Next generation single-photon detectors are becoming available. Research grade SPAD arrays promise camera-like detectors with dozens to hundreds of thousands of pixels, providing both spatial and temporal information about single photon events. Concurrently, other detector technologies are under development, such as crossed-delay line detectors. We evaluate the application of SPAD arrays and NCam, a crossed-delay line detector, towards quantum ghost imaging applications. We discuss implementation challenges and performance differences in the context of ghost imaging.
The near infrared (NIR) and mid-infrared (MIR) spectral regions contain absorption features that can identify specific molecular bonds and chemical species in a sample. For example, lignan and proteins in plants have specific absorption signatures in the NIR. However, because detectors are inefficient in the NIR and MIR, infrared spectroscopy requires high light levels to overcome detector limitations. Cameras in particular do not perform well in this spectral range, and microscopy methods such as Fourier transform infrared spectroscopy (FT-IR) typically rely on scanning confocal arrangements with single-element detectors to spatially map chemical information. To overcome these limitations, we have developed and exploited a new quantum ghost imaging microscope for obtaining absorption measurements in the NIR without the need of scanning or high light intensities. We report on the use of a novel detector–NCam–in quantum ghost imaging using non-degenerate photon pairs generated by spontaneous parametric down conversion (SPDC). NCam records single-photon arrival events with ∼100 ps resolution, enhancing the correlation window of SPDC pairs over previous wide-field ghost imaging by 30-fold. This permits ghost imaging of living and intact plant samples at light levels lower than what the plants would experience from starlight. For photosynthesizing organisms, this low-light imaging method enables the study of plants without disturbing or eliciting responses from the plant due to the measurement itself.
NIR imaging of biological samples can reveal details about the chemical makeup of the sample. To overcome the difficulties of current imaging devices, which are most sensitive in the visible region of the spectrum, we use ghost imaging with quantum entangled photons. A non-degenerate photon pair is used to probe a sample, revealing structures with fewer photons per second than starlight.
Modern instruments for imaging biological samples often use high-power lasers or fluorescent dyes that can disturb sensitive processes within living organisms such as plants. Additionally, many interesting plant processes have absorption bands within the near-infrared (NIR), a spectral region hard to efficiently and cost-effectively detect using current camera technology. We present a quantum ghost imaging (QGI) protocol using a proprietary time and space-resolving photon-counting visible camera, NCam, and a highly nondegenerate source of entangled photon pairs. The combination of these two technologies allows for low-noise, high-resolution non-destructive imaging in the NIR, while using a camera sensitive for visible wavelengths.
We discuss two novel entanglement sources utilizing spontaneous parametric downconversion in periodically poled waveguides. Using quasi-phase matched KTP crystals, we have demonstrated a post-selection-based polarization entangled degenerate source at 810 nm, as well as a post-selection-free non-degenerate collinear source producing entangled photons at 810 nm and 1550 nm. The sources exhibit high brightness and state quality – with the non-degenerate source achieving fidelities and purities up to 99% – with clear paths for further improvement. Furthermore, they are compact, stable, and need little alignment when set, critical for practical quantum communication and network applications. Lastly, their small size, weight, and power (SWaP) makes them an attractive option for mobile platforms, e.g., with drones or satellites.
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