Protecting Quantum Spin Coherence of Nanodiamonds in Living Cells

Because of its superior coherent and optical properties at room temperature, the nitrogen-vacancy ($\mathrm{N}$-$V$) center in diamond has become a promising quantum probe for nanoscale quantum sensing. However, the application of $\mathrm{N}$-$V$-containing nanodiamonds to quantum sensing suffers from their relatively short spin coherence times. Here we demonstrate energy-efficient protection of $\mathrm{N}$-$V$ spin coherence in nanodiamonds using concatenated continuous dynamical decoupling, which exhibits excellent performance with a less-stringent microwave-power requirement. When this is applied to nanodiamonds in living cells, we are able to extend the spin coherence time by an order of magnitude to the ${T}_{1}$ limit of $30\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$. Further analysis demonstrates concomitant improvements of sensing performance, which shows that our results provide an important step toward in vivo quantum sensing using $\mathrm{N}$-$V$ centers in nanodiamond.