Experimental Investigation On Continuous Variable Quantum Logical Operation

Quantum computing is expected to be more efficient than its classical counterpart. In recent years, more and more scientists have joined in the investigation of quantum information science. Quantum communication and quantum computation are being rapidly developed, which will prompt the development of science and technology and the progress of society. The dream to enter the quantum time will become to realization in the near future.Continuous variable (CV) quantum entanglement between amplitude and phase quadratures of optical fields is one of the most important quantum resources to develop quantum computation and quantum information networks. For demonstrating the quantum computing based on utilizing optical continuous variables. We have to prepare CV entangled states of light with higher entanglement degree, firstly. During my Ph. D period, the first experimental research subject finished by our group was to generate 6dB EPR entanglement state of optical field experimentally. Then, we experimentally demonstrated the controlled-X operation based on continuous-variable four-partite cluster states and quantum teleporters. We also theoretically designed the generation systerms of optical CV cluster states with six and eight entangled submodes. These accomplished works provide the fundamental and useful references for implementing more complex CV quantum computation.The completed main research works are as following:1 We experimentally demonstrated that the quantum correlations of amplitude and phase quadratures between signal and idler beams produced from a non-degenerate optical parametric amplifier (NOPA) can be significantly improved by using a mode cleaner in the pump field and reducing the phase fluctuations in phase locking systems. Based on the two technical improvements the quantum entanglement measured with a two-mode homodyne detector is enhanced from 4 dB achieved by our previous system to present 6 dB below the quantum noise limit (QNL) with the same NOPA and nonlinear crystal.2 We accomplished an experimental study toward demonstrating the controlled-X operation. This is a two-mode quantum logical gate, in which continuous variable (CV) four-partite cluster states of optical modes are utilized. Two quantum teleportation elements are used for achieving the gate operation of the quantum state transformation from input target and control states to output states. By means of the optical cluster state prepared off-line, the homodyne detection and electronic feeding forward, the information carried by the input control state is transformed to the output target state. The presented scheme of the controlled-X operation based on teleportation can be implemented nonlocally and deterministically. The distortion of the quantum information resulting from the imperfect cluster entanglement is estimated with the fidelity.3 We propsed the scheme for preparing six-partite and eight-partite linear CV cluster states of optical field based on utilizing phase squeezed states of light and linear optical elements. By comparing four-partite weighted and unweighted linear cluster states, we show that the effect of anti-squeezing components to quantum correlations of cluster states can be eliminated by means of choosing the optimum splitting ratio of beamsplitter.The creative works are as follows:1 we obtained CV optical entangled states with amplitude and phase quadrature quantum correlations of about 6 dB, which are the highest correlations generated from NOPA devices so far to the best of our knowledge. Comparing with our previous NOPA systems, two technical improvements are made:1. Both amplitude and phase noises of the pump laser of NOPA are reduced to the QNL level with a suitable mode cleaner.2. The relative phase fluctuations of phase locking systems are minimized. Our experiment shows that the implementation of NOPAs on entanglement generation can be significantly enhanced by means of some technical improvements on the pump laser and the phase locking systems even using the same NOPA configuration and a same nonlinear crystal.2 An experimental study toward demonstrating the CV controlled-X logical operation based on four-partite cluster states of electro-magnetic fields is presented. Although the entanglement between output states was not observed in the present experiment due to the absence of better cluster resources, the proof-of-principle experiment proves that the controlled-X operation of one-way QC can be unconditionally demonstrated with the designed system if CV cluster states of higher entanglement are available.3 We propose the experimental schemes to realize the six-partite and eight-partite linear cluster states with continuous variables, based on utilizing phase squeezed states of light and linear optical elements.