  State-of-the-art microscopes use intense lasers that can severely disturb
biological processes, function and viability. This introduces hard limits on
performance that only quantum photon correlations can overcome. Here we
demonstrate this absolute quantum advantage, achieving signal-to-noise beyond
the photodamage-free capacity of conventional microscopy. We achieve this in a
coherent Raman microscope, which we use to image molecular bonds within a cell
with both quantum-enhanced contrast and sub-wavelength resolution. This allows
the observation of nanoscale biological structures that would otherwise not be
resolved. Coherent Raman microscopes allow highly selective biomolecular
finger-printing in unlabelled specimens, but photodamage is a major roadblock
for many applications. By showing that this roadblock can be overcome, our work
provides a path towards order-of-magnitude improvements in both sensitivity and
imaging speed.
