A new type of optical imaging is described, which has undoubted advantages in certain applications in comparison with traditional optical methods. The use of nonlinear optical quantum processes makes it possible to obtain entangled photon pairs rigidly correlated in space and time in the course of spontaneous parametric three-photon scattering and counter-propagating four-photon mixing. This correlation can be used to construct ghost images. A fiber-optic version of the formation of phantom images is also described, which is suitable for the study of hard-to-reach cavities and organs of the human body, which allow the introduction of a thin fiber-optic bundle.
A new type of optical imaging is described, which has undoubted advantages in certain applications in comparison with traditional optical methods. The use of nonlinear optical quantum processes makes it possible to obtain entangled photon pairs rigidly correlated in space and time in the course of spontaneous parametric three-photon scattering and counter-propagating four-photon mixing. This correlation can be used to construct ghost images. A fiber-optic version of the formation of phantom images is also described, which is suitable for the study of hard-to-reach cavities and organs of the human body, which allow the introduction of a thin fiber-optic bundle.
A generalization of the calculation of the formation of ghost images in the course of backward four-photon mixing to the real situation of non-planar spatially limited pumping, which has a Gaussian profile of the fundamental laser mode, is developed, taking into account the specifics of the formation of phantom images. Computer images have been obtained that make it possible to estimate the spatial resolution of the systems. A relatively simple equivalent scheme for describing the process is proposed, which shows that the effect of diffraction in nonlinear systems is similar to its role in ordinary linear optics with a limited aperture. A fiber-optic version of the formation of ghost images is proposed, suitable for the study of hard-to-reach cavities and organs of the human body, which allow the introduction of a thin fiber bundle there. Keywords: quantum entangled states, quantum images, optical systems, ghost images, quantum measurements.
“Ghost” imaging is a process utilizing entangled photon pairs, by which an image is produced indirectly, using photons that have never interacted with the object being imaged. We can do this by taking advantage of correlations inherent in Spontaneous Parametric Down Conversion (SPDC), which is used to produce these entangled photon pairs. SPDC is a classically forbidden process in which a photon of a high frequency is converted into a pair of photons. The change in frequency makes this a nonlinear process. Processes like absorption, reflection, refraction, and polarization are linear processes. These can affect the properties of a light field but cannot change the frequency. SPDC allows us to produce a pair of correlated photons. By utilizing a coincidence-counter, we will be able to measure photon pairs from the source beam arriving within a narrow time interval. With this restriction, we can limit our attention to these correlated photons. One of these photons, which does interact with the object to be imaged, is used to turn on a “camera” detecting the other photon. It is the correlations between these photons that allow for ghost imaging, the reconstruction of the image of an object, even though the object never interacts directly with those single photons that are going to our camera.
A generalization of the calculation of the formation of ghost images in the course of backward four-photon mixing to the real situation of non-planar spatially limited pumping, which has a Gaussian profile of the fundamental laser mode, is developed, taking into account the specifics of the formation of ghost images. Computer images have been obtained that make it possible to estimate the spatial resolution of the systems. A relatively simple equivalent scheme for describing the process is proposed, which shows that the effect of diffraction in nonlinear systems is similar to its role in ordinary linear optics with a limited aperture. A fiber-optic version of the formation of ghost images is proposed, suitable for the study of hard-to-reach cavities and organs of the human body, which allow the introduction of a thin fiber bundle there.
The process of image recording is arguably one of the most prevalent technology in modern society and continues to inspire vast swathes of research due to its widespread applications spanning military, medical and consumer spheres. The danger present in a field so broad is that separate niches of research can become isolated with critical advancements struggling to traverse the gulfs between. Unifying the field is the omnipresent drive to acquire an ever increasing quality of images at the lowest possible cost, a goal which warrants continual fundamental research. Quantum entanglement exhibits many intriguing characteristics which make it a suitable tool for such fundamental investigations. The process of spontaneous parametric down- conversion has offered a yet unbeaten strength of photon correlations with the quantum nature of their production providing a reliable and controllable source of single-photons. Arising from these attributes is the technique known as ghost imaging. Though now known to be classically possible, the strength of entanglement generated correlations is yet to be surpassed. This thesis implemented this technique in tandem with the cutting edge detector technology in order to probe the fundamentals of image formation. This form of imaging allowed us to subject an object to a known number of photons whilst acquiring structural information from spatially separate, correlated photons which never interact with the object. The strength of the produced correlations allow us to acquire low background, high resolution images with far less light than traditional techniques and affords many novel benefits. The possibility of incorporating this technique with pre-existent regimes allowed me to draw from advancements made across the landscape of imaging research. Although referred to as “quantum ghost imaging” throughout this work, it should be noted that the intrinsic quantum nature of the correlations was not directly relied upon but provided an ideal source of strongly correlated photons. In order to determine the limits of a traditional imaging system this thesis first sought to answer the question: “can an image of an object be reconstructed from fewer photons than ii pixels in the image?” In chapter 3 I approached this from the perspective of compression, which minimises redundant information within a signal. This lead to the development of an imaging regime capable of imaging with far fewer photons than pixels in the image. By employing assumptions about the sparsity of natural images I was able to reconstruct an image of a biological sample containing an average of less than one photon per image pixel. Having reduced the number of photons necessary to form an image I then considered alternative methods for reducing the optical energy impinging on a sample. I sought to answer the question: “can non-degenerate ghost imaging reduce the optical energy impinged upon an object during imaging”. The photons produced in SPDC need not be of similar wavelengths, however may be chosen far from degeneracy, i.e. non-degenerate. In Chapter 4 I presented a ghost imaging system which illuminated the object with infrared light whilst recording the structural information via entangled visible photons. This allowed for objects opaque to visible light to be imaged in high quality without the need for a spatially resolving infrared detector, the state of the art of which lags behind their silicon based visible counterparts. I presented the systems capabilities by imaging objects which were etched into a gold substrate layered on to silicon, both of which are opaque to visible light. Not only did a reduction in energy deposition arise from the lower energy probe wavelength but applying the reconstruction techniques from the previous chapter brought that down to as low as ≈16 nJcm−2s−1. Seeking to expand the repertoire of applications, the low-light capabilities of my ghost imaging were applied to the technique of phase-contrast microscopy in chapter 5. Typically applied to translucent objects, phase-contrast imaging transfers phase information, i.e. the refractive index changed within the object, into an intensity distribution through the use of a phase-filter. In many of these applications the objects tend to be biological in nature, where high optical exposure can result in bleaching or damage. By applying the phase-filter non- locally, i.e. to the photons correlated to those probing the object, I acquired edge-enhanced images of a phase object whilst illuminating with significantly fewer photons than standard phase-contrast techniques. Having displayed the broad applicability of our low-light ghost imaging system, I then sought to determine the optical resolution in chapter 6. The resolution limits of ghost imaging are not clear at first glance owing to the resolutions dependence upon the strength of spatial correlations. As the length over which the spatial correlations are produced can be brought below the standard diffraction limit, it would seem the resolution of the system could be brought similarly low. To clarify this I artificially restricted the number of spatial modes in each of the correlated beams to uncover the physically realisable resolution. I show that although the resolution of a ghost imaging system to be fundamentally determined by the strength of the correlations, this can never be reached due to the inherent limitations of the intervening imaging system.
Ghost imaging functions achieved by means of the spatial correlations between two photons is a new modality in imaging systems. With a small number of photons, ghost imaging is usually realized based on the position correlation of photon pairs produced from the spontaneous parametric down-conversion process. Here we demonstrate a way to realize multi-path ghost imaging by introducing an additional time correlation. Different delays of paths will induce the shift of the coincidence peak, which carries the information about objects. By choosing the suitable coincidence window, we obtain images of three objects simultaneously, with a visibility of 87.2%. This method provides insights and techniques into multi-parameter ghost imaging. It can be applied to other correlated imaging systems, for example, quantum spiral imaging .
Nonlinear optical metasurfaces, which relax the phase-matching constraints of bulk nonlinear crystals and allow for precise engineering, are opening new possibilities in the field of quantum photonics. Recent advancements have experimentally demonstrated high-resolution 2D imaging using a 1D detector array by combining quantum ghost imaging and all-optical scanning with spatially entangled photon pairs generated from a nonlinear metasurface. These findings establish metasurfaces as a promising platform for quantum imaging, communications, and sensing applications.