Light Quantum Entangled States With Applications

Quantum entangled states are at the heart of quantum information science and play a crucial role in quantum physics. Quantum information science is an emerging field with the potential to cause revolutionary advances in the fields of science and engineering involving computation, communication, precision measurement, and fundamental quantum science. This thesis describes in detail our studies on the preparation and application of two-photon entangled states.First, we discuss the schemes for generating two-photon entangled states through spontaneous parametric down-conversion (SPDC) in a nonlinear crystal. Many experiments about entangled states are based on the two-photon field generated by SPDC which is inefficient for generating two-photon entangled states. We also introduce a method based on parametric mixing processes in quasi-periodically poled quasi-phase matched nonlinear crystals. It can greatly improve the generating efficiency of two-photon entangled states.We describe various applications of two-photon entangled states. A new scheme for a random number generator and the absolute calibration of the quantum efficiency of single photon detectors are given in detail. A new experimental set-up for the absolute self-calibration of a single photon detector, i.e. without using a second detector, is presented. The security of information is the most important above all and true random number sequences must be introduced into the emerging field of quantum cryptography. A scheme for the generation of a true random number sequence using two-photon entangled states is also presented in this thesis.Experiments related to these two aspects have been performed. We have set up a source of two-photon entangled states based on parametric down-conversion in a BBO crystal pumped by a He-Cd laser. Using that source a new scheme for a random number generator based on quantum entangled photon pairs is demonstrated. Signal photons produced by optical parametric down-conversion are detected at two single-photon detectors after transmission or reflection at a 50/50% beamsplitter, to form a truly random binary sequence. Their arrival is signaled by their twin idler photons, so that a cw laser source has been usedinstead of attenuated laser pulses. Coincidence measurement is employed to obtain the bit sequences, which are shown to fully satisfy the standard tests for randomness. We also have conducted experiments to calibrate the quantum detection efficiency of our Si single photon counting module.There are for chapters in this thesis. The first chapter is summarization, which chiefly introduces the development of quantum entangled states. The second chapter introduces the schemes for generating two-photon entangled states. A scheme for the generation of a true random number sequence using two-photon entangled states is given in the third chapter. The forth chapter present the theory of absolute calibration of the quantum efficiency of single photon detectors.