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Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences

An introduction to ghost imaging: quantum and classical

Miles J. Padgett, Robert W. Boyd

Published Jun 26, 2017• 286 citations
Publisher

Abstract

Ghost imaging has been a subject of interest to the quantum optics community for the past 20 years. Initially seen as manifestation of quantum spookiness, it is now recognized as being implementable in both single- and many-photon number regimes. Beyond its scientific curiosity, it is now feeding novel imaging modalities potentially offering performance attributes that traditional approaches cannot match.This article is part of the themed issue 'Quantum technology for the 21st century'.

DOI
10.1098/rsta.2016.0233

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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.

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Ghost imaging has been a subject of interest to the quantum optics community for the past 20 years. Initially seen as manifestation of quantum spookiness, it is now recognized as being implementable in both single- and many-photon number regimes. Beyond its scientific curiosity, it is now feeding novel imaging modalities potentially offering performance attributes that traditional approaches cannot match.This article is part of the themed issue 'Quantum technology for the 21st century'.
The term Ghost Imaging embeds the notion that images can be formed by photons that have not interacted directly with an object. The first demonstration of ghost imaging turns 30, and the rich physics contained therein remains inspiring, fuelling advances in computational imaging with classical light, and novel imaging modalities with quantum light. Three decades have passed since the first demonstration of Ghost Imaging by Pittman and collaborators. This comment reflects on the initial intellectual challenges that emerged at the time of the experiment, and discusses how such challenges fuelled the development of three decades of imaging technologies.
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.
One of the most surprising consequences of quantum mechanics is the entanglement of two or more distant particles. Although questions regarding fundamental issues of quantum theory still exist, quantum entanglement has started to play important roles in practical engineering applications. Quantum imaging is one of these exciting areas. Quantum imaging has demonstrated two peculiar features: (1) reproducing ghost images in a nonlocal manner, and (2) enhancing the spatial resolution of imaging beyond the diffraction limit. In this article, we start with the review of classical imaging to establish the basic concepts and formalisms of imaging. We then analyze two-photon imaging with particular emphasis on the physics of spatial resolution enhancement and the ghost imaging phenomenon.
One of the most surprising consequences of quantum mechanics is the entanglement of two or more distant particles. Although questions regarding fundamental issues of quantum theory still exist, quantum entanglement has started to play important roles in practical engineering applications. Quantum imaging is one of these exciting areas. Quantum imaging has demonstrated two peculiar features: (1) reproducing "ghost" images in a "nonlocal" manner, and (2) enhancing the spatial resolution of imaging beyond the diffraction limit. In this article, we start with the review of classical imaging to establish the basic concepts and formalisms of imaging. We then analyze two-photon imaging with particular emphasis on the physics of spatial resolution enhancement and the "ghost" imaging phenomenon.
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.