Ghost imaging (GI) has been under development for several years and is now achievable in both single- and multiphoton regimes with both quantum and classical light. Here, we address the experimental challenge of using sunlight as a pump beam to excite spontaneous parametric down-conversion and generate photon pairs. Remarkably, our investigations reveal that photon pairs excited from sunlight exhibit strong positional correlation, enabling their application in GI with an efficiency comparable to that achieved using a laser with similar pump power. This finding demonstrates the potential of employing an incoherent beam as the pump source in GI systems. Our research holds substantial significance as it expands the range of viable illumination sources—including scattered light and nontraditional artificial incoherent light—for imaging applications. A key potential use is in space-based quantum information systems, where this approach enables operation independent of laser sources.
Ghost imaging (GI) has been under development for several years and is now achievable in both single- and multiphoton regimes with both quantum and classical light. Here, we address the experimental challenge of using sunlight as a pump beam to excite spontaneous parametric down-conversion and generate photon pairs. Remarkably, our investigations reveal that photon pairs excited from sunlight exhibit strong positional correlation, enabling their application in GI with an efficiency comparable to that achieved using a laser with similar pump power. This finding demonstrates the potential of employing an incoherent beam as the pump source in GI systems. Our research holds substantial significance as it expands the range of viable illumination sources—including scattered light and nontraditional artificial incoherent light—for imaging applications. A key potential use is in space-based quantum information systems, where this approach enables operation independent of laser sources.
Initially, ghost imaging (GI) was demonstrated with entangled light from parametric down conversion. Later, classical light sources were introduced with the development of thermal light GI concepts. State-of-the-art classical GI light sources rely either on complex combinations of coherent light with spatially randomizing optical elements or on incoherent lamps with monochromating optics, however suffering strong losses of efficiency and directionality. Here, a broad-area superluminescent diode is proposed as a new light source for classical ghost imaging. The coherence behavior of this spectrally broadband emitting opto-electronic light source is investigated in detail. An interferometric two-photon detection technique is exploited in order to resolve the ultra-short correlation timescales. We thereby quantify the coherence time, the photon statistics as well as the number of spatial modes unveiling a complete incoherent light behavior. With a one-dimensional proof-of-principle GI experiment, we introduce these compact emitters to the field which could be beneficial for high-speed GI systems as well as for long range GI sensing in future applications.
In traditional ghost imaging, the entangled photon pairs produced from the spontaneous parametric down conversion (SPDC) process are used. There is an intrinsic disadvantage that the utilization efficiency of the photon pairs is very low. Inasmuch as all the correlated photon pairs produced by the degenerate SPDC process can be used to record the image of an object, the ghost imaging scheme we present here has a higher utilization efficiency of the photon pairs. We also investigate the robustness of our experimental scheme. The experimental results show that, no matter whether the photon-pair source is two light cones or two beam-like spots, the clear image of the object can be obtained. The slight rotation of the nonlinear crystal has no influence on the imaging quality. Our experimental results also demonstrate that when the part of the photon-pair source in the signal path or the idler path is blocked by unwanted things, the clear ghost image of the object can still be recorded.
Spontaneous parametric down-conversion (SPDC) can produce entangled photons from a nonlinear medium driven by a pump beam, and has thus been a key resource in photonic quantum technologies. Historically, the low efficiency of SPDC prompted the use of lasers, which are high-intensity light sources, as pump sources. However, the practical implementation of laser-driven systems faces an imminent energy bottleneck. Lasers typically have suboptimal energy efficiencies due to lasing thresholds and the technical demands of thermal management. As a result, laser-pumped entangled photon sources suffer from significant power overhead, rendering quantum devices less sustainable. Over the past few decades, light-emitting diodes (LEDs) have emerged as inexpensive and energy-efficient alternatives for high-intensity light sources. Since 2010, studies have shown that LEDs can drive SPDC and generate photon pairs. These results suggests the potential to replace lasers with ubiquitous and natural light sources to build environmentally friendly quantum devices. However, replacing lasers with other light sources entails a lack of coherence in all degrees of freedom (DoFs) of the pump beam, including spatial, temporal-spectral, and polarization, which may influence the resulting entanglement generated from SPDC. While earlier studies established that pump coherence in a specific DoF sets an upper bound for entanglement in that same DoF, the cross-influence between different DoFs has not been fully investigated. In this thesis, we study the influence of the pump's spatiotemporal coherence on the polarization entanglement generated from SPDC. First, we experimentally demonstrate that polarization-entangled two-photon states can be generated from SPDC pumped by spatiotemporally incoherent light emitted from an LED. Although the resulting entanglement appears lower than that of laser-pumped SPDC, we theoretically show that spatiotemporal incoherence does not fundamentally constrain polarization entanglement; rather, the reduction arises from a coupling between spatiotemporal and polarization DoFs introduced by the experimental setup. We determined experimentally that this cross-influence acts as a deterministic mapping from the spatial components of the pump to the two-photon polarization state. Because this mapping depends on technical aspects of the setup, we envisioned that it could be either (i) engineered to structure entanglement in higher dimensions, or (ii) compensated to reduce spatial distinguishability. Building on the latter, we show that by placing the nonlinear medium in a phase-compensated interferometric setup, SPDC pumped by spatially incoherent light can produce high polarization entanglement that violates local realism, even without postselecting the SPDC field for a single spatial mode. Finally, as a proof-of-principle demonstration of a truly environmentally friendly quantum light source, we demonstrate the generation of polarization-entangled two-photon states from SPDC pumped by a natural source - sunlight.
Correlated-photon imaging, popularly known as ghost imaging, is a technique whereby an image is formed from light that has never interacted with the object. In ghost imaging experiments two correlated light fields are produced. One of these fields illuminates the object, and the other field is measured by a spatially resolving detector. In the quantum regime, these correlated light fields are produced by entangled photons created by spontaneous parametric down-conversion. To date, all correlated-photon ghost-imaging experiments have scanned a single-pixel detector through the field of view to obtain the spatial information. However, scanning leads to a poor sampling efficiency, which scales inversely with the number of pixels, N, in the image. In this work we overcome this limitation by using a time-gated camera to record the single-photon events across the full scene. We obtain high-contrast images, 90%, in either the image plane or the far-field of the photon pair source, taking advantage of the EPR-like correlations in position and momentum of the photon pairs. Our images contain a large number of modes, >500, creating opportunities in low-light-level imaging and in quantum information processing.
We examine experimentally how the degree of position-momentum entanglement of photon pairs depends on the transverse coherence of the pump beam that excites them in a process of spontaneous parametric down-conversion. Using spatially incoherent light from a light-emitting diode, we obtain strong position correlation of the photons, but we find that transverse momentum correlation, and thus entanglement, is entirely absent. When we continuously vary the degree of spatial coherence on the pump beam, we observe the emergence of stronger momentum correlations and entanglement. We present theoretical arguments that explain our experimental results. Our results shed light on entanglement generation and can be applied to control entanglement for quantum information applications.
Ghost Imaging (GI) is a fascinating imaging technique that exploits second-order correlations of light to investigate samples without directly observing them with a camera [1]. Quantum GI (QGI) version exploits correlations stemming from photon pairs generated by spontaneous parametric down-conversion (SPDC) [2]. It retrieves images from two-path correlation measurements, where single-path detection is not sufficient to obtain an image of the target. Since early demonstrations, this technique has suffered in terms of scalability due to the difficulty of efficiently performing spatially resolved single-photon detection. Early demonstrations suffered from detection efficiency limitations, due to raster scanning spatial detection. More recent camera-based experiments relying on intensified CCD cameras obtain only a limited frame rate and need complex image-preserving delay-lines [3]. More recently, SPAD arrays, two-dimensional arrangement of single-photon avalanche detectors, have been developed and employed in a number of quantum imaging and spectroscopy schemes, dramatically improving detection performances [4]. Here we report on a QGI implementation exploiting a 2D SPAD array detector for the spatially resolving path, solving limitations in terms of frame-rate and the need for optical delay lines. Our experiment enables the acquisition of an image in under one minute, and with much higher frame rate compared to the state-of-the-art. We further take advantage of non-degenerate SPDC to interrogate a Si-Au target. While light interaction with the sample happens at infrared wavelengths, no infrared cameras are needed to obtain an image, and spatial detection is performed in the visible region. This two-color scheme allows to make use of the advanced and cost-efficient silicon-technology while still investigating a sample in spectral regions where cameras are too expensive or not yet available [5]. Our findings advance the state-of-the-art of QGI schemes towards practical applications.