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Circular dichroism (CD), which detects differential absorption with left- and right-circularly polarized light, is widely used for characterizing molecular chirality. Despite its similarity to absorption spectroscopy, CD spectroscopy typically requires a significantly longer time to acquire a spectrum due to the low intensity of the CD signal. The CD measurements can be accelerated if the noise level in the spectra can be reduced, which facilitates the detection of the weak CD signal. Here we show that such noise suppression is indeed possible using entangled photon pairs as the light source. By taking advantage of the photon number correlation of entangled photon pairs, we suppressed the noise contained in the CD spectra by 30% below the shot-noise limit which is a fundamental limit in conventional CD measurements. As a consequence, sub-shot-noise CD spectroscopy developed in this study is capable of characterizing the molecular chirality twice as fast as conventional CD spectroscopy.
Matsuzaki et al. (Tue,) studied this question.