Conventional ghost imaging aims at reconstructing any given object by means of spatially correlated photons. Our work highlights the importance of the selection of the quantum states that are used for such reconstructions, showing how the object can be rotated, or even doubled, by properly engineer the spatial correlations each of the photons possess.
Conventional ghost imaging aims at reconstructing any given object by means of spatially correlated photons. Our work highlights the importance of the selection of the quantum states that are used for such reconstructions, showing how the object can be rotated, or even doubled, by properly engineer the spatial correlations each of the photons possess.
Since its first demonstration in 1995, ghost imaging has provided amazing insights into both classical and quantum physics as well as having found application in, for example, microscopy and imaging under low light conditions. Traditional ghost imaging uses correlations between two photons to reconstruct an image of an object from two systems which each individually know nothing about the object. In the quantum case, the state of the two photons is typically a symmetric, entangled state. Here we investigate the effect that changing the two-photon state's symmetry has on the reconstructed object, by using Dove prisms and a Hong-Ou-Mandel filter. Interestingly, it appears that post-selecting on the anti-symmetric Bell state results in a `double image': a juxtaposition of the original image rotated both clockwise and anti-clockwise. Furthermore, we consider a 4-photon experiment in which two photons, which originate from different entanglement sources and are hence completely independent initially, acquire correlations by way of entanglement swapping via appropriate post-selection on the remaining two photons. In such a setup, post-selecting on the symmetric Bell states results in the original object, but post-selecting on the anti-symmetric Bell state results in a contrast-reversed image of the object. These studies highlight the fundamental importance that state symmetry plays in quantum imaging.
Abstract Traditional ghost imaging experiments exploit position correlations between correlated states of light. These correlations occur directly in spontaneous parametric down-conversion, and in such a scenario, the two-photon state usually used for ghost imaging is symmetric. Here we perform ghost imaging using an anti-symmetric state, engineering the two-photon state symmetry by means of Hong–Ou–Mandel interference. We use both symmetric and anti-symmetric states and show that the ghost imaging setup configuration results in object-image rotations depending on the state selected. Further, the object and imaging arms employ spatial light modulators for the all-digital control of the projections, being able to dynamically change the measuring technique and the spatial properties of the states under study. Finally, we provide a detailed theory that explains the reported observations.
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.
Abstract Traditional ghost imaging requires correlated but spatially separated photons and has been observed in many physical situations, spanning both the quantum and classical regimes. Here we observe ghost imaging in a new system—a system based on entanglement swapping, the key feature of a quantum network. We detail how the exact form of quantum interference between independent photons dictates the precise nature of the ghost imaging, for example, for an anti-symmetric projection, the recorded image is the contrast-reversed version of the object—where the object is bright, the image is dark, and vice versa. The results highlight the importance of state projection in this ghost-imaging process and provide a pathway for the teleportation of two-dimensional spatial states across a quantum network. Our results also indicate that ghost images with new image properties could be achieved in conventional settings through a variety of new signal processing procedures.
Ghost imaging is the remarkable process where an image can be formed from photons that have not "seen" the object. Traditionally this phenomenon has required initially correlated but spatially separated photons, e.g., one to interact with the object and the other to form the image, and has been observed in many physical situations, spanning both the quantum and classical regimes. To date, all instances of ghost imaging record an image with the same contrast as the object, i.e., where the object is bright, the image is also bright, and vice versa. Here we observe ghost imaging in a new system - a system based on photons that have never interacted. We utilise entanglement swapping between independent pairs of spatially entangled photons to establish position correlations between two initially independent photons. As a consequence of an anti-symmetric projection in the entanglement swapping process, the recorded image is the contrast reversed version of the object, i.e., where the object is bright, the image is dark, and vice versa. The results highlight the importance of state projection in this ghost imaging process and provides a pathway to teleporting images across a quantum network.