Quantum and classical coincidence imaging.

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.

Relating to magnified image formation systems, we consider two processes in this work: quantum and classical incoherent imaging. There are similarities and differences between them. A similarity is that the optical transfer functions (OTFs) of both systems are obtained by calculating a correlation function. Concerning the differences, only two were found. The first difference is that, unlike classical incoherent imaging, quantum imaging with photon pairs generated by spontaneous parametric down-conversion is sensitive to the phase in the function which describes the object. The second difference is that the OTF of the quantum imaging system is a cross-correlation function of the OTFs of the signal and idler arms, while the OTF of the classical incoherent imaging system is an autocorrelation of the only pupil function existing in this case. We explore experimentally the second difference. We report the observation of a highly resolved image produced by a quantum light source in a setup where an incoherent light source is unable to spatially resolve the image of the object. The spatial resolution of the image is studied in a situation where the size of the object is magnified at the image.

A critical review is presented of recent progress in classical diffraction theory. Both scalar and electromagnetic problems are discussed. The report may serve as an introduction to general diffraction theory although the main emphasis is on diffraction by plane obstacles. Various modifications of the Kirchhoff and Kottler theories are presented. Diffraction by obstacles small compared with the wavelength is discussed in some detail. Other topics included are: variational formulation of diffraction problems, the Wiener-Hopf technique of solving integral equations of diffraction theory, the rigorous formulation of Babinet's principle, the nature of field singularities at sharp edges, the application of Mathieu functions and spheroidal wave functions to diffraction theory. Reference is made to more than 500 papers published since 1940.

HIGH-FREQUENCY SCATTERING BY AN IMPENETRABLE SPHERE

High-frequency scattering by an impenetrable sphere

Entangled photon sources play a pivotal role in most of the optical systems that utilize quantum effects. Those include quantum key distribution, quantum computing, metrology, sensing and imaging. Photon pair generation using spontaneous parametric down conversion (SPDC) is one of the most popular routes for producing entangled photons. While SPDC has become more popular, the creation of entangled photon pairs in a degree of freedom, such as polarization, is more challenging. The difficulty is related in no small part to the birefringence of the structure where the generation takes place. Birefringence results in distinguishing information that hinders the production of entanglement in the platforms where SPDC has been most popular including ferroelectric crystals.Despite this limitation, entangled sources have been realized in bulk crystals via well designed interference techniques, which often include additional compensating optics. As the boundaries of optical quantum information science continue to be extended, multi-photon entanglement is now required. Solutions based on SPDC have been attempted, and while some outstanding results have been achieved, the interferometers and compensation procedures become increasingly complex as the size of the entangled state grows. Such scalability and stability issues are directly addressed by integrated entangled photon sources, and while still in their infancy, they promise to help a great deal towards the practical utilization and the production of entanglement on a larger scale.Semiconductors are an ideal platform to fabricate such integrated entangled photon sources. Recently, it was demonstrated that the gallium-arsenide (GaAs) based Bragg refection waveguide (BRW), could efficiently produce photon pairs via SPDC. This platfrom was shown to have a distinct advantage over other semiconductor sources, due largely to its monolithic architecture and its layered epitaxy, which underpins many photonic devices. In this work we demonstrate yet another advantage: the intrinsic capability of the BRW to directly produce polarization entangled photon pairs, without any additional interferometry, spectral filtering, compensation or post-selection. Not only do we show that the BRW can produce entangled photons, but we wish to emphasize that the traditional compensation and interferometric methods used to create entanglement, which presumably would have occurred on chip, may no longer be necessary. Our experimental results confirm that the BRW has the potential to be one of the very first self-contained integrated room temperature resources of entanglement.In this talk various approaches for utilizing SPDC processes in various integration platforms will be discussed. The root cause of entanglement generation is the lack of birefringence in these material systems, which otherwise makes exact phase matching impossible in bulk crystals. Because of this, cross-polarized photons propagate at almost identical group velocities, making off-chip path compensation unnecessary.

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.

The metascreen-based acoustic passive phased array provides a new degree of freedom for manipulating acoustic waves due to their fascinating properties, such as a fully shifting phase, keeping impedance matching, and holding subwavelength spatial resolution. We develop acoustic theories to analyze the transmission/reflection spectra and the refracted pressure fields of a metascreen composed of elements with four Helmholtz resonators (HRs) in series and a straight pipe. We find that these properties are also valid under oblique incidence with large angles, with the underlying physics stemming from the hybrid resonances between the HRs and the straight pipe. By imposing the desired phase profiles, the refracted fields can be tailored in an anomalous yet controllable manner. In particular, two types of negative refraction are exhibited, based on two distinct mechanisms: one is formed from classical diffraction theory and the other is dominated by the periodicity of the metascreen. Positive (normal) and negative refractions can be converted by simply changing the incident angle, with the coexistence of two types of refraction in a certain range of incident angles.

A full detail model of the point spread function (PSF) is presented for the complete optical system of aberration-free lightfield cameras with low to moderate numerical apertures. This serves as an applicable model for both plenoptic 1.0 and 2.0 systems with an arbitrarily defocused main lens. New analytical expressions for the microlens pupil distributions are formulated in terms of the main lens defocus as a function of object depth and microlens array to main lens distance, with further analytical treatment for the computation of the lightfield PSF over state of the art numerical methods. Simulations validate the defocus parameter in full accordance with geometrical optics and the sensor PSF for a circle of confusion centred on the microlenses is shown to be consistent with classical diffraction theory.

The electron density of transient plasma is measured with a Michelson type CO2 laser interferometer. The frequency and amplitude stabilities of the Baser oscillation and the spatial distribution of the laser beam are investigated. The directivity of the laser beam is in accord with the classical diffraction theory in the Fraunhofer region. Although the experimental apparatus is not stabilized against the ambient temperature change, the error of measurement coming from fluctuation of interference signal is less than 0.1 fringe which corresponds to the phase shift produced by a plasma of electron density of 3×1014cm-3 and of length of 3 cm.

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.

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.

Photonic entanglement is central resource to quantum information sciences, such as quantum communication and quantum computation. The entangled photons generated in conventional spontaneous parametric down-conversion usually yield THz bandwidth which becomes very dim in many applications call for narrow-band entanglement sources. Here we demonstrate the polarization-entanglement photon source with counterpropagating phase matching, which results in an inherent bandwidth of 7.1 GHz at telecom wavelength. The entanglement is measured to violate the Bell inequality by up to 18.5 standard deviations, with Clauser–Horne–Shimony–Holt S-parameter of 2.720±0.039. The quantum state tomography further characterizes the entanglement, with fidelity F=(95.71±0.61)%.

An interdisciplinary quantum communications and sensing research effort has been underway at the NASA Glenn Research Center since the summer of 2000. Researchers in the Communications Technology, Instrumentation and Controls, and Propulsion and Turbomachinery Divisions have been working together to study and develop techniques that use the principle of quantum entanglement (QE). This work is supported principally by the Nanotechnology Base R&T program at Glenn. As applied to communications and sensing, QE is an emerging technology that holds promise as a new and innovative way to communicate faster and farther, and to sense, measure, and image environmental properties in ways that are not possible with existing technology. Quantum entangled photons are inseparable as described by a wave function formalism. For two entangled photons, the term inseparable means that one cannot describe one photon without completely describing the other. This inseparability gives rise to what appears as spooky, or nonintuitive, behavior because of the quantum nature of the process. For example, two entangled photons of lower energy can be created simultaneously from a single photon of higher energy in a process called spontaneous parametric down-conversion. Our research is focused on the use of polarization-entangled photons generated by passing a high-energy (blue) photon through a nonlinear beta barium borate crystal to generate two red photons that have orthogonal, but entangled, polarization states. Although the actual polarization state of any one photon is not known until it is measured, the act of measuring the polarization of one photon completely determines the polarization state of its twin because of entanglement. This unique relationship between the photons provides extra information about the system. For example, entanglement makes it easy to distinguish entangled photons from other photons impinging on a detector. For many other applications, ranging from quantum computation and information to quantum sensing, the entanglement property is critical.

Energy-time entangled photon pairs (EPPs), which are at the heart of numerous quantum light applications, are commonly generated in nonlinear crystals. Some highly sensitive quantum applications require the use of ultra-broadband entangled photons that cannot be generated in nonlinear crystals due to phase-matching requirements. Here, we investigate the possibility of using metallic nanoparticles (MNPs) as a means for generating entangled photons through spontaneous parametric down-conversion (SPDC). MNPs are known for their strong light-matter coupling at their localized surface plasmon resonance, and since the propagation length through them is negligible relative to optical wavelengths, we consider them as excellent candidates to serve as non-phase matched sources of ultra-broadband entangled photons. To that end, we report experimental results of classical-light second-harmonic generation in silver nanotriangles and nanocubes embedded in polyvinyl alcohol. Based on the results of our experiments, performed using the reference-free hyper-Rayleigh scattering method [A. Ashkenazy et al., J. Phys. B 145401, 2019], we present an estimation of the characteristics of SPDC in MNPs. We show that, despite of the metals' centrosymmetric structure, MNPs exhibit second order nonlinearity that is mainly of an electric dipole nature, and so they are suitable for SPDC. Moreover, we show that localized surface-plasmon resonance can play a significant role in enhancing the generated EPPs flux. Finally, we compare the SPDC capabilities of MNPs to that of commonly used nonlinear crystals and show that the expected EPPs flux from MNPs is weaker but the EPPs have very large bandwidth, which could be helpful for advanced quantum sensing, spectroscopy and communication applications.