We report the demonstration of phase coherence and control for the recently developed "light storage" technique. Specifically, we use a pulsed magnetic field to vary the phase of atomic spin excitations which result from the deceleration and storing of a light pulse in warm Rb vapor. We then convert the spin excitations back into light and detect the resultant phase shift in an optical interferometric measurement. The coherent storage of photon states in matter is essential for the practical realization of many basic concepts in quantum information processing.The realization of scalable quantum networks for long-distance quantum communication and quantum computation requires the use of photons as quantum information carriers and matter (e.g., spins) as quantum memory elements [1]. For example, intermediate memory nodes will be essential for quantum communication over lossy photonic channels [2,3]. When successfully implemented, such a technique will facilitate secure transmission of secret messages over long distances [4,5]. Likewise, quantum memory elements linked by light are desirable for scalable quantum computation [6].The quantum carrier/memory interface is a key component that should be capable of reversibly transferring quantum states between light pulses and long-lived matter states. In contrast to ordinary "destructive" techniques which convert light into, e.g., electrical signals by photoabsorption, a quantum memory should be free from dissipation and, most importantly, should preserve phase coherence in the process of information transfer to and from the carrier.Recently, proposals have been made to accomplish the quantum transfer of photon states to individual atoms [7] and atomic ensembles [8,9]. Active experimental efforts toward the realization of these ideas are currently under way [10,11]. In particular, recent experiments employing cold Na atoms [12] and warm Rb vapor [13] demonstrated the basic principle of the "storage of light" in atomic ensembles by the dynamic and reversible reduction of the light pulse group velocity to zero.In this Letter we present the first experimental evidence that this light storage technique is phase coherent. Although anticipated from theoretical predictions [8,9], this essential feature of a quantum memory for light has not been verified experimentally up to now. In addition, we demonstrate that the phase of the stored coherence can be accurately manipulated during the storage interval and then mapped coherently onto the released light pulse. These results show that the present technique should be suitable for applications in quantum information processing. For example, a specific "quantum repeater" protocol, which allows for scalable quantum communication over very long distances using this technique, has already been proposed [14]. Scalable quantum computation using atomic ensembles coupled by light has also been suggested [15].The light storage technique is based on the phenomenon of Electromagnetically Induced Transparency (EIT) [16], in which an external o...

We use a modified Thomas-Fermi approximation to estimate analytically the critical velocity for the formation of vortices in harmonically trapped BEC. We compare this analytical estimate to numerical calculations and to recent experiments on trapped alkali condensates.

We investigate theoretically the collective excitations of trapped Bose condensates with energies of the order of the chemical potential of the system. For the MIT sodium Bose condensate in a Cloverleaf trap, we find interesting level crossing behavior for high energy excitations and calculate the spatial magnetic dipole moments for selective creation of condensate excitations.

We report observations of novel dynamic behavior in resonantly-enhanced stimulated Raman scattering in Rb vapor. In particular, we demonstrate a dynamic hysteresis of the Raman scattered optical field in response to changes of the drive laser field intensity and/or frequency. This effect may be described as a dynamic form of optical bistability resulting from the formation and decay of atomic coherence. We have applied this phenomenon to the realization of an all-optical switch.