Today, we are witnessing the second quantum revolution, where quantum states, which can exhibit superposition and entanglement, are harnessed for quantum technological applications. Quantum Sensing exploits the high sensitivity of quantum systems to external disturbances to develop highly sensitive sensors.
Breaking the limitations of today’s established imaging schemes is a goal many researchers try to realize all around the globe. This addresses resolution, SNR, contrast, and spectral range. Exploiting the quantum properties of light is one way to overcome some of those limitations.
Quantum imaging is a new sub-field of quantum optics that exploits quantum correlations such as quantum entanglement of the electromagnetic field in order to image objects with a resolution or other imaging criteria that is beyond what is possible in classical optics.
Quantum science opens up the possibility of detecting details in images beyond the standard wavelength limit, with low light levels, or in the presence of strong background illumination. Taking the quantum nature of light into account, it becomes possible to envision new imaging schemes, which may be realized in actual quantum-enhanced microscopes or imaging devices.
Quantum imaging technology could be used to provide high-quality X-ray images, helping emergency services get a more accurate live image before embarking on rescue attempts. It could also be used to see through snowstorms, around corners, and map hidden underground hazards.
Quantum-inspired imaging techniques combined with computational approaches and artificial intelligence are changing our perspective of what constitutes an image and what can or cannot be imaged. Steady progress is being made in the direction of building cameras that can see through fog or directly inside the human body with groundbreaking potential for self-driving cars and medical diagnosis.
Light is typically detected at relatively high intensities, in the spectral range and with frame rates comfortable to the human eye. However, emerging technologies are now relying on sensors that can detect just one single photon, the smallest quantum out of which light is made. These detectors provide a “click,” just like a Geiger detector that clicks in the presence of radioactivity.
We have now learned to use these “click” detectors to make cameras that have enhanced properties and applications. For example, videos can be created at a trillion frames per second, making a billion-fold jump in speed with respect to standard high-speed cameras. These frame rates are sufficient, for example, to freeze light in motion in the same way that previous photography techniques were able to freeze the motion of a bullet—although light travels a billion times faster than a supersonic bullet, write UK researchers led by Yoann Altmann.
By fusing this high temporal resolution together with single-photon sensitivity and advanced computational analysis techniques, a new generation of imaging devices is emerging, together with an unprecedented technological leap forward and new imaging applications that were previously difficult to imagine.
Examples of quantum imaging are quantum ghost imaging, quantum lithography, sub-shot-noise imaging, and quantum sensing. Furthermore, by utilizing certain quantum states of light and their photon number statistics, sensing and imaging beyond classical limitations, like the shot noise level, become possible
Other cameras are being developed that can form 3D images from information with less than one photon per pixel. Single-photon cameras have already made their way into widely sold smartphones where they are currently used for more mundane purposes such as focusing the camera lens or detecting whether the phone is being held close to one’s ear. This technology is already out of the research laboratories and is on the way to delivering fascinating imaging systems, write UK researchers led by Yoann Altmann.
For example, full three-dimensional (3D) images can be taken of a scene that is hidden behind a wall, the location of a person or car can be precisely tracked from behind a corner, or images can be obtained from a few photons transmitted directly through an opaque material. Inspired by quantum techniques, it is also possible to create cameras that have just one pixel or that combine information from multiple sensors, providing images with 3D and spectral information that was not otherwise possible to obtain.
Correlations between quantum light beams also enable new modes of imaging such as ‘ghost imaging’, in which an image of an object illuminated by one beam is acquired by a camera looking at a different beam, that did not impinge upon the object.
The ghost imaging satellite would have two cameras, one aiming at the targeted area of interest with a bucket-like, single pixel sensor while the other camera measured variations in a wider field of light across the environment. The target could be illuminated by almost any light source such as the sun, moon or even a fluorescent light bulb. Alternatively, a pair of physically “entangled” or “correlated” laser beams could be generated from the satellite to light up the object and its surroundings.
By analysing and merging the signals received by the two cameras with a set of sophisticated algorithms in quantum physics, scientists could conjure up the imaging of an object with extremely high definition previously thought impossible using conventional methods. Gong said darkness, cloud, haze and other negative elements impairing visibility would no longer matter. “A ghost imaging satellite will reveal more details than the most advanced radar satellite,” the research director said.
To date, the most widely used approach to generate photon pairs is based on spontaneous parametric downconversion (SPDC) in second-order nonlinear crystals such as bismuth borate (BBO), lithium borate (LBO), lithium niobate (LN), and potassium titanyl phosphate (KTP). Here, the two photons of a correlated photon pair generated by SPDC are split into two separate beams. One of these beams, for example, the signal, is used to illuminate the object, the transmitted signal photons are subsequently detected using a so-called bucket detector. The idler beam, on the other hand, is characterized by a spatially resolving single-photon detector, which can be either implemented using an array detector or a single scanning detector.
Because quantum imaging can collect data from a wide spectrum of light, the images they produce would look “more natural” to human eyes than the black-and-white radar images based on the echo of high-frequency electromagnetic waves of narrow bandwidths, he said.
The ghost camera could also identify the physical nature or even chemical composition of a target, according to Gong. This meant the military would be able to distinguish decoys such as fake fighter jets on display in an airfield or missile launchers hidden under a camouflage canopy.
Recently, it was shown that the principles of ‘Compressed-Sensing’ can be directly utilized to reduce the number of measurements required for image reconstruction in ghost imaging. This technique allows an N pixel image to be produced with far less than N measurements and may have applications in LIDAR and microscopy.
Ghost imaging is being considered for application in remote-sensing systems as a possible competitor with imaging laser radars (LADAR). A theoretical performance comparison between a pulsed, computational ghost imager and a pulsed, floodlight-illumination imaging laser radar identified scenarios in which a reflective ghost-imaging system has advantages.
China developing Ghost imaging Satellite
China is developing a new type of spy satellite using ghost imaging technology that can achieve unprecedented sensitivity by detecting not just the extremely small amount of light straying off a dim target, but also its interactions with other light in the surrounding environment to obtain more information than traditional methods. A satellite equipped with the new quantum sensor would be able to identify and track targets that are currently invisible from space, such as stealth bombers taking off at night, according to researchers.
Tang Lingli, a researcher with the Academy of Opto-Electronics, Chinese Academy of Sciences in Beijing, said numerous new devices had been built, tested in the field and were ready for deployment on ground-based radar stations, planes and airships. “Satellite is the next step,” she said. Tang said ghost imaging could be achieved using different methods in either quantum or classical physics, and it would work best with other intelligence gathering methods including optical cameras and synthesised aperture radars.
“Each detection method has its unique advantages. It depends on the circumstances and nature of the mission as to which one should be used, if not all [of them],” said Tang, who is also the general secretary of the National Committee on Remote Sensing Technology Standardisation and a supervisor of the national ghost imaging project.
Gong Wenlin, research director at the Key Laboratory for Quantum Optics, Chinese Academy of Sciences in Shanghai – whose team is building the prototype ghost imaging device for satellite missions – said their technology was designed to catch “invisibles” like the B-2s ( US stealth bomber). He said his lab, led by prominent quantum optics physicist Han Shensheng, would complete a prototype by 2020 with an aim to test the technology in space before 2025. By 2030 he said there would be some large-scale applications.
While ghost imaging has already been tested on ground-based systems, Gong’s lab is in a race with overseas competitors, including the US Army Research Laboratory, to launch the world’s first ghost imaging satellite. The team showed the engineering feasibility of the technology with a ground experiment in 2011. Three years later the US army lab announced similar results. “We have beat them on the ground. We have confidence to beat them again in space,” Gong said.
Quantum Imaging technique can have military applications
Researchers at Vienna Center for Quantum Science and Technology, Austria, have created a quantum imaging technique, by creating the image of the object that was never illuminated with light, through which it was detected. The quantum entanglement was the phenomenon that made it possible, a link that has been shown to exist between particles even if they are separated by vast distances.
The experiment published in Nature , Scientists created a pair of “entangled” pairs of photons, of different wavelengths, by illuminating two separate crystals by a laser, consisting of one infrared photon and a “sister” red photon and then split apart. The process called spontaneous parametric down-conversion (SPDC). The object (e.g. the contour of cat) is placed in between the two crystals. The arrangement is such that if a photon pair is created in the first crystal, only the infrared photon passes through the imaged object. Its path then goes through the second crystal where it fully combines with any infrared photons that would be created there.
However, due to the quantum correlations of the entangled pairs the information about the object is now contained in the red photons – although they never touched the object. Stunningly, all of the infrared photons (the only light that illuminated the object) are discarded; the picture is obtained by only detecting the red photons that never interacted with the object
The researchers are confident that their new imaging concept is very versatile and could find applications in biological or medical imaging, in low light imaging or hard-to-see situations. It can also be useful in military applications to image an object at different wavelengths, where it is more easily detected or can image target better.
Quantum imaging products on display at Laser World of Photonics
The UK Quantum Technology Hub in Quantum Enhanced Imaging (Quantic) demonstrated a number of quantum enhanced imaging project in trade fair held in June 2017, including a methane gas camera being built in collaboration with M Squared Lasers, and a low-cost 3D imager developed with aerospace and defence company Leonardo.
The prototype methane sensing imager is a single-pixel infrared camera with an estimated cost of less than £500 once produced in volume. This is far cheaper than conventional infrared cameras with InGaAs pixel arrays. Single-pixel cameras differ from traditional cameras in that the image sensor is replaced with a pixelated transmission mask encoding a series of binary patterns. The light is measured with a single photon detector and, combined with knowledge of the patterns, the image can be deduced through data inversion.
The technique has the potential to fill a niche for low-cost, non-visible imaging; it uses the same technology as that found in data projectors. The M Squared Lasers device operates at a resolution of 32 x 32 pixels at 20 frames per second, and can produce real-time video of methane gas, sensing at 1.65µm, at a distance of 1 metre. Oil and gas, construction, food processing, and water treatment would all benefit from a low-cost, low-power and portable gas detection system; the global gas sensing market is projected to be worth $2.32 billion by 2018.
Quantic and Leonardo’s low-cost 3D ranging camera operates on the same single-pixel camera approach. The camera has a range of 1-10 metres with a depth resolution of millimetre precision at between 1 and 10 frames per second.