Matter-wave interferometer for large molecules

We demonstrate a near-field Talbot-Lau interferometer for C-70 fullerene molecules. Such interferometers are particularly suitable for larger masses. Using three free-standing gold gratings of one micrometer period and a transversally incoherent but velocity-selected molecular beam, we achieve an interference fringe visibility of 40 % with high count rate. Both the high visibility and its velocity dependence are in good agreement with a quantum simulation that takes into account the van der Waals interaction of the molecules with the gratings and are in striking contrast to a classical moire model.Comment: revtex4, 4 pages, 3 figure

Nonclassicality of pure two-qutrit entangled states

We report an exhaustive numerical analysis of violations of local realism by two qutrits in all possible pure entangled states. In Bell type experiments we allow any pairs of local unitary U(3) transformations to define the measurement bases. Surprisingly, Schmidt rank-2 states, resembling pairs of maximally entangled qubits, lead to the most noise-robust violations of local realism. The phenomenon seems to be even more pronounced for four and five dimensional systems, for which we tested a few interesting examples.Comment: 6 pages, journal versio

Experimental realization of Dicke states of up to six qubits for multiparty quantum networking

We report the first experimental generation and characterization of a six-photon Dicke state. The produced state shows a fidelity of F=0.56+/-0.02 with respect to an ideal Dicke state and violates a witness detecting genuine six-qubit entanglement by four standard deviations. We confirm characteristic Dicke properties of our resource and demonstrate its versatility by projecting out four- and five-photon Dicke states, as well as four-photon GHZ and W states. We also show that Dicke states have interesting applications in multiparty quantum networking protocols such as open-destination teleportation, telecloning and quantum secret sharing.Comment: 4 pages, 4 figures, RevTeX

On the origin of bursts in blue compact dwarf galaxies: clues from kinematics and stellar populations

Blue compact dwarf galaxies (BCDs) form stars at, for their sizes, extraordinarily high rates. In this paper, we study what triggers this starburst and what is the fate of the galaxy once its gas fuel is exhausted. We select four BCDs with smooth outer regions, indicating them as possible progenitors of dwarf elliptical galaxies. We have obtained photometric and spectroscopic data with the FORS and ISAAC instruments on the VLT. We analyse their infrared spectra using a full spectrum fitting technique, which yields the kinematics of their stars and ionized gas together with their stellar population characteristics. We find that the stellar velocity to velocity dispersion ratio ((nu/sigma)(star)) of our BCDs is of the order of 1.5, similar to that of dwarf elliptical galaxies. Thus, those objects do not require significant (if any) loss of angular momentum to fade into early-type dwarfs. This finding is in discordance with previous studies, which however compared the stellar kinematics of dwarf elliptical galaxies with the gaseous kinematics of star-forming dwarfs. The stellar velocity fields of our objects are very disturbed and the star formation regions are often kinematically decoupled from the rest of the galaxy. These regions can be more or less metal rich with respect to the galactic body and sometimes they are long lived. These characteristics prevent us from pinpointing a unique trigger of the star formation, even within the same galaxy. Gas impacts, mergers, and in-spiraling gas clumps are all possible star formation igniters for our targets

Correspondence between continuous variable and discrete quantum systems of arbitrary dimensions

We establish a mapping between a continuous variable (CV) quantum system and a discrete quantum system of arbitrary dimension. This opens up the general possibility to perform any quantum information task with a CV system as if it were a discrete system of arbitrary dimension. The Einstein-Podolsky-Rosen state is mapped onto the maximally entangled state in any finite dimensional Hilbert space and thus can be considered as a universal resource of entanglement. As an explicit example of the formalism a two-mode CV entangled state is mapped onto a two-qutrit entangled state.Comment: 4 pages, 1 figure, revised version, an example adde

High-fidelity entanglement swapping with fully independent sources

Entanglement swapping allows to establish entanglement between independent particles that never interacted nor share any common past. This feature makes it an integral constituent of quantum repeaters. Here, we demonstrate entanglement swapping with time-synchronized independent sources with a fidelity high enough to violate a Clauser-Horne-Shimony-Holt inequality by more than four standard deviations. The fact that both entangled pairs are created by fully independent, only electronically connected sources ensures that this technique is suitable for future long-distance quantum communication experiments as well as for novel tests on the foundations of quantum physics.Comment: added technical details and extended introduction and conclusion, slightly modified the abstract, corrected a mistake in the affiliation

High-Fidelity Teleportation of Independent Qubits

Quantum teleportation is one of the essential primitives of quantum communication. We suggest that any quantum teleportation scheme can be characterized by its efficiency, i.e. how often it succeeds to teleport, its fidelity, i.e. how well the input state is reproduced at the output, and by its insensitivity to cross talk, i.e. how well it rejects an input state that is not intended to teleport. We discuss these criteria for the two teleportation experiments of independent qubits which have been performed thus far. In the first experiment (Nature {\bf 390},575 (1997)) where the qubit states were various different polarization states of photons, the fidelity of teleportation was as high as 0.80 $\pm$ 0.05 thus clearly surpassing the limit of 2/3 which can, in principle, be obtained by a direct measurement on the qubit and classical communication. This high fidelity is confirmed in our second experiment (Phys. Rev. Lett. {\bf 80}, 3891 (1998)), demonstrating entanglement swapping, that is, realizing the teleportation of a qubit which itself is still entangled to another one. This experiment is the only one up to date that demonstrates the teleportation of a genuine unknown quantum state.Comment: 13 pages, Latex, 5 figures(eps), to appear in Journal of Modern Optic

A feasible "Kochen-Specker" experiment with single particles

We present a simple experimental scheme which can be used to demonstrate an all-or-nothing type contradiction between non-contextual hidden variables and quantum mechanics. The scheme, which is inspired by recent ideas by Cabello and Garcia-Alcaine, shows that even for a single particle, path and spin information cannot be predetermined in a non-contextual way.Comment: 4 pages, 3 figure

Experimental Demonstration of Decoherence-Free One-Way Information Transfer

We report the experimental demonstration of a one-way quantum protocol reliably operating in the presence of decoherence. Information is protected by designing an appropriate decoherence-free subspace for a cluster state resource. We demonstrate our scheme in an all-optical setup, encoding the information into the polarization states of four photons. A measurement-based one-way information-transfer protocol is performed with the photons exposed to severe symmetric phase-damping noise. Remarkable protection of information is accomplished, delivering nearly ideal outcomes.Comment: 5 pages, 3 figures, RevTeX

Operationally Invariant Information in Quantum Measurements

A new measure of information in quantum mechanics is proposed which takes into account that for quantum systems the only feature known before an experiment is performed are the probabilities for various events to occur. The sum of the individual measures of information for mutually complementary observations is invariant under the choice of the particular set of complementary observations and conserved if there is no information exchange with an environment. That operational quantum information invariant results in N bits of information for a system consisting of N qubits.Comment: 4 pages, 1 figur