Giant-atom Quantum Acoustodynamics Achieves 40x Purcell Factor in Hybrid Circuits

The pursuit of increasingly complex quantum systems has led scientists to explore novel platforms for manipulating quantum information, and recent work demonstrates a significant advance in this field. Lintao Xiao, Bo Zhang, and Yu Zeng, along with colleagues, have created a ‘giant atom’ by intricately coupling a superconducting circuit to a lithium niobate phononic waveguide. This innovative approach allows for the controlled manipulation of sound waves to influence quantum states, resulting in a system exhibiting unusual behaviour, including a four-fold variation in decay rate over a narrow frequency range and a substantial Purcell factor exceeding 40. By achieving these results, the team establishes phononic integrated circuits as a powerful and versatile platform for building highly tunable quantum devices with potential applications in advanced quantum information processing.

Giant Atom Dynamics via Phononic Waveguide Coupling

Scientists engineered a giant atom by integrating a superconducting qubit with a lithium niobate phononic waveguide, coupling them at points separated by approximately 600 acoustic wavelengths, resulting in a propagation delay of 125 nanoseconds. This setup allows for the study of non-Markovian relaxation dynamics, characterised by phonon backflow and a frequency-dependent decay rate that varies fourfold over a mere 4 megahertz. The team meticulously probed this modulation of the decay rate by applying excitation pulses to the qubit and precisely measuring the resulting excitation probability, employing a method also used for determining the qubit’s intrinsic relaxation time. They demonstrated the ability to switch phonon emission with a mere 4 megahertz qubit frequency tuning, achieving a fourfold enhancement of the decay rate.

Fitting the observed decay rates with a theoretical model, the team determined a qubit intrinsic decay rate of 0.07 megahertz and an oscillation visibility of 0.78, further validating the giant atom’s unique properties. Beyond characterizing the decay rate, the study revealed the influence of phonon backflow by examining qubit excitation decay at four frequencies near maximal coupling, confirming the contribution of phonons emitted from one transducer coupling back to the qubit after a propagation time of 125 nanoseconds. This detailed analysis confirms the non-Markovian behavior of the giant atom, showcasing its potential for advanced quantum information processing. The achieved Purcell factor exceeds 40, substantially surpassing previous demonstrations using surface acoustic waves.

Circuit Quantum Acoustodynamics and Phononic Integration

This compilation of research papers details the rapidly developing field of Circuit Quantum Acoustodynamics (cQAD), which combines superconducting circuits with acoustic phonons in integrated circuits to create new quantum devices and explore quantum phenomena. The references cover the design, fabrication, and application of these circuits, with a strong focus on materials like Lithium Niobate on Sapphire (LNOS) for high-frequency phonon control. Central to this research are giant atoms, artificial atoms created using superconducting circuits and acoustic resonators, exhibiting enhanced light-matter interaction and used for exploring quantum optics and quantum information processing. The studies explore various aspects of superconducting qubits and circuits, surface acoustic waves, and quantum information processing, with a particular emphasis on quantum transduction, the conversion of quantum information between different carriers.

The research encompasses several key areas, including the fabrication and characterization of thin-film lithium niobate, the design of complex phononic circuits, and the exploration of giant atom physics, waveguide quantum electrodynamics, and phonon-qubit interaction. Scientists are actively investigating quantum bath engineering to control qubit environments and improve performance, and developing applications such as quantum memories, quantum random access memory, quantum sensors, and quantum simulators. Specific device types and techniques under investigation include Brillouin integrated circuits, traveling-wave phononic cavities, and phononic resonators, alongside theoretical and computational aspects such as Lindblad master equations, quantum simulation, and device modeling. Recent trends highlight the development of scalable phononic integrated circuits, hybrid quantum systems, high-frequency phonon control, and quantum metrology and sensing. In summary, this body of work represents a comprehensive overview of Circuit Quantum Acoustodynamics, demonstrating the potential of combining superconducting circuits and acoustic phonons to create new quantum devices and explore fundamental quantum phenomena. The research is highly interdisciplinary, drawing on expertise in condensed matter physics, quantum optics, electrical engineering, and materials science.

Tunable Phononic Waveguide Creates Giant Atom

This research demonstrates the creation of a “giant atom” by strongly coupling a superconducting qubit to a lithium niobate phononic waveguide at two distinct points, separated by a significant distance along the waveguide. This unique architecture results in non-standard relaxation dynamics, characterised by phonon backflow and a frequency-dependent decay rate that varies considerably over a narrow frequency range, exceeding a Purcell factor of 40. The team successfully exploited this frequency-dependent dissipation to prepare quantum superposition states with high purity, showcasing precise control over qubit states. The achievement establishes phononic integrated circuits as a versatile platform for exploring giant atom physics, offering a highly tunable system for advanced quantum information processing. By carefully adjusting the qubit and drive frequencies, the researchers demonstrated the ability to prepare high-purity steady states without the need for additional resonators. The authors acknowledge that further improvements to the coupling strength, through design modifications to the waveguide, and scaling to multiple qubits represent potential avenues for future work, potentially leading to phonon-mediated entanglement and exploring decoherence-free interactions.