Researchers Present Wide Variety of New Quantum Tools

Optical antennae, new breakthroughs in cavity QED, and a new twist on three-dimensional diagnostic imaging were among the many technical highlights at the 2005 APS March Meeting in Los Angeles. All three represent valuable new quantum tools for the medical, quantum computing, and quantum communication fields, among other potential applications.

Michael Barnes of the University of Massachusetts-Amherst described the construction of a pair of "nantennae," small posts just 10 nm tall, about 100 million times smaller than a car antenna. The two nantennae interact with each other much like conventional antennae do, as a transmitter/receiver pair. In addition to providing insights into the behavior of light at small distances, the nantennae could be important for photonic-based quantum-information processing applications.

These nanoscale antennae are made with semiconducting polymers, which are already used in LEDs and photovoltaics, for example. Polymers don’t normally conduct but they can be made semiconducting if the molecules are properly ordered and aligned. The problem is morphological control: researchers must find a means to impose order on a jumbled mess of polymer molecules. Barnes’ team confines single molecules of conjugated polymers in microdroplets on a glass surface and then allows the droplet to evaporate. This can cause the polymers to fold in ways that change their properties so that they become conducting.

Potential applications include quantum computing and tabletop nanoscale photonics, such as phased nantennae arrays and photonic networks, as well as novel light sources for integrated nanoscale optoelectronics. Advantages to be gained from optical antennae include being able to get amplification on a very small scale, such as through linear arrays.

Malvin Teich of Boston University presented a new twist in a 3D diagnostic imaging technique known as optical coherence tomography (OCT), widely used in opthalmology and in creating cross-section images of biological tissue for noninvasive optical biopsy. By replacing the broadband light source used in traditional OCT with pairs of entangled photons, the BU researchers have performed demonstrations of "quantum optical coherence tomography" (QOCT)–imaging the surfaces of fused silica windows while increasing the axial resolution of the resulting images by a factor of five.

The investigators produce photon pairs by passing laser light through a nonlinear optical crystal, in this case a krypton-ion laser beam directed at a crystal made of lithium iodate. The twin photons that emerge continue to be linked even as they are directed along different paths: one toward the sample under investigation, the other toward a mirror.

Both ultimately reach photon detectors. Observing a signal in both detectors requires that the path lengths of the two photons be the same. Changing the mirror’s position changes the depth from which a reflection is observed, so the image of the sample’s interior is much more accurate.

Teich plans to test these technologies on biological samples such as salamander retinas. The salamander retina has a layered structure so it is not smooth, and there is more scattering of light. Teich wants to know how this extra scattering will affect the technologies’ resolution. Potential applications include learning more about the structure of the retina and its many layers; dermatological imaging (such as skin tumors); and small devices inserted into a catheter to look for plaque in vivo in blood vessels.

Jeff Kimble of Caltech presented his group’s latest experimental breakthroughs in cavity QED, in which a single atom is trapped in an optical resonator formed by two mirrors separated by 40 microns. Such a setup is a promising building block for quantum computation and communication, since the energy levels of the atom could constitute a useful "quantum bit" and the atom-field interaction can enable quantum logic operations between pairs of atoms or photons.

Kimble’s group has demonstrated what he considers the first "quantum protocol" for cavity QED, and also discovered a "photon blockade" for light traveling through the cavity. Trap one atom in a small cavity and then add photons. The atom should absorb and emit, absorb and emit, achieving some form of coherence. By this means the cavity can emit photons without the atoms going into an excited state. The flaw is it’s difficult to control, so sometimes more than one atom end up in the cavity. This single photon generation is both coherent and reversible. Kimble hopes to use this technology to build a simple quantum optical network.