  Quantum metrology enables some of the most precise measurements. In the life
sciences, diamond-based quantum sensing has enabled a new class of biophysical
sensors and diagnostic devices that are being investigated as a platform for
cancer screening and ultra-sensitive immunoassays. However, a broader
application in the life sciences based on nanoscale nuclear magnetic resonance
spectroscopy has been hampered by the need to interface highly sensitive
quantum bit (qubit) sensors with their biological targets. Here, we demonstrate
a new approach that combines quantum engineering with single-molecule
biophysics to immobilize individual proteins and DNA molecules on the surface
of a bulk diamond crystal that hosts coherent nitrogen vacancy qubit sensors.
Our thin (sub-5 nm) functionalization architecture provides precise control
over protein adsorption density and results in near-surface qubit coherence
approaching 100 {\mu}s. The developed architecture remains chemically stable
under physiological conditions for over five days, making our technique
compatible with most biophysical and biomedical applications.
