In this dissertation, a twin-beam squeezed state combined with various experiments and computational techniques were applied to investigate quantum systems including quantum sensing and quantum coherence. These quantum ideas can provide us with methodologies to achieve un-precedented sensitivity and resolution for sensing and imaging, which is far beyond the classical measurement could achieve. These new methods based on quantum physics are revolutionizing the scales at which one can do sensing, imaging and microscopy with applications in diverse fields from physics and engineering to chemical, biology and medicine. The quantum sensing has clear bearing on many future technologies. In our lab, the quantum light source is a twin-beam squeezed state, which is generated via four- wave mixing nonlinear processes in a 85Rb vapor cell. Our goal is to develop novel quantum sensing techniques by engineering quantum light sources and by improving the quantumness of these sources, so that they can be utilized in proof of principle applications as well as other related applications, such as the weak absorption measurement, decoherence investigation, and two-photon microscopy. To investigate the quantum system, we first measured the quantumness of the twin-beam squeezed state, which has 6.5dB squeezed noise below the shot-noise limit. We demonstrated that an electron-multiplying charge-coupled-device EMCCD camera can capture the temporal images which provides the dynamic information. Thus, EMCCD camera can be used to measure the temporal quantum noise of twin beams squeezed light, and we observed ~ 25% of temporal quantum noise reduction with respect to the shot-noise limit, which was given by coherent state. To the best of our knowledge, this is the first experimental showcase that an EMCCD camera can be used to acquire quantum properties of light in the temporal domain. One of the most serious challenges to the implementation of quantum technology, such as quantum communication and quantum methodology, is decohenrence, which due to the quantum state interacting with environment. The quantum decoherence has been extensively investigated theoretically. Thus, we report a novel experimental scheme on the study of decoherence of a two-mode squeezed vacuum state via its second harmonic generation signal. One of the most important properties of squeezed light is the nonzero quantum correlation ab between two entangled modes a and b, our scheme can directly extract the decoherence of this quantum correlation. This is the first experimental study on the decoherence effect of a squeezed vacuum state, and also the first experimental study on the decoherence effect of a squeezed vacuum state without full density matrix tomography. We obtain a good agreement between the experimental results and the theoretical calculation. Based on the quantumness of the twin-beam squeezed state, we demonstrated that direct absorption measurement can be performed with sensitivity beyond the shot-noise limit. By using laser beam, the theoretical absorption measurement limit which be called the shot noise, is due to the fundamental Poisson distribution of photon number of laser radiation. With the twin-beam squeezed state, we present detailed theoretical analysis for the expected quantum advantage, which well agrees with experimental result. More than 1.2 dB quantum advantage for the measurement sensitivity is obtained at faint absorption levels (<= 10%). The observed quantum advantage when corrected for optical loss would be equivalent to 3 dB. Another quantum application of the twin-beam squeezed state is two-photon absorption (TPA) fluorescence sensing or imaging. For a classical illumination, the TPA is extremely small and the quadratic dependence on the input photon flux, which means that high peak-intensity pulsed laser are used to get a faithful result. However, the bio-sample or biological specimen may be damaged by such strong peak-intensity pulsed laser. Her
