We report an experimental demonstration of optimal storage and retrieval of heralded single-photon wave packets using electromagnetically induced transparency (EIT) in cold atoms at a high optical depth. We obtain an optimal storage efficiency of (49 ± 3)% for single-photon waveforms with a temporal likeness of 96%. Our result brings the EIT quantum light-matter interface closer to practical quantum information applications.

We report the observation of Sommerfeld-Brillouin optical precursors generated from a long squaremodulated laser pulse propagating through a cold atomic ensemble with electromagnetically induced transparency. The optical depth ( 0 L) of the medium can be varied from 0 up to 50. We demonstrated that the step-on rising and step-off falling edges propagate with the speed of light in vacuum without a slow light effect. At high 0 L, the precursor is separated from the delayed main pulse at the rising edge, while at the falling edge, we observe damped oscillatory structures resulting from the interference between the precursor and main field.

We describe the apparatus of a dark-line two-dimensional (2D) magneto-optical trap (MOT) of 85Rb cold atoms with high optical depth (OD). Different from the conventional configuration, two (of three) pairs of trapping laser beams in our 2D MOT setup do not follow the symmetry axes of the quadrupole magnetic field: they are aligned with 45° angles to the longitudinal axis. Two orthogonal repumping laser beams have a dark-line volume in the longitudinal axis at their cross over. With a total trapping laser power of 40 mW and repumping laser power of 18 mW, we obtain an atomic OD up to 160 in an electromagnetically induced transparency (EIT) scheme, which corresponds to an atomic-density-length product NL = 2.05 × 1015 m−2. In a closed two-state system, the OD can become as large as more than 600. Our 2D MOT configuration allows full optical access of the atoms in its longitudinal direction without interfering with the trapping and repumping laser beams spatially. Moreover, the zero magnetic field along the longitudinal axis allows the cold atoms maintain a long ground-state coherence time without switching off the MOT magnetic field, which makes it possible to operate the MOT at a high repetition rate and a high duty cycle. Our 2D MOT is ideal for atomic-ensemble-based quantum optics applications, such as EIT, entangled photon pair generation, optical quantum memory, and quantum information processing.

We report the generation of nondegenerate narrow-bandwidth paired photons with time-frequency and polarization entanglements from laser cooled atoms. We observe the two-photon interference caused by Rabi splitting with a coherence time of about 30 ns and a visibility of 81.8% which verifies the timefrequency entanglement of the paired photons. The polarization entanglement is confirmed by polarization correlation measurements which exhibit a visibility of 89.5% and characterized by quantum-state tomography with a fidelity of 90.8%. Taking into account the transmission losses and duty cycle, we estimate that the system generates hyperentangled paired photons into opposing single-mode fibers at a rate of 320 pairs per second. DOI: 10.1103/PhysRevLett.106.033601 PACS numbers: 42.50.Dv, 03.67.Bg, 03.67.Mn, 42.65.Lm Entangled paired photons, a hallmark tool of quantum optics, play a critical role in quantum information processing, quantum computation and quantum communication [1]. For many decades, spontaneous parametric downconversion (SPDC) in nonlinear crystals has been used as the standard method to generate nonclassical correlated photons [2,3]. Recently, much research interest has been drawn to the production of photons that are simultaneously entangled in more than one degree of freedom, or hyperentangled [4][5][6][7][8], because of the increasing information carrying capacity of these paired photons. The hyperentangled photons can be implemented as building blocks for efficient quantum computation [9,10]. However, the SPDC sources typically have wide bandwidths ($ THz) and short coherence time ($ ps), and thus are not suitable for use in entanglement storage based long distance quantum communication protocols. By putting the nonlinear crystal inside a cavity and using two-photon interference outside the cavity, generation of narrow-band degenerate polarizationentangled photon pairs has been demonstrated [11].Following the protocol of long distance quantum communication proposed by Duan et al. [12], nonclassical photon pairs have been generated from atomic ensembles using ''writing-reading'' technique [13,14]. Polarization entanglement has been achieved using this technique [15], but the photons are not entangled in time-frequency due to the fact that ''writing'' and ''reading'' are timeseparated pulsed processes. Subsequently, Harris group [16-18] generated narrow-bandwidth time-frequency entangled paired photons with the two driving lasers running in continuous-wave (cw) mode, making use of electromagnetically induced transparency [19] and spontaneous four-wave mixing (SFWM). With the long coherence time of these biphotons, it was possible to shape and modulate their single-and two-photon waveforms [20,21]. So far, paired photons with entanglement in both time-frequency and polarization have not been produced from atomic ensembles, even though this possibility has been suggested theoretically [22].In this Letter, we report the experimental demonstration of generation of narrow-bandwidth nondegenerate pai...