FastGhost tackles the challenges for implementing fast quantum ghost imaging in the mid-IR. In the first 12 months of the project, great progress was achieved for all components as well as for the complete imaging scheme.
In detail, for realizing a single-photon single-pixel detector in the mid-IR, essential first steps have been implemented. The detection system is based on superconducting nanowires. There, a superconducting thin film is the linchpin of the system and is normally not sensitive for the targeted spectral range. In FastGhost, such superconducting films could be developed exhibiting a high performance in the mid-IR range for the first time. Based on these films, a first detection system could be implemented. It demonstrated mid-IR detection with 70% system detection efficiency and < 15 ps timing. Both are superior values and are signs of the extreme boost FastGhost brings into the field of single-photon detection in the low-energy regime.
The same steep progress holds for the SPAD-array development. Starting with the implementation of a theoretical model to validate the “look back” correlation method, schematized in Figure 1. The SPAD trigger is delayed by almost the same propagation delay of the trigger bucket detector. The correlation network generates a correlation window starting from the SPAD trigger with a duration of few ns and the correlation is detected if the correlation window and the bucket detector are overlapped. Figure 2 shows the results of the theoretical model with 100k acquisitions including also the SPAD dark count noise. For experimental realization two approaches have been considered: First, in-pixel correlation architecture having the correlation logic at each pixel. And second, event-driven architecture, where the correlation happens in the periphery. Three different pixel architectures (the two described above and a mix of both) are shown in Figure 3.
The development of dedicated SPAD arrays, combined with the “look-back principle” allowed to perform fast widefield quantum ghost imaging at 1.4 µm, see Figure 4. A larger SPAD imager with 472x456 was developed, and quantum correlations were observed on it. In parallel, a scanning approach was also pursued, to make use of readily usable SNSPDs working up to 2 µm. This led to the successful implementation of quantum ghost imaging of relevant bio-medically relevant samples such as neck tissue and HEK 293T cell samples (Figure 5), and thanks to the wavelength tunability of the employed photon-pair source, a lithium niobate waveguide, we could demonstrate hyperspectral scanning ghost imaging (Figure 6).
For ghost imaging implementations deeper in the MIR, a SNSPD at 4 µm with an efficiency between 5% and 15%, corresponding to dark count rates (DCR) of respectively 2.33 MHz and 5.20 MHz. This detector was integrated in a ghost imaging setup at IOF, however doe to technical issues in terms of:
• Single-mode fiber-coupling of a highly diverging Mid-IR beam.
• Noise contribution from thermal radiation.
• Losses through the Mid-IR optical path.
• The obtainable signal is lower than the fluctuations in the detector.
For the same reasons, a SNSPD detector up to 7 µm was not developed.
