Quantum imaging of biological specimens using entangled photons

Over the last decade, two-photon microscopy and optical coherence tomography (OCT) have emerged as two of the most widespread techniques for performing high-resolution, three-dimensional optical imaging of the internal structure of biological specimens Two-photon microscopy relies on the use of dye molecules as markers that only fluoresce when they accidentally absorb two laser photons simultaneously. To enhance the probability of absorption, and thus provide more efficient imaging; expensive femtosecond-pulsed lasers are typically employed. OCT, on the other hand, is a form of range finding that makes use of photons one-at-a-time and the second-order coherence properties of a classical laser source to enable sectioning of specimens. Each imaging modality is enabled by distinct properties of the light source and ultimately yields information about structure and/or refractive index properties of the specimen. An ability to measure changes in these physical properties provides a means to track pathological processes or evaluate the viability and integrity of biological specimens.; In this work, we explore the merits and challenges of using a source of entangled photons to enable analogous non-classical two-photon imaging techniques. The configuration for two-photon microscopy is a case in which the object interacts with both beams simultaneously. However, since entangled photons are guaranteed to be emitted pair wise, it has been hypothesized that they should be absorbed more readily so that imaging can be accomplished at reduced light levels. This would permit extended functional imaging of light-sensitive specimens. Alternatively, using photon pairs where one of the photons scatters from the object then interferes at a beam splitter with the second photon of the pair used as a reference, describes a configuration analogous to OCT but makes use of fourth-order, rather then second-order interference effects. This method, which we call quantum optical coherence tomography (QOCT), has the distinct advantage of cancelling even-order dispersion from the specimen. We further provide a method for polarization-sensitive QOCT, in which a change in the polarization state of light scattered from a specimen due to birefringence is detected. This provides added contrast for identifying structure or pathology in thick samples where dispersion plagues conventional imaging modalities.