Propagation of light through small clouds of cold interacting atoms

We demonstrate experimentally that a cloud of cold atoms with a size comparable to the wavelength of light can induce large group delays on a laser pulse when the laser is tightly focused on it and is close to an atomic resonance. Delays as large as -10 ns are observed, corresponding to "superluminal" propagation with negative group velocities as low as -300 m/s. Strikingly, this large delay is associated with a moderate extinction owing to the very small size of the cloud and to the light-induced interactions between atoms. It implies that a large phase shift is imprinted on the continuous laser beam, and opens interesting perspectives for applications to quantum technologies.Comment: 5 pages, 3 figures Supplemental Material : 2 pages, 2 Figure

Taxol: A complex diterpenoid natural product with an evolutionarily obscure origin

Taxol, a diterpenoid natural product first isolated from Taxus brevifolia, is one of today’s better known anticancer drugs. Despite its clinical efficacy, the difficulty of establishing a secure and cost-effectivesupply of taxol has limited its use. However, its unique mode of action and efficacy against multiple forms of cancer has ensured continual efforts to achieve total and semisynthesis, as well as biotechnological production methods. Total synthesis is now possible but inefficient, so the production of taxol and related taxoids remains completely dependent on biomass derived from Taxus sp, with cell suspensions and collected plant materials as sources. The key to improving the supply of taxol and other clinically useful taxoids is the detailed elucidation of the taxoid biosynthesis pathway, which has been the subject of intense research. Many genes and enzymes in the Taxus pathway for taxoidbiosynthesis have now been identified, although gaps remain. In addition to Taxus sp, taxoids are also synthesized by various endophytic fungi, which often live in association with Taxus trees, thus raising questions about the evolutionary origin of this complex diterpenoid pathway. In the future, it may be possible to improve taxoid synthesis through the genetic modification of Taxus cell cultures, byculturing endophytic fungi or by transferring the entire pathway into a heterologous expression host, such as Saccharomyces cerevisiae

Observation of suppression of light scattering induced by dipole-dipole interactions in a cold atomic ensemble

We study the emergence of collective scattering in the presence of dipole-dipole interactions when we illuminate a cold cloud of rubidium atoms with a near-resonant and weak intensity laser. The size of the atomic sample is comparable to the wavelength of light. When we gradually increase the atom number from 1 to 450, we observe a broadening of the line, a small red shift and, consistently with these, a strong suppression of the scattered light with respect to the noninteracting atom case. Numerical simulations, which include the internal atomic level structure, agree with the data.Comment: 5 pages, 5 figure

Experimental test of nonlocal realistic theories without the rotational symmetry assumption

We analyze the class of nonlocal realistic theories that was originally considered by Leggett [Found. Phys. 33, 1469 (2003)] and tested by us in a recent experiment [Nature (London) 446, 871 (2007)]. We derive an incompatibility theorem that works for finite numbers of polarizer settings and that does not require the previously assumed rotational symmetry of the two-particle correlation functions. The experimentally measured case involves seven different measurement settings. Using polarization-entangled photon pairs, we exclude this broader class of nonlocal realistic models by experimentally violating a new Leggett-type inequality by 80 standard deviations.Comment: Published versio

Quantum key distribution over 30km of standard fiber using energy-time entangled photon pairs: a comparison of two chromatic dispersion reduction methods

We present a full implementation of a quantum key distribution system using energy-time entangled photon pairs and functioning with a 30km standard telecom fiber quantum channel. Two bases of two orthogonal states are implemented and the setup is quite robust to environmental constraints such as temperature variation. Two different ways to manage chromatic dispersion in the quantum channel are discussed.Comment: 10 pages, 4 figure

Secure quantum channels with correlated twin laser beams

This work is the development and analysis of the recently proposed quantum cryptographic protocol, based on the use of the two-mode coherently correlated states. The protocol is supplied with the cryptographic control procedures. The quantum noise influence on the channel error properties is examined. State detection features are proposed

Experimental delayed-choice entanglement swapping

Motivated by the question, which kind of physical interactions and processes are needed for the production of quantum entanglement, Peres has put forward the radical idea of delayed-choice entanglement swapping. There, entanglement can be "produced a posteriori, after the entangled particles have been measured and may no longer exist". In this work we report the first realization of Peres' gedanken experiment. Using four photons, we can actively delay the choice of measurement-implemented via a high-speed tunable bipartite state analyzer and a quantum random number generator-on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one definite of two mutually exclusive quantum states in which either the photons are entangled (quantum correlations) or separable (classical correlations). This can also be viewed as "quantum steering into the past"

A Compact Solid State Detector for Small Angle Particle Tracking

MIDAS (MIcrostrip Detector Array System) is a compact silicon tracking telescope for charged particles emitted at small angles in intermediate energy photonuclear reactions. It was realized to increase the angular acceptance of the DAPHNE detector and used in an experimental program to check the Gerasimov-Drell-Hearn sum rule at the Mainz electron microtron, MAMI. MIDAS provides a trigger for charged hadrons, p/pi identification and particle tracking in the region 7 deg < theta < 16 deg. In this paper we present the main characteristics of MIDAS and its measured performances.Comment: 13 pages (9 figures). Submitted to NIM

Conditioned Unitary Transformation on biphotons

A conditioned unitary transformation ($90^o$ polarization rotation) is performed at single-photon level. The transformation is realized by rotating polarization for one of the photons of a polarization-entangled biphoton state (signal photon) by means of a Pockel's cell triggered by the detection of the other (idler) photon after polarization selection. As a result, polarization degree for the signal beam changes from zero to the value given by the idler detector quantum efficiency. This result is relevant to practical realization of various quantum information schemes and can be used for developing a new method of absolute quantum efficiency calibration