Abstract
Light is widely used for a variety of measurements. Often the light intensity should be weak in order not
to affect the object. Even if the light itself is of sufficient intensity, the change in the probe light can be
quite small when, for example, the amount of the specimen is very small or the generated light is weak as
in cases of nonlinear spectroscopy. In such cases, the noise in the signal becomes a critical issue. The
use of quantum states and/or quantum properties of light is expected to improve the signal-to-noise ratio.
In this special issue, we focus on the measurements using such quantum light.
