  Absorption spectroscopy is routinely used to characterise chemical and
biological samples. For the state-of-the-art in absorption spectroscopy,
precision is theoretically limited by shot-noise due to the fundamental
Poisson-distribution of photon number in laser radiation. In practice, the
shot-noise limit can only be achieved when all other sources of noise are
eliminated. Here, we use wavelength-correlated and tuneable photon pairs to
demonstrate sub-shot-noise absorption spectroscopy. We measure the absorption
spectra of spectrally-similar biological samples---oxyhaemoglobin and
carboxyhaemoglobin---and show that obtaining a given precision in resolution
requires fewer heralded single probe photons compared to using an ideal laser.
