A team of researchers at the Fraunhofer Institute for Optoelectronics, Systems Technology and Image Development and Karlsruhe Institute of Technology are using single-photon avalanche diode (SPAD) arrays to achieve three-dimensional (3D) quantum ghost imaging.
The new method, called "asynchronous detection," produces the lowest photon dose of any measurement and can be used to image light-sensitive tissues or drugs that are toxic when exposed to light without causing damage.
"Our institute specializes in remote sensing, and when the Fraunhofer Society launched its quantum Sensing lighthouse project (called 'QUILT') in 2018, we wanted to explore whether remote sensing could be achieved through quantum ghost imaging," says Carsten Pitsch, a researcher at the Fraunhofer and Karlsruhe Institute of Technology.
His colleague Dominik Walter came up with the idea of using the time-stamp function of the SPAD camera to perform imaging, rather than relying on complex optical Settings.
"To do imaging at longer distances, we had to come up with an alternative to the traditional quantum ghost imaging setup, and an image reconstruction algorithm using only time stamps as input was a challenge, but it was the best answer to all the problems at the time," Walter said. "With a parallel project, I have the right tools at hand to quickly prove the concept of the algorithm and to disprove any doubts that our new approach might not work."
Quantum ghost imaging meets SPAD
Quantum ghost imaging is an eerie method of creating images by entangling pairs of photons in which only one photon actually interacts with an object. The researchers relied on photon detection times to identify the entangled pairs in the first place, which allowed them to reconstruct the image from the properties of the entangled photons. As an added bonus, this method allows imaging at extremely low light levels.
The team noted that the previous quantum ghost imaging device could not handle 3D imaging because it used an enhanced charge-coupled device (ICCD) camera. The ICCD camera provides spatial resolution but has a time gating function that does not allow independent time detection of single photons.
To solve this problem, the researchers built a device based on a SPAD array that borrows techniques from the fields of light detection and ranging (LiDAR) and medical imaging. These detectors have multiple independent pixels and dedicated timing circuits that record the detection time of each pixel at picosecond resolution.
Their device relies on spontaneous parametric downconversion (SPDC) as a source of correlated photon pairs and has a special periodically polarized crystal. Potassium titanium oxyphosphate (KTP) crystals are nonlinear optical crystals that are highly transparent for wavelengths between 350 and 2770 nm, producing entangled photons.
It enables efficient quasi-phase matching for virtually any triplet or pump signal idle signal, providing a wide range of wavelength combinations for entangled photon pairs, "says Pitsch. "This allows us to adjust our Settings to suit other wavelengths or applications."
For example, it makes it possible to pump a photon from blue to produce a green photon and an infrared photon. "The wavelength/color combination is given by the energy conservation constraint," adds Peach.
The researchers illuminated the scene with infrared photons and detected backscattered photons with a single-pixel single-photon detector (also SPAD). At the same time, green photons are recorded by the SPAD array, which acts as a single-photon camera. By harnessing the properties of entanglement, they can reconstruct the lighting scene from the green photons detected by the camera.
How exactly does this part work? Two entangled photons (the signal photon and the idle photon) can be used to obtain 3D images through single-photon illumination. Idle photons are directed to the object, whose backscattered photons are detected, recording their arrival times. The signal photons are sent to a dedicated camera that detects as many photons as possible in time and space. To reconstruct entanglement, the detection time of each pixel is compared to the detection time of a single pixel detector. This makes it possible to determine the time of flight of the interacting idle photons, so that the depth of the object can be calculated.
"This method is called quantum ghost imaging, and it allows imaging over a wide spectral range without the need for a camera in the spectral range we want to image, but we still need a simple barrel detector to record the arrival of idle photons," Peach said. "For imaging, we can often customize the system so that the signal photons are best suited for silica detection - the most mature cameras and single-photon detection materials."
The in-pixel timing circuit enables SPAD not only to perform conventional intensity imaging, but also to add time stamps to single photons. "This is a big advantage for every system that relies on photon time of flight, such as lidar," says Peach. "But it's also very good for many quantum applications, because they tend to rely on identifying photon pairs by time of flight." We use it to temporarily record infrared and green light photons, and then identify the pair of photons after measurement to get an image of the infrared scene."
The timestamp of the entangled partner photons "gives us a timely secure quantum key that helps us determine whether the detection event is part of the 3D image or just noise," Walter said. "This greatly improves the signal-to-noise ratio. But keeping the "clocks" of the two SPAD detectors running at the same rate (allowing the detection results to be referenced without any synchronization signals) is a huge challenge. Every time we lose sync due to some unknown error, we have to automatically correct it, which is not easy."
Keeping every frame of the array in sync was another challenge for the team. "We do this by analyzing the camera's time behavior and correcting/estimating the timestamps lost by individual frames," said Peach.
Peach added that it was surprising to find out how well the light source needed to actually perform the imaging needed to be tuned, as this is their first quantum imaging device.
The team demonstrated their asynchronous detection method using two different Settings. Their first device was similar to a Michelson interferometer and acquired images using two spatially separated arms, which allowed them to analyze SPAD performance and improve coincidence detection. Their second setup uses free-space optics, and instead of imaging with two separate arms, they image two objects within the same arm.
Both Settings work well as proof-of-concept demonstrations, Peach said, noting that asynchronous detection can be used for remote detection and could be useful for atmospheric measurements.
The researchers are currently working on improving the SPAD camera, focusing on pixel count and duty cycle. "For the current project, we're making a custom upgrade to our setup to make it easier to 'maintain' synchronization between detectors," Pitsch said. "We are exploring the application of spectral entanglement properties in mid-infrared spectroscopy and hyperspectral imaging - an area of high interest in biology and medicine."
Their approach could also have security and military applications, as asynchronous detection has the potential to make observations without being detected while reducing the effects of excessive lighting, turbulence, or scattering.
"It has some advantages that stem from the use of single-pixel detectors and classical ghost imaging, while some further advantages come from the use of quantum light," Pitsch added. "For example, the system is very resistant to interference due to SPDC's random continuous-wave lighting."
Source: Laser Network
