Quantum ghost imaging was less than two decades old when the U.K. government launched the £1 billion Quantum Technologies Program in 2016. The program established four hubs dedicated to various fields in quantum research and development. One of these hubs, QuantIC, is focused on furthering advancements in quantum-enhanced imaging with 120 full-time researchers spread across eight partner institutions. QuantIC’s director, Miles Pladgett, recalled how when the hub was launched, “most industries and companies in the U.K. would have regarded quantum as something to do with cats that may be dead or alive, or some of the spooky science fiction-y thing.” Technische Universität Darmstadt and Fraunhofer IOF developed QUANCER, a simulation tool for tumor diagnostics that leverages two correlated laser beams created via spontaneous parametric down conversion and nonlinear interferometers. One beam illuminates a sample to generate a signal for detection while the other beam is sent to a camera. Using the coincidence detection events from both detectors reveals an image of the tumor. Courtesy of TU Darmstadt. The “spooky science” he alluded to includes ghost imaging, which is a correlation-based quantum method that can image an object through the detection of light that never interacted with the object. As Pladgett explains, the general attitude among many businesses in 2016 was that quantum imaging was “not something from which they were going to make money.” Nine years later, that attitude has changed. Several QuantIC partner institutions have successfully translated quantum imaging innovations into real-world applications. This includes a camera, developed by the University of Bristol and marketed by the spinout QLM Technology, that leverages tunable diode lidar for methane leak detection. Another QuantIC system, designed at the University of Edinburgh, employed single-photon avalanche diodes (SPADs) to produce a time-resolved image sensor that has since been incorporated into Horiba’s new wide-field fluorescence lifetime imaging camera. A third product that emerged from QuantIC is a correlated photon-pair source tool that was pioneered by the University of Bristol spinoff Raycal and is now manufactured by Thorlabs. QLM Technology’s tunable diode lidar gas camera uses correlation-based quantum imaging for methane gas leak detection. The system produces a clear and intuitive visualization of the methane concentration in the field of view. The camera was recently field tested at the Bacton Gas Terminal in North Norfolk, U.K. Courtesy of QLM Technology. “Now, here we are nine years later, and U.K. industry is investing bigtime — tens of millions, possibly even hundreds of millions in the computing area, but in other areas too,” Pladgett said. Gas detection and especially biomedical diagnostics are the fields with the most potential for quantum imaging with undetected light, said Markus Gräfe, a Technische Universität (TU) Darmstadt professor at the Institute of Applied Physics. “There are already ongoing projects in that direction. It needs some coworking of quantum physicists and photonic engineers to bring this approach out of the optics lab and into the biolab in the form of microscopes and spectrometers.” Quantum imaging and its applications Quantum imaging involves techniques that generate very sensitive images through the use of closely controlled photons and other particles of light. According to Klea Dhimitri, a Hamamatsu applications engineer, there are three general categories into which quantum imaging techniques fall: correlation-based, entanglement-based, and interference-based imaging. Researchers at the University of Glasgow have developed a prototype time-of-flight 3D imaging endoscope that clocks photons going out and back to build an image with a single fiber that is the width of a human hair. The endoscope can produce frames at 5 Hz with a depth resolution of ~5 mm that each contain up to ~4000 independently resolvable features. Courtesy of University of Glasgow. A wide-field fluorescence lifetime imaging camera features a multiplexed, time-correlated, single-photon timing fluorescence system designed by the University of Edinburgh. Each pixel is in an array that contains a single photon avalanche diode with time-to-digital converter-based timing electronics that enable fast fluorescence lifetime determination based on the time-correlated single photon counting. Courtesy of Horiba. A quantitative CMOS camera can resolve the number of photoelectrons per pixel from incoming photons due to low read noise. Courtesy of Hamamatsu. Correlation-based quantum imaging — such as ghost imaging and quantum ghost spectroscopy — detects one photon of an entangled pair while the other entangled photon interacts with a sample. Entanglement-based quantum imaging — such as photon-pair fluorescence microscopy and quantum interference microscopy — detects both photons in an entangled pair, and both photons interact with the sample. Interference-based quantum imaging detects one photon of a photon pair that has been generated by a probabilistic source, such as spontaneous parametric down-conversion. Representative appli- cations include quantum imaging with undetected photons and quantum spectroscopy. QLM’s tunable-diode-lidar camera is an example of correlation-based quantum imaging. It was one of the first commercially available quantum imaging systems offered under the U.K.’s Quantum Technologies Program. Last November, the company completed a three-year project that included R&D, field trials at the Bacton Gas Terminal in North Norfolk, U.K., subsequent production optimization, and a commercial presentation of an industry-ready, single-photon lidar gas camera. Through the combined use of diode lasers that have a methane absorption wavelength of 1651 nm and SPAD detectors, the camera can measure the path-integrated concentrations of methane with a sensitivity as low as 150 ppm and above ambient at a range of up to 200 m. This system works by combining the lidar point cloud with path-integrated concentration measurements to produce a clear and intuitive visualization of the methane concentration in the field of view. “Combined with a simple trigonometric algorithm, this allows the camera to calculate highly accurate leak-flow rates based on the shape and concentration density of the methane plume in the prevailing wind conditions,” said QLM CEO Murray Reed. A joint venture between the U.K. companies QLM, Redwave Labs, and Bay Photonics, called Q3MD, is working to detect smaller methane concentrations over longer distances with shorter response times — all without the need for higher laser powers or the use of reflective surfaces. Such improvements could be achieved through the development of SPADs that detect light in the mid-infrared (MIR) around methane’s fundamental absorption wavelength at 3300 nm. Detection at that wavelength is ~1000× stronger than overtone absorptions in the near infrared. Using mid-infrared SPADs will also allow QLM to expand detection to other gas species and applications that are currently not measurable with detectors operating in the shortwave-infrared, Reed said. Detector challenges Ghost imaging requires the rapid acquisition of many coincident photons per pair of pixels. According to Hugo Defienne, a researcher at the Institute of Nanosciences of Paris at Sorbonne University, achieving this necessitates critical parameters for candidate detectors, such as a high quantum efficiency to catch every photon; a high temporal resolution and low noise to minimize accidental coincidences; a high frame rate to accumulate photon coincidences quickly to generate reliable statistics; and a high pixel count to detect all the spatial modes to provide favorable image resolution. “Today, there are no sensors that can meet all these criteria,” Defienne said. Though he added that sensor types such as electron-multiplying charge-coupled devices, CMOS, SPADs and others come close. Hamamatsu, for example, recently delivered its first camera to be equipped with its quantitative CMOS (qCMOS) imaging sensor that, due to a low read noise, can resolve the number of photoelectrons per pixel from incoming photons. The sensor can also quantify the number of photons in a pulse of light, ranging from a single photon to up to 200 photons. The potential improvements of correlated-based quantum imaging to differentiate between one- and two-photon events could come from lower read noise and higher detection efficiency, both of which could provide enhancements to contrast imaging, Dhimitri said. Horiba’s SPAD-based CMOS sensor is also capable of single-photon detection, which applies to the recording of time-resolved fluorescence decay. This analytical technique uses the light from a laser or LED to elicit fluorescence in a sample, such as biological tissue. A detector registers the arrival times of the individual photons from this fluorescence signal to reveal the fluorescence decay curve and the characteristic lifetime value of the sample. This is a promising technique to aid surgeons with demarcating the boundaries of tumors without the use of dyes. QuantIC researchers have also developed a prototype for a time-of-flight 3D imaging endoscope that is the width of a human hair. Typical endoscopes require tens of thousands of fibers that each image one pixel, whereas this new device clocks photons going out and back to build an image with a single fiber. The 40-cm-long optical fiber can produce frames at 5 Hz with a depth resolution of ~5 mm that each contain up to ~4000 independently resolvable features. In Germany, TU Darmstadt and the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) are working with a team of photonics manufacturers to develop a simulation tool for tumor diagnostics using SPDC and nonlinear interferometers. Their QUANCER tool uses two correlated photon beams via SPDC. One beam illuminates a sample to generate a signal that is detected by a bucket detector, while the other beam is sent to a camera. Neither of the detectors can obtain an image of the object by themselves, but using the coincidence detection events from both detectors will reveal the object picture, TU Darmstadt’s Gräfe said. TU Darmstadt is a part of the Fraunhofer’s Quantum Methods for Advanced Imaging Solutions (QUILT) project. Recent project achievements include a simulation tool for SPDC and nonlinear interferometers, a quantum ghost imaging system without an optical delay line, and the first demonstration of THz sensing with undetected light based on nonlinear interferometers, according to Gräfe. Into the mid-infrared Gräfe said QUILT researchers are also focusing on detection in the mid-infrared because it allows chemically selective imaging with the need to label tissue. Defienne further regarded the valuable ability of mid-infrared light to penetrate deeper inside tissue for biological microscopy applications. Many quantum imaging schemes use an SPDC process that produces correlated or squeezed photons from a laser beam, he said. During this process, the beam’s wavelength is down converted. For example, light from a 405-nm pump laser would be transformed into two photons of 810 nm. “The option to choose a pump laser at any frequency is very useful because it then becomes possible to produce photon pairs at any wavelength, from the visible into the MIR,” Defienne said. Gräfe said he does not see laser technology causing a bottleneck in the advancement of quantum imaging. However, having a miniaturized, cost-efficient, single-frequency source that is tunable across many wavelengths would help to advance quantum imaging much further. “The laser companies are already working on miniaturization and making more wavelengths available.” Technical challenges QUANTIC’s Pladgett said the biggest technical challenge for quantum imaging lies in the detector technology. “The camera in your mobile phone is absolutely incredible and nothing like that exists at other wavelengths,” he said. “As soon as you move away from the visible range, you are paying orders of magnitude more for something that is actually worse. It’s staggering.” Sophisticated devices, such as super-conducting nanowire single-photon detectors and transition edge sensors, are not practical for quantum-enhanced imaging devices due to their cost, size, and use of cryogenics, according to Dhimitri. For quantum imaging via coincidence detection, SPAD arrays are a top contender, although commercially available options need improvements in the number of pixels, their detection efficiency, and their ability to practically correlate all the pixels together. Another promoting technology, according to Dhimitri, is scientific CMOS cameras, which have made great strides over the past decade in terms of lowering read noise, and offer a dependable balance between performance, footprint, and price for future quantum imaging devices, she said. Quantum imaging techniques that do not utilize coincidence detection often rely on more mature detection technologies and therefore might require improvements to the underlying imaging schemes themselves. One weakness of many quantum imaging techniques, according to Omar Magana-Loaiza, an assistant professor of physics at Louisiana State University, is their vulnerability against ambient light. “Some of these quantum signals are too weak and the detectors can be easily overwhelmed by environmental light,” he said. “However, in theory, it is possible to isolate different kinds of light. Consequently, it would be interesting to see applications such as quantum radars or quantum schemes for remote sensing operating in realistic environments of noise and brightness.”