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 demonstrate coherent control of single-photon absorption and reemission in a two-level cold atomic ensemble. This is achieved by interfering the incident single-photon wave packet with the emission (or scattering) wave. For a photon with an exponential growth waveform with a time constant equal to the excited-state lifetime, we observe that the single-photon emission probability during the absorption can be suppressed due to the perfect destructive interference. After the incident photon waveform is switched off, the absorbed photon is then reemitted to the same spatial mode as that of the incident photon with an efficiency of 20%. For a photon with an exponential decay waveform with the same time constant, both the absorption and reemission occur within the waveform duration. Our experimental results suggest that the absorption and emission of a single photon in a two-level atomic ensemble may possibly be manipulated by shaping its waveform in the time domain. DOI: 10.1103/PhysRevLett.109.263601 PACS numbers: 42.50.Nn, 03.65.Ta, 32.80.Qk, 42.50.Ct Photon absorption and emission are two key probes to study the light-matter quantum interaction, which lays down the foundations for atomic, molecular, and optical physics [1][2][3]. When a coherent optical pulse propagates through a medium, the absorption and emission can coherently modify its spectral components and lead to many interesting and important optical phenomena, such as attenuation, amplification, distortion, and slow and fast light effects [4][5][6][7][8]. For a single photon, causality requires that the absorption and reemission follow the right time order: the reemission can only occur following the absorption. Although this quantum time order has been observed as the antibunching effect in resonance fluorescence measured by two-photon correlations [9,10], it is not controllable on demand in these experiments. When a single photon enters a two-level atomic medium, the absorption and emission usually both occur within the photon pulse duration. Recently, Scully et al. proposed a scheme for observing directed ''spontaneous'' emission excited by a single photon [11].In this Letter, we demonstrate coherent control of singlephoton absorption and reemission in a two-level cold atomic ensemble in free space by shaping the singlephoton waveform. Making use of the destructive interference between the emission (or scattering) and the incident photon wave packet, we show that the probability of reemitting the photon during the absorption can be completely suppressed when the incident photon has an exponential growth waveform with a time constant equal to the excitedstate lifetime. The reemission process only starts after the incident photon waveform is switched off and thus can be controlled on demand. This technique can be used to efficiently excite atoms with a given single atom. Our result may have potential applications in the quantum networks that require efficient conversion between flying single-photon states and local atomic states [12]. Figure 1 i...

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 experimentally investigate optical storage with electromagnetically induced transparency in a dense cold 85 Rb atomic ensemble. By varying the optical depth (OD) from 0 to 140, we observe that the optimal storage efficiency has a saturation value of 50% as OD > 50. Our result is consistent with that obtained from hot vapor cell experiments.