Quantum Measurement And Its Backactions

Measurements in classical theory are roughly viewed as an innocent visit of the subjective physical realities. In contrast with that, there is no uncontested physical re-ality, and the measurements necessarily disturb the system being measured. This is so-called the backaction of quantum measurements. The various differences between the classical and the quantum, represented by uncertainty, non-locality, contextuality and so on, exhaust the spirits of physicists, meanwhile predicting colorful quantum phenomena and basing future technologies. In this thesis, we shall take a glimpse of them from three directions:quantum weak measurements, error-disturbance tradeoff of quantum measurements, and the laser cooling in an optomechanical system.Quantum weak measurement is specific as measurement realized with a slight in-teraction followed by postselection. Rather than the expectation values generated in or-dinary projective measurements, weak measurements produce weak values, from which all the mysteries origin. In the names, the word "weak" emphasizes the neglectable backaction during the measurements. By definition, weak values could be complex numbers, and simply break the spectrum of observables being measured. True under-standing of weak values and measurements with postselection calls an exactly analytical approach. By slightly modifying a trivial part of the definition of weak value, we find that the essential information therein can be exactly extracted independent to the mea-surement strength. Thus, the backaction should not be an issue in this research. In this sense, the names named by "weak" are quite misleading. This result also solves a series of problems of the practical applications of weak value theory.It is widely believed that the backaction of quantum measurements may imply a tradeoff between precision of measurements and the consequent disturbance to the systems. In fact, the historic idea of such tradeoff has never been exactly formulated until very recently. Some theories were proposed and caused heated debate in the com-munity. As the second theme of this thesis, we shall theoretically and experimentally propose a completely new theory of this tradeoff, and unveil its close relationship with the quantum uncertainty. We find that if quantifying error and disturbance by the prob-ability distributions inherent to quantum mechanics, the existence of such tradeoff is determined by the relation of quantum uncertainties of the relevant observables.Finally, we shall focus on a concrete quantum system, the optomechanics, which consists of lasers and macroscopic oscillators. Optomechanics arises as the basic model of the gravitational wave detection project of LIGO. Now it has been developed to be an important methods for precise measurements, quantum information processing and the study of quantum foundations. We shall focus on the passive laser cooling of the oscillators, a building brick of almost all interesting applications of optomechanics. And this cooling relies on the backaction of the optics field to the mechanical oscillator. The conventional sideband cooling scheme fails in the unresolved sideband regime. We shall propose a new scheme to solve this problem.