The basic principles behind ghost imaging is well established and researchers around the world have been looking into the technique for more than two decades now. At its most basic, the concept is in many ways the reverse of a traditional camera, with a pair of sensors coupled with a computer algorithm working together to essentially look for what isn’t there rather than what is. The complete arrangement looks for subtle differences in the interaction between two distinct sets of photons, particles of light, that otherwise operate identically regardless of how far they are apart, a phenomenon called quantum entanglement .

“We have beat them [the United States] on the ground,” Gong boasted, noting that the U.S. military was working along similar lines . “We have confidence to beat them again in space.”

On Nov. 26, 2017, the South China Morning Post reported that scientists at multiple research institutions were working on a ghost imaging sensor that would work on a satellite. Gong Wenlin, the director of research at the Key Laboratory for Quantum Optics at the Chinese Academy of Sciences in Shanghai, said he expected his team to build a prototype by 2020, test it in space by 2025, and have a finalized, operational design ready by 2030.

Chinese scientists claim they are working towards building an advanced spy satellite that could use something of a physics trick to spot stealth planes from space. This complex "ghost imaging" camera is the latest in a string of reportedly “game changing” developments in China to counter low-observable aircraft , all of which have significant limitations and face serious developmental challenges.

Putting the system on a satellite could impose significant limitations from the start. Many spy satellites by their very nature follow regular orbits and it can take a long to reposition them. This could result in significant expenditure or resources to build a broad constellation covering a wide area. But such a system is more likely to be placed in geostationary orbit, providing persistent coverage over a certain region of the globe. American surveillance satellites, including infrared missile warning systems like SBIRS , use a similar concept.

The same principle could, at least in theory, provide an option for monitoring the movements of stealth aircraft, even under the added cover of darkness, giving extra notice of potential attacks. But as simple as this might sound, turning this concept into a viable sensor is easier said than done.

Presently, the alternatives include synthetic aperture radar and laser imaging, both of which can produce relatively detailed images at long ranges. These systems are both limited to producing still imagery.

One key goal of research into such a system for more widespread field use is to develop optical cameras that can see through clouds, smoke, dust, and other obscuring particles. This is a particular issue for electro-optical imaging satellites, where overcast skies and other environmental factors can easily blur or block the desired view.

In a traditional camera, photons of light will bounce off an object, pass through the lens, and then land on either a strip of physical film or a digital sensor. In experimental ghost imaging cameras, there are two beams of laser light split from a single source emitting particles at the same speed, with one passing over an object before hitting a “bucket” that records the impact. The other beam goes to a regular camera. A computer program looks at both data sets, but only records only the photons that hit both the bucket and the camera at the same time, creating a silhouette or “ghost” of the object in question.

CNN's War In Space special, which you can watch in full below, remains one of the better explainers available on the growing potential for conflict in that domain.

China itself has tested a number of anti-satellite ballistic missiles. These already include physical interceptors, but reports suggest that various countries, including Russia and the United States , are developing small autonomous mini-satellites that could either repair or destroy space-based assets. Electronic warfare and cyber attacks against any associated ground-based infrastructure, especially data links from the satellite to ground control stations and other communications nodes, could slow or halt the distribution of any ghost imagery.

Since a ghost imaging system still relies on optical cameras and light sources, it is possible that an opponent could try and blind or otherwise confuse the sensors with their own laser beams or other photon emitters.

This in turn would delay the ability of command centers, air defense headquarters, or other elements to examine the information and initiate an appropriate response. And these are just the systems we know about. But perhaps most importantly, the underlying science remains largely theoretical. Xiong Jun, a professor of physics who studied quantum optics at Beijing Normal University, told the South China Morning Post that a space-based ghost imaging sensor relying on natural light would need to scan the entire target area in nanoseconds to create an accurate picture. If it used a laser as in typical laboratory setups, the system could need a substantial amount of power to make sure the beams of light could even reach the target area from orbit, he added.

USAF A B-2 bomber seen from almost directly above.

The same issues could apply to a ground-based system using similar physics principles. In September 2016, China Electronics Technology Group Corporation (CETC) claimed it had developed a so-called counter-stealth "quantum radar." A traditional radar emits a beam of electromagnetic energy, which then bounces off the target, registering their position. A quantum radar does the same thing, but with streams of quantum entangled photons, which existing radar absorbent materials and low observable features would not be able to defeat. In theory, the light particles might even be able to record other aspects of the object, including the density of its component materials. Again, even Chinese researchers were skeptical about the practicality of such system outside of a laboratory environment. “[I have] not seen anything like this in an open report,” Ma Xiaosong, a physics professor at Nanjing University, told the South China Morning Post at the time. Though it would not be impossible to build such a system, Ma said basic physics issues could be difficult to scientists and engineers to overcome. The biggest issue is the tendency of photons to break free of their quantum entanglement at long distnace, another physics phonemenon known as "decoherence."

USAF A B-2 bomber.

On top of that, fixed-position, land-based ghost imaging sensors and quantum radars would be a prime target during the opening salvos of an enemy attack. A time-sensitive strike using hypersonic weapons, another area of steady advancement both in China and elsewhere, or even more traditional stealthy cruise missiles could neutralize these defenses before they could come into play. Any ground-based system linked to a networked air defense system would be susceptible to electronic warfare and cyber attacks just like a satellite, too. And many of these same limitations apply to low frequency radars, which the Chinese, along with the Russians and the Iranians, have increasingly touted as anti-stealth tools. The large arrays have limited if any mobility and present largely static targets for stand-off attacks. The long wavelengths may be able to detect the presence of a stealth aircraft within a broad area, but would not necessarily be able to plot a particularly accurate position to make the information useful yet alone provide an engagement quality radar track of the target, either.

Imaginechina via AP China's JY-27A, a low frequency radar it says can readily detect stealth aircraft.

Even with the low frequency modifications they made to their targeting radars, the Serbian air defenders who shot down an F-117 Nighthawk – a stealth design dating to the 1970s – over Serbia in 1999 only claimed they could detect the plane when its bomb bay doors opened. In addition, they insisted that they had intercepted communications transmissions that allowed them to position their surface-to-air missiles in the best possible locations to intercept the aircraft. The B-2 bomber, which Gong, the research director at Key Laboratory for Quantum Optics, told the South China Morning Post, would be a key target of the future ghost imaging system, has a stealth shape and low-observable features that Northrop Grumman devised in the 1980s. As the firm’s top secret B-21 Raider stealth bomber will undoubtedly incorporate new technological advances to reduce the aircraft’s radar, infrared, acoustic, and other signatures even more. Still, “the theory of ghost imaging has been well established and understood,” Xiong, the physics professor at Beijing Normal University, noted to the South China Morning Post. “The speed of application very much depends on the [Chinese] government and the amount of money it’s willing to spend.” But the U.S. Air Force expects to have the first operational B-21 unit ready by the mid-2020s. Gong says that even if everything goes according to his plan, China’s prototype ghost imaging satellite will still be in testing at that point. Contact the author: joe@thedrive.com