Various systems and methods for detecting electron spins using electron paramagnetic resonance are described. An excitation signal generator configured to generate an excitation signal of varying amplitude and phase as compared to a reference signal may be present. A crossed loop resonator configured to isolate a detection signal produced by the excitation signal exciting an object with a magnetic field may also be preset. Further, a detection device configured to detect electron spins of the object using the detection signal isolated by the crossed loop resonator may be present.