Quantum imaging of an obscured object is achieved by the gated coincidence measurement of first the thermal source transverse intensity and second the measured count of scattered and reflected photons that have traversed the obscurant. Interestingly, the CCD camera is looking at the pseudo thermal source of photons not the object. The image does not exist in the mean, but only in the coincidence and has a higher image quality metric than that achieved by a camera viewing the object through the obscurant.
Quantum imaging of an obscured object is achieved by the gated coincidence measurement of first the thermal source transverse intensity and second the measured count of scattered and reflected photons that have traversed the obscurant. Interestingly, the CCD camera is looking at the pseudo thermal source of photons not the object. The image does not exist in the mean, but only in the coincidence and has a higher image quality metric than that achieved by a camera viewing the object through the obscurant.
Distinct from previous principle-demonstrations, this practical quantum ghost imaging experiment presents the first set of two-photon images captured by a photon counting CCD camera by means of jointly counting 'reflected' photons from the object. In fact, the CCD camera was not 'looking' at the object at all. Rather, the CCD camera was facing the chaotic light source. The output of the CCD camera was used for coincidence registration of the two-photon joint-detection events with another photon counting detector which simply collects all randomly reflected photons from the surface of the object. It is also interesting to find that the observed two-photon images are 'distortion-free', i.e., any disturbances made along the light path has no effect on the quality of the image. These experimental observations are not only useful for practical field-applications, but also important from fundamental point of view. The experiment evidences a rejection of the 'projection shadow' idea in a non-deniable way and further explores the two-photon interference nature of thermal light ghost imaging.
Imaging is the estimation of the spatial distribution of a physical object by measuring the optical radiation it emits, or by making use of an optical wave that interacts with the object, via reflection or transmission, for example, before being measured by a detector. The resolution of an imaging system is limited by the inability to localize the optical field at points of the object. Under otherwise ideal conditions, resolution is limited by diffraction. The sensitivity of an imaging system is limited by the uncertainty in the measurement. Under ideal conditions, this is determined by photon noise, which depends on the statistical fluctuations of the light. In conventional imaging systems, an extended detector, such as a CCD camera or an array detector, measures the spatial distribution of the optical intensity, which is proportional to the photon flux density. In interferometric systems, the spatial distribution of the optical field is inferred from measurements of the optical intensity. With the emergence of coherence theory, imaging systems based on measurements of the second-order coherence function at pairs of points in the detection plane were developed. An example is the imaging of an incoherent object based on the van Cittert-Zernike theorem. Imaging systems based on measurement of intensity correlation, or the photon coincidence rate, at pairs of points, were developed in the 1960s. A classic example of the photon-correlation imaging of an object emitting thermal light is stellar imaging using a Hanbury-Brown-Twiss intensity-correlation interferometer. More recently, two-photon light, which may be generated via spontaneous parametric downconversion in a second-order nonlinear optical crystal, has been used for imaging. This type of two-photon (or biphoton) imaging, which has come to be called quantum imaging, is also based on the measurement of photon coincidence by the use of photon-counting array detectors or by scanning two photon-counting detectors at pairs of points. To compare the resolution and sensitivity of imaging systems based on the aforementioned types of measurements, it is necessary to derive expressions for the measured quantities in terms of the object distribution. The point-spread functions based on such expressions can be used to assess the resolution. One measure of the sensitivity of the imaging process is the signal-to-noise ratio (SNR) of the measured variables. The statistical nature of the light source must be known in order to determine the SNR. The purpose of this chapter is to compare the resolution and sensitivity of photon-correlation imaging systems that make use of thermal light and two-photon light. We will henceforth refer to these two imaging modalities as classical and quantum photon-correlation imaging, or simply classical and quantum imaging, respectively. Clearly, light in other quantum states can also be used for imaging.
We show an experimental study of two-photon imaging using thermal light sources. It is interesting to see that the thermal source behaves like a mirror producing an equal size reproduction of a chosen object. We refer to such mirror as "magic" because the ghost image produced by the mirror is real in the sense that it lies in the real space and can be further imaged by a camera or an equivalent optical system. We also show that it is possible to overcome the main limitation towards the actual implementation of thermal light two-photon imaging, i.e. the poor contrast of the imaging pattern, by designing an appropriate correlation measurement scheme that is insensitive to the uncorrelated background noise.
Abstract Losses should be accounted for in a complete description of quantum imaging systems, and yet they are often treated as undesirable and largely neglected. In conventional quantum imaging, images are built up by coincidence detection of spatially entangled photon pairs (biphotons) transmitted through an object. However, as real objects are non-unitary (absorptive), part of the transmitted state contains only a single photon, which is overlooked in traditional coincidence measurements. The single-photon part has a drastically different spatial distribution than the two-photon part. It contains information about both the object, and, remarkably, the spatial entanglement properties of the incident biphotons. Here, we image the one- and two-photon parts of the transmitted state using an electron multiplying CCD array, treating it both as a traditional camera and as a massively parallel coincidence counting apparatus, and demonstrate agreement with theoretical predictions of non-unitary quantum optics. This work may prove useful for photon-number imaging and may lead to techniques for entanglement characterization that do not require coincidence measurements.
We utilize a single-photon sensitive electron multiplying CCD camera as a massively parallel coincidence counting apparatus to study spatial entanglement of photon pairs. This allows rapid measurement of transverse spatial entanglement in a fraction of the time required with traditional point-scanning techniques. We apply this technique to quantum experiments on entangled photon pairs: characterization of the evolution of entanglement upon propagation, and measurement of one- and two-photon portions of the state transmitted through non-unitary (lossy) objects, and quantum phase imaging.
Ghost imaging allows extracting information about an object from the spatial correlation function between the photons that interacted with an object and the photons in the reference arm(s). The reference beam does not interact with the object; however, it is measured by a CCD matrix. The mathematical method of measurement reduction allows using both the mutual correlations of multiplexed ghost images and prior information about how the image of the research object can look in order to improve the quality of the reconstructed image. The information about the object is the sparsity of its transparency distribution under a given transform. Computer modelling of image reconstruction using multiplexed ghost images formed by four-mode entangled quantum light states confirms the efficiency of the proposed algorithm even with a small number of photons that illuminate the research object.