  Quantum sensing, using quantum properties of sensors, can enhance resolution,
precision, and sensitivity of imaging, spectroscopy, and detection. An
intriguing question is: Can the quantum nature (quantumness) of sensors and
targets be exploited to enable schemes that are not possible for classical
probes or classical targets? Here we show that measurement of the quantum
correlations of a quantum target indeed allows for sensing schemes that have no
classical counterparts. As a concrete example, in case where the second-order
classical correlation of a quantum target could be totally concealed by
non-stationary classical noise, the higher-order quantum correlations can
single out a quantum target from the classical noise background, regardless of
the spectrum, statistics, or intensity of the noise. Hence a
classical-noise-free sensing scheme is proposed. This finding suggests that the
quantumness of sensors and targets is still to be explored to realize the full
potential of quantum sensing. New opportunities include sensitivity beyond
classical approaches, non-classical correlations as a new approach to quantum
many-body physics, loophole-free tests of the quantum foundation, et cetera.
