Biological measurement beyond the quantum limit

Quantum metrology allows high sensitivity measurements to proceed with a lower light intensity than classically possible [1]. An important frontier for this technology is in biological measurements, where photochemical interactions often disturb biological processes and can damage the specimen [2]. Here we report the first demonstration of biological measurement with precision surpassing the quantum noise limit [3]. This was enabled through the development of a new microscopy system which extended previous methods used to track the motion of highly reflective mirrors with non-classical light to measurements of microscopic particles with non-paraxial fields (see Fig. 1). Biological dynamics in the critical Hz-kHz frequency range were made accessible by applying a quantum optical lock-in technique for the first time. This straightforward technique allowed quantum enhancement over a frequency range which reached as low as the range reported for squeezed light sources developed for gravity wave interferometers [4].