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>109 at 46mK. 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 9.6zN/Hz and a thermal decoherence time of 0.38s which bode well for future applications such as nanomechanical force sensing and beyond.
@article{arxiv.2112.03730,
title = {Soft-clamped silicon nitride string resonators at millikelvin temperatures},
author = {Thomas Gisler and Mohamed Helal and Deividas Sabonis and Urs Grob and Martin Héritier and Christian L. Degen and Amir H. Ghadimi and Alexander Eichler},
journal= {arXiv preprint arXiv:2112.03730},
year = {2022}
}