Circuit Quantum Acoustodynamics in a Scalable Phononic Integrated Circuit Architecture

Abstract

Previous demonstrations of quantum acoustic systems have been limited to isolated devices, with limited capability to route phonons and interconnect multi-port acoustic elements for further extension. Here, we demonstrate a scalable architecture for circuit quantum acoustodynamics (cQAD) by integrating superconducting qubits with suspension-free phononic integrated circuits (PnICs). Coherent coupling between tunable transmon qubits and waveguide-integrated phononic cavities, including Fabry-Perot cavities via monolithic integration and microring cavities via flip-chip assembly, has been achieved, producing a pronounced enhancement of phonon emission with a Purcell factor of ~19. These devices represent elementary building blocks for scalable phononic circuits, establishing the foundation for phonon-based quantum information processors and the testbed for novel quantum acoustic phenomena.

Figures (3)

• Figure 1: Principle of phononic integrated circuits (PnICs)-based circuit quantum acoustodynamics (cQAD) platform.a, Schematic of the cQAD platform with a transmon coupled to a Fabry-Perot (FP) cavity (b) or a microring cavity (c). d, Cross-sectional schematic of the lithium niobate (LN) waveguide on a sapphire substrate with an interdigital transducer (IDT). e, Simulated mode profile showing tightly confined Love-like acoustic wave in the waveguide, excited via the IDT. f, Principle of the Purcell effect in cQAD devices. Yellow regions denote the engineered phononic density of states (DOS) and solid lines represent the qubit transition frequencies. The transition linewidth broadens significantly when its frequency aligned with a cavity resonance (high DOS). Inset: The corresponding qubit decay dynamics is drastically accelerated by a phonon resonance.
• Figure 2: Monolithically integrated cQAD device based on an FP phononic cavity.a, False-color image of the device, which comprises a straight phononic waveguide (green), two mirrors (blue), and an IDT coupler (purple in b) for the transmon qubit. b, Magnified SEM picture of the IDT and a mirror. c, Simulated standing-wave mode profile confined by the two mirrors, using a re-scaled FP cavity structure for better visualization. d, Simulated qubit-cavity coupling strength and mirror reflectivity versus probe frequency. e, Qubit spectroscopy as a function of the flux bias. Three frequencies marked I, II, and III correspond to near-zero measured populations. These frequencies are located within the lossy-cavity regime [regime (i), discussed in the main text]. The frequencies at positions II and III correspond to the two peaks of the coupling coefficient depicted in d. f, Qubit decay dynamics at transition frequencies of $5.013\,\mathrm{GHz}$, $5.032\,\mathrm{GHz}$, and $5.281\,\mathrm{GHz}$, with fitted energy relaxation times of $4.70\,\mathrm{\mu s}$, $1.79\,\mathrm{\mu s}$, and $0.35\,\mathrm{\mu s}$, respectively. g, Measured qubit dissipation rate $\gamma_e/2\pi$ (red dots) and theoretical prediction (line) as functions of qubit frequency. Inset: expanded view near $5.27\,\mathrm{GHz}$ with a pronounced modulation due to phonon modes.
• Figure 3: cQAD device based on a microring cavity.a, Schematic of the flip-chip assembly. The phononic microring cavity is fabricated on a top chip of 11 mm by 11 mm, while the superconducting qubit along with its readout and control circuitry are fabricated on a bottom chip of 15 mm by 15 mm. b, Schematic of qubit-ring cavity coupling. An IDT is placed on the ring cavity, with its two electrodes connected to two large pads. These pads form a coupling capacitance with the pads of the floating transmon, enabling the interaction between the qubit and the cavity mode. c, Optical micrograph of the bottom chip. d, Quality factor of the phononic microring cavity as a function of the input power to the chip. e, Qubit spectroscopy as a function of flux bias. f, Qubit decay dynamics at two distinct frequencies: 3.867 GHz (resonant, blue points) with $T_1=0.67\,\mathrm{\mu s}$ and 3.872 GHz with $T_1=2.36\,\mathrm{\mu s}$ (off-resonant, orange points). g, Effective qubit dissipation rate as a function of transition frequency. Experimental data are shown as dots; the solid line represents the theoretical fit.