  Sensors that are able to detect and track single unlabelled biomolecules are
an important tool both to understand biomolecular dynamics and interactions at
nanoscale, and for medical diagnostics operating at their ultimate detection
limits. Recently, exceptional sensitivity has been achieved using the strongly
enhanced evanescent fields provided by optical microcavities and nano-sized
plasmonic resonators. However, at high field intensities photodamage to the
biological specimen becomes increasingly problematic. Here, we introduce an
optical nanofibre based evanescent biosensor that operates at the fundamental
precision limit introduced by quantisation of light. This allows a four
order-of-magnitude reduction in optical intensity whilst maintaining
state-of-the-art sensitivity. It enable quantum noise limited tracking of
single biomolecules as small as 3.5 nm, and surface-molecule interactions to be
monitored over extended periods. By achieving quantum noise limited precision,
our approach provides a pathway towards quantum-enhanced single-molecule
biosensors.
