  Quantum spectroscopy with single two level systems has considerably improved
our ability to detect weak signals. Recently it was realized that for classical
signals, precision and resolution of quantum spectroscopy is limited mainly by
coherence of the signal and stability of the clock used to measure time. The
coherence time of the quantum probe, which can be significantly shorter, is not
a major limiting factor in resolution measurements. Here, we address a similar
question for spectroscopy of quantum signals, for example a quantum sensor is
used to detect a single nuclear spin. We present and analyze a novel
correlation spectroscopy technique with performance that is limited by the
coherence time of the target spins and the stability of the clock.
