Chasing ghost images: Chinese scientists report breakthrough in a quantum camera for satellite use

Mainland scientists have achieved a significant breakthrough which could help them build the first quantum camera for satellites within five years.

In a paper in the latest issue of the Scientific Reports journal, professor Gong Wenlin and Han Shensheng detailed a revolutionary method to achieve high-resolution far-field imaging with a quantum camera, which could help snap images an ordinary lens cannot see, such as objects obscured by clouds or smoke.

This is commonly dubbed “ghost imaging”.

“We have overcome a major hurdle for the quantum camera’s application. Ghost-imaging long distance object is not a dream anymore,” said Gong, quantum optics researcher with the Chinese Academy of Sciences’ Shanghai Institute of Optics and Fine Mechanics.

“The construction of a passive quantum camera for satellite [use] can be finished in five years. I see no problem with it,” he said.

A quantum camera is significantly smaller and lighter than traditional optical cameras. Its sensor has just one pixel, and it has no need for expensive, bulky telescope lenses. The magic lies in quantum physics.

Instead of trying to “see” an object, the quantum camera simply “feels” the rise and fall of photonic waves in quantum fields around the camera.

In a sense, it is like a Buddhist monk in meditation. If some photons hit an object and bounce towards the camera, the quantum fields will be disturbed to create a “ghost image” of the object.

Though scientists had discovered the phenomenon nearly two decades ago, most quantum cameras remain in laboratories.

A main reason holding back their application is their “short sight”. The existing quantum cameras are only capable of making near-field observations, or detecting the fluctuation of quantum fields near the object.

Far field or long-distance observation was difficult to achieve.

As the distance increased, the quantum field “ripples” became sparse and the ghost image was easily lost in a cloud of noises.

In their paper, Gong and Han demonstrated a successful experiment of far fiend ghost imaging.

They reproduced some relatively sharp images of objects using the signal of sparse quantum waves, which in the past only led to a blurred glare.

The breakthrough was a result of “coincidence”, Gong said.

They came across an algorithm called “sparsity constraint” which was often used in digital image compression and found the mathematical model fit perfectly to the physical model of ghost imaging.

“This is a marriage of love between math and physics. The model created by mathematicians in mind found an exact match to a real phenomenon discovered by physicists in the real world,” Gong said.

The researchers were impressed by the results of their experiment. With the help of the algorithm they could make far-field snapshots of a quality almost as good as near-field ones.

The new technology’s application has enormous potential, the scientists say.

“The performance of optical telescopes are determined by their size. The larger the lenses, the more things they can see. But that also makes good telescopes very large and expensive. The quantum camera has no need for a lens and its sensor uses only one pixel, which makes it very attractive for use in space with its compact size and being lightweight,” Gong said.

The technology could also be used on microscopes. Picturing extremely small objects was, in theory, the same as distant stars.

The Shanghai team was a leader in the global race on quantum camera development. In 2013, they produced the world’s first holographic quantum camera that could capture the full dimensions of an object.

There is some work to do before quantum cameras can be launched in space, the scientists said. The algorithm, for instance, still has room for improvement.

Due to its mathematical complexity, the camera takes a few minutes to generate an image, which would be too long for real applications. The team’s goal was to reduce the processing time to a few seconds by refining the mathematical model.

They were also working on some new methods to reduce the impact of negative environments on image quality. The satellite-mounted quantum camera was expected to “see” through cloud and smog.