Quantum-enhanced absorption spectroscopy

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.