During the past decade the interaction of light with multiatom ensembles has attracted much attention as a basic building block for quantum information processing and quantum state engineering. The field started with the realization that optically thick free space ensembles can be efficiently interfaced with quantum optical fields. By now the atomic ensemble-light interfaces have become a powerful alternative to the cavity-enhanced interaction of light with single atoms. Various mechanisms used for the quantum interface are discussed, including quantum nondemolition or Faraday interaction, quantum measurement and feedback, Raman interaction, photon echo, and electromagnetically induced transparency. This review provides a common theoretical frame for these processes, describes basic experimental techniques and media used for quantum interfaces, and reviews several key experiments on quantum memory for light, quantum entanglement between atomic ensembles and light, and quantum teleportation with atomic ensembles. The two types of quantum measurements which are most important for the interface are discussed: homodyne detection and photon counting. This review concludes with an outlook on the future of atomic ensembles as an enabling technology in quantum information processing.

Quantum interface between light and atomic ensembles

Quantum optics: Dedication

Many applications of quantum communication crucially depend on reversible transfer of quantum states between light and matter. Motivated by rapid recent developments in theory and experiment, we review research related to quantum memory based on a photon-echo approach in solid state material with emphasis on use in a quantum repeater. After introducing quantum communication, the quantum repeater concept, and properties of a quantum memory required to be useful in a quantum repeater, we describe the historical development from spin echoes, discovered in 1950, to photon-echo quantum memory. We present a simple theoretical description of the ideal protocol, and comment on the impact of a non-ideal realization on its quantum nature. We extensively discuss rare-earth-ion doped crystals and glasses as material candidates, elaborate on traditional photon-echo experiments as a test-bed for quantum state storage, and describe the current state-of-the-art of photon-echo quantum memory. Finally, we give a brief outlook on current research.

Photon-echo quantum memory in solid state systems

As stated in the text, Figs. 3 and 4 (transmission versus probe laser detuning) were taken at an energy of 10 nJ in a 0.5-mm-diam beam. We should have noted that when the probe energy is increased to 150 n3 (— 10" W/cm ) the absorption cross section at the transmission maximum increases by a factor of approximately 2. At this energy density, with or without the coupling laser present, transmitted probe energy is no longer linear with incident probe energy.

Observation of Electromagnetically Induced Transparency in Collisionally Broadened Lead Vapor

Rare-earth-doped materials for applications in quantum information storage and signal processing

LETTER TO THE EDITOR: Electron spectrometry at the µeV level and the electron affinities of Si and F

Photodetachment microscopy of the negative ions Si- and F- has made it possible to measure the electron affinities of silicon and fluorine with improved accuracy. The new recommended electron affinities of 28Si and 19F are 11 207.252(18) and 27 432.446(19) cm-1, i.e. 1.389 5220(24) and 3.401 1895(25) eV, respectively.

https://iopscience.iop.org/article/10.1088/0953-4075/34/9/101/pdf

Electron spectrometry at the µeV level and the electron affinities of Si and F

A photodetachment experiment is performed on the v=0→v=0 OH− detachment threshold. The weak O and S branches provide a signal strong enough to make amplitude measurements on all five O, P, Q, R, and S branches possible, which are used to fix the formulas for their relative intensities. Photodetachment microscopy is applied to 15 different thresholds of the P, Q, and R branches. The quantitative analysis of the interference patterns obtained does not show any effect of the dipole moment of OH, but yields a new measurement of the rotational parameters of OH−(v=0) and of the electron affinity of the molecule. The new recommended value for the electron affinity of 16O1H is 14 740.982(7) cm−1 or 1.827 648 7(11) eV.

Photodetachment microscopy of the P, Q, and R branches of the OH(-)(v=0) to OH(v=0) detachment threshold.

A three-level $\ensuremath{\Lambda}$ system in ${\mathrm{Tm}}^{3+}$ doped YAG crystal is experimentally investigated in the prospect of quantum information processing Zeeman effect is used to lift the nuclear spin degeneracy of this ion In a previous paper [de Seze et al, Phys Rev B 73, 085112 (2006)] we measured the gyromagnetic tensor components and concluded that adequate magnetic field orientation could optimize the optical connection of both ground state sublevels to each one of the excited state sublevels, thus generating $\ensuremath{\Lambda}$ systems Here we report on the direct measurement of the transition probability ratio along the two legs of the $\ensuremath{\Lambda}$ Measurement techniques combine frequency selective optical pumping with optical nutation or photon echo processes

https://arxiv.org/pdf/quant-ph/0606048.pdf

Branching ratio measurement of a Λ system in Tm 3+ :YAG under a magnetic field

It is well known that ultranarrow electromagnetically induced transparency (EIT) resonances can be observed in atomic gases at room temperature. We report here the experimental observation of another type of ultranarrow resonances, as narrow as the EIT ones, in a \Lambda-system selected by light polarization in metastable 4He at room temperature. It is shown to be due to coherent population oscillations in an open two-level system (TLS). For perpendicular linearly polarized coupling and probe beams, this system can be considered as two coupled open TLSs, in which the ground state populations exhibit anti-phase oscillations. We also predict theoretically that in case of two parallel polarizations, the system would behave like a closed TLS, and the narrow resonance associated with these oscillations would disappear.

F. Goldfarb

Papers

LETTER TO THE EDITOR: Electron spectrometry at the µeV level and the electron affinities of Si and F

Electron spectrometry at the µeV level and the electron affinities of Si and F

Photodetachment microscopy of the P, Q, and R branches of the OH(-)(v=0) to OH(v=0) detachment threshold.

Branching ratio measurement of a Λ system in Tm 3+ :YAG under a magnetic field

Ultranarrow resonance due to coherent population oscillations in a Λ-type atomic system