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.

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In recent years, technologies on quantum-entangled photon pairs have made significant progress. This paper introduces applications of quantum entangled photon pairs in optical sensing. The first example is quantum optical coherence tomography with the tolerance for the effect of dispersion in the medium, which has been an obstacle to the high resolution of conventional optical coherence tomography. The second example is quantum infrared spectroscopy. This technology enables infrared spectroscopy with a visible detector. The latest status of our recent works on the above-mentioned photonic quantum sensing using entangled photons will be reviewed.

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Absorption spectroscopy is one of the most fundamental and widely used spectroscopic methods. Its sensitivity is usually limited by shot noise, which arises from the statistical fluctuation of the number of photons. When entangled photon pairs are used as the light source, however, it is possible to suppress the shot noise and increase the sensitivity of absorption spectroscopy beyond the shot-noise limit. Recently, experimental realizations of ultrasensitive absorption spectroscopy using entangled photon pairs have been reported by two groups including us. Furthermore, we demonstrated that ultrasensitive absorption spectroscopy exhibits an increased capability to identify and quantify the chemical constitution of sample solutions, which are the two principal purposes of using absorption spectroscopy as an analytical tool.

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Stimulated Raman scattering (SRS) microscopy is regarded as a sensitive molecular-vibrational imaging method. Its quantum enhancement (QE) is attracting attention for achieving sub-shot-noise sensitivity, while previous QE-SRS suffered from low optical power, which limits the sensitivity. We present a QESRS microscope based on quantum-enhanced balanced detection (QE-BD), which allows us to operate QE-SRS in high-power regimes (＞30 mW) that are comparable to classical SRS microscopes. We observed a squeezing level of 3.6 dB without an SRS microscope, and the demonstrated QE-SRS imaging shows 2.89 dB noise reduction compared with classical balanced detection SRS microscopy. These results confirm that QE-SRS with QE-BD can work in the high-power regime, paving the way for breaking the sensitivity of classical SRS microscopes.

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In this paper, instead of generating and using quantum entangled states for quantum measurement, we extract and detect the components that correspond to quantum entangled states by post-selection from classical optical states. This scheme is derived from a systematic method based on the time-reversal symmetry of quantum mechanics and allows us to efficiently reproduce the same measurement results when using quantum entangled states, but with high intensity classical states. We successfully obtained two-photon interferometric phenomena, such as phase super-resolution and automatic dispersion cancellation in the Hong-Ou-Mandel interference, with classical optical pulses. The proposed time-reversal method will be a step toward a new boundary between quantum and classical physics.

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Optical magnetometry with ultracold atoms can achieve excellent magnetic field sensitivity with μmscale spatial resolution. In addition, the readily available quantum control on an ultra-cold-atom cloud makes it an ideal platform for studying quantum measurement. We achieved 5.0 pT/Hz 1/ 2 sensitivity with 1.4 × 102 μm 2 spatial resolution by magnetometry using a Bose-Einstein condensate (BEC). In this article, we overview optical magnetometry using ultra-cold atoms, present our results on a sensitive BEC magnetometer and the application of quantum technologies to BEC magnetometry, and describe prospects for ultra-cold-atom magnetometers.

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When a quantum entanglement is distributed to atoms consisting of optical clocks in a network of optical clocks by a quantum network, measurement precision can be obtained that approaches the limit of quantum mechanics. Such a system is called a quantum network of optical clocks. We provide an overview of the present studies toward a quantum network of ion-trap optical clocks. These studies address iontrap optical clocks, optical clock networks, quantum networks of trapped ions, quantum networks of ion trap optical clocks and quantum frequency conversion. We find that many missing building blocks are present to implement a quantum network of ion-trap optical clocks and that basic research and development remain important.

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