A mechanical quantum memory for microwave photons

Long-lived mechanical oscillators are actively pursued as critical resources for quantum storage, sensing, and transduction. However, achieving deterministic quantum control while limiting mechanical dissipation remains a persistent challenge. Here, we demonstrate strong coupling between a transmon superconducting qubit and an ultra-long-lived nanomechanical oscillator ($T_\text{1} \approx 25 \text{ ms}$ at 5 GHz, $Q \approx 0.8 \times 10^9$) by leveraging the low acoustic loss in silicon and phononic bandgap engineering. The qubit-oscillator system achieves large cooperativity ($C_{T_1}\approx 1.5\times10^5$, $C_{T_2}\approx 150$), enabling the generation of non-classical states and the investigation of mechanisms underlying mechanical decoherence. We show that dynamical decoupling$\unicode{x2014}$implemented through the qubit$\unicode{x2014}$can mitigate decoherence, leading to a mechanical coherence time of $T_2\approx 1 \text{ ms}$. These findings extend the exceptional storage capabilities of mechanical oscillators to the quantum regime, putting them forward as compact bosonic elements for future applications in quantum computing and metrology.