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Physical Review Applied

Piezoelectric loss of superconducting microwave resonators integrated with thin-film lithium niobate

Likai Yang, Yuntao Xu, Chunzhen Li, Jiacheng Xie, Mohan Shen, Hong X. Tang

Published Nov 13, 2023• 4 citations
Publisher

Abstract

Thin-film lithium niobate (LN) has become an enabling platform in the development of quantum integrated circuits, for applications including microwave-to-optical converters and electromechanical transducers. Incorporation of high-quality planar superconducting resonators will further elevate the performance of these devices. However, alongside the desired interactions originated from LN, its piezoelectric nature also introduces parasitic coupling between microwave and phononic modes, resulting in excessive dielectric loss. In this work, we investigate such coupling by studying niobium nitride resonators directly deposited on patterned LN thin films. By varying device configuration, we pinpoint that overlapping electric field with LN can inadvertently excite bulk acoustic modes within the substrate, inducing microwave loss independent of the acoustic dissipation rate. The experimental results are supported by simulation and theoretical modeling. Our approach can be applied to different piezoelectric systems and provide insights into the design of microwave components for quantum transduction devices.

DOI
10.1103/physrevapplied.20.054026

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Thin-film lithium niobate (LN) has become an enabling platform in the development of quantum integrated circuits, for applications including microwave-to-optical converters and electromechanical transducers. Incorporation of high-quality planar superconducting resonators will further elevate the performance of these devices. However, alongside the desired interactions originated from LN, its piezoelectric nature also introduces parasitic coupling between microwave and phononic modes, resulting in excessive dielectric loss. In this work, we investigate such coupling by studying niobium nitride resonators directly deposited on patterned LN thin films. By varying device configuration, we pinpoint that overlapping electric field with LN can inadvertently excite bulk acoustic modes within the substrate, inducing microwave loss independent of the acoustic dissipation rate. The experimental results are supported by simulation and theoretical modeling. Our approach can be applied to different piezoelectric systems and provide insights into the design of microwave components for quantum transduction devices.
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Quantum networks of superconducting qubits linked by optical channels could leverage both the quantum information processing capabilities of superconducting circuits and the long communication distances provided by optical photons. Such networks would require high efficiency, low noise, and wide bandwidth transducers between microwave and optical frequencies. Transducers based on the Pockels electro-optic (EO) effect are particularly promising in this application for their direct conversion mechanism and potential for strong performance. EO transducers could also be used for sensitive optical modulators and low-noise detection of microwave and millimeter-wave signals. This dissertation presents recent work to create cavity EO transducers in thin-film lithium niobate, an integrated photonics platform that provides low optical loss and strong EO coupling. I first describe the theory of cavity electro-optics and how it can be used to generate high-efficiency transduction between microwave and optical fields. An initial device is presented and characterized, demonstrating per-photon on-chip transduction efficiencies of up to (2.7 ± 0.3) × 10^−5. A key benefit of the device described here, which is based on photonic molecule modes, is the ability to use a static electro-optic bias to trim the transducer into resonance. However, we find the migration of free carriers in thin-film lithium niobate reduces the EO response of the device to static fields, making such trimming ineffective. This carrier migration is a key challenge for enabling the promise of low-power electro-optics provided by thin-film lithium niobate devices. I characterize the low-frequency electro-optic response, which suggests that conduction occurs on the etched surface of lithium niobate. I show how this conduction can be reduced – and low-frequency EO performance improved – by changing the electrode design and annealing the devices. Following this, I describe the design and initial characterization of an improved transducer device. Finally, I describe a scheme by which even relatively low-efficiency transducers could be used to generate remote entanglement using an optically heralding scheme. Demonstration of such a system appears possible with current devices, suggesting that small opticallymediated quantum networks of superconducting qubits may be feasible soon.
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.
Acoustic waves are versatile excitations that couple to many different physical systems. They interact with optical fields in optomechanics, with spin defects through strain, and with superconducting circuits via capacitive or piezoelectric mechanisms. This versatility has motivated their use in hybrid quantum systems for quantum transduction, qubit control, and precision sensing, among other applications. In this thesis, the focus is on the optimization of microwave-to-acoustic coupling between superconducting circuits and Lamb modes in suspend thin-film lithium niobate (LiNbO₃). We leverage the strong piezoelectricity of this material to engineer strong interactions.The first part of the work addresses linear transduction. IDTs require impedance matching to operate efficiently, but this generally imposes a trade-off between efficiency and bandwidth. An optimal design can be identified in theory but is difficult to reach with standard microwave engineering. Here, high-impedance SQUID arrays are employed as linear, tunable inductors to realize the necessary impedance transformation. This approach enables transducers that operate close to the efficiency-bandwidth limit imposed by the piezoelectric coupling strength of LiNbO₃. The design strategy is applied to different types of devices: standard bi-directional IDTs, unidirectional IDTs where acoustic mirrors recover backward-emitted waves, and single-mode IDTs where additional transverse confinement produces quasi-one-dimensional propagation. Among these, unidirectional IDTs achieve unprecedented coupling, with efficiency–bandwidth products of ≈440 MHz and peak efficiencies of 62%. By leveraging flux tunability of SQUIDs we get tunable couplers between microwave circuits and Lamb-wave resonators.The second part investigates the nonlinear regime, where transmon qubits are coupled to Lamb-wave resonators. Unlike the high-Q but weakly coupled resonators often used in the literature, LiNbO₃ offers strong piezoelectric interactions that enable access to very strong coupling even without extremely high quality factors.Overall, this thesis establishes efficient linear transducers and explores strong nonlinear interactions, advancing superconducting-acoustic interfaces based on LiNbO₃. These results extend the superconducting circuit toolbox with sensitive acoustic elements and open pathways toward hybrid applications such as microwave-optical transduction, acoustic spectroscopy, and fundamental experiments exploring the interplay of quantum mechanics and gravity.
We couple a superconducting qubit to a piezoelectric nanome-chanical resonator fabricated from thin film lithium niobate (LN). The sizeable piezoelectric coefficient of LN leads to a coupling rate g/2π>15MHzg/2\pi > 15 \text{MHz}g/2π>15MHz allowing quantum operations.