Study On Quantum Information Acquisition In Quantum Control

Since the birth of quantum theory at the beginning of the twentieth century, it has changed and affected human society profoundly: it provides a theoretical basis for people to understand and rebuild the micro-world; also it promotes the laser, semiconductor, nuclear, and other high-tech development. With the rapid development quantum theory has integrated with other subjects. Quantum information science is a new discipline generated from the integration of quantum theory and information science, and its rapid development will bring another revolution in scientific community.Quantum state is an information unit in quantum information science, which need be manipulated in order to deal with quantum information, so the quantum control theory was born to study the active control of quantum systems. With the continuous improvement, quantum control has become a hot international research focus and many gratifying results have been achieved in recent years. Closed-loop control has been the core of classical control, so quantum control began to gradually move to closed-loop control from the initial open-loop control. Closed-loop control is also known as feedback control, in which the information acquisition, the basis for understanding the system status and making control decision, is the key. Quantum system follows the laws of quantum mechanics which is essentially different from classical mechanics, so the classic information acquisition method is no longer applicable to quantum system. We need to explore new technology and method for information acquisition in quantum feedback control.The main contents on quantum control and the latest development of quantum feedback control are summarized at the beginning of this paper. Then, combined with the character of quantum feedback control, the feasibility of measuring quantum states by quantum tomography is analyzed, and further quantum control system identification based on the quantum process tomography is proposed and quantum feedback control system based on quantum tomography is designed. This paper includes main aspects as following:(1) Quantum state measurement based on quantum tomography is proposed.Quantum state as a unit of quantum information is main research object in quantum control. Since quantum states follow the laws of quantum mechanics, quantum non-cloning theorem and uncertainty relationship result in "quantum collapse" phenomenon when quantum states are measured, which makes quantum information acquisition much difficult. We attempt to solve this problem by quantum tomography technology. The term, quantum tomography, comes from medical X-ray computer-aided tomography (CT), and it is a statistical measurement method. According to quantum mechanics we know that a quantum system density matrix contains all the system information, so the system information is totally obtained if the system density matrix can be acquired. Quantum tomography can reconstruct density matrix by measuring a large number of unknown quantum state in the same status to gain quantum state information. Taking example for polarized photon state, we design the experiment of quantum state density matrix reconstruction through quantum tomography, and validate the feasibility of experiment by computer simulation technology. The results show that quantum tomography is an effective and feasible method to obtain quantum state information.(2) The scheme for quantum control system identification based on quantum process tomography is designed.In order to design better quantum control systems and achieve more precise control, the performance and parameters of control system need to be understood well. To solve this problem, we propose a scheme of quantum system identification based on quantum process tomography. Quantum process tomography is a main application based on quantum tomography, which is widely used to determine quantum channel parameter in the field of quantum communication. The main process of quantum process tomography is as follows: first, let a set of selected complete input quantum statesρin pass through the quantum channel, and then quantum output statesρout are measured by quantum tomography, and quantum channel parameter is determined according to relationship between input and output states. Following this idea, taking example for magnetic field control of electron spin, we design the control system identification program which is simulated by computer simulation technology. The simulation results proved that this scheme is able to effectively identify quantum control system parameter.(3) Quantum feedback control based on quantum tomography is proposed.Quantum feedback control has gradually become a research focus and achieved some results in recent years. However, these feedback control strategy has some flaws, in Markovian quantum feedback, Bayesian quantum feedback and non-Markovian quantum feedback with delay, feedback signals are all classical information, so they can not be regarded as purely quantum feedback control; and for quantum coherent control, feedback control based on quantum cloning and teleportation, although quantum information is used as feedback signal and coherence of quantum systems is maintained in some extent, these systems are very difficult to realize. The realization of these systems is depended on further improvement of experimental techniques. We propose quantum feedback control strategy based on quantum tomography, in which quantum state measurement and quantum control system identification are realized, and quantum feedback control system using quantum information as feedback is formed. In this system, control strategy can be adjusted according to current system status and deviation of control system can be corrected according to control system parameters form identification so as to effectively reduce external interference and realize more precise control of quantum systems.