Quantum Interference Of Single Photons From Cold Atomic Ensembles

Single photons of high purity are the basis of quantum interference,and the interference of photons from independent sources is a requirement for scalable quantum network.For decades,single-photon sources based on various media from various physical mechanisms have been reported.They have unique advantages of their own,but also different disadvantages in the properties such as narrow band,long coherence time and time-frequency entanglement.Utilizing the property of coherence and the strong third-order nonlinear effect within the cold85 Rb atoms in a two-dimensional magneto-optical trap(MOT),we achieved narrow-band photon pairs with coherence time of even 2.34?s from four-wave mixing(FWM)process.We for the first time experimentally proved that the heralded single photon produced from a photon pair has a temporal purity of near unity,with the time-frequency entanglement maintained.We studied the temporal interference between single photons from two independent cold atomic sources not only in continuous and pulsed cases,but also in frequency-matched and-unmatched cases,and obtained very high visibilities.We tried to reveal the physics nature of different cases,and indicate the potential applications.This thesis includes the following work:1.We built two cold atomic systems in two-dimensional(2D)85Rb magneto-optical traps.We assembled all the components for the vacuum systems;built the rectangular coils and the optical paths of six cooling beams;made many electronic controlling instruments for frequency locking,module driving and laser spectral filtering,etc.,and programmed the softwares with Labview for high-speed data acquiring and processing.We built up the whole laboratory from an empty room.2.Employing the heralded single photons of narrow bandwidth and long temporal length produced from two independent cold atomic systems,we for the first time experimentally realized the Hong-Ou-Mandel interference based on cold atoms in continuous excitation fields,and obtained a high visibility of 80%,which proves that the heralded single photons are in pure temporal states.Also,we performed the interference of single photons retrieved from stored spin waves in pulsed excitation fields,and obtained a visibility of 91%.The time-frequency entanglement between the photon pair in the latter case is proved to be eliminated.3.With the two cold atomic systems talked above,we studied the quantum beat between photons from two independent sources,using the frequency controllability of heralded photons from the spontaneous FWM in a double-? four-level scheme.The frequency difference between the interfering photons is as short as megahertz-level which is time-resolvable,and the output state of the interference is actually a frequency-bin entangled state.