Researchers use SPAD detector for 3D quantum ghost imaging

Researchers have reportedly acquired 3D measurements with quantum ghost imaging. The new technique enables 3D imaging on a single photon level, yielding the lowest photon dose possible for any measurement. Researcher Carsten Pitsch from the Karlsruhe Institute of Technology said 3D imaging with single photons could be used for various biomedical applications, such as diagnostics. “It can be applied to image materials and tissues that are sensitive to light or drugs that become toxic when exposed to light without any risk of damage,” Pitsch said.

In the journal Applied Optics, the researchers described their approach, which incorporates new single photon avalanche diode (SPAD) array detectors. They applied the new imaging scheme, which they call asynchronous detection, to perform 3D imaging with quantum ghost imaging. “We also want to investigate its use in hyperspectral imaging, which could allow multiple spectral regions to be recorded simultaneously while using a very low photon dose. This could be very useful for biological analysis,” Pitsch said.

Quantum ghost imaging creates images using entangled photon-pairs in which only one member of the photon pair interacts with the object. The detection time for each photon is then used to identify entangled pairs, which allows an image to be reconstructed. This approach allows imaging at low light levels and also ensures that the objects being imaged do not have to interact with the photons used for imaging. Previous set-ups for quantum ghost imaging were not capable of 3D imaging because they relied on intensified charge-coupled device (ICCD) cameras; although these cameras have good spatial resolution, they are time-gated and don’t allow the independent temporal detection of single photons.

The researchers developed a set-up based on new SPAD arrays developed for LiDAR and medical imaging. These detectors have multiple independent pixels with dedicated timing circuitry, which allows them to record the detection time of every pixel with picosecond resolution. The new approach uses two entangled photons — a signal and an idler — to obtain 3D images with single photon illumination. This involves directing the idler photons onto the object and then detecting the backscattered photons in time. Meanwhile, the signal photons are directed to a dedicated camera that detects as many photons as possible in both time and space. The researchers then compared the time of detection of every pixel with the detection of the single-pixel detector to reconstruct the entanglement. This allowed them to determine the time of flight of the interacting idler photons, and with that, the depth of the object.

The researchers demonstrated the 3D capabilities of the asynchronous detection scheme using two different set-ups. One, which resembled a Michelson interferometer, acquired images using two spatially separated arms. This set-up allowed the researchers to analyse the SPAD performance and improve the coincidence detection. The other set-up used free-space optics and was more application centred. Instead of imaging with two separated arms, two objects in the same arm were imaged.

Both set-ups worked well as a proof-of-concept demonstration for the new technique. The experiments also showed that asynchronous detection could be used for remote detection, which could be useful for atmospheric measurements.

Image caption: Researchers developed a new approach that allows 3D measurements to be acquired with a computational imaging approach known as quantum ghost imaging. A SPAD array detector, shown on the left of the optical set-up, is key to the new technique. Image credit: Fraunhofer IOSB/indigo.