Mobile Electron Spin Resonance with Spins in Optically Trapped Nanodiamonds

The nitrogen-vacancy (NV) color center in diamond has emerged as a powerful, optically addressable, spin-based probe of electromagnetic fields and temperature through the precise quantum control of a single electron spin state. For nanoscale sensing applications, the NV center’s atom-like nature enables the close-range interactions necessary for both high spatial resolution and the detection of fields generated by proximal nuclei, electrons, or molecules. Using a custom-designed optical tweezers apparatus, we demonstrate three-dimensional position control of nanodiamonds in solution with simultaneous optical measurement of electron spin resonance (ESR) [1]. Single crystal diamond nanoparticles containing NV centers were microfabricated and subsequently suspended in the optical trap, enabling the observation of distinct peaks in the ESR spectra from the ground-state spin transitions. We model the ESR spectra observed in an applied magnetic field and estimate the dc magnetic sensitivity based on the ESR line shapes to be ~10 μT/√Hz.

Moreover, using dynamical decoupling protocols to convert thermally induced shifts in the NV center’s spin resonance frequencies into large changes in its fluorescence, we demonstrate fluorescence thermometry techniques with sensitivities approaching 10 mK√Hz based on the spin-dependent photoluminescence of nitrogen vacancy (NV) centers in diamond [2]. These techniques use dynamical decoupling protocols to convert thermally induced shifts in the NV center’s spin resonance frequencies into large changes in its fluorescence. We show that these quantum-based measurement techniques can be applied over a broad temperature range and in both finite and near-zero magnetic field environments. This versatility suggests that the quantum coherence of single spins could be practically leveraged for sensitive thermometry in a wide variety of biological and microscale systems.

Optically trapped nanodiamonds are also used to probe the local environment within microfluidic circuits, providing a pathway to spin-based sensing in fluidic environments and biophysical systems that are inaccessible to existing scanning probe techniques, such as the interiors of living cells.

[1] V.R. Horowitz, B.J. Alemán, D.J. Christle, A.N. Cleland, and D.D. Awschalom, Proc. Natl. Acad. Sci. USA, 109, 13493 (2012).

[2] D.M. Toyli, C.F. de las Casas, D.J. Christle, V.V. Dobrovitski, D.D. Awschalom, Proc. Natl. Acad. Sci. 110, 8417 (2013).