Quantum Logic Gate Sequence And Quantum Error Correction With Continuous Variables

Quantum information science is a new subject recently developed via combining quantum mechanics, information science and computer science and has become the frontier of international research. Quantum information based on applying quantum properties of matter has special information function, which is able to break through the limit of classical information systems, such as to improve the computing speed, to ensure information security, to expand the capacity of information and to increase the detection accuracy. Quantum information mainly includes two research directions of quantum communication and quantum computation.It has been expected that the quantum computation has the capacity to work out some difficult problems which are intractable in the classical computation. Originally, Quantum computing research is focused on the field of discrete variables based on quantum bits. In recent years, the continuous variable (CV) quantum information processing based on the optical field has been promptly developed and used as a new tool to implement quantum protocols. CV quantum computation can effectively simulate the continuous evolution of the quantum systems. One of the important resources for CV quantum computation and quantum communication networks is the multipartite entangled states with continuous quantum variables. The one-way quantum computation utilizing CV cluster entangled states is one of the important quantum computation protocols. Quantum logic operation and quantum error correction are two necessary performances in quantum computation. Our group achieved the experimental studies on CV quantum logic gate sequence and quantum error, in which multipartite entangled states of optical field are used as the quantum resources.The completed main research works are as following:1. Single-mode squeezing and Fourier transformation operation are two essential logical gates in CV quantum computation. We efficiently realized the two logical operations in a simpler scheme based on the use of Einstein-Podolsky-Rosen two-mode entangled states. The theoretical calculations and the experimental results demonstrate that the presented scheme not only decreases the requirement to the resource quantum states at the largest extent but also enhances significantly the squeezing degree and the fidelity of the resultant modes under an identical resource condition. The gate operations applying two-mode entanglement can be utilized as a basic element in the future quantum computer involving a large-scale cluster state.2. Measurement-based one-way quantum computation using cluster states as resources provides an efficient model to perform computation and information processing of quantum codes. We implemented the first CV gate sequence consisting of a single-mode squeezing gate and a two-mode controlled-phase gate based on applying a six-mode cluster state of optical field. The quantum property of this gate sequence is confirmed by the fidelities and the quantum entanglement of two output modes, which depend on both the squeezing and controlled phase gates. The experiment demonstrates the feasibility of implementing Gaussian quantum computation by means of accessible gate sequences.3. Quantum error correction protects the quantum state against noise and decoherence in quantum communication and quantum computation, which enables one to perform fault-torrent quantum information processing. We experimentally demonstrate a quantum error correction scheme with a five-wave-packet code against a single stochastic error. Five sub-modes of a C V cluster entangled state of light are used for five encoding channels. Especially, in our encoding scheme the information of the input state is only distributed on three of the five channels and thus any error appearing in the remained two channels never affects the output state, i.e. the output quantum state is immune from the error in the two channels. The stochastic error on a single channel is corrected for both vacuum and squeezed input states and the achieved fidelities of the output states are beyond the corresponding classical limit.The creative works are as follows:1. Using EPR source to achieve two single-mode Gaussian operations of single mode squeezing and Fourier transform, with a simpler program and less quantum resource.2. Realizing CV quantum logic gate sequence based on the six cluster entangled state of optical field, the gate sequence includes a squeezing operation and a controlled phase gate. The experiment provides technical reference for developing CV one-way quantum computer consisting of many logical gates.3. We designed and accomplished an efficient and compact quantum error correction system of five wave packet coding for continuous, in which a new partial encoding method based on five-mode entanglement of light is utilized.