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] .

We demonstrate a heralded quantum memory based on mapping of a photon polarization state onto a single collective-spin excitation (magnon) shared between two spatially overlapped atomic ensembles. The polarization fidelity is measured by quantum state tomography to be above 90(2)% for any input polarization, which far exceeds the classical limit of 2 3. The process also constitutes a quantum non-destructive probe that detects and regenerates a photon without touching itspotentially undetermined -polarization.The power of quantum communication can be boosted by quantum memories [1,2,3,4,5,6,7,8,9,10,11] that can receive, store, and release a quantum state typically carried by a photon. The advantages memories offer, however, are often thwarted by photon losses [1,12,13,14]. Such unpredictable failure may be largely remedied by a heralding feature that announces photon arrival and successful storage without destroying or revealing the stored quantum state. Heralded storage may thus advance long-distance quantum communication [1], linear-optics quantum computing [15], or schemes aimed at breaking quantum encryption [16].Quantum state storage has been investigated in various systems [2,3,4,5,6,7,8,9,10]. Atomic-ensemble quantum memories have been pursued both for continuous variables of electromagnetic fields [17,18], and for quantized photonic excitations [3,4,6,7,8,10,11]. In an elegant experiment, Julsgaard et. al.[18] mapped the quadrature variables of a weak coherent field onto an atomic ensemble through a field measurement and subsequent feedback onto the ensemble. Other advancements towards a continuous-variable memory include the recent demonstration of storage and retrieval of squeezed vacuum [19,20].Much progress has been made in the storage and retrieval of individual photons. Early work demonstrated capture and release of single photons of fixed polarization using electromagnetically induced transparency [6,7], as well as their adiabatic transfer between two ensembles via an optical resonator [21]. Matsukevich and Kuzmich showed that two atomic ensembles can serve as a twolevel system whose state can be prepared by a projective measurement [4]. Recently, Choi et. al. mapped photonic entanglement created by a polarizing beamsplitter onto two ensembles, and later retrieved the photon [11], realizing unheralded, but relatively high-efficiency, polarization storage. In work by Chen et. al., a successful Bell measurement between two photons resulted in probabilistic teleportation of a photon polarization state onto two atomic ensembles [22]. This can be viewed as a partially heralded quantum memory, where a two-photon coincidence between two beams with Poissonian statistics sometimes, but not always, heralds a successful Bell measurement and teleportation [22].In this Letter, we demonstrate a system where a single photon announces polarization storage in the form of a single collective-spin excitation (magnon) that is shared between two spatially overlapped atomic ensembles. The heralded storage occurs rarely...

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The orientation of polar molecules is demonstrated by the combination of electrostatic and nonresonant, nanosecond Nd:YAG laser fields. The orientation is probed by Coulomb exploding the molecules with a femtosecond laser pulse and detecting the fragment ions with the time-of-flight mass spectrometer. A significant asymmetry is observed in the signal magnitudes of the forward and the backward fragments, which is well explained in terms of the above-mentioned combined-fields scheme proposed by Friedrich and Herschbach [J. Phys. Chem. A 103, 10280 (1999)]. The degree of orientation is enhanced by increasing the peak intensity of the laser field and the magnitude of the electrostatic field, or by lowering the initial rotational temperature. The experimental results obtained are compatible with our numerical simulations.

The molecular alignment technique utilizing the interaction between the intense nonresonant laser field and the induced dipole moment is applied to the homonuclear rare gas dimers Rg2 (Rg=Ar, Kr, and Xe). The degree of alignment is investigated by Coulomb exploding Rg2 and by measuring the angular distributions of the fragment ions. At the same peak intensity of the laser field, the degree of alignment ≪cos2 θ ≫ becomes larger in order of Ar2, Kr2, and Xe2, reflecting the order of magnitudes of their polarizability anisotropy Δα. By taking I2 molecules as a reference, Δα of Ar2, Kr2, and Xe2 are estimated to be 0.5, 0.7, and 1.3 Å3, respectively.

We describe the mapping of quantum states between single photons and an atomic ensemble.In particular, we demonstrate a heralded quantum memory based on the mapping of a photon polarization state onto a single collective-spin excitation (magnon) shared between two atomic ensembles. The polarization fidelity above 90(2)% for any input polarization far exceeds the classical limit of 2 3 . The process also constitutes a quantum non-destructive probe that detects and regenerates a photon without measuring its polarization.