Quantum science and technology are attracting world-wide attention due to the impacts they will have on computing and communications which have no classical counterpart. However, quantum also impacts imaging and sensing. Leaving aside how new detection technologies can sense both single photons and measure their arrival time with pico-second precision, the quantum nature of light enables new types of imaging system, which again have no easy classical implementation. This is brief overview of the historical development of quantum imaging focuses on how the photon pairs created through spontaneous parametric down-conversion lead to unusual imaging systems. Referring to the work from across the global community and some work of my own group we will consider which of these imaging approaches might be considered truly quantum and which might have classical analogies. In all cases I will emphasize those systems which seem to offer practical advantage over traditional approaches giving performance benefits in terms of resolution, signal to noise or wavelength coverage.
Quantum science and technology are attracting world-wide attention due to the impacts they will have on computing and communications which have no classical counterpart. However, quantum also impacts imaging and sensing. Leaving aside how new detection technologies can sense both single photons and measure their arrival time with pico-second precision, the quantum nature of light enables new types of imaging system, which again have no easy classical implementation. This is brief overview of the historical development of quantum imaging focuses on how the photon pairs created through spontaneous parametric down-conversion lead to unusual imaging systems. Referring to the work from across the global community and some work of my own group we will consider which of these imaging approaches might be considered truly quantum and which might have classical analogies. In all cases I will emphasize those systems which seem to offer practical advantage over traditional approaches giving performance benefits in terms of resolution, signal to noise or wavelength coverage.
The UK is continuing its drive to commercialise Quantum Technology in Imaging, Sensing, Communications and Computing. In the first part of my talk I will give an overview of the work of QuantIC, the UK hub tasked with bringing quantum technology to bear on real world imaging challenges, across the industrial, scientific, security and consumer markets.
In the second part of my talk I will feature some of my own group’s work on using quantum illumination to both increase image resolution and suppress background and detector noise. Pairs of photons that are quantum entangled with each other are correlated in many degrees of freedom, creation-time, energy, lateral position etc. Fundamentally this means that measuring one of the photons gives near-perfect prediction of the corresponding property of the other photon. In an imaging context, this second photon with known properties can then be used to probe an object. Using the correlations inherent in this entanglement we are able to increase the resolution of an imaging system and suppress both sensor noise and background light. Our immediate goal is to recover an image of the object without interference from any thermal background or other light sources that might otherwise obscure or confuse what we are seeing. This work in the optical regime has strong parallels with proposals around quantum radar and it is an interesting exercise to consider what one might inform with respect to the other.
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.
The field of quantum imaging has recently seen an explosion of new methods and techniques. At the heart of most quantum imaging approaches are three enabling technologies: the source of the quantum light, the detectors that turn sensing into imaging, and the method by which the quantum nature of light is leveraged to improve the desired measurement. This review addresses the dominant and emerging technologies in each of these components as well as details concerning the implementations of various quantum imaging methods.
Since the second quantum revolution, the growing exploitation of quantum states led to many sophisticated and novel applications [Phil. Trans. R. Soc. Lond. A 361, 1655 (2003). Former, mainly academic research is more and more transferred into real-world quantum technology ready to serve practical tasks. Today, the quantum computer is within reach, satellite based quantum communication already started, and the field of sensing and imaging was revolutionized, too. Non-classical states of light promise a phase sensitivity beyond any classical possibility [PRL 112, 103604 (2014)] and super-resolution capability [Nature Commun. 8, 14786 (2017)]. Moreover, based on quantum correlations, which are rooted in the very heart of quantum mechanics, the imaging of samples with photons that have never interacted with the object is feasible. This science fiction like phenomenon was first investigated by Mandel [PRA 44, 4614 (1991)] and later implemented for actual imaging purposes in the Zeilinger group [Nature 512, 409 (2014)]. We are going to present the transfer of this approach into applicable quantum technology within the realm of the Fraunhofer Key Research Initiative Quantum Methods for Advanced Imaging Solutions (QUILT). The method itself is based on induced coherence without induced emission [PRA 44, 4614 (1991)]. We built a novel compact implementation of this scheme based on only one nonlinear crystal. The crystal is coherently pumped by a laser from two sides such that signal and idler photon pairs can be collinearly emitted in either of two opposite directions (to the right or to the left). On the right side the idler beam will be separated by a dichroic mirror (DM) and interacts with an object (directly placed in front of a mirror or reflective object). On the left side the signal beam will be separated by another dichroic mirror and hit the camera, which is either a sCMOS or an EMCCD. Additional lenses provide the particular imaging. Sine the two possibilities for the idler beam – going right and interact with the object or going left from the nonlinear crystal – interfere the object could be observed by idler detection. However, the same interference can be observed for the signal beams although the never interacted with the object. Hence, the object can be imaged by detecting the signal light only. The crucial point here is the lack of wich-path information of the idler beams in this type of Michelson interferometric setup. The following points should be emphasized: (i) the signal channels never interact with the object, (ii) there is no induced emission due to the idler beam that interacted with the object, and (iii) there is no coincidence detection involved in this scheme. The obvious advantage of this technique is that the wavelength of the idler photons can be tailored to match the interesting spectral range for interaction with the object. At the same time, the signal photons, which are actually detected, can stay in the VIS range where, e.g., Si-based detectors are optimized. Besides the application for life science imaging, we are comparing the quantum imaging properties utilizing momentum correlations or spatial correlations, where the ladder is a modus that was never investigated before. As an outlook we work on significantly enhance SPDC sources in scope of quantum performance and wavelength separation. Obviously, pushing the short-wavelength idler photons further into the deep UV or XUV broadens the range of applications, but must be implemented by non-SPDC photon pair sources.
Quantum imaging protocols have traditionally relied on spatially resolving detector arrays or multi-pixel cameras whose noise floors, limited detection efficiencies, and timing jitter fundamentally constrain image quality. Here we report a fibre-integrated architecture that reconstructs full two-dimensional quantum images using only a single superconducting nanowire single-photon detector, replacing spatial resolution with temporal multiplexing and compressive measurement. Correlated photon pairs generated by parametric down-conversion illuminate the target through one arm while the partner photon is retained as a temporal reference; non-local dispersion engineered across the two arms suppresses uncorrelated background while preserving pair timing. A compressive sensing algorithm exploiting sparsity in the Fourier domain then recovers 50×50-pixel images from far fewer measurements than conventional raster scanning would require. Operating over several kilometres of standard single-mode fibre at the eye-safe wavelength of 1550 nm, the system produces images of comparable fidelity to classical illumination while using 95 dB less optical power, achieves sub-shot-noise performance with a 7.9 dB signal-to-noise ratio improvement, and sustains useful image quality under injected noise exceeding the signal by 10 dB. The same temporal correlation data yield a timing precision of 1.2 ps, corresponding to sub-millimetre ranging resolution. These results establish single-detector temporal multiplexing as a viable and scalable route to practical quantum imaging over deployed telecommunications infrastructure.