In this thesis I discuss the experimental demonstration of quantum-enhanced imaging and sensing schemes able to surpass the performance of their classical counterparts. This is achieved by exploiting the spatial properties of quantum correlated biphotons. Over the next chapters I first discuss the production and detection of quantum correlated photons using a type-I nonlinear crystal and a single-photon sensitive electron-multiplying CCD camera. I then provide a simple yet powerful description of the spatially resolved detection of biphotons, allowing to accurately model and assess the performance of the quantum-enhanced schemes featured in this thesis. These consist of a shadow-sensing and an imaging scheme able to respectively beat the shot-noise-limit in the optical measurement of the position of a shadow and the diffraction limit in the full-field imaging of real-world objects. A combination of simulated and experimental results are used to investigate both the achieved and theoretically available quantum advantage. Optical losses and detector noise are found to limit the better-than-classical performance of the schemes, which rely on the ability to jointly detect an as high as possible number of spatially correlated biphotons.
In this thesis I discuss the experimental demonstration of quantum-enhanced imaging and sensing schemes able to surpass the performance of their classical counterparts. This is achieved by exploiting the spatial properties of quantum correlated biphotons. Over the next chapters I first discuss the production and detection of quantum correlated photons using a type-I nonlinear crystal and a single-photon sensitive electron-multiplying CCD camera. I then provide a simple yet powerful description of the spatially resolved detection of biphotons, allowing to accurately model and assess the performance of the quantum-enhanced schemes featured in this thesis. These consist of a shadow-sensing and an imaging scheme able to respectively beat the shot-noise-limit in the optical measurement of the position of a shadow and the diffraction limit in the full-field imaging of real-world objects. A combination of simulated and experimental results are used to investigate both the achieved and theoretically available quantum advantage. Optical losses and detector noise are found to limit the better-than-classical performance of the schemes, which rely on the ability to jointly detect an as high as possible number of spatially correlated biphotons.
The spatial resolution of an optical system is limited by diffraction. Various schemes have been proposed to achieve resolution enhancement by employing either a scanning source/detector configuration or a two-photon response of the object. Here, we experimentally demonstrate a full-field resolution-enhancing scheme, based on the centroid estimation of spatially quantum-correlated biphotons. Our standard-quantum-limited scheme is able to image a general non-fluorescing object, using low-energy and low-intensity infrared illumination (i.e., with <0.001 photon per pixel per frame at 710 nm), achieving 41% of the theoretically available resolution enhancement. Images of real-world objects are shown for visual comparison, in which the classically bound resolution is surpassed using our technically straightforward quantum-imaging scheme.
Abstract In this review we present the potentialities and the achievements of the use of non-classical photon-number correlations in twin-beam states for many applications, ranging from imaging to metrology. Photon-number correlations in the quantum regime are easily produced and are rather robust against unavoidable experimental losses, and noise in some cases, if compared to the entanglement, where losing one photon can completely compromise the state and its exploitable advantages. Here, we will focus on quantum enhanced protocols in which only phase-insensitive intensity measurements (photon-number counting) are performed, which allow probing the transmission/absorption properties of a system, leading, for example, to innovative target detection schemes in a strong background. In this framework, one of the advantages is that the sources experimentally available emit a wide number of pair-wise correlated modes, which can be intercepted and exploited separately, for example by many pixels of a camera, providing a parallelism, essential in several applications, such as wide-field sub-shot-noise imaging and quantum enhanced ghost imaging. Finally, non-classical correlation enables new possibilities in quantum radiometry, e.g. the possibility of absolute calibration of a spatial resolving detector from the on-off single-photon regime to the linear regime in the same setup.
Spontaneous downconversion is a versatile source for correlated biphotons that has been employed in many quantum sensing and imaging experiments. Spatially-resolved photon-counting detectors allow to access a large number of modes, posing the challenge of an accurate description of such systems. We propose a simple model to generate images as though acquired by a photon-counting camera, and allow to simulate and characterise the detection of quantum correlations. We derive quantitative parameters characteristic of the spatial correlations for a given experiment, comparing the images produced by our model to the frames acquired by an electron-multiplying CCD camera. Moreover we accurately predict the decreased detection of spatially correlated biphotons caused by introducing a variable amount of optical loss after the nonlinear crystal, even though the total number of detected events is kept constant, showing excellent agreement between model and experiment.
The quantised nature of the electromagnetic field sets the classical limit to the sensitivity of position measurements. However, techniques based on the properties of quantum states can be exploited to accurately measure the relative displacement of a physical object beyond this classical limit. In this work, we use a simple scheme based on the split-detection of quantum correlations to measure the position of a shadow at the single-photon light level, with a precision that exceeds the shot-noise limit. This result is obtained by analysing the correlated signals of bi-photon pairs, created in parametric downconversion and detected by an electron multiplying CCD (EMCCD) camera employed as a split-detector. By comparing the measured statistics of spatially anticorrelated and uncorrelated photons we were able to observe a significant noise reduction corresponding to an improvement in position sensitivity of up to 17% (0.8dB). Our straightforward approach to sub-shot-noise position measurement is compatible with conventional shadow-sensing techniques based on the split-detection of light-fields, and yields an improvement that scales favourably with the detector's quantum efficiency.
Entangled photons have the remarkable ability to be more sensitive to signal and less sensitive to noise than classical light. Joint photons can sample an object collectively, resulting in faster phase accumulation and higher spatial resolution, while common components of noise can be subtracted. Even more, they can accomplish this while physically separate, due to the nonlocal properties of quantum mechanics. Indeed, nearly all quantum optics experiments rely on this separation, using individual point detectors that are scanned to measure coincidence counts and correlations. Scanning, however, is tedious, time consuming, and ill-suited for imaging. Moreover, the separation of beam paths adds complexity to the system while reducing the number of photons available for sampling, and the multiplicity of detectors does not scale well for greater numbers of photons and higher orders of entanglement. We bypass all of these problems here by directly imaging collinear photon pairs with an electron-multiplying CCD camera. We show explicitly the benefits of quantum nonlocality by engineering the spatial entanglement of the illuminating photons and introduce a new method of correlation measurement by converting time-domain coincidence counting into spatial-domain detection of selected pixels. We show that classical transport-of-intensity methods are applicable in the quantum domain and experimentally demonstrate nearly optimal (Heisenberg-limited) phase measurement for the given quantum illumination. The methods show the power of direct imaging and hold much potential for more general types of quantum information processing and control.
Quantum-enhanced optical sensing leverages correlations and entanglement in multiphoton states to surpass the limitations of classical measurement. However, the widespread deployment of quantum imaging and polarimetry has been hindered by bulky nonlinear crystals, alignment-sensitive free-space optics, and limited control over the spatial and polarization structure of quantum light. This thesis investigates how engineered nanophotonic metasurfaces, together with two-photon probing schemes, can overcome these constraints and enable compact, versatile, and high-performance quantum sensing platforms. The first component of the thesis introduces a metasurface-enabled quantum imaging system based on spatially entangled photon pairs generated from a subwavelength-thick lithium niobate (LiNbO3) nonlinear metasurface. Exploiting nonlocal guided-mode resonances with strong angular dispersion, the system realizes a hybrid imaging protocol that combines quantum ghost imaging with all-optical wavelength-controlled scanning. A theoretical framework linking momentum-space biphoton correlations to image formation is developed, including the effects of pump-beam waist and metasurface quality factor. Numerical simulations and proof-of-principle experiments demonstrate 2D imaging with strong contrast and establish that metasurface-based photon-pair sources can achieve fields of view and resolution-cell counts far beyond what is possible with bulk-crystal SPDC. Building on this platform, the thesis presents the first quantum phase imaging system that integrates metasurfaces for both photon-pair generation and phase-gradient detection. A silicon (Si) metasurface with a near-linear optical transfer function directly maps phase gradients of transparent samples into spatial quantum correlations, eliminating the need for interferometers or long propagation paths. Experiments show that phase gradients up to 25rad/mm can be retrieved with high fidelity, and the system supports switchable operation between amplitude and phase imaging within a significantly more compact scale. The final component of the thesis develops a two-photon probing quantum polarimetry (TPQP) protocol for characterizing scattering media. By combining Monte Carlo modeling of microscopic photon trajectories with Mueller-matrix decomposition and quantum state tomography, the framework links entanglement degradation to macroscopic polarization observables and resolves ambiguities in Mueller-matrix extraction through tailored probe states. Experiments using tissue-mimicking phantoms confirm that TPQP exhibits significantly higher sensitivity to depolarization than one-photon probing, validating its potential for quantum-enhanced polarimetric sensing and imaging. Overall, this thesis advances quantum optical sensing through two distinct but complementary directions. The first two parts demonstrate how engineered metasurfaces enable compact quantum imaging and phase-gradient detection by integrating photon-pair generation and spatial processing within ultrathin nanophotonic structures. Independently, the two-photon probing quantum polarimetry framework, implemented using a conventional BBO-based entangled source, shows how nonlinear two-photon correlations can enhance sensitivity to depolarization and improve Mueller-matrix reconstruction in scattering media. Although realized on different physical platforms, all components of this work exploit quantum correlations to extract amplitude, phase, and polarization information beyond classical approaches, contributing to the development of practical quantum sensing methodologies.