Quantum optical coherence tomography of a biological sample

A number of nonclassical (quantum) sources of light have come to the fore in recent years [1], but few practical applications have emerged. One such application is quantum optical coherence tomography (QOCT) [2, 3], a fourth-order interferometric optical-sectioning scheme that makes use of frequency-entangled photon pairs generated via spontaneous optical parametric down-conversion (SPDC). A particular merit of QOCT is that it is inherently immune to group-velocity dispersion (GVD) by virtue of the frequency entanglement of the photon pairs [4]-[6]. Conventional optical coherence tomography (OCT) [7], in contrast, is a second-order interferometric scheme that provides high-resolution axial sectioning by employing ultra-broadband light. Unfortunately, however, this leads to GVD, which degrades resolution. Here we present the first experimental QOCT images of a biological sample: an onion-skin tissue coated with gold nanoparticles. Three-dimensional images are displayed in the form of transverse sections at different depths. The results reveal that QOCT is a viable biological imaging technique.