Experimental Preparation And Manipulation Of Optical Quantum Entangled States

Quantum information is one new-rising and fast-developing subject, as a successful combination information science and quantum mechanics, which can be used to solve problems that classical computer cannot address. Quantum information is attracting more and more attention due to the great potential applications and significant scientific influence.Entanglement plays an important role in quantum information processing, such as quantum cryptography, quantum communication, and quantum computation. Entanglement also serves as a one of the greatest features that distinguish quantum information from classical information theory. Entanglement is so important that how to define, quantify, and detect entanglement both in theory and experiment becomes the basic task and hot topic in quantum information.The dissertation are focusing on the characterization of quantum states and their evolving dynamical process, as well as the concept of entanglement, the detection and quantification of entanglement. The main results of the dissertation are as follows:1. Entanglement operation.Quantum systems will unavoidably interact with external environment leading to decoherence. In general, the practical quantum state is non-maximally entangled states or mixed states, or both. In order to make full use of entanglement states, we have to operate on the practical entangled states at hand, i.e. purify entanglement, such that the entanglement can meet our criteria in practical application.Generally speaking, there are two kinds of entanglement purification or distillation protocols. The first kind of protocols involves the operation on single copy of entangled states, which is also called as Filtering Protocols. We experimentally implement the optimal filtering protocol and show that filtering operations can effectively increase entanglement. The second kind of protocols need the operation on many copies of quantum states, such as simultaneous operation on two copies, Recurrence Protocols.The importance of entanglement purification exists in not only quantum information theory, but also in the practical applications. It can build unconditional quantum key distribution protocols, constitute quantum repeaters, and improve the error threshold in quantum computation. 2. Entanglement detection and quantification.Entanglement detection and quantification is another key problems in quantum information. Combined with our works, we introduce the common entanglement detection and quantification methods which is frequently used in experiments, such as the local decomposition of entanglement witness, optical interferometer for direct observation of entanglement witness, the uncertainty relations, and the projecting into the antisymmetric subspace consisting of two identical copies. These methods working in the special experimental scenario can effectively reduce the measurement times and estimate the information about the quantum systems directly. In practical cases they may have important applications.We not only discuss entanglement detection and quantification for discrete variables entanglement, but also for one special kind of continuous variables entanglement, which rises from the transverse momentum entanglement between the signal and idler photon produced in Spontaneous Parametric Down Conversion process. It involves the relation between the spatial and spectral information of the pump light and that of the down-conversioned photons, which can help to control the distribution of down-conversioned photons and its frequency correlation type.3. The application in quantum dynamics.Entanglement is also helpful in characterizing the state and dynamical process of quantum systems. We introduce two methods in characterizing dynamical process of quantum systems. One method is quantum process tomography which requires quantum state tomography on a set of input states and subsequent reconstruction of quantum process matrix on these data; another method is Direct Characterization of Quantum Dynamics (DCQD), which is based on input entangled states and Bell states measurement at the output. We experimentally perform quantum dynamical process measurement with the two methods and show it can reduce the experimental configurations by a factor of 2 compared with that of standard quantum processing tomography (SQPT). The algorithm works better when ideal Bell state measurement is adopted since in optical experiment we can only perform partial Bell state measurement.