QKD, XFELs Highlight 2004 DAMOP Meeting

The latest research results in quantum key distribution, quantum entanglement, and next- generation free electron lasers (FELs) were among the technical highlights at the 35^th annual meeting of the APS Division of Atomic Molecular and Optical Physics (DAMOP). It was held May 25-29 in Tucson, Arizona, in conjunction with the corresponding division of the Canadian Association of Physicists.

Among the special events was a welcoming reception Tuesday evening at the Arizona Historical Society Museum, and an after-dinner lecture by Rice University's Neal Lane, former director of both the National Science Foundation and the Office of Science and Technology Policy.

The conference also featured a public lecture on Wednesday evening by JILA's Eric Cornell, winner of the 2001 Nobel Prize in Physics for his contributions to realizing Bose-Einstein Con-densation.

Searching for a Quantum Key. Quantum key distribution (QKD) uses single-photon communications to generate the shared, secret random number sequences that are used to encrypt and decrypt secret communications. The secret to the technique's security is based on the interplay between quantum physics and information theory, according to Richard Hughes (Los Alamos National Laboratory).

"An adversary can neither successfully tap the transmissions nor evade detection," he said, since eavesdropping raises the key error rate above a set threshold value. Hughes described a recent QKD experiment performed over multi-kilometer line-of-sight paths, serving as a model for a satellite-to-ground key distribution system. His system uses single-photon polarization states, with active switching, and is capable of continuous operation through day and night.

FEL's Generation X. Stanford Linear Accelerator Center's planned Linac Coherent Light Source is an example of the next generation of X-ray free electron lasers (XFELs).

These instruments will offer users the ability to study ultrafast time-dependent phenomena with resolutions at atomic length scales. But in order to take full advantage of this time resolution, users will need single-shot measurements, in real time, of the temporal characteristics of the bunches of electrons that power such sources, according to speakers at a Saturday morning session on new techniques for studying ultracold molecules. Electro-optic sampling can be used to accomplish such characterization of electron bunches at SLAC, which are then used to produce ultrafast x-rays at the Sub-Picoscond Pulse Sourceexperiment.

Entangled Photons. Entanglement consists of the correlations between quantum systems that cannot be described by a local hidden variable model, and is now widely recognized as a resource for information processing tasks in communication and computation.

Scientists at the University of Michigan reported observation of quantum entanglement between a single atom and a single photon.

The experiment constituted the first direct observation of entanglement between stationary and so-called "flying" qubits, and the team accomplished this without using the standard cavity-QED technique or a prepared nonclassical light source. The Michigan team's technique provides an entanglement source that could be used for a variety of quantum communication protocols, as well as for seeding large-scale entangled states of trapped ion qubits for scalable quantum computing.

John Chiaverini of NIST in Boulder, Colorado, is confining atomic ions in radio frequency traps, cooled and addressed with laser pulses. He reported that this constitutes a scalable system for bringing about and exploring quantum entanglement and information processing.

He is currently experimenting with superdense coding, quantum teleportation, and entangled state spectroscopy.

At the same Friday afternoon session, Eugene Polzik of the Niels Bohr Institute at Copenhagen University in Denmark described how he and his colleagues have recently demonstrated entanglement of two atomic ensembles at a distance of 0.5 meters. He believes it is possible to extend this distance to tens or even hundreds of meters.

Exposing Molecular Dynamics. The University of Maryland's Ill Hill has found that Coulomb explosion imaging provides a unique window into molecular structure and dynamics because it can capture all fragment ions after they've been dissociated from a highly-stripped molecular ion.

The technique was originally used in experiments with fast ion beams traversing thin foils.

At a Thursday morning session, Hill discussed some of the latest "tricks of the trade" using ultrafast lasers to increase the flexibility of preparing the initial state of the system and provide a way of inducing new dynamics before the explosion.