Comparing quantum imaging with classical second-order incoherent imaging

Quantum imaging promises increased imaging performance over classical protocols. However, there are a number of aspects of quantum imaging that are not well understood. In particular, it has been unknown so far how to compare classical and quantum imaging procedures. Here, we consider classical and quantum imaging in a single theoretical framework and present general fundamental limits on the resolution and the deposition rate for classical and quantum imaging. The resolution can be estimated from the image itself. We present a utility function that allows us to compare imaging protocols in a wide range of applications.

We propose and demonstrate a temporally and spatially incoherent spectrum-sliced light source for scintillation mitigation in free-space optical (FSO) communications. For the temporal incoherence, we utilize the spectrum-sliced incoherent light (SSIL) modulated by gain-saturated reflective semiconductor optical amplifiers for data modulation and the suppression of excess intensity noise inherent in a narrowband incoherent light source. We then employ offset launching from a single-mode fiber to a multi-mode fiber to add the spatial incoherence to the SSIL source. The experimental demonstration shows that this temporally and spatially incoherent light is effective in mitigating scintillation. For example, this light source reduces the transmitter power required to achieve a bit-error ratio of 10-3 by ∼6 dB in comparison with the laser source. This is an ∼3 dB larger reduction in the transmitter power than the reduction achieved by a temporally incoherent light source.

One of the most surprising consequences of quantum mechanics is the entanglement of two or more distant particles. Even though we still have questions in regard to fundamental issues of the entangled quantum systems, quantum entanglement has started to play important roles in practical applications. Quantum imaging is one of the hot topics. Quantum imaging has many interesting features which are useful for different applications. For example, quantum imaging can be nonlocal, which is useful for secure two-dimensional information transfer. Quantum imaging can reach a much higher spatial resolution comparing with classical imaging, even beyond the diffraction limit, which is useful for lithography and other microsystem fabrication technology. It is not a violation of the uncertainty principle, however, a quantum mechanical multi-particle phenomenon.

Quantum imaging utilizing entangled photon pairs takes advantage of their strong quantum correlations to surpass the limitations of classical imaging techniques in terms of spatial resolution and signal-to-noise ratio. However, due to the low brightness of entangled photon sources, an enormous amount number of intensity images (\(\sim 10^7\)) is required for imaging, posing a challenge for practical implementation. In this study, we propose the use of a BiBO crystal for generating entangled photons to enhance the efficiency of quantum imaging. A BiBO crystal has large nonlinear optical coefficient, so it has been expected to serve as a high-brightness entangled photon source. However, a BiBO crystal has never been applied to quantum imaging because it's relatively recently discovered. Here, we investigate the potential of a BiBO crystal as a light source for high-efficiency quantum imaging by performing parallel intensity correlation measurements and implementing quantum image distillation as an example in comparison with the commonly used a BBO crystal. As a result, we achieved a tenfold reduction in measurement time of the conventional method and a significant improvement in imaging efficiency.

Phase imaging normally employs coherent a light source while an incoherent light source is not preferred due to its random wavefront. Another challenge for practical phase imaging is imaging through scattering media, which scatter the photons in a random manner and lead to seriously distorted images of speckles. Based on the convolutional neural network (CNN), this paper presents an approach for phase imaging through scattering media using an incoherent light source. A CNN was trained and utilized to reconstruct the target images from the captured images of speckles. Similarities of over 90% between the reconstructed images and their target images have been achieved. It was concluded that an incoherent light source can be used as an illumination source for scattering phase imaging with the assistance of deep learning technology. This phase imaging approach with an incoherent light source through scattering media can be used to record the refractive indices of transparent samples, which might lead to its application in biomedical imaging.

A single-photon state was generated by heralding cavity-enhanced spontaneous parametric downconversion in a PPKTP optical parametric oscillator. The Wigner distribution was reconstructed by quantum state tomography, using photon-number-resolving measurements with a system efficiency of 58±2%. © 2019 The Author(s)

Properties of quantum states have disclosed new technologies, ranging from quantum information to quantum metrology. Among them a recent research field is quantum imaging, addressed to overcome limits of classical imaging by exploiting spatial properties of quantum states of light . In particular quantum correlations between twin beams represent a fundamental resource for these studies. One of the most interesting proposed scheme exploits spatial quantum correlations between parametric down conversion light beams for realizing sub-shot-noise imaging of the weak absorbing objects, leading ideally to a noise-free imaging. Here we present the first experimental realisation of this scheme, showing its capability to reach a larger signal to noise ratio (SNR) with respect to classical imaging methods. This work represents the starting point of this quantum technology that can have relevant applications, especially whenever there is a need of a low photon flux illumination (e.g. as with certain biological samples).

We explore the use of nonlinear optical methods for important applications in quantum information science. One example is the use nonlinear optical methods in the context of “quantum imaging.” We describe some of the conceptual possibilities afforded by the marriage of quantum and classical imaging methods, and we describe recent work that illustrates some of these features. We also address the complementary issue of how advanced imaging protocols can allow one to utilize and manipulate the enormous information content residing in the transvers structure of a quantum light field.

Purpose: To demonstrate a method of measuring the modulation transfer function (MTF) of digital imaging systems using the common star pattern test tool. The goal of this research is to develop a technique that brings precise and objective evaluation of spatial resolution to routine quality control in radiography and fluoroscopy. Method and Materials: Images of a 2‐degree star pattern were obtained on a variety of digital radiographic imaging systems, spanning a range of spatial resolution performance levels. One‐dimensional profiles from concentric circles around the center of the star were obtained from each star image. The circular profiles were unwrapped to produce a series of square waves, which were analyzed to produce the square wave response function (SWRF). Droege's method was used to compute the MTF from the SWRF. The MTF determined using the star pattern method was compared to published and quality control measurements of the MTF for several digital mammography and digital radiography systems. In implementing this method, important parameters that must be considered include the radial and angular sampling rates, the method for determining the zero‐frequency normalization and correction for beam nonuniformity. Results: The MTF computed from star pattern profile measurements closely matched the MTF obtained using other methods over the range of spatial frequencies between the Nyquist frequency and 1/3 of the Nyquist frequency. The star pattern MTF, which is a system measurement and includes the effects of detector and geometric unsharpness, was within 6–8% of reference values obtained from the scientific literature for two models digital mammography equipment. Conclusion: Images of a 2‐degree star pattern can produce estimates of the two‐dimensional system MTF. This method may offer a convenient means of assessing the spatial resolution of digital imaging systems for acceptance testing and quality control.

To derive a theoretical model for the modulation transfer function (MTF) of the complete imaging chain of a cardiovascular x-ray system. A sufficiently accurate MTF model may be used in quantitative coronary arteriographic applications to decrease the systematic overestimation of small coronary vessels. The model could be used to restore blurred images in image restoration applications and also may be used for image quality assurance measurements. Digital images of a step wedge phantom were acquired at different modes (5-, 7-, and 9-inch) of the image intensifier and at different kilovolt levels of the x-ray generator. From these images the MTFs that were used to estimate the parameters of the model were assessed. The total MTF of the cardiovascular x-ray imaging chain consisting of image intensifier, video camera, optical elements, and analog/digital converter can be modeled by the simple formula MTF = e-(w/f)R, where w is the frequency in linepairs per millimeter. The constants frequency f and the device index n are parameters that can be derived for an individual x-ray system using the phantom. In this formula, the contribution from the focus of the x-ray source has been excluded. The mean square error (MSE) between the measured values and those assessed from the theoretical model of the MTF for the horizontal direction was found to be equal to 0.00003 for the 5-inch mode of image intensifier, and for the vertical resolution at the 9-inch mode this MSE was equal to 0.00038. In addition, we have demonstrated that the contributions of the video camera and the image intensifier to the total MTF can be replaced by an imaginary electron-optical device. An accurate representation for the MTF of a cardiovascular x-ray system has been derived. This MTF model can be used in x-ray image quality assurance measurements and in quantitative image processing applications.

We started to research quantum illumination radar and quantum imaging by utilizing high quality continuous-wave two-mode squeezed light source as a quantum entanglement resource. Two-mode squeezed light is a macroscopic quantum entangled state of the electro-magnetic field and shows strong correlation between quadrature phase amplitudes of each optical field. One of the most effective methods to generate two-mode squeezed light is combining two independent single-mode squeezed lights by using a beam splitter with relative phase of 90 degrees between each optical field. As a first stage of our work we are developing two-mode squeezed light source for exploring the possibility of quantum illumination radar and quantum imaging. In this article we introduce current development of experimental investigation of single-mode squeezed light. We utilize a sub-threshold optical parametric oscillator with bow-tie configuration which includes a periodically-polled potassium titanyl phosphate crystal as a nonlinear optical medium. We observed the noise level of squeezed quadrature −3.08±0.13 dB and anti-squeezed quadrature at 9.29±0.13 dB, respectively. We also demonstrated the remote tuning of squeezing level of the light source which leads to the technology for tuning the quantum entanglement in order to adapt to the actual environmental condition.

Non-phase-matched spontaneous parametric down-conversion (SPDC) in atomically thin materials provides new degrees of freedom and enhanced quantum information capacity compared to conventional phase-matched sources. These systems emerged as promising platforms for quantum computing, communication, and imaging, with the potential to support higher-order nonlinear processes. However, direct observation of photon-pair emission from a monolayer has remained experimentally challenging. In this work, we theoretically modeled SPDC emission across the full angular space from a monolayer GaSe film and experimentally validated the model through measurements of both co- and counter-propagating photon pairs. We demonstrated two-photon quantum correlations in the telecom C-band from a monolayer SPDC source. The spatially symmetric, broadband emission predicted by theory was confirmed experimentally. Furthermore, we observed high-fidelity Bell states in the counter-propagating configuration. Our results revealed the emission characteristics of SPDC in the deeply subwavelength, non-phase-matched regime, and introduced atomically thin, counter-propagating SPDC as a scalable and integrable platform for programmable quantum state generation, extendable via moiré superlattice engineering.

Nonlocal realism tests and quantum state tomography in Sagnac-based type-II polarization-entanglement SPDC-source

This paper concerns an experimental method to quantify the spatial resolution of an experimental DPC-CT system via modulation transfer function (MTF) measurement. Note that conventional metal wire-based MTF measurement methods are no long applicable to DPC-CT, because: (i) The refractive signal generated by the high density metal is out of the dynamic range of DPC-CT, and (ii) A DPC-CT system is not sensitive to input pulse finer than a detector pixel. These technical challenges were overcome by a new experimental design, in which a graphite rod was used as the probe to simultaneously measure the MTFs of the DPC-CT and the associated absorption CT. An appreciable difference in MTFs between DPC-CT and absorption CT was observed in our experimental benchtop system: At the 10% MTF level, the MTFs of the DPC-CT and the absorption CT are 4.7 cycles/mm and 5.2 cycles/mm, respectively. Such a difference in MTF leads to the conclusion that the grating interferometer used by the DPC-CT system has a frequency-dependent response to input DPC-CT signals.

Wavefront coding technique can extend the depth of filed (DOF) of the incoherent imaging system. Several rectangular separable phase masks (such as cubic type, exponential type, logarithmic type, sinusoidal type, rational type, et al) have been proposed and discussed, because they can extend the DOF up to ten times of the DOF of ordinary imaging system. But according to the research on them, researchers have pointed out that the images are damaged by the artifacts, which usually come from the non-linear phase transfer function (PTF) differences between the PTF used in the image restoration filter and the PTF related to real imaging condition. In order to alleviate the image artifacts in imaging systems with wavefront coding, an optimization model based on the PTF was proposed to make the PTF invariance with the defocus. Thereafter, an image restoration filter based on the average PTF in the designed depth of field was introduced along with the PTF-based optimization. The combination of the optimization and the image restoration proposed can alleviate the artifacts, which was confirmed by the imaging simulation of spoke target. The cubic phase mask (CPM) and exponential phase mask (EPM) were discussed as example.