Creation and control of multi-phonon Fock states in a bulk acoustic wave resonator

Abstract: Quantum states of mechanical motion can be important resources for quantum information, metrology, and studies of fundamental physics. Recent demonstrations of superconducting qubits coupled to acoustic resonators have opened up the possibility of performing quantum operations on macroscopic motional modes, which can act as long-lived quantum memories or transducers. In addition, they can potentially be used to test for novel decoherence mechanisms in macroscopic objects and other modifications to standard quantum theory. Many of these applications call for the ability to create and characterize complex quantum states, putting demanding requirements on the speed of quantum operations and the coherence of the mechanical mode. In this work, we demonstrate the controlled generation of multi-phonon Fock states in a macroscopic bulk-acoustic wave resonator. We also perform Wigner tomography and state reconstruction to highlight the quantum nature of the prepared states. These demonstrations are made possible by the long coherence times of our acoustic resonator and our ability to selectively couple to individual phonon modes. Our work shows that circuit quantum acousto-dynamics (circuit QAD) enables sophisticated quantum control of macroscopic mechanical objects and opens the door to using acoustic modes as novel quantum resources.

• The paper demonstrates the controlled generation of multi-phonon Fock states with high fidelity (F|1⟩ = 0.87) and robust qubit-phonon coupling.
• It employs circuit quantum acousto-dynamics using an HBAR and a superconducting transmon qubit to achieve optimal phonon coherence and a coupling strength of 2π×350 kHz.
• The study paves the way for quantum information storage and transduction by utilizing mechanical resonators as novel quantum resources.

Creation and Control of Multi-Phonon Fock States in a Bulk Acoustic Wave Resonator

This paper investigates the potential of macroscopic mechanical systems in the field of quantum information, focusing on the realization and characterization of complex quantum states. The authors present a significant advancement in this field by demonstrating controlled generation of multi-phonon Fock states in macroscopic bulk-acoustic wave resonators. Utilizing advances in circuit quantum acousto-dynamics (circuit QAD), a promising platform for quantum acoustics, this work paves the way for acoustic modes to function as novel quantum resources.

The experimental setup employs a high-overtone bulk acoustic wave resonator (HBAR) coupled to a superconducting transmon qubit using a flip-chip device geometry. The separation of the qubit and acoustic resonator onto different chips allows for independent optimization, enhancing phonon coherence and mode selectivity. The qubit-phonon coupling within this system is robust, with a measured strength of $g_0 = 2\pi \times 350$ kHz. This strong coupling regime is crucial for the manipulation of quantum states within the acoustic domain.

The authors successfully conduct Wigner tomography and full quantum state reconstruction on multi-phonon Fock states, extending previous work limited to electromagnetic systems. This study exemplifies the utility of circuit QAD for exploiting mechanical degrees of freedom in quantum systems. The reported phonon lifetimes, with a $T_1$ of up to 113 μs, rival state-of-the-art superconducting qubits, underlining the potential for such resonators in long-lived quantum information storage.

The fidelity of the prepared states, such as $F_{|1\rangle} = 0.87$, highlights the system's capability to maintain high-quality quantum states. The entropy and fidelity metrics, as well as the specific limitations identified in phonon coherence and qubit-induced decoherence, provide a detailed framework for future improvements.

Looking forward, the implications of this research are substantial. This approach could herald a new class of quantum devices that leverage mechanical resonators for tasks typically reserved for photonic systems, including simulating many-body quantum phenomena and providing more efficient bosonic encoding for quantum information protocols. Additionally, the work holds promise for quantum information transduction between disparate quantum systems, such as microwave and optical domains, through enhanced electromechanical coupling.

The paper also opens avenues for testing fundamental questions about quantum mechanics related to decoherence in massive objects, potentially setting a stage for exploring quantum mechanics modifications at macroscopic scales. Altogether, this research enriches the toolkit available to quantum technologists and theorists, fortifying the bridge between acoustic and photonic quantum regimes.