Cold Atom And Biphoton Generation

Non-classical correlated photon pairs have been important tools in the field of quantum optics.As well as for important applications in quantum cryptography and key distribution,quantum teleportation,quantum computation.Traditional methods of producing biphotons include spontaneous parametric down conversion(SPDC).Photons from these nonlinear processes in free space forward-wave configuration usually have broad bandwidth(>THz),these broadband biphotons are not suitable for recently proposed protocols of long-distance quantum communication and quantum network based on photon-atom interaction.Therefore it motivated the search for methods for generating narrowband correlated photons in atomic ensembles.we established one cold 85Rb atoms ensemble in two-dimensional magneto-optical trap with OD about 100.The most important character of two-dimensional magneto-optical trap have a zero magnetic field line and own long ground-state coherence time without switching off the MOT magnetic fields.With spontaneous four-wave mixing(SFWM)process in cold atom ensembles,we could produce narrow-band paired photons with long coherence time.Secondly,we generate biphotons from the DLCZ scheme in cold atoms with high optical depths of 90.a pulsed pumping beam excites the atom cloud and an idler photon with a collective atomic excitation(spin wave)is written into the atom cloud.With a controllable delay time,the atomic excitation is retrieved by another pump pulse into a signal photon.Thirdly,we report the narrow-band biphotons with a coherence time of 2.34?s generated from spontaneous four-wave mixing(SFWM)in a dense cold atomic cloud,in which the anti-Stokes photons go through a narrow electromagnetically-induced transparency(EIT)window.A number of factors limiting the coherence time are analyzed in detail.We find the EIT coherence plays an essential role in determining the coherence time for paired photons.The EIT dephasing rate is the ultimate limit to the coherence time,and an ultra-long coherence time above ten microseconds is possible by further improvement of the dephasing rate below 100 kHz.