Giant atom dynamics in a hybrid superconducting-phononic circuits

Encoding quantum information in acoustic waves offers significant advantages over electromagnetic waves, particularly due to the five orders of magnitude slower propagation speed of acoustic waves. This slower propagation speed permits a higher integration density of components and more precise control over quantum information, which are crucial for building scalable quantum information processing architectures. In this work, we introduce a novel hybrid platform that combines a superconducting circuit with a lithium niobate phononic circuit on a sapphire substrate. We demonstrate a giant artificial atom enabled by strongly coupling a transmon qubit to the acoustic waveguide at two points separated by an acoustic propagation delay of approximately 125ns. This engineered delay results in dynamic behaviors distinct from those of a conventional transmon, including non-exponential decay and a frequency-dependent effective relaxation rate. The frequency-dependent characteristics enable multiple applications, such as reservoir-engineered state preparation and on-demand single-phonon source. Our system demonstrates a practical hybrid circuit quantum acoustodynamics platform, laying the foundation for integrated phononic quantum circuits.