A quantum memory, for storing and retrieving flying photonic quantum states, is a key interface for realizing long-distance quantum communication and large-scale quantum computation. While many experimental schemes of high storage-retrieval efficiency have been performed with weak coherent light pulses, all quantum memories for true single photons achieved so far have efficiencies far below 50%, a threshold value for practical applications. Here, we report the demonstration of a quantum memory for single-photon polarization qubits with an efficiency of >85% and a fidelity of >99%, basing on balanced two-channel electromagnetically induced transparency in laser-cooled rubidium atoms. For the singlechannel quantum memory, the optimized efficiency for storing and retrieving single-photon temporal waveforms can be as high as 90.6%. Our result pushes the photonic quantum memory closer to its practical applications in quantum information processing.

We demonstrate an efficient experimental scheme for producing polarization-entangled photon pairs from spontaneous four-wave mixing (SFWM) in a laser-cooled 85 Rb atomic ensemble, with a bandwidth (as low as 0.8 MHz) much narrower than the rubidium atomic natural linewidth. By stabilizing the relative phase between the two SFWM paths in a Mach-Zehnder interferometer configuration, we are able to produce all four Bell states. These subnatural-linewidth photon pairs with polarization entanglement are ideal quantum information carriers for connecting remote atomic quantum nodes via efficient light-matter interaction in a photon-atom quantum network. DOI: 10.1103/PhysRevLett.112.243602 PACS numbers: 42.50.Dv, 03.67.Bg, 42.65.Lm The connectivity of a long-distance photon-atom quantum network strongly depends on efficient interactions between flying photonic quantum bits and local long-lived atomic matter nodes [1,2]. Such efficient quantum interfaces, which convert quantum states (such as time-frequency waveform and polarizations) between photons and atoms, require the photons to have a bandwidth sufficiently narrower than the natural linewidth of related atomic transitions (such as 6 MHz for rubidium D1 and D2 lines). As a standard method for producing entangled photons, spontaneous parametric down-conversion (SPDC) in a nonlinear crystal usually has a wide bandwidth (larger than terahertz) and very short coherence time (less than picosecond). Many efforts have been investigated in the past more than one decade ago to narrow down the SPDC photon bandwidth by using optical cavities [3][4][5][6][7][8]. However, the bandwidth of SPDC polarization-entangled photon pairs is still wider than most atomic transitions and leads to a very low efficiency of storing these polarization states in a quantum memory [5,9].Our motivation was stimulated by the recent progress in generating subnatural-linewidth biphotons by using continuous-wave spontaneous four-wave mixing (SFWM) in a laser-cooled atomic ensemble with electromagnetically induced transparency (EIT) [10,11]. Photons produced from this method not only have narrow bandwidth but also automatically match the atomic transitions. The applications of these narrow-band photons include the demonstration of a single-photon memory with a storage efficiency of about 50% [12], a single-photon precursor [13], and coherent control of single-photon absorption and reemission [14]. However, while this method provides a natural entanglement mechanism in the time-frequency domain, it is extremely difficult to produce polarization entanglement because of the polarization selectivity of EIT in a nonpolarized atomic medium [15]. It is possible to generate the polarization entanglement by scarifying the EIT effect, but the photon generation efficiency is low and the bandwidth is not narrower than the atomic natural linewidth [16]. The "writing-reading" technique with optical pumping provides a solution to polarization entanglement but results in reducing time-frequency entanglement [17].In...

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We report the direct characterization of energy-time entanglement of narrowband biphotons produced from spontaneous four-wave mixing in cold atoms. The Stokes and anti-Stokes two-photon temporal correlation is measured by single-photon counters with nano second temporal resolution, and their joint spectrum is determined by using a narrow linewidth optical cavity. The energy-time entanglement is verified by the joint frequency-time uncertainty product of 0.063 ± 0.0044, which does not only violate the separability criterion but also satisfies the continuous variable Einstein-Podolsky-Rosen steering inequality.

Light's wave-particle duality is at the heart of quantum mechanics and can be well illustrated by Wheeler's delayed-choice experiment: The choice of inserting or removing the second classical (quantum) beam splitter in a Mach-Zehnder interferometer determines the classical (quantum) wave-particle behavior of a photon. In this paper, we report our experiment on directly observing simultaneous wave and particle behavior in a narrowband single-photon wave packet by classically inserting or removing the second beam splitter when part of the wave packet passes through it. Our experiment demonstrates that the produced wave-particle state can be utilized in encoding quantum information.

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