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arXiv:2307.05544 · Applied Physics Letters

Coupling high-overtone bulk acoustic wave resonators via superconducting qubits

Wayne Crump, Alpo Välimaa, Mika A. Sillanpää

Published Sep 25, 2023• 10 citations
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Abstract

In this work, we present a device consisting of two coupled transmon qubits, each of which are coupled to an independent high-overtone bulk acoustic wave resonator (HBAR). Both HBAR resonators support a plethora of acoustic modes, which can couple to the qubit near resonantly. We first show qubit–qubit interaction in the multimode system and, finally, quantum state transfer where an excitation is swapped from an HBAR mode of one qubit to an HBAR mode of the other qubit.

DOI
10.1063/5.0166924

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Applied Physics Letters•Oct 28, 2024•3 citations
In this work, we present a device consisting of two coupled transmon qubits, each of which are coupled to an independent high-overtone bulk acoustic wave resonator (HBAR). Both HBAR resonators support a plethora of acoustic modes, which can couple to the qubit near resonantly. We first show qubit–qubit interaction in the multimode system and, finally, quantum state transfer where an excitation is swapped from an HBAR mode of one qubit to an HBAR mode of the other qubit.
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.
Collective dynamics in engineered quantum systems offer a unique and versatile platform for exploring how many-body correlations bridge microscopic entanglement and macroscopic behavior. In this work, we report collective Dicke dynamics of acoustic modes in a macroscopic high-overtone bulk acoustic resonator (HBAR). To achieve this, we engineer a hybrid quantum acoustodynamic system comprising an HBAR strongly coupled to a superconducting transmon qubit. The HBAR device is distinctive in the sense that its narrow mode spacing, together with enhanced qubit-mode coupling strength, gives rise to efficient coupling between the transmon and clusters of near-resonant modes. By harnessing the system properties, we observe collective dynamics involving clusters composed by two or three mechanical modes, where their non-resonant spectrum allows for the observation of the transition between the Dicke static regime to dynamically induced timed-Dicke one. The coherent collective behavior of the system is supported by time-domain measurements of the qubit's purity, indicating the quantum nature of the collective dynamics. Overall, our work establishes HBAR-based hybrid quantum system as a promising platform for exploring many-body collective dynamics in macroscopic mechanical systems.
Mechanical resonators are a promising way for interfacing qubits in order to realize hybrid quantum systems that offer great possibilities for applications. Mechanical systems can have very long energy lifetimes, and they can be further interfaced to other systems. Moreover, integration of a mechanical oscillator with qubits creates a potential platform for the exploration of quantum physics in macroscopic mechanical degrees of freedom. The utilization of high overtone bulk acoustic resonators coupled to superconducting qubits is an intriguing platform towards these goals. These resonators exhibit a combination of high-frequency and high-quality factors. They can reach their quantum ground state at dilution refrigeration temperatures and they can be strongly coupled to superconducting qubits via their piezoelectric effect. In this paper, we demonstrate our system where bulk acoustic phonons of a high overtone resonator are coupled to a transmon qubit in a planar circuit architecture. We show that the bulk acoustic phonons are interacting with the qubit in a simple design architecture at the quantum level, representing further progress towards the quantum control of mechanical motion.
Achieving sufficiently low residual excited-state populations remains a key challenge in superconducting quantum circuits, particularly for protocols operating close to noise limits or requiring repeated qubit initialization. Existing protocols primarily address this challenge through sophisticated control, engineered dissipation, or feedback mechanisms. Here, we demonstrate an alternative approach in which a superconducting qubit is reset using a physically distinct, intrinsically colder phononic bath. Specifically, we interface a transmon with a high-overtone bulk acoustic resonator (HBAR), enabling cooling of the qubit into GHz-frequency modes. Using this approach, we achieve a residual excited-state population of the qubit below 10−410^{-4}10−4, representing an improvement of one to two orders of magnitude compared to existing reset schemes. These results highlight the potential of phononic baths as a resource for high-fidelity qubit initialization in superconducting circuits.
Quantum acoustics is an emerging platform for hybrid quantum technologies enabling quantum coherent control of mechanical vibrations. High-overtone bulk acoustic resonators (HBARs) represent an attractive mechanical implementation of quantum acoustics due to their potential for exceptionally high mechanical coherence. Here, we demonstrate an implementation of high-coherence HBAR quantum acoustics integrated with a planar superconducting qubit architecture, demonstrating an acoustically-induced-transparency regime of high cooperativity and weak coupling, analogous to the electrically-induced transparency in atomic physics. Demonstrating high-coherence quantum acoustics with planar superconducting devices enables new applications for acoustic resonators in quantum technologies.
Quantum acoustics is an emerging platform for hybrid quantum technologies enabling quantum coherent control of mechanical vibrations. High-overtone bulk acoustic resonators (HBARs) represent an attractive mechanical implementation of quantum acoustics due to their potential for exceptionally high mechanical coherence. Here, we demonstrate an implementation of high-coherence HBAR quantum acoustics integrated with a planar superconducting qubit architecture, demonstrating an acoustically induced-transparency regime of high cooperativity and weak coupling, analogous to the electrically induced transparency in atomic physics. Demonstrating high-coherence quantum acoustics with planar superconducting devices enables interesting applications for acoustic resonators in quantum technologies.