optomechanical coupling and piezo-electromechanical coupling to a superconducting qubit in one geometry. We can align the optical cavity mode to pass through the ring-shaped antenna of the superconducting qubit without affecting the optical finesse. This allows us to measure optomechanical coupling in this device as a next milestone towards high-efficiency, low noise microwave-to-optical transduction.

DOI
10.3929/ethz-c-000788761

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As superconducting quantum computing devices grow more and more powerful [1–3], the quest to connect them over large-scale distances [4] becomes increasingly relevant. Microwave-to-optical transducers offer a pathway for such connections via a low-loss and thermal noise-free optical fiber network. However, achieving simultaneously high efficiency and low added noise is currently limited by a trade-off between both quantities due to pump absorption heating [5–9]. High-overtone bulk acoustic-wave resonators (HBARs) promise a solution to this problem due to their crystalline material and low surface-to-bulk ratio. In this thesis, we introduce an HBAR-based microwave-to-optical transducer to achieve quantum state transfer. As a first milestone, we build a Brillouin-optomechanical cavity that self-aligns when cooled down and is relatively insensitive to vibrations, which allows optomechanical coupling in an environment that is compatible with superconducting circuits. We measure the phonon mode occupation of the HBAR to be below 0.5, demonstrating the heaviest mechanical object measured in the quantum ground state to date. In addition, we show that the mode occupations are insensitive to laser pump power, indicating negligible absorption heating and confirming the unique advantage of this system. We then build a transducer device which combines Brillouin-optomechanical coupling and piezo-electromechanical coupling to a superconducting qubit in one geometry. We can align the optical cavity mode to pass through the ring-shaped antenna of the superconducting qubit without affecting the optical finesse. This allows us to measure optomechanical coupling in this device as a next milestone towards high-efficiency, low noise microwave-to-optical transduction.
Quantum networks of superconducting circuit quantum processors are an exciting prospect but are unfeasible without an efficient quantum interface between optical light and superconducting qubits. This problem motivates the development of a quantum microwave-to-optical transducer, a device that can faithfully transfer the state of a superconducting circuit to a photonic flying qubit and vice versa. This thesis describes our progress towards building a microwave-to-optical transducer mediated by phonons in a high-overtone bulk acoustic resonator (HBAR). Novel aluminum nitride-on-quartz HBARs are designed, fabricated, and tested. The acoustic loss rates are found to be near the state-of-the-art. Transduction-compatible transmon qubits are designed and tested. These qubits are capable of operating inside an optical cavity as well as achieving electromechanical interactions in the strong-coupling regime. An initial experiment studied how superconducting qubits recover after exposure to infrared light and demonstrated that the transduction-compatible qubits should be able to operate in a transducer with a reasonable duty cycle. The concept of microwave-to-optical transduction using multiple acoustic modes is established, and new simulation methods are developed to rapidly optimize HBARs. These tools are used to create a transduction-compatible qubit-HBAR device operating in the multi-mode strong coupling regime. Separately, a transduction compatible optical cavity was built and used to perform optomechanical measurements of a transduction-compatible HBAR and confirm that this device can reach the high-cooperativity regime. Finally, a measurement for verifying that a transducer is operating as a source of single photons is discussed. This can be accomplished by measuring the optical second-order correlation function of transduced photons. A filter cavity cascade is constructed to perform this measurement without being overwhelmed by optical pump leakage. These results establish bulk acoustic transduction as a promising platform for low-noise quantum microwave-to-optical transduction.
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.
A device capable of converting single quanta of the microwave field to the optical domain is an outstanding endeavor in the context of quantum interconnects between distant superconducting qubits, but likewise can have applications in other fields, such as radio astronomy or, in the classical realm, microwave photonics. A variety of transduction approaches, based on optomechanical or electro-optical interactions, have been proposed and realized, yet the required vanishing added noises and an efficiency approaching unity, have not yet been attained. Here we present a transduction scheme that could in theory satisfy the requirements for quantum coherent bidirectional transduction. Our scheme relies on an intermediary mechanical mode, a high-overtone bulk acoustic resonance (HBAR), to coherently couple microwave and optical photons through the piezoelectric and strain-optical effects. Its efficiency results from the combination of integrated Si3N4{\mathrm{Si}}_{3}{\mathrm{N}}_{4}Si3​N4​ photonic circuits with ultra-low loss sustaining high intracavity photon numbers with the highly efficient microwave to mechanical transduction offered by piezoelectrically coupled HBAR. We develop a quantum theory for this multipartite system by first introducing a quantization method for the piezoelectric interaction between the microwave mode and the mechanical mode from first principles (which to our knowledge has not been presented in this form) and link the latter to the conventional Butterworth-Van Dyke model. The HBAR is subsequently coupled to a pair of hybridized optical modes from coupled optical ring cavities via the strain-optical effect. We analyze the conversion capabilities of the proposed device using signal flow graphs, and demonstrate that near quantum coherent transduction is possible, with realistic experimental parameters. Combined with the high thermal conduction via the device bulk, heating effects are mitigated, and the approach does not require superconducting resonators that are susceptible to absorption of optical photons.
A transducer capable of converting single microwave and optical photons with low loss and added noise could lay the foundation for a quantum network. Our strategy for building such a device uses an acoustic resonator as an intermediary between a superconducting transmon qubit and an optical cavity. We outline our progress and share insights into some obstacles that we have encountered when engineering a cryogenic-compatible optical cavity suitable for Brillouin optomechanics. We also discuss the materials and design choices for acoustic resonators in our transduction scheme.
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.