  We demonstrate a sensor scheme for nanoscale target displacement that relies
on a single Quantum Cascade Laser (QCL) subject to optical feedback. The system
combines the inherent sensitivity of QCLs to optical re-injection and their
ultra-stability in the strong feedback regime where nonlinear frequency mixing
phenomena are enhanced. An experimental proof of principle in the micrometer
wavelength scale is provided. We perform real-time measurements of displacement
with {\lambda}/100 resolution by inserting a fast-shifting reference etalon in
the external cavity. The resulting signal dynamics at the QCL terminals shows a
stroboscopic-like effect that relates the sensor resolution with the reference
etalon speed. Intrinsic limits to the measurement algorithm and to the
reference speed are discussed, disclosing that nanoscale ranges are attainable.
