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arXiv:2505.13721 · Optics Express

Non-degenerate SPDC photon-pair source for UV-A illumination

Preetisha Goswami, Marta Gilaberte Basset, Jorge Fuenzalida, Markus Gräfe, Valerio Flavio Gili

Published Oct 13, 2025• 2 citations
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Abstract

We present a frequency-correlated non-degenerate photon-pair source consisting of a second-order nonlinear crystal that generates ultraviolet UV-A and infrared light via spontaneous parametric down-conversion (SPDC). Quantum imaging and sensing techniques like quantum imaging with undetected light and quantum ghost imaging leverage wavelength correlations between down-converted photon pairs to decouple sensing and detection wavelengths, thereby exploiting established camera technology within the visible spectrum. Our results open up quantum sensing application scenarios in the ultraviolet domain, with potential implications for advancements in biomedical and non-destructive testing fields.

DOI
10.1364/oe.568162

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We present a frequency-correlated non-degenerate photon-pair source consisting of a second-order nonlinear crystal that generates ultraviolet UV-A and infrared light via spontaneous parametric down-conversion (SPDC). Quantum imaging and sensing techniques like quantum imaging with undetected light and quantum ghost imaging leverage wavelength correlations between down-converted photon pairs to decouple sensing and detection wavelengths, thereby exploiting established camera technology within the visible spectrum. Our results open up quantum sensing application scenarios in the ultraviolet domain, with potential implications for advancements in biomedical and non-destructive testing fields.
Metrology with undetected photons is an emerging technique that leverages quantum effects and photon correlations (entanglement) to retrieve valuable information in a target spectral range (e.g., mid-infrared, mid-IR) using measurements in an easily accessible domain (e.g., visible, near-IR). The underlying quantum process of spontaneous parametric down-conversion (SPDC) is utilized to generate non-degenerate correlated signal and idler photons to serve as detection and probing photons, respectively. Sensing with undetected photons enables important advantages, such as ultra-low probe powers, room-temperature operation, and shot-noise-limited detection. In this contribution, we apply a quantum nonlinear interferometer based on an SPDC source to perform applied mid-IR spectroscopy, mid-IR microscopy, and mid-IR optical coherence tomography (OCT) as among the most promising techniques for quantum-based routine non-destructive testing. Moreover, we characterize the system, benchmark it against classical systems, and provide a prospective outlook for this new technology.
Quantum states of light are opening new paradigm for imaging, enabling imaging with photons that didn’t interact with the object. Quantum imaging methods discussed here explore quantum correlated photon pairs, generated via spontaneous parametric down conversion (SPDC) in the nonlinear crystal (NLC). We demonstrate generation of highly non-degenerate photon pairs, via SPDC in AGS crystal, with the photon interacting with the object in the mid-infrared (MIR) and the non-interacting in the near-infrared spectral region. Results revealed that the wavelength of the interacting photon can be tuned in a wide spectral range, between 5 μm and 11 μm, covering most of "fingerprint" spectral region. This makes quantum imaging with AGS an alternative to more costly FTIR spectrometers.
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.
Efficient entangled photon pair sources are the main component for several applications based on quantum imaging. Specifically for ghost imaging, different wavelengths of signal (imaging photons) and idler (interaction with the object) photons are desired. An efficient and narrowband generation of entangled photons exploiting spontaneous parametric down-conversion using periodically poled (pp) nonlinear crystals is therefore a fundamental preliminary requirement to achieve (the process of) ghost imaging. This work presents the design and implementation of a precise and efficient crystalheater as a variable photon pair source and compares the achieved experimental values of the SPDC-wavelengths with theoretical calculations. A periodically poled nonlinear crystal from potassium titanyl phosphate (ppKTP) can generate various non-degenerate wavelengths from a pump radiation of 405 nm by temperature changes and satisfaction of energy conservation and quasi-phase-matching conditions. For this purpose, the crystal is securely housed in a custom-built mechanical mount. A computation and adjustment of various control parameters, as well as a precise determination of the current temperature via two temperature sensors allow the heater to set the target temperature with an accuracy of 0.1 °C±0.015 °C. A method for the theoretical determination of the temperature-dependent shift of the nondegenerate wavelengths, provides a foundation from which experimental verification of achievable wavelengths and intensities can be compared. By experimental verification, the efficiency and functionality of the photon pair source and SPDC-process is verified. These presented investigations and the design of the crystal-heater provide the basis for a precise and effective photon pair source, for subsequent studies in the field of ghost imaging.
Time-correlated photons can be used to demonstrate emerging technical applications such as ghost imaging, and they can demonstrate quantum-based phenomena in order to interest undergraduate students in engineering applications involving optics and quantum mechanics. In this work, we use spontaneous parametric down-conversion to generate time-correlated photons. To achieve this, we simulated laboratory parameters with MATLAB and designed optical and electrical setups. Specifically, we used a 405 nm diode laser, various common optical components, and a nonlinear crystal designed to generate time-correlated photons, and we used single-photon detectors, time-to-amplitude converters, custom flash-conversion analog-to-digital converters, a Teensy 4.1 microcontroller, and the Arduino IDE to detect the time-correlated photons. We present two significant results. First, we measured >100,000 counts per second 5 ± 1° on either side of our input laser which was within our theoretical expectations of 4–9° and much greater than our < 2000 dark counts per second. Our second major result was finding 15.8 ± 0.7 time-correlated counts per second with an SNR of 2.7. We also outline future extensions of our work including ghost imaging and generating polarization-entangled photons.