A. S. Sheremet

Large Bragg Reflection from One-Dimensional Chains of Trapped Atoms Near a Nanoscale Waveguide

We report experimental observations of a large Bragg reflection from arrays of cold atoms trapped near a one-dimensional nanoscale waveguide. By using an optical lattice in the evanescent field surrounding a nanofiber with a period nearly commensurate with the resonant wavelength, we observe a reflectance of up to 75% for the guided mode. Each atom behaves as a partially reflecting mirror and an ordered chain of about 2000 atoms is sufficient to realize an efficient Bragg mirror. Measurements of the reflection spectra as a function of the lattice period and the probe polarization are reported. The latter shows the effect of the chiral character of nanoscale waveguides on this reflection. The ability to control photon transport in 1D waveguides coupled to spin systems would enable novel quantum network capabilities and the study of many-body effects emerging from long-range interactions.

Reversible optical memory for twisted photons

We report on an experiment in which orbital angular momentum is mapped at the single photon level into and out of an atomic ensemble, opening the possibility to the storage of qubits encoded in OAM.

Experimental investigation of the transition between Autler-Townes splitting and electromagnetically-induced transparency models

Summary form only given. If in general the transparency of an initially absorbing medium for a probe field is increased by the presence of a control field on an adjacent transition, two very different processes can be invoked to explain it. One of them is a quantum Fano interference between two paths in the three-level system, which occurs even at low control intensity and gives rise to electromagnetically-induced transparency (EIT), the other one is the appearance of two dressed states in the excited level at higher control intensity, corresponding to the Autler-Townes splitting (ATS). This distinction is particularly critical for instance for the implementation of slow light or optical quantum memories. In a recent paper, P. M. Anisimov, J. P. Dowling and B. C. Sanders proposed a quantitative test to objectively discerning ATS from EIT. We experimentally investigated this test with cold atoms and demonstrated that it is very sensitive to the specific properties of the medium. In this study, we use an ensemble of cold Cesium atoms trapped in a MOT, interacting with light via a Λ-type scheme on the D2 line. Absorption profiles are obtained for various values of the control Rabi frequency Ω between 0.1Γ and 4Γ, where Γ is the natural linewidth.

Electromagnetically induced transparency in an inhomogeneously broadenedΛtransition with multiple excited levels

Electromagnetically induced transparency (EIT) has mainly been modeled for three-level systems. In particular, considerable interest has been dedicated to the {Lambda} configuration, with two ground states and one excited state. However, in the alkali-metal atoms, which are commonly used, the hyperfine interaction in the excited state introduces several levels which simultaneously participate in the scattering process. When the Doppler broadening is comparable with the hyperfine splitting in the upper state, the three-level {Lambda} model does not reproduce the experimental results. Here we theoretically investigate the EIT in a hot vapor of alkali-metal atoms and demonstrate that it can be strongly reduced by the presence of multiple excited levels. Given this model, we also show that well-designed optical pumping enables us to significantly recover the transparency.

Stimulated Raman process in a scattering medium applied to the quantum memory scheme

We consider the coherent stimulated Raman process developing in an optically dense disordered atomic medium, which can also incoherently scatter the light over all outward directions. The Raman process is discussed in the context of a quantum memory scheme and we point out the difference in its physical nature from a similar but not identical protocol based on the effect of electromagnetically induced transparency (EIT). We show that the Raman and EIT memory schemes do not compete but complement one another and each of them has certain advantages in the area of its applicability. We include in our consideration an analysis of the transient processes associated with switching the control pulse off or on and follow how they modify the probe pulse dynamics on the retrieval stage of the memory protocol.

Highly-efficient quantum memory for polarization qubits in a spatially-multiplexed cold atomic ensemble

Quantum memory for flying optical qubits is a key enabler for a wide range of applications in quantum information. A critical figure of merit is the overall storage and retrieval efficiency. So far, despite the recent achievements of efficient memories for light pulses, the storage of qubits has suffered from limited efficiency. Here we report on a quantum memory for polarization qubits that combines an average conditional fidelity above 99% and efficiency around 68%, thereby demonstrating a reversible qubit mapping where more information is retrieved than lost. The qubits are encoded with weak coherent states at the single-photon level and the memory is based on electromagnetically-induced transparency in an elongated laser-cooled ensemble of cesium atoms, spatially multiplexed for dual-rail storage. This implementation preserves high optical depth on both rails, without compromise between multiplexing and storage efficiency. Our work provides an efficient node for future tests of quantum network functionalities and advanced photonic circuits.Future quantum networks will require quantum memories with effective storage-and-retrieval capabilities. Here, the authors use electromagnetically-induced transparency in a high optical-depth, spatially-multiplexed cold atom ensemble to store and retrieve polarization qubits with high efficiency.

Enhancement of electromagnetically induced transparency in room temperature alkali metal vapor

Electromagnetically induced transparency (EIT) has led to several quantum optics effects such as lasing without inversion or squeezed light generation. More recently quantum memories based on EIT have been experimentally implemented in different systems such as alkali metal atoms. In this system the excited state of the optical transition splits into several sublevels due to the hyperfine interaction. However, most of the theoretical models used to describe the experimental results are based on a Λ-system with only one excited state. In this article, we present a theoretical model for the Λ-type interaction of two light, fields and an atomic system with multiple excited state. In particular we show that if the control and probe fields are orthogonally circularly polarized the EIT effect in an alkali-metal vapor can almost disappears. We also identify the reasons of this reduction and propose a method to recover the transparency via velocity selective optical pumping.

Large Bragg reflection from 1D chains of trapped atoms near an optical nanofiber

Reversible light-matter interfaces are crucial elements in quantum optics and quantum information networks. In particular, the coupling of one-dimensional bosonic nanoscale waveguides and cold atoms appears as a promising pathway to build strong light-matter interaction thanks to the tight transverse confinement of light.

High-efficiency quantum memory for photonic polarization qubits in a spatially-multiplexed dense cold atomic ensemble

Quantum storage of flying optical qubits, namely coherently mapping photonic states into and out of an optically controlled memory on demand, constitutes an essential component in the optical quantum information processing science, with applications to long-distance optical communication [1]. In this context, practical protocols require sufficiently large storage efficiency to achieve a performance better than direct transmission. Theoretically, the storage and retrieval efficiency can only be improved with the increase of the optical depth (OD). Recent progresses have shown that high OD media enable to reach high-efficiency electromagnetically induced transparency (EIT)-based optical storage, albeit without the demonstration of qubit storage [2, 3]. Here we experimentally implement an optical quantum memory with single-photon-level probes, showing a EIT-based storage efficiency around 70%. Furthermore, thanks to the spatial multiplexing of the dense atomic ensemble, highly efficient storage of polarization qubit is performed.

Storage and Controlled Transport of Single-Photon Pulses

We report both the experimental realization of an optical memory and the experimental observation of a large Bragg reflection by combining cold neutral atoms and a one-dimensional nanoscale waveguide.