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Quantum Enhanced Microscope
Diving into the World of "Ghost Imaging"
Seeing is believing, as the adage goes, and imaging systems have the fantastic ability to convince and inform. Over the course of history, humans have developed a wide range of imaging systems, from x-ray microscopes to space telescopes, with the aim to understand the world through high-resolution images and measurements.
Usually, high-accuracy measurements require the production of images with a high signal-to-noise ratio, which is typically achieved by high input flux (longer exposure). However, for living cells, when imaged using x-rays, a high dose complicates the image by inducing radiation damage, leading to unwanted artifacts or loss of image resolution. This issue represents the essential compromise made in designing a biological imaging experiment: the experimenter must choose between precision of the image and damage to the sample.
Therefore, we, a team of multidisciplinary researchers at Brookhaven National Laboratory, have set our eyes on the quantum properties of light, in our case x-rays, to tackle this challenge. We are developing a new type of microscope that uses quantum correlations of a two-photon system to retrieve an image; this process is called “ghost imaging.”
Ghost Imaging with a Quantum Enhanced X-ray Microscope
Ghost Imaging has powerful implications in applications where the sample would normally require high intensity beams to be imaged, but would also be destroyed by them. With ghost imaging, a sample could be illuminated by less intense beams, with a more suitable energy, without being modified during the experiment. Our current efforts are directed at the development of a mechanism to perform ghost imaging of biological samples using x-rays.
This form of quantum imaging, with undetected photons has been successfully performed with visible light entangled photons. We are proposing to expand this line of research by creating an equivalent microscope that uses entangled x-ray beams, produced by the National Synchrotron Light Source II (NSLS-II), instead of visible light. The key to this microscope is the use of coherent x-rays from NSLS-II, the recent successful research in non-linear media for entangled x-ray creation, and the use of ultrafast pixelated detectors with excellent detection efficiency. The use of quantum enhanced x-ray microscopy methods has the, currently unexplored, potential to provide low-dose images of unprecedented resolution and low radiation damage.
Goals and Objectives
The applications of quantum imaging with visible light are numerous. More recently, some demonstrations using x-rays, using pairs of non-locally correlated photons, have started to emerge. While most of the visible light experiments use entangled photons to perform type-1 ghost imaging, the first demonstrations of ghost imaging with x-rays did not use entanglement, per se, but a split photon beam, which still has non-local correlations of quantum nature associated with the fact that two indistinguishable realizations of photon states are being obtained through the beam splitting process. This form of ghost imaging is commonly referred to as “type-2” and has been demonstrated in several x-ray experiments.
In this project, we describe a research program with the ultimate, long-term goal of establishing a novel experimental method to perform quantum x-ray scattering imaging. The potential applications of these quantum-enhanced imaging methods with x-rays are vast and can be designed to access many length scales. We will first focus on performing quantum-enhanced scattering and imaging of single, approximately micron sized objects. We plan to demonstrate the applicability of quantum phase-sensitive scattering with hard x-rays to take advantage of a favorable scaling of the detection signal versus the intensity (dose) impinging on the sample and, hence, mitigate beam damaging effects while pushing the imaging resolution to the nanometer range. We will demonstrate the transformative nature of these methods by performing imaging of the metal transport and sequestration process in nitrogen-fixing symbiotic microbes. The quantum phase-sensitive diffraction imaging experiments described here have the clear potential of pushing the limits of capabilities in imaging of mesoscale biological single objects in terms of imaging resolution and irradiation dose beyond what is possible today with other techniques.
The First Step Towards Ghost Imaging
The first goal will be to perform type-2 ghost imaging on microfabricated samples. In doing so, we will be using a relatively straightforward and low-risk experimental approach to develop the most basic capability for ghost imaging, namely, establish the data analysis methods and pipelines to calculate second order correlations exploiting non-local correlations between the two photon beams. The complexity of the experiments we undertake will gradually increase towards performing type- 1 ghost projection imaging and type-2 scattering and diffraction experiments on the same test objects.
To achieve photon counting x-ray sensitivity, we will employ a novel imaging device based on the Timepix detector. This detector allows timestamping of incident photons with one nanosecond (ns) time resolution and will be used in combination with planar and 3D sensors. The processing electronics in each pixel records the time of arrival of hits and stores it as a timecode in memory inside the pixel. Brookhaven Lab has expertise in the use of the Timepix electronics and have already employed a fast camera based on this readout chip for a variety of quantum imaging applications in the visible spectrum.
Ghost Imaging and Biology
To provide the biological context for our developments, the collaboration with Brookhaven’s Biology Department is essential. Our effort will be focused on developing a better understanding of the relationship between plant roots and environmental microbes. To begin with, we will work to enable imaging of the metal transport and sequestration process in nitrogen-fixing symbiotic microbes. The metals of interest will include cadmium (Cd), mercury (Hg), and zinc (Zn). As a first target, we would examine metal ion distribution in the rhizobial microbe Sinorhizobium medicae, which has a symbiotic interaction with the model plant Medicago truncatula. This model system has the advantage of multiple genomics capabilities in both host-plant and symbiotic microbes due to the use of high-quality reference genomes and annotations, transcriptomics, genome-wide single-nucleotide polymorphism (SNP) data, and functional genetics tools. We can, therefore, incorporate studies targeting candidate genes and molecular pathways that co-co-principal investigators (co-PIs) have identified to be involved in the transport and compartmentalization of both essential and toxic metal ions in the plant and microbe and use imaging to establish the role of symbiotic microbes in the cellular localization of these ions.