EPR, or ESR as it is also known, utilizies the resonance observed at an electron's own Larmor frequency when precessing about a static magnetic field. To gain a good understanding, I invite you to read two well-organized books on the subjects:
[1] Electron Paramagnetic Resonance: Elementary Theory and Practical Applications
John A. Weil and James R. Bolton, Wiley & Sons 2006
[2] Electron Paramagnetic Resonance: A Practitioners Toolkit
Marina Brustolon and Elio Giamello (editors), Wiley & Sons 2009
The spectra recorded by EPR machines is in essence the averaging over an almost incomprehensible amount of electrons. Therefore, these spectra may differ dramatically from that of a single electron, or even a single few. With our sensor, we can implement a direct method of probing the sample's electron spin, and also perform tests, which tell us whether this is a single spin or a larger spin bath. This method is called, again borrowing from the EPR community - Double Electron Electron Resonance, or DEER.
There have already been reports on the sensing of a single electron in diamond [3] due to a paramagnetic impurity, as well as the use of DEER to detect the spin-label from a single protein [4]. Both show the feasibility of sensing single electron spins. However, there is no direct proof, so far, of truly sensing a single electron spin from an external sample (i.e., not diamond).
[3] Grinolds et al., Nat. Nanotech. 9, 279 (2014)
[4] Shi et al., Science 347, 1135 (2015)
One of the foci of our group, is to use a combined Atomic Force Microscope (AFM) and a Confocal Fluorescence Microscope (CFM) to perform DEER measurements (and their "offsprings"). The samples are typically spin-labels, which are the markers used by the traditional EPR community to infer structural information about molecules. We aim to do the same, but in the single-molecule limit.
Here is an exemplar of how a DEER measurement looks in our setup:
electron spin resonance of spin labels (top and middle) and dangling bond electron spins measured by the nitrogen-vacancy center in diamond using the double-electron-electron-resonance protocol