We demonstrate that soft-clamped silicon nitride strings with large aspect ratio can be operated at \si{ \milli\kelvin} temperatures. The quality factors ($Q$) of two measured devices show consistent dependency on the cryostat temperature, with soft-clamped mechanical modes reaching $Q > 10^9$ at $46~\mathrm{ mK}$. For low optical readout power, $Q$ is found to saturate, indicating good thermalization between the sample and the stage it is mounted on. Our best device exhibits a force sensitivity of $9.6~\mathrm{ zN}/\sqrt{ \mathrm{ Hz}}$ and a thermal decoherence time of $0.38~\mathrm{ s}$ which bode well for future applications such as nanomechanical force sensing and beyond.
Categories: cond-mat.mes-hall physics.app-ph