  The principles of quantum optics have yielded a plethora of ideas to surpass
the classical limitations of sensitivity and resolution in optical microscopy.
While some ideas have been applied in proof-of-principle experiments, imaging a
biological sample has remained challenging mainly due to the inherently weak
signal measured and the fragility of quantum states of light. In principle,
however, these quantum protocols can add new information without sacrificing
the classical information and can therefore enhance the capabilities of
existing super-resolution techniques. Image scanning microscopy (ISM), a recent
addition to the family of super-resolution methods, generates a robust
resolution enhancement without sacrificing the signal level. Here we introduce
quantum image scanning microscopy (Q-ISM): combining ISM with the measurement
of quantum photon correlation allows increasing the resolution of ISM up to
two-fold, four times beyond the diffraction limit. We introduce the Q-ISM
principle and obtain super-resolved optical images of a biological sample
stained with fluorescent quantum dots using photon antibunching, a quantum
effect, as a resolution enhancing contrast mechanism.
