Two-photon quantum entanglement.

摘要

Entangled states of two or more particles have attracted a great deal of attention even in the early days of quantum mechanics and they lie at the heart of many deep questions on the foundations of quantum theory.; In this thesis, various aspects of two-photon entangled states generated in spontaneous parametric down-conversion are studied. Through a series of experimental studies, we show that an attempt to interpret the experimental results based on single-photon picture for an entangled two-photon state fails and results in paradoxes. These experiments include a set of Young-type two-slit experiments, in which the two interfering two-photon amplitudes originate from two spatial domains, and the time-equivalent, in which the interfering amplitudes originate from two time domains. In both cases, interference disappears when the which-path information is made available.; The concept of two-photon effective wavefunction is then introduced to explain the experimental results. By treating the entangled photon pair as a single, nonlocal, indivisible object, the experimental results can be explained without any paradoxes.; We then discuss how one can prepare a bright source of maximally entangled polarization states, or Bell states, via femtosecond-pulse pumped spontaneous parametric down-conversion without amplitude and spectral post-selection. The Bell state preparation scheme is then applied to the first experimental demonstration of quantum teleportation with a complete Bell state measurement, which is based on nonlinear interactions. The unknown polarization state of Alice's photon is teleported to Bob's photon by utilizing both classical and quantum channels.