Quantum-enhanced balanced detection for ultrasensitive transmission measurement

Abstract

Balanced detection is a popular method to cancel out the effect of laser intensity noise in optical measurements and spectroscopy. However, the signal-to-noise ratio (SNR) that can be achieved with balanced detection is constrained by the standard quantum limit (SQL). Here, we propose quantum-enhanced balanced detection (QBD), which allows us to improve the SNR beyond the SQL to realize ultrasensitive transmission measurement. In QBD, squeezed vacuum is injected to one of the input ports of a beamsplitter (BS) used in balanced detection to produce a pair of light waves whose shot noises are entangled with each other. Compared with previous quantum-enhanced measurement methods, QBD is advantageous because it can handle a higher optical power without sacrificing the degree of sensitivity enhancement. We present the theory of QBD and discuss the effects of the splitting ratio of the BS and the optical loss caused by the sample under test. We also describe the application of QBD to the sensitivity enhancement of molecular vibrational imaging based on stimulated Raman scattering microscopy.