Quantum Measurement In Quantum Information Protection

As a burgeoning interdisciplinary science, quantum information theory has received much attention from worldwide since its birth in 1980 s. Based on quantum theory, scientists try to realize nontrivial tasks such as the design of fast computation, the unconditionally secure private communication. Quantum coherence and quantum entanglement play important roles in quantum information processing and give rise to many interesting phemomonia. Howerver, they are both fragile to quantum noises and are easy to destroy by the bad interactions between the quantum system and its environment. so, it is of great meaning to protect the quantum coherence and quantum entanglement against environmental noises. Many methods have been proposed for decoherence protection including decoherence-free subspace,quantum error correction code and dynamical decoupling.Based on the theory of quantum measurement, we proposed the following two schemes for decoherence suppression.Firstly, we proposed the operator quantum Zeno effect theory based on which a new scheme for quantum coherence protection is given. According to the typical quantum Zeno effect(QZE), the evolution of a quantum system can be frozen by frequent measurements.Based on QZE, some methods for quantum entanglement have been proposed. However,limited by the requirement of the commuting between the measurement elements, most of the proposals call for many-particle operations which are difficult to realize. We studied the case that the measurements do not commute with each other. In such a situation, we find that certain physical quantities can be frozen through frequent measurements although the quantum states may change with time as a result of the measurements. We coin this effect as operator quantum Zeno effect. Based on the new theory, we have designed a new technique to suppress decoherence. Besides, we give a 3-qubit encoding method as an example.Secondly, we studied how to optimally protect quantum states and quantum entanglement in finite temperature environments using weak measurement. Dynamical decoupling is one of the most powerful techniques to battle against quantum decoherence. However,it is not willing to work because of the lack of memory when the time scale characterizing the unwanted interaction is too short. In such cases, it has been shown that quantum reversal scheme using weak measurement has some advantages. But unfortunately, the extentstudy focus mostly on the zero-temperature case. We studied how to battle against the quantum decoherence when the environmental temperature is nonzero. The optimal measurement strengths are given in explicit formulas. In addition, we reexamine the efficiency of the weak measurement scheme for many common encountered noise, such as bit-flip noise, phase-flip noise and depolarizing noise.