Love songs. African indigobirds reproduce and then lay their eggs in the nests of firefinches, where the young hatchlings learn the songs of their adoptive parents. Male indigobirds later sing these songs as part of their courtship rituals, and this behavior leads to the development of new indigobird species, according to recent research by Michael Sorenson, a CAS assistant professor of biology.

In earlier experiments Sorenson’s colleague, Robert Payne of the University of Michigan, Ann Arbor, showed that when indigobird eggs were transplanted from the nests of firefinches to another finch species, the chicks learned the song of the new species. When ready to lay their eggs, the female indigobirds laid them in the nests of the new species — the one in which they had hatched, not the species that their parents had parasitized.

The new studies by Sorenson, Krishna Secf, a postdoctoral student who worked with Sorenson on the research, and Payne indicate that certain gene sequences are more common in some species of indigobirds than others, suggesting that interbreeding is rare. They attribute this to females having a preference for males singing a familiar song. A characteristic pattern of spots around the mouths of firefinch chicks helps the parent finches recognize their young. Since indigobird chicks with the same markings are more likely to survive in the host nests, the researchers believe that female indigobirds use the song to find mates with genes that will produce chicks with markings that help them best blend in with the finch chicks.

According to Sorenson and Payne, the adopted melodies and the pressure to produce chicks that will survive in the hosts’ nests contribute to the rapid evolution of a new species of indigobirds, even within the same habitat. It is the only example of sympatric speciation — the development of a new species within the same geographic area — found thus far among birds.

This research was reported in the August 21 issue of the journal Nature.

Quantum leap. Researchers at the College of Engineering have added a new twist to a three-dimensional diagnostic imaging technique known as optical coherence tomography (OCT). This technology is widely used in ophthalmology 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, Bahaa Saleh and Malvin Teich, ENG professors of electrical and computer engineering, Alexander Sergienko, an ENG associate professor of electrical and computer engineering, and graduate students Ayman Abouraddy, a postdoctoral researcher, and Magued Nasr (ENG’04) from the department of electrical and computer engineering have increased 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. The differences in the amount of time that it takes for the photons in the pair to reach the detector are used to generate a highly accurate image of the interior of the sample under investigation.

The researchers used both techniques to image a piece of fused silica sandwiched between two zinc selenide windows. The improved resolution of the new technique, dubbed QOCT (quantum optical coherence tomography), results from enhanced sensitivity of the photon pairs as a depth probe and from the elimination of dispersion effects created by the wider bandwidth needed in conventional OCT.

This research has been a centerpiece of the Boston University National Science Foundation Engineering Research Center CenSSIS (Center for Subsurface Sensing and Imaging Systems) and earned Nasr the Berman Future of Light Award at Science and Technology Day 2003.

For more information about quantum imaging, see http://www.bu.edu/qil/.