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High-frequency mechanical oscillators with long coherence times are essential for realizing a variety of high-fidelity quantum sensors, transducers and memories. However, the coherence times needed for quantum applications require advances in probing and mitigating the origins of phonon decoherence in the materials used for mechanical oscillators. Here we identify key sources of phonon decoherence in crystalline media by combining non-invasive laser spectroscopy with materials analysis. Using micro-fabricated high-overtone bulk acoustic-wave resonators as an experimental test bed, we find that phonon–surface interactions are the dominant source of phonon decoherence in crystalline quartz. The probable causes are lattice distortion, subsurface damage and a high concentration of elemental impurities. We use an optimized polishing process to remove the compromised surface layer and produce resonators with quality factors exceeding 240 million at 12 GHz, corresponding to phonon coherence times of over 6 ms. We verify that these mechanical oscillators have negligible dephasing. Building on these results, we propose a path that could reach coherence times beyond 100 ms as the basis for high-frequency quantum memories. These findings demonstrate that enhanced control over surfaces enables a substantial reduction in dissipation and noise. 

Diamond optomechanical crystal (OMC) devices with embedded color center spins are promising platforms for a broad range of applications in quantum sensing, networking, and computing, offering an interface between a GHz-frequency mechanical mode and both optical photons and coherent spins. A crucial but elusive step toward realizing this platform is to engineer a device with a high-quality factor mechanical mode while preserving the bulk-like coherence of embedded spins. Here, we demonstrate sideband-resolved diamond OMCs with mechanical quality factors in excess of 10⁶at cryogenic temperatures and find coherence times up toT₂=227µs for embedded nitrogen-vacancy (NV) centers. Furthermore, we measure these devices across five orders of magnitude in intracavity optical power, demonstrating robust power handling and a high optomechanical cooperativity (C≫1) at cryogenic temperatures, which is essential for a broad range of quantum protocols requiring strong, coherent interactions between photons and phonons. These results are enabled by a robust, high-throughput method for forming single-crystal diamond membranes in combination with chemical vapor deposition (CVD) diamond overgrowth with nitrogenδ-doping. We discuss the prospects of this platform for hybrid spin-mechanical devices in the quantum regime. 

Fusion-based quantum computing with dual-rail qubits is a leading candidate for scalable quantum computing using linear optics. This paradigm requires single photons which are entangled into small resource states before being fed into a fusion network. The most common sources for single optical photons and for small entangled states are probabilistic and heralded. The realization of a single reliable deterministic source requires many redundant probabilistic sources and a complex optical network for rerouting and retiming probabilistic outputs. In this work, we show how optomechanics enables reliable production of resources for photonic quantum computing without the redundancy of the all-optical approach. This is achieved by using acoustic modes as caches of quantum resources, ranging from single-particle states to small entangled states, with on-demand readout. The advantages of acoustic modes as optical quantum memories, compared to other technologies, include their intrinsically long lifetimes and that they are solid state, highly tailorable, and insensitive to electromagnetic noise. We show how the resource states can be prepared directly in the acoustic modes using optical controls. This is still probabilistic and heralded, as in the all-optical approach, but the acoustic modes act as a quantum memory which is integrated into the production of the states. The quantum states may be deterministically transferred from acoustic modes to optical modes, on demand, with another optical drive. 

Recognized as a potential resource for quantum technologies and a possible test bed for fundamental physics, the control and preparation of nonclassical states of mechanical oscillators have been explored extensively. Within optomechanics, quantum state synthesis can be realized by entangling photonic and phononic degrees of freedom followed by optical detection. Single-photon detection enables one of the most powerful forms of such heralded quantum state preparation, permitting the creation of single-phonon states when applied to conventional cavity optomechanical systems. As the complexity of optomechanical systems increases, single-photon detection may provide heralded access to a larger class of exotic quantum states. Here, we examine the quantum dynamics of optomechanical systems that permit forward Brillouin scattering, where a single-phonon mode mediates transitions between a collection of equally spaced optical resonances. Solving both the Schrodinger equation and the Lindblad master equation for this system, we find that initial states comprised of single photons or weak laser pulses evolve into complex quantum states where the frequency of single-photon states and the phonon occupation number are entangled. Physically, these interactions permit a single photon to scatter to lower frequencies, where phonon excitation occurs for each scattering event. Combining this result with frequency filtering, we show how single-photon detection can herald selected multiphonon Fock states, even in the presence of optical losses. We also present an approach for quantum tomography of the heralded phonon states, which in the form of a state swap from the phononic to photonic domain enables on-demand generation of multiphoton Fock states.