Theoretical And Experimental Investigation For The Entanglement Characteristics Of Output Optical Fields From NOPO

As well known, the phenomenon of entanglement is one of the quintessential features in quantum mechanics. Utilizing quantum entanglement, we can accomplish the impossible tasks in the frame of classical physics. In recent years, some important experiments in quantum information such as unconditional quantum teleportation, quantum dense coding and quantum entanglement swapping have been realized with quantum entanglement. Quantum computation with entanglement also becomes an advanced research topic in modern science and technology. Designing and experimentally realizing entanglement sources have the significance not only for the fundamental research but also for the potential application requirement.Entanglement between amplitude-quadrature variables and phase-quadrature variables of light fields, is one of main research areas for quantum entanglement with continuous variable (CV). CV entanglement can be accomplished by coupling squeezed states on a beam splitter, also can be directly generated from an non-degenerate optical parametric amplifier (NOPA). The optical parametric oscillator below threshold without injecting the seed light can generate vacuum squeezed states. If injecting seed light, bright entangled states can be generated from a NOPA. However, the power of the entangled optical beams is only in the order of tens of microwatts, which is not suitable for some applications. NOPO operating above threshold can generate several milliwatts or even higher intensity entangled beams easily. Beacause the frequencies of two down-converted optical beams from NOPO operating above threshold is nondegenerate, the phase correlation between the signal and idler beams did not be detected for a long time due to the absence of the local osccilator. We measured the noise power spectrum of the phase correlation of twin beams utilizing a pair of the unbalanced fiber Mach-Zehnder interferometers. The quantum entanglement of the twin beams with higher intensity was experimentally demonstrated. The main contents presented in the thesis are as following: 1. The methods to measure the entangled beams generated from NOPO above threshold are introduced. There are two types of methods: one is the balanced homodyne detection, in which the frequencies of two down-conversion optical beams have to be made degenerate by inserting a birefringent plate inside NOPO or to generate two local oscillators corresponding to the frequencies of signal and idler lights respectively by means of acousto-optic modulators; the other one is the self-homodyne detection, which can be accomplished by scanning optical cavity or using a pair of unbalanced Mach-Zehnder (M-Z) interferometers.2. The classical and quantum characteristics of NOPO above threshold are analyzed with semiclassical theory. The results show that the intensity-difference noise between the output fields has no relationship with the phase noises of the pump filed. However the phase-sum noise of them not only depends on the physical parameters of the down-conversion fields, but also depends on that of the pump field. We considered that there are three factors influencing the phase correlation of the down-conversion fields: 1.The finesse of the pump field in NOPO. 2. The excess phase noise in the pump field. 3. The amplification of the spurious phase noises of pump field inside NOPO. Based on the analyses, the formulas expressing the intensity-difference and phase-sum of twin beams are derived in which only the experimentally measurable parameters are involved. According to the formulas, accomplished experimental results can be explained reasonably.3. A set of unbalance fiber M-Z interferometers for measuring the noise power spectra at the analytical frequencies of 2MHz, 5MHz, 10MHz are constructed and the influence of the fiber coupling loss on the noise measurement are analyzed theoretically. The correlation noise spectra of intensity-difference and phase-sum between twin beams are experimentally detected at 2MHz, 5MHz and 10MHz. The influence of the excess phase fluctuation of the pump laser on the phase correlation between the signal and idler modes of the output fields was experimentally demonstrated.The creative works are as follows:A. We derived the formulas expressing quantum correlations of amplitudeand phase quadratures between twin beams, in which the finesse of NOPO for the pump field, the excess phase noise of the pump laser and the amplification of the spurious pump phase noises inside OPO are considered and only experimentally measurable parameters are involved.B. We constructed a set of unbalanced fiber Mach-Zehnder interferometers and analyzed the influence of the loss in the interferometer to the noise measurement.C. We obtained the entangled optical beams with nondegenerate frequencies and higher intensity from a NOPO above the threshold. Both quantum correlations of intensity-difference and phase-sum were experimentally demonstrated and the correlation noise spectra were experimentaaly detected.