Techniques to facilitate controlled interactions between single photons and atoms are now being actively explored. These techniques are important for the practical realization of quantum networks, in which multiple memory nodes that utilize atoms for generation, storage and processing of quantum states are connected by single-photon transmission in optical fibres. One promising avenue for the realization of quantum networks involves the manipulation of quantum pulses of light in optically dense atomic ensembles using electromagnetically induced transparency (EIT, refs 8, 9). EIT is a coherent control technique that is widely used for controlling the propagation of classical, multi-photon light pulses in applications such as efficient nonlinear optics. Here we demonstrate the use of EIT for the controllable generation, transmission and storage of single photons with tunable frequency, timing and bandwidth. We study the interaction of single photons produced in a 'source' ensemble of 87Rb atoms at room temperature with another 'target' ensemble. This allows us to simultaneously probe the spectral and quantum statistical properties of narrow-bandwidth single-photon pulses, revealing that their quantum nature is preserved under EIT propagation and storage. We measure the time delay associated with the reduced group velocity of the single-photon pulses and report observations of their storage and retrieval.

We describe a technique for generating pulses of light with controllable photon numbers, propagation direction, timing, and pulse shapes. The technique is based on preparation of an atomic ensemble in a state with a desired number of atomic spin excitations, which is later converted into a photon pulse. Spatio-temporal control over the pulses is obtained by exploiting long-lived coherent memory for photon states and electromagnetically induced transparency (EIT) in an optically dense atomic medium. Using photon counting experiments we observe generation and shaping of few-photon sub-Poissonian light pulses. We discuss prospects for controlled generation of high-purity n-photon Fock states using this technique.PACS numbers: 42.50. Gy, 42.50.Dv, In recent years much effort has been directed toward generating quantum-mechanical states of the electromagnetic field with a well-defined number of light quanta (i.e., photon-number or Fock states). In addition to being of fundamental interest, these states represent an essential resource for the practical implementation of many ideas from quantum information science such as quantum communication [1]. Over the past decade, tremendous progress has been made in generating singlephoton states by using photon pairs in parametric downconverters [2], single emitters [3,4], and single atoms in high-finesse cavities [5,6]. While parametric downconversion techniques have recently been used to generate multi-photon states [7], it remains experimentally challenging to implement schemes that allow for simultaneous control over both photon number and spatiotemporal properties of the pulse.In this Letter we describe a novel technique for generating pulses of light with controllable, well-defined photon numbers, propagation direction, timing, and pulse shapes by exploiting long-lived coherent memory for photon states in an optically dense atomic medium [8]. We experimentally demonstrate key elements of this technique in photon-counting experiments. This approach combines different aspects of earlier studies on "light storage" [9,10] and Raman preparation and retrieval of atomic excitations [11,12,13]. It is particularly important in the contexts of long-distance quantum communication [14], and EIT-based quantum nonlinear optics [15,16,17].In our approach we first optically pump a large ensemble of N atoms with a three-state "lambda" configuration of atomic states (see Fig.1a) in the ground state |g . Spontaneous Raman scattering [18] is induced by a weak, off-resonant laser beam with Rabi frequency Ω W and detuning ∆ W , referred to as the write laser. This twophoton process flips an atomic spin into the metastable state |s while producing a correlated frequency-shifted photon (a so-called Stokes photon). Energy and momentum conservation ensure that for each Stokes photon emitted in certain direction there exists exactly one flipped spin quantum in a well-defined spin-wave mode. The number of spin wave quanta and the number of photons in the Stokes field thus exhibit strong correlations...

We review recent experiments [M. D. Eisaman et al., Nature 438 (2005) 837] demonstrating the generation of narrow-bandwidth single photons using a room-temperature ensemble of 87 Rb atoms. Our method involves creation of an atomic coherence via Raman scattering and projective measurement, followed by the coherent transfer of this atomic coherence onto a single photon using electromagnetically induced transparency (EIT). The single photons generated using this method are shown to have many properties necessary for quantum information protocols, such as narrow bandwidths, directional emission, and controllable pulse shapes. The narrow bandwidths of these single photons (∼MHz), resulting from their matching to the EIT resonance (∼MHz), allow them to be stored in narrow-bandwidth quantum memories. We demonstrate this by using dynamic EIT to store and retrieve the single photons in a second ensemble for storage times up to a few microseconds. We also describe recent improvements to the single-photon fidelity compared to the work by M. D. Eisaman in Nature 438 (2005) 837. These techniques may prove useful in quantum information applications such as quantum repeaters, linear-optics quantum computation, and daytime free-space quantum communication.

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We describe a technique for generating pulses of light with controllable, welldefined photon numbers, propagation direction, timing, and pulse shapes. The technique is based on preparation of an atomic ensemble in a state with a desired number of atomic spin excitations, which is later converted into a photon pulse. Spatio-temporal control over the pulses is obtained by exploiting long-lived coherent memory for photon states and electromagnetically induced transparency in an optically dense atomic medium. Using photon counting experiments we observe collective behavior of atomic spins in a room-temperature ensemble, enabling controlled generation and shaping of few-photon sub-Poissonian light pulses. We discuss the prospects for controlled generation of n-photon Fock states.