We study the performance and limitations of a coherent interface between collective atomic states and single photons. A quantized spin-wave excitation of an atomic sample inside an optical resonator is prepared probabilistically, stored, and adiabatically converted on demand into a sub-Poissonian photonic excitation of the resonator mode. The measured peak single-quantum conversion efficiency of χ=0.84(11) and its dependence on various parameters are well described by a simple model of the mode geometry and multilevel atomic structure, pointing the way towards implementing highperformance stationary single-photon sources.PACS numbers: 42.50. Dv, 03.67.Hk, 42.50.Fx, 32.80.Pj A quantum-coherent interface between light and a material structure that can store quantum states is a pivotal part of a system for processing quantum information [1]. In particular, a quantum memory that can be mapped onto photon number states in a single spatio-temporal mode could pave the way towards extended quantum networks [2,3] and all-optical quantum computing [4]. While light with sub-Poissonian fluctuations can be generated by a variety of single-quantum systems [5,6,7], a point emitter in free space is only weakly, and thus irreversibly, coupled to an electromagnetic continuum.To achieve reversible coupling, the strength of the emitter-light interaction can be enhanced by means of an optical resonator, as demonstrated for quantum dots in the weak- [8,9], trapped ions in the intermediate- [10], and neutral atoms in the strong-coupling regime [11,12]. By controlling the position of a single atom trapped inside a very-high-finesse resonator, McKeever et al. have realized a high-quality deterministic single-photon source [12]. This source operates in principle in the reversiblecoupling regime, although finite mirror losses presently make it difficult to obtain full reversibility in practice.Alternatively, superradiant states of an atomic ensemble [13] exhibit enhanced coupling to a single electromagnetic mode. For three-level atoms with two stable ground states these collective states can be viewed as quantized spin waves, where a spin-wave quantum (magnon) can be converted into a photon by the application of a phasematched laser beam [3]. Such systems have been used to generate [14,16], store and retrieve single photons [18,19], to generate simultaneous-photon pairs [17,25], and to increase the single-photon production rate by feedback [21,22,23]. The strong-coupling regime between magnons and photons can be reached if the sample's optical depth OD exceeds unity. However, since the failure rate for magnon-photon conversion in these free-space [14,15,16,17,18,19,20,21,22,23] or moderate-finessecavity [24,25] systems has been around 50% or higher, which can be realized with OD ≤ 1, none of the ensemble systems so far has reached the strong, reversible-coupling regime. In this Letter, we demonstrate for the first time the strong-coupling regime between collective spin-wave excitations and a single electromagnetic mode. This is evidenc...

We demonstrate that molecules with a moderate permanent dipole moment can be oriented with combined electrostatic and pulsed, nonresonant laser fields. We use OCS molecules as a sample. The degree of orientation can be increased by increasing the magnitude of electrostatic field and the peak intensity of the laser field or by decreasing the rotational temperature of the molecules.

Generation of non-classical correlations (or entanglement)between atoms 1-7 , photons 8 or combinations thereof 9-11 is at the heart of quantum information science. Of particular interest are material systems serving as quantum memories that can be interconnected optically 3,6,7,9-11 . An ensemble of atoms can store a quantum state in the form of a magnon-which is a quantized collective spin excitation-that can be mapped onto a photon 12-18 with high efficiency 19 . Here, we report the phasecoherent transfer of a single magnon from one atomic ensemble to another via an optical resonator serving as a quantum bus that in the ideal case is only virtually populated. Partial transfer deterministically creates an entangled state with one excitation jointly stored in the two ensembles. The entanglement is verified by mapping the magnons onto photons, whose correlations can be directly measured. These results should enable deterministic multipartite entanglement between atomic ensembles.A quantum memory, that is, a device for storing and retrieving quantum states, is a key component of any quantum information processor. Optical memory access is highly desirable, as it is intrinsically fast and single photons are robust, easily controlled carriers of quantum states. Although a bit of quantum information (qubit) can be stored in a single two-level system, it can be expedient to instead use long-lived collective spin excitations of an atomic ensemble 12 . The ensemble can then be viewed as a 'macroatom' , whose excitations are quantized spin waves (magnons), such that transitions between its energy levels (magnon number states) correspond to highly directional (superradiant 20 ) photon emission or absorption 6,7,[12][13][14][15][16][17][18][19] .