QUANTUM OCT (first order correlations)

As optical coherence tomography (OCT) approaches two decades of existence, the number of proven and potential applications continues to grow.  At the same time, paradigm shifts in classical OCT technology seem to be slowing down.  However, as in other fields of optics (and physics in general), quantum mechanics has the potential of producing paradigm shifting advances in the technology.  Almost all OCT theoretical work to date is classical, while our work focuses on quantum OCT mechanisms.  While in some areas of optics, the electromagnetic field (EM) can be treated as semi-classically (the field is classical plus vacuum fluctuations but matter is quantum mechanical), but in many instances full quantization is necessary.  This is the case with OCT.  For full quantization of OCT, basic concepts for light quantization are needed including treating the EM field as a ‘sea’ of harmonic operators, as well as basic mathematical tools that includes annihilation/creation (and the related electric field operators) and density operators.  The central concept is that first order correlations with OCT represent a single photon interference depending on indistinguishable paths, the foundations of which date back to Dirac and Feynman among others.  We have extended this concept to low coherence interferometry (LCI) and OCT with appropriate superpositions of single photon wavepackets to yield an interference pattern.  From this we have developed concepts for potential future OCT development and understanding.  For example, we have demonstrated that path indistinguishability and interference are complimentary concepts.  Fringe visibility and maximum contrast are dependent on single photon path indistinguishability.  Similarly, quantum noise sources enter primarily through vacuum fluctuation and photon count errors (PCE), (excluding photon interactions for now), which can be treated in the context of the single photon interference.  Techniques for reducing this source of error involve interferometer design, determining optimal power, and squeezing.  Second (and higher) order coherence (particularly entanglement and indistinguishable paths for thermal biphotons), and position probability amplitude uncertainty, among other topics, are dealt with separately as this is a complex topic.  Together, by treating OCT from a field quantization approach, we provide a framework for potentially new insights into advances in the technology.