Backaction is defined as the effect of a detector on the measurement process, where the detector is not just making a measurement but also affecting the measured system. The contribution from backaction becomes significant when measuring very small quantities (like very weak forces). Backaction-evading measurements were envisioned first in the context of detecting gravitational waves in LIGO experiments. The problem we are interested in is the monitoring of a time-varying electric field using a molecular vibrational mode as a probe. This is particularly useful for studying local electrostatic environment in biological environments like proteins. Specifically, the time variation of the electric field can contain signatures of the dynamics of the macromolecule’s conformational dynamics. Standard methods for this include vibrational Stark spectroscopy, which relies on second-order effects to study changes in frequency of the probe vibration. However, these methods may be affected by backaction when the vibration experiences a sufficiently weak electric field. Here, we propose cavity-assisted backaction-evading Raman spectroscopy. This method allows for backaction-free, quantitative determination of the strength and frequency of a time-dependent classical force acting on a high-frequency vibrational mode. The setup involves a single molecule placed inside a cavity, which is coupled to the high-frequency vibrational mode. Raman spectroscopy is performed on the cavity by driving the cavity at its sidebands. The reflected output current is then collected to produce a spectrum. Experimental feasibility of this scheme would enable bypassing backaction-induced uncertainty in quantum measurements.

Detection of time-varying weak electric fields using cavity-assisted backaction-evading Raman spectroscopy

ACS Spring 2022

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