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Chinese Physics Letters

Robust Ghost Imaging Based on Degenerate Spontaneous Parametric Down-Conversion

Ling‐Jun Kong, Yu Si, Rui Liu, Zhou-Xiang Wang, Wen-Rong Qi, Chenghou Tu, Yongnan Li, Hui‐Tian Wang

Published May 1, 2017• 0 citations
Publisher

Abstract

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.

DOI
10.1088/0256-307x/34/5/054206

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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.
“Ghost” imaging is a process utilizing entangled photon pairs, by which an image is produced indirectly, using photons that have never interacted with the object being imaged. We can do this by taking advantage of correlations inherent in Spontaneous Parametric Down Conversion (SPDC), which is used to produce these entangled photon pairs. SPDC is a classically forbidden process in which a photon of a high frequency is converted into a pair of photons. The change in frequency makes this a nonlinear process. Processes like absorption, reflection, refraction, and polarization are linear processes. These can affect the properties of a light field but cannot change the frequency. SPDC allows us to produce a pair of correlated photons. By utilizing a coincidence-counter, we will be able to measure photon pairs from the source beam arriving within a narrow time interval. With this restriction, we can limit our attention to these correlated photons. One of these photons, which does interact with the object to be imaged, is used to turn on a “camera” detecting the other photon. It is the correlations between these photons that allow for ghost imaging, the reconstruction of the image of an object, even though the object never interacts directly with those single photons that are going to our camera.
In this work, pairs of entangled photons are aimed at two spatially separated detectors: the path to one detector contains a physical object, while the path to the second detector does not. Yet, the second detector can be used to create an image of the object, even though none of the photons arriving at that detector have ever interacted with the object! This “ghost” imaging takes advantage of information that is shared between entangled photon pairs. This work aims to explore the underlying principles in a ghost imaging system that utilizes spontaneous parametric down-conversion (SPDC) as the source of entangled photons, with the larger goal of understanding the principles and operation of key parts of the system.
Quantum ghost imaging approaches have been proposed to enhance biological microscopy, for example, using 2D visible detectors to provide IR images or providing additional dimensions of spatial or spectral information. Toward the goal of making such imaging schemes practical, we compare image quality and depth-of-field between traditional images and ghost images at the same excitation levels. We measure how image quality and depth-of-field depend on the parameters of the entangled light produced using type-I spontaneous parametric down-conversion (SPDC). We use a pair of time-synchronized, photon-timing single-photon avalanche diode (SPAD) array detectors to capture two distinct microscope imaging paths simultaneously on a photon-pair-by-photon-pair basis: one in a traditional imaging pathway and the other a quantum ghost imaging pathway. We calculate the depth-of-field, resolution, contrast, and signal-to-noise ratio (SNR) through the parameter space of a β-Barium Borate (BBO) type-I bulk non-linear crystal length and angle. Our results provide a basis for choosing parameters for quantum ghost imaging with type-I SPDC sources.
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.
Ghost imaging can be performed using either quantum or classical states of light that possess strong spatial correlations. In both cases, the image is formed by averaging over many optical events. Here we show that it is possible to distinguish an object from a preestablished basis set of objects by using a small number of position-correlated photon pairs produced by spontaneous parametric down-conversion. The signal photon is incident on one member of a set of spatially nonoverlapping objects. The "ghost" image information is impressed upon the spatially separated idler photon and is extracted by means of holographic filtering and coincidence detection. We were able to distinguish among sets of two and four spatially nonoverlapping objects with confidence levels higher than 87% and 81%, respectively. This method of ghost imaging can be performed in situations requiring extremely low light levels.
The quantum correction of photon pairs inspired the development of quantum ghost imaging principles. It offers fundamental advantages compared to conventional imaging, including the operation at ultra-low photon flux and the potential for higher resolution [1]. Over the last year, it was demonstrated experimentally that nonlinear metasurfaces with a thickness of only a few hundred nanometers can facilitate strongly enhanced generation of photon pairs through spontaneous parametric down-conversion (SPDC). Furthermore, the quantum photon state can be tailored to feature spatial [2] and spectral [3] entanglement. However, the potential of metasurface-based photon-pair sources for quantum imaging was not previously investigated.