Collaborative Research: Quantum acoustics for optomechanical transduction and entanglement of solid-state spin qubits

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Future quantum computers will need to utilize different physical types of qubits that need to communicate and convert between each other with high fidelity and high efficiency. While photons are ideal for quantum communication, different qubit systems couple to photons of vastly different frequency ranges. The strain field generated by the mechanical wave in a solid-state material is a promising approach to enable coupling with a broad range of qubits with theoretically high efficiency. With a traveling velocity of five orders of magnitude lower than photons, acoustic waves are ideal for quantum interconnect between multiple qubits. The quantum acoustic technology developed in this project and the integration with NV-defect center qubits is an essential first step toward a chip-scale hybrid, multiple qubit systems. The proposed research both addresses the imminent issue of frequency inhomogeneity that has been plaguing solid-state optical qubits and explores the frontier of strong coupling of mechanical modes with spin qubits. The project will make significant advances from previous studies of discrete systems to realizing a monolithic quantum system that includes waveguides, optical and acoustic cavities, and acoustic transducers to directly interface with qubits, all integrated on a novel material platform. The approach offers a path to the realization of the integrated quantum computing system based on hybrid solid-state qubits interconnected with photons and phonons. The research leverages the tremendous technological development in the acoustic MEMS technology and advances it to the quantum regime, with the potential outcome that can impact both quantum information science and microwave photonics for classical communication. Education and outreach activities aim to increase the participation of students from underrepresented groups and improve the diversity of the STEM workforce and include course development in advanced quantum computing and K-12 science outreach programs with publicly accessible online courses. Technical Abstract:The project aims to develop a novel integrated quantum acoustic device platform for optomechanical transduction and quantum state manipulation of solid-state spin qubits based on defect centers in diamond. The integrated devices will be built on the high-performance heterogeneous material platform of gallium phosphide (GaP) on the crystalline diamond. The platform uniquely utilizes the layer of piezoelectric GaP for the dual functions of optical waveguiding and acoustic wave generation and guiding, thereby to achieve tremendously enhanced acousto-optic interaction. The effort will include three main thrusts. The first thrust will realize integrated acousto-optic frequency shifter (AOFS) to address the optical frequency inhomogeneity problem of qubits based on defect centers. AOFS can achieve single-sideband, carrier-suppressed frequency shift of photons from qubits freely over a range of ±3GHz and with an efficiency better than 80%. The second thrust will investigate the coupling of itinerant acoustic waves to ensembles and single defect centers. The acoustic coupling strength will be enhanced to reach the strong coupling regime and realize time-dependent control over the states of the qubits. The final thrust will realize the strong coupling of acoustic modes confined in a high-Q cavity with single defect centers embedded therein. Quantum state manipulation and quantum entanglement of the qubits by utilizing the acoustic mode will be achieved. Ensembles of NV-centers coupled to the cavity acoustic mode in the strong-coupling regime and novel physics effects in this regime will be explored.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.