  Electron paramagnetic resonance (EPR) spectroscopy is an important technology
in physics, chemistry, materials science, and biology. Sensitive detection with
a small sample volume is a key objective in these areas, because it is crucial,
for example, for the readout of a highly packed spin based quantum memory or
the detection of unlabeled metalloproteins in a single cell. In conventional
EPR spectrometers, the energy transfer from the spins to the cavity at a
Purcell enhanced rate plays an essential role and requires the spins to be
resonant with the cavity, however the size of the cavity (limited by the
wavelength) makes it difficult to improve the spatial resolution. Here, we
demonstrate a novel EPR spectrometer using a single artificial atom as a
sensitive detector of spin magnetization. The artificial atom, a
superconducting flux qubit, provides advantages both in terms of its quantum
properties and its much stronger coupling with magnetic fields. We have
achieved a sensitivity of $\sim$400 spins/$\sqrt{\mathrm{Hz}}$ with a magnetic
sensing volume around $10^{-14} \lambda^3$ (50 femto-liters). This corresponds
to an improvement of two-order of magnitude in the magnetic sensing volume
compared with the best cavity based spectrometers while maintaining a similar
sensitivity as those spectrometers . Our artificial atom is suitable for
scaling down and thus paves the way for measuring single spins on the nanometer
scale.
