Physicists at Boston University in the US have improved the axial resolution of optical coherence tomography (OCT) by a factor of five. Malvin Teich, Bahaa Saleh and colleagues used a two-photon source in what they claim is the first experimental demonstration of quantum OCT (M B Nasr et al. 2003 Phys. Rev. Lett. 91 083601).
OCT is an imaging technique that uses a broadband light source to generate an optical cross-section of biological tissue. It is becoming increasingly widespread in commercial medical applications, particularly for ophthalmology. But unlike the conventional method, the new quantum technique uses two “entangled” photons that are produced when a 406 nm beam from a krypton-ion laser strikes a lithium iodate crystal. Entanglement is a property of quantum theory that allows two particles to display much stronger correlations than are possible in classical physics.
In the experiment one photon from each pair is directed along a beam path towards the sample, and the other is sent towards a mirror. These beams are then recombined via a beam splitter onto a pair of photon-counting detectors to generate an interferogram. In this way, the technique resembles a two-photon interferometer, with the sample placed in one arm of the apparatus.
Teich and colleagues imaged a piece of fused silica sandwiched between two zinc selenide windows to demonstrate the effect on resolution. With standard OCT, the silica windows could be imaged to a resolution of 92 microns. However, this improved to 18.5 microns when quantum OCT was used.
Using entangled photons improves the resolution in two ways, explains Teich. First, there is an automatic improvement simply by using two photons for imaging instead of one. When the photons are entangled, it improves axial resolution by a factor of two.
The second improvement is due to the elimination of dispersion effects. In conventional OCT, resolution is enhanced by increasing the bandwidth of the illuminating light source. However, the broader bandwidth introduces more group-velocity dispersion, which has a detrimental effect on resolution. Using entangled photons automatically cancels out the dispersion effects. The overall effect is to improve the resolution by a factor of five.