We report an experimental investigation of electromagnetically induced transparency in a multilevel cascade system of cold atoms. The absorption spectral profiles of the probe light in the multi-level cascade system were observed in cold 85 Rb atoms confined in a magneto-optical trap, and the dependence of the spectral profile on the intensity of the coupling laser was investigated.The experimental measurements agree with the theoretical calculations based on the density matrix equations of the rubidium cascade system.Electromagnetically induced transparency (EIT)[1] is a quantum interference effect that permits propagation of light through an opaque atomic medium without attenuation, it was first proposed in 1989 [2] and experimentally verified in 1991 [3]. Since then, theoretical and experimental studies of EIT have attracted great attentions due to their potential applications in many fields, such as low light nonlinear optics [4], quantum information [5], atomic frequency standard [6], and so on. Early studies were carried out with hot atoms in vapor cells. In the hot atomic medium, the interaction time between the atoms and the laser fields is short which leads to the transient broadening. Also, the collisions in the hot atomic medium may severely shorten the coherence decay time. Recently, many groups explored the EIT phenomena using the laser cooled atoms. There are several advantages in the cold atoms [7]. Firstly, because of the low temperature of the cold atoms, the Doppler broadening effect is effectively minimized, which renders it possible to explore EIT-type nonlinear optical phenomena involving odd number of photons. Secondly, the lower collision rates in the cold atomic sample reduce the decoherence rate.Early experimental studies of EIT in the cold atoms were mainly carried out in rubidium atoms [8,9,10]. Subsequently, the EIT based nonlinear optical phenomena were studied [4,11], which led to the recent experiments on the resonant nonlinear optics at low light intensity. A very steep slope of refractive index and the extremely low group velocity of probe light have been obtained in the cold EIT mediums [12], which have been used to demonstrate light storage and recall based on the coherent excitation transfer between the photons and the atoms [13]. Recently, electromagnetically induced grating (EIG) [14,15] was realized in the cold atoms. Jason et al. experimentally compared the EIT phenomena between the hot atoms and the cold atoms [16], and Ahufinger et al. compared the EIT phenomena between the cold atoms above and below the transition temperature for Bose-Einstein condensation [17]. These studies on EIT and the related phenomena in the cold atoms provided intensive understanding of the atomic coherence and interference in the fundamental interaction between the light field and the atoms [18,19,20,21,22,23].EIT in the simple three-level system have been extensively studied, but EIT in the multilevel cascade systems and their possible applications have not been fully explored. Although essenti...

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In this article the recent experimental works on cold atoms carried out at Wuhan Institute of Physics and Mathematics (WIPM) are reported. These include the experimental realization of Bose-Einstein condensation (BEC), different type of cold atom interferometers, and bichromatic electromagnetically-induced transparency (EIT). We have realized Bose-Einstein condensates of 87 Rb dilute atomic gases. The apparatus consists of two horizontally mounted magneto-optic-traps (MOTs) and a QUIC magnetic trap. Nearly 3×10 8 atoms were trapped in the second MOT, and up to 1.2×10 8 atoms were adiabatically transferred to the QUIC trap. A pure condensate with about 1.1×10 5 atoms at about 30nK was achieved. We also demonstrated two type of cold atom interferometers, the Sagnac and Ramsey interference fringes were recorded with contrast of up to 37%.