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Fast quantum ghost microscopy in the mid-infrared

Periodic Reporting for period 1 - FastGhost (Fast quantum ghost microscopy in the mid-infrared)

Reporting period: 2020-10-01 to 2021-09-30

Imaging in the mid-IR will allow chemical-selective monitoring of biomolecules such as proteins and lipids. They can be identified by their characteristic absorption spectra in that specific spectral fingerprint range. Hence, biomedical imaging can highly benefit from a development in that field. However, there is a substantial lack of efficient high-performance mid-IR cameras hindering such exploitation.
This is exactly where FastGhost will bring new perspectives. Based on the scheme of quantum ghost imaging, a new quantum imaging approach for the mid-IR will be developed, in particular the essential components for implementation. Quantum ghost imaging relies on photon pair sources which deliver correlated photon pairs. These can have different spectral properties. In FastGhost, an object will be illuminated by mid-IR photons, which are afterwards detected not by a camera but by a single-pixel detector. In parallel, the visible partner photon (from the mid-IR-visible photon pair) will be detected by a highly sensitive spatially resolving camera. The camera is correlated with the single pixel detector and thus, an image of the object is obtained. This is sketched in an attached figure. In order to allow efficient, compact, and reliable quantum ghost imaging in the mid-IR, further development on the essential components such as the photon pair source, the camera, and the single pixel detector are indispensable. Hence, FastGhost tackles these developments and verifies their impact by demonstrating fast quantum ghost imaging in the mid-IR.
The objectives of FastGhost are:
1. Demonstrating quantum ghost imaging in the mid-IR.
2. Development of photon pair sources with one photon in the visible and its partner in the mid-IR.
3. Development of single-photon detector in the mid-IR
4. Development of SPAD-camera for asynchronous detection.
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. Both approaches have already been implemented in reduced-scale arrays with 100×100 pixels. The chips are currently fabricated in foundry based on a 110 nm CMOS process including variations in filling factor and thus photon detection efficiency. The three different pixel architectures (the two described above and a mix of both) are shown in Figure 3.
On the photon pair source, an essential challenge is to reach the large spectral splitting of one photon in the mid-IR and its partner in the visible. Periodically-poled potassium titanyl phosphate (ppKTP) is a commonly used nonlinear crystal for spontaneous parametric down-conversion for photon pair generation. However, its transparency drops above 3.5 µm, thus, limiting its suitability for mid-IR applications. Nevertheless, in FastGhost a source based on ppKTP was implemented to make use of the mid-IR spectral range until its cut-off and rely on the already existing great expertise with this material. Additionally, a new material was tested. A silver thiogallate (AGS) crystal was characterized. It features photon pair generation with one photon going up to 12 µm in center wavelength, which enables a whole new spectral region for quantum imaging and sensing applications.
Combining first iterations on single-pixel detector, SPAD-array, and photon pair source, the first quantum ghost images have been taken.
FastGhost will realize quantum ghost imaging in the mid-IR, which enables a variety of applications, especially in the biomedical imaging sector. Along these lines, the development of the essential components themselves will foster a broader application of photonic quantum technology far beyond quantum ghost imaging.
In detail, the realization of high-performance mid-infrared single-photon detectors in terms of efficiency and timing resolution will enable applications such as environmental sensing or long wavelength (quantum) communication. So far, such systems have been limited to efficiently work below 1700 nm. In FastGhost, the superconducting film used to fabricate the single-photon detectors was optimized to extend the wavelength range of the superconducting nanowire detectors. Furthermore, the newly developed mid-IR detectors efficiently working beyond 2 µm. In fact, the recent demonstration goes beyond the-state-of-the art in terms of efficiency: with 70% system detection efficiency, the FastGhost consortium partner Single Quantum holds the current record for system detection efficiency at 2 µm. Moreover, our detectors exhibit very low jitter (< 15 ps).
Photon pair sources covering the mid-IR can be harnessed for various quantum sensing applications such as quantum imaging with undetected light or hyperspectral correlation-based imaging. Till now, such sources are mostly based on ppKTP (or lithium niobate with similar properties) and have been limited to ~3.5 µm. With the already measured spectral coverage up to 12 µm this limit is tremendously exceeded.
SPAD-arrays are used as highly sensitive camera in both domains, for classical imaging and sensing as well as for quantum imaging. With their timing and thus correlation capability, they are a crucial tool for all correlation-based imaging approaches. Hence, their advancement will beneficially impact the imaging market, especially in the quantum regime and in the low-light biophotonics regime. The current results from FastGhost are a 17 µm pitch embedding correlation electronics, which is well beyond the state of the art.
Simulation of the ghost-imaging experiment with 100k acquisitions and 5ns correlation window. From l
Scheme of quantum ghost imaging in mid-IR in FastGhost
Layout of the three architectures: (a) All-in pixel, (b) Shared correlation network between four pix
Temporal waveform of the "look back" method. The SPAD trigger is delayed to compensate for the propa